Lipid droplets are intracellular mechanical stressors that impair hepatocyte function.

Matrix stiffening and external mechanical stress have been linked to disease and cancer development in multiple tissues, including the liver, where cirrhosis (which increases stiffness markedly) is the major risk factor for hepatocellular carcinoma. Patients with nonalcoholic fatty liver disease and lipid droplet-filled hepatocytes, however, can develop cancer in noncirrhotic, relatively soft tissue. Here, by treating primary human hepatocytes with the monounsaturated fatty acid oleate, we show that lipid droplets are intracellular mechanical stressors with similar effects to tissue stiffening, including nuclear deformation, chromatin condensation, and impaired hepatocyte function. Mathematical modeling of lipid droplets as inclusions that have only mechanical interactions with other cellular components generated results consistent with our experiments. These data show that lipid droplets are intracellular sources of mechanical stress and suggest that nuclear membrane tension integrates cell responses to combined internal and external stresses.

Regulation of nuclear architecture, mechanics, and nucleocytoplasmic shuttling of epigenetic factors by cell geometric constraints.

Cells sense mechanical signals from their microenvironment and transduce them to the nucleus to regulate gene expression programs. To elucidate the physical mechanisms involved in this regulation, we developed an active 3D chemomechanical model to describe the three-way feedback between the adhesions, the cytoskeleton, and the nucleus. The model shows local tensile stresses generated at the interface of the cell and the extracellular matrix regulate the properties of the nucleus, including nuclear morphology, levels of lamin A,C, and histone deacetylation, as these tensile stresses 1) are transmitted to the nucleus through cytoskeletal physical links and 2) trigger an actomyosin-dependent shuttling of epigenetic factors. We then show how cell geometric constraints affect the local tensile stresses and subsequently the three-way feedback and induce cytoskeleton-mediated alterations in the properties of the nucleus such as nuclear lamina softening, chromatin stiffening, nuclear lamina invaginations, increase in nuclear height, and shrinkage of nuclear volume. We predict a phase diagram that describes how the disruption of cytoskeletal components impacts the feedback and subsequently induce contractility-dependent alterations in the properties of the nucleus. Our simulations show that these changes in contractility levels can be also used as predictors of nucleocytoplasmic shuttling of transcription factors and the level of chromatin condensation. The predictions are experimentally validated by studying the properties of nuclei of fibroblasts on micropatterned substrates with different shapes and areas.

Correction: Long-range mechanical signaling in biological systems.

Correction for 'Long-range mechanical signaling in biological systems' by Farid Alisafaei et al., Soft Matter, 2020, DOI: 10.1039/d0sm01442g.

Long-range mechanical signaling in biological systems.

Cells can respond to signals generated by other cells that are remarkably far away. Studies from at least the 1920's showed that cells move toward each other when the distance between them is on the order of a millimeter, which is many times the cell diameter. Chemical signals generated by molecules diffusing from the cell surface would move too slowly and dissipate too fast to account for these effects, suggesting that they might be physical rather than biochemical. The non-linear elastic responses of sparsely connected networks of stiff or semiflexible filament such as those that form the extracellular matrix (ECM) and the cytoskeleton have unusual properties that suggest multiple mechanisms for long-range signaling in biological tissues. These include not only direct force transmission, but also highly non-uniform local deformations, and force-generated changes in fiber alignment and density. Defining how fibrous networks respond to cell-generated forces can help design new methods to characterize abnormal tissues and can guide development of improved biomimetic materials.

Mechanisms of Local Stress Amplification in Axons near the Gray-White Matter Interface.

Diffuse axonal injury is a primary neuropathological feature of concussion and is thought to greatly contribute to the classical symptoms of decreased processing speed and memory dysfunction. Although previous studies have investigated the injury biomechanics at the micro- and mesoscale of concussion, few have addressed the multiscale transmission of mechanical loading at thresholds that can induce diffuse axonal injury. Because it has been recognized that axonal pathology is commonly found at anatomic interfaces across all severities of traumatic brain injury, we combined computational, analytical, and experimental approaches to investigate the potential mechanical vulnerability of axons that span the gray-white tissue interface. Our computational models predict that material heterogeneities at the gray-white interface lead to a highly nonuniform distribution of stress in axons, which was most amplified in axonal regions near the interface. This mechanism was confirmed using an analytical model of an individual fiber in a strained bimaterial interface. Comparisons of these collective data with histopathological evaluation of a swine model of concussion demonstrated a notably similar pattern of axonal damage adjacent to the gray-white interface. The results suggest that the tissue property mismatch at the gray-white matter interface places axons crossing this region at greater risk of mechanical damage during brain tissue deformation from traumatic brain injury.

Nuclear Mechanics within Intact Cells Is Regulated by Cytoskeletal Network and Internal Nanostructures.

The mechanical properties of the cellular nucleus are extensively studied as they play a critical role in important processes, such as cell migration, gene transcription, and stem cell differentiation. While the mechanical properties of the isolated nucleus have been tested, there is a lack of measurements about the mechanical behavior of the nucleus within intact cells and specifically about the interplay of internal nuclear components with the intracellular microenvironment, because current testing methods are based on contact and only allow studying the nucleus after isolation from a cell or disruption of cytoskeleton. Here, all-optical Brillouin microscopy and 3D chemomechanical modeling are used to investigate the regulation of nuclear mechanics in physiological conditions. It is observed that the nuclear modulus can be modulated by epigenetic regulation targeting internal nuclear nanostructures such as lamin A/C and chromatin. It is also found that nuclear modulus is strongly regulated by cytoskeletal behavior through a robust mechanism conserved in different culturing conditions. Given the active role of cytoskeletal modulation in nearly all cell functions, this work will enable to reveal highly relevant mechanisms of nuclear mechanical regulations in physiological and pathological conditions.

Fibrous nonlinear elasticity enables positive mechanical feedback between cells and ECMs.

In native states, animal cells of many types are supported by a fibrous network that forms the main structural component of the ECM. Mechanical interactions between cells and the 3D ECM critically regulate cell function, including growth and migration. However, the physical mechanism that governs the cell interaction with fibrous 3D ECM is still not known. In this article, we present single-cell traction force measurements using breast tumor cells embedded within 3D collagen matrices. We recreate the breast tumor mechanical environment by controlling the microstructure and density of type I collagen matrices. Our results reveal a positive mechanical feedback loop: cells pulling on collagen locally align and stiffen the matrix, and stiffer matrices, in return, promote greater cell force generation and a stiffer cell body. Furthermore, cell force transmission distance increases with the degree of strain-induced fiber alignment and stiffening of the collagen matrices. These findings highlight the importance of the nonlinear elasticity of fibrous matrices in regulating cell-ECM interactions within a 3D context, and the cell force regulation principle that we uncover may contribute to the rapid mechanical tissue stiffening occurring in many diseases, including cancer and fibrosis.

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.

An ex vivo culture model of kidney podocyte injury reveals mechanosensitive, synaptopodin-templating, sarcomere-like structures.

Chronic kidney diseases are widespread and incurable. The biophysical mechanisms underlying them are unclear, in part because material systems for reconstituting the microenvironment of relevant kidney cells are limited. A critical question is how kidney podocytes (glomerular epithelial cells) regenerate foot processes of the filtration apparatus following injury. Recently identified sarcomere-like structures (SLSs) with periodically spaced myosin IIA and synaptopodin appear in injured podocytes in vivo. We hypothesized that SLSs template synaptopodin in the initial stages of recovery in response to microenvironmental stimuli and tested this hypothesis by developing an ex vivo culture system that allows control of the podocyte microenvironment. Results supported our hypothesis. SLSs in podocytes that migrated from isolated kidney glomeruli presented periodic synaptopodin-positive clusters that nucleated peripheral, foot process-like extensions. SLSs were mechanoresponsive to actomyosin inhibitors and substrate stiffness. Results suggest SLSs as mechanobiological mediators of podocyte recovery and as potential targets for therapeutic intervention.

The nuclear piston activates mechanosensitive ion channels to generate cell migration paths in confining microenvironments.

Cell migration in confining microenvironments is limited by the ability of the stiff nucleus to deform through pores when migration paths are preexisting and elastic, but how cells generate these paths remains unclear. Here, we reveal a mechanism by which the nucleus mechanically generates migration paths for mesenchymal stem cells (MSCs) in confining microenvironments. MSCs migrate robustly in nanoporous, confining hydrogels that are viscoelastic and plastic but not in hydrogels that are more elastic. To migrate, MSCs first extend thin protrusions that widen over time because of a nuclear piston, thus opening up a migration path in a confining matrix. Theoretical modeling and experiments indicate that the nucleus pushing into the protrusion activates mechanosensitive ion channels, leading to an influx of ions that increases osmotic pressure, which outcompetes hydrostatic pressure to drive protrusion expansion. Thus, instead of limiting migration, the nucleus powers migration by generating migration paths.

Surface-directed engineering of tissue anisotropy in microphysiological models of musculoskeletal tissue.

Here, we present an approach to model and adapt the mechanical regulation of morphogenesis that uses contractile cells as sculptors of engineered tissue anisotropy in vitro. Our method uses heterobifunctional cross-linkers to create mechanical boundary constraints that guide surface-directed sculpting of cell-laden extracellular matrix hydrogel constructs. Using this approach, we engineered linearly aligned tissues with structural and mechanical anisotropy. A multiscale in silico model of the sculpting process was developed to reveal that cell contractility increases as a function of principal stress polarization in anisotropic tissues. We also show that the anisotropic biophysical microenvironment of linearly aligned tissues potentiates soluble factor-mediated tenogenic and myogenic differentiation of mesenchymal stem cells. The application of our method is demonstrated by (i) skeletal muscle arrays to screen therapeutic modulators of acute oxidative injury and (ii) a 3D microphysiological model of lung cancer cachexia to study inflammatory and oxidative muscle injury induced by tumor-derived signals.

The Balance between Actomyosin Contractility and Microtubule Polymerization Regulates Hierarchical Protrusions That Govern Efficient Fibroblast-Collagen Interactions.

Fibroblasts undergo a critical transformation from an initially inactive state to a morphologically different and contractile state after several hours of being embedded within a physiologically relevant three-dimensional (3D) fibrous collagen-based extracellular matrix (ECM). However, little is known about the critical mechanisms by which fibroblasts adapt themselves and their microenvironment in the earliest stage of cell-matrix interaction. Here, we identified the mechanisms by which fibroblasts interact with their 3D collagen fibrous matrices in the early stages of cell-matrix interaction and showed that fibroblasts use energetically efficient hierarchical micro/nano-scaled protrusions in these stages as the primary means for the transformation and adaptation. We found that actomyosin contractility in these protrusions in the early stages of cell-matrix interaction restricts the growth of microtubules by applying compressive forces on them. Our results show that actomyosin contractility and microtubules work in concert in the early stages of cell-matrix interaction to adapt fibroblasts and their microenvironment to one another. These early stage interactions result in responses to disruption of the microtubule network and/or actomyosin contractility that are opposite to well-known responses to late-stage disruption and reveal insight into the ways that cells adapt themselves and their ECM recursively.

Multiscale reverse engineering of the human ocular surface.

Here we present a miniaturized analog of a blinking human eye to reverse engineer the complexity of the interface between the ocular system and the external environment. Our model comprises human cells and provides unique capabilities to replicate multiscale structural organization, biological phenotypes and dynamically regulated environmental homeostasis of the human ocular surface. Using this biomimetic system, we discovered new biological effects of blink-induced mechanical forces. Furthermore, we developed a specialized in vitro model of evaporative dry-eye disease for high-content drug screening. This work advances our ability to emulate how human physiological systems interface with the external world, and may contribute to the future development of novel screening platforms for biopharmaceutical and environmental applications.

Compressive force induces reversible chromatin condensation and cell geometry-dependent transcriptional response.

Fibroblasts exhibit heterogeneous cell geometries in tissues and integrate both mechanical and biochemical signals in their local microenvironment to regulate genomic programs via chromatin remodelling. While in connective tissues fibroblasts experience tensile and compressive forces (CFs), the role of compressive forces in regulating cell behavior and, in particular, the impact of cell geometry in modulating transcriptional response to such extrinsic mechanical forces is unclear. Here we show that CF on geometrically well-defined mouse fibroblast cells reduces actomyosin contractility and shuttles histone deacetylase 3 (HDAC3) into the nucleus. HDAC3 then triggers an increase in the heterochromatin content by initiating removal of acetylation marks on the histone tails. This suggests that, in response to CF, fibroblasts condense their chromatin and enter into a transcriptionally less active and quiescent states as also revealed by transcriptome analysis. On removal of CF, the alteration in chromatin condensation was reversed. We also present a quantitative model linking CF-dependent changes in actomyosin contractility leading to chromatin condensation. Further, transcriptome analysis also revealed that the transcriptional response of cells to CF was geometry dependent. Collectively, our results suggest that CFs induce chromatin condensation and geometry-dependent differential transcriptional response in fibroblasts that allows maintenance of tissue homeostasis.