A general model of focal adhesion orientation dynamics in response to static and cyclic stretch

Focal adhesion and actin orientation regulated by cellular geometry determine stem cell differentiation via mechanotransduction

Tuning cell adhesion geometry can affect cytoskeleton organization and the distribution of cytoskeleton forces, which play critical roles in controlling cell functions. To elucidate the geometrical relationship with cytoskeleton force distribution, it is necessary to control cell morphology. In this study, a series of dextral vortex micropatterns were prepared to precisely control cell morphology for investigating the influence of the curvature degree of adhesion curves on intracellular force distribution and stem cell differentiation at a sub-cellular level. Peripherial actin filaments of micropatterned cells were assembled along the adhesion curves and showed different orientations, filament thicknesses and densities. Focal adhesion and cytoskeleton force distribution were dependent on the curvature degree. Intracellular force distribution was also regulated by adhesion curves. The cytoskeleton and force distribution affected the osteogenic differentiation of mesenchymal stem cells through a YAP/TAZ-mediated mechanotransduction process. Thus, regulation of cell adhesion curvature, especially at cytoskeletal filament level, is critical for cell function manipulation. STATEMENT OF SIGNIFICANCE: In this study, a series of dextral micro-vortexes were prepared and used for the culture of human mesenchymal stem cells (hMSCs) to precisely control adhesive curvatures (0°, 30°, 60°, and 90°). The single MSCs on the micropatterns had the same size and shape but showed distinct focal adhesion (FA) and cytoskeleton orientations. Cellular nanomechanics were observed to be correlated with the curvature degrees, subsequently influencing nuclear morphological features. As a consequence, the localization of the mechanotransduction sensor and activator-YAP/TAZ was affected, influencing osteogenic differentiation. The results revealed the pivotal role of adhesive curvatures in the manipulation of stem cell differentiation via the machanotransduction process, which has rarely been investigated.

Physical biology of cell-substrate interactions under cyclic stretch

Mechanosensitive focal adhesion (FA) complexes mediate dynamic interactions between cells and substrates and regulate cellular function. Integrins in FA complexes link substrate ligands to stress fibers (SFs) and aid load transfer and traction generation. We developed a one-dimensional, multi-scale, stochastic finite element model of a fibroblast on a substrate that includes calcium signaling, SF remodeling, and FA dynamics. We linked stochastic dynamics, describing the formation and clustering of integrins to substrate ligands via motor-clutches, to a continuum level SF contractility model at various locations along the cell length. We quantified changes in cellular responses with substrate stiffness, ligand density, and cyclic stretch. Results show that tractions and integrin recruitments varied along the cell length; tractions were maximum at lamellar regions and reduced to zero at the cell center. Optimal substrate stiffness, based on maximum tractions exerted by the cell, shifted toward stiffer substrates at high ligand densities. Mean tractions varied biphasically with substrate stiffness and peaked at the optimal substrate stiffness. Cytosolic calcium increased monotonically with substrate stiffness and accumulated near lamellipodial regions. Cyclic stretch increased the cytosolic calcium, integrin concentrations, and tractions at lamellipodial and intermediate regions on compliant substrates. The optimal substrate stiffness under stretch shifted toward compliant substrates for a given ligand density. Stretch also caused cell deadhesions beyond a critical substrate stiffness. FA's destabilized on stiff substrates under cyclic stretch. An increase in substrate stiffness and cyclic stretch resulted in higher fibroblast contractility. These results show that chemomechanical coupling is essential in mechanosensing responses underlying cell-substrate interactions.

Cell orientation under stretch: A review of experimental findings and mathematical modelling

The key role of electro-chemical signals in cellular processes had been known for many years, but more recently the interplay with mechanics has been put in evidence and attracted substantial research interests. Indeed, the sensitivity of cells to mechanical stimuli coming from the microenvironment turns out to be relevant in many biological and physiological circumstances. In particular, experimental evidence demonstrated that cells on elastic planar substrates undergoing periodic stretches, mimicking native cyclic strains in the tissue where they reside, actively reorient their cytoskeletal stress fibres. At the end of the realignment process, the cell axis forms a certain angle with the main stretching direction. Due to the importance of a deeper understanding of mechanotransduction, such a phenomenon was studied both from the experimental and the mathematical modelling point of view. The aim of this review is to collect and discuss both the experimental results on cell reorientation and the fundamental features of the mathematical models that have been proposed in the literature.

The effect of cyclic stretch on aortic viscoelasticity and the putative role of smooth muscle focal adhesion

Due to its viscoelastic properties, the aorta aids in dampening blood pressure pulsatility. At the level of resistance-arteries, the pulsatile flow will be transformed into a continuous flow to allow for optimal perfusion of end organs such as the kidneys and the brain. In this study, we investigated the ex vivo viscoelastic properties of different regions of the aorta of healthy C57Bl6/J adult mice as well as the interplay between (altered) cyclic stretch and viscoelasticity. We demonstrated that the viscoelastic parameters increase along the distal aorta and that the effect of altered cyclic stretch is region dependent. Increased cyclic stretch, either by increased pulse pressure or pulse frequency, resulted in decreased aortic viscoelasticity. Furthermore, we identified that the vascular smooth muscle cell (VSMC) is an important modulator of viscoelasticity, as we have shown that VSMC contraction increases viscoelastic parameters by, in part, increasing elastin fiber tortuosity. Interestingly, an acute increase in stretch amplitude reverted the changes in viscoelastic properties induced by VSMC contraction, such as a decreasing contraction-induced elastin fiber tortuosity. Finally, the effects of altered cyclic stretch and VSMC contraction on viscoelasticity were more pronounced in the abdominal infrarenal aorta, compared to both the thoracic ascending and descending aorta, and were attributed to the activity and stability of VSMC focal adhesion. Our results indicate that cyclic stretch is a modulator of aortic viscoelasticity, acting on VSMC focal adhesion. Conditions of (acute) changes in cyclic stretch amplitude and/or frequency, such as physical exercise or hypertension, can alter the viscoelastic properties of the aorta.

Sonomechanobiology: Vibrational stimulation of cells and its therapeutic implications

All cells possess an innate ability to respond to a range of mechanical stimuli through their complex internal machinery. This comprises various mechanosensory elements that detect these mechanical cues and diverse cytoskeletal structures that transmit the force to different parts of the cell, where they are transcribed into complex transcriptomic and signaling events that determine their response and fate. In contrast to static (or steady) mechanostimuli primarily involving constant-force loading such as compression, tension, and shear (or forces applied at very low oscillatory frequencies (≤ 1 Hz) that essentially render their effects quasi-static), dynamic mechanostimuli comprising more complex vibrational forms (e.g., time-dependent, i.e., periodic, forcing) at higher frequencies are less well understood in comparison. We review the mechanotransductive processes associated with such acoustic forcing, typically at ultrasonic frequencies (> 20 kHz), and discuss the various applications that arise from the cellular responses that are generated, particularly for regenerative therapeutics, such as exosome biogenesis, stem cell differentiation, and endothelial barrier modulation. Finally, we offer perspectives on the possible existence of a universal mechanism that is common across all forms of acoustically driven mechanostimuli that underscores the central role of the cell membrane as the key effector, and calcium as the dominant second messenger, in the mechanotransduction process.

A Statistical Mechanics Approach to Describe Cell Reorientation Under Stretch

Experiments show that when a monolayer of cells cultured on an elastic substratum is subject to a cyclic stretch, cells tend to reorient either perpendicularly or at an oblique angle with respect to the main stretching direction. Due to stochastic effects, however, the distribution of angles achieved by the cells is broader and, experimentally, histograms over the interval [Formula: see text] are usually reported. Here we will determine the evolution and the stationary state of probability density functions describing the statistical distribution of the orientations of the cells using Fokker-Planck equations derived from microscopic rules for describing the reorientation process of the cell. As a first attempt, we shall use a stochastic differential equation related to a very general elastic energy that the cell tries to minimize and, we will show that the results of the time integration and of the stationary state of the related forward Fokker-Planck equation compare very well with experimental results obtained by different researchers. Then, in order to model more accurately the microscopic process of cell reorientation and to shed light on the mechanisms performed by cells that are subject to cyclic stretch, we consider discrete in time random processes that allow to recover Fokker-Planck equations through classical tools of kinetic theory. In particular, we shall introduce a model of reorientation as a function of the rotation angle as a result of an optimal control problem. Also in this latter case the results match very well with experiments.

Zyxin and actin structure confer anisotropic YAP mechanotransduction

Tissues and the embedded cells experience anisotropic deformations due to their functions and anatomical locations. The resident cells, such as tenocytes and muscle cells, are often restricted by their extracellular matrix and organize parallel to their major loading direction, yet most studies on cellular responses to strains use isotropic substrates without predetermined organizations. To understand how confined cells sense and respond to anisotropic loading, we combine cell patterning and uniaxial stretch to have precise geometric control. Dynamic stretch parallel to the long axis of the cell activates YAP nuclear translocation, but not when stretched in the perpendicular direction. Looking at the initial cytoskeleton response, parallel stretch leads to actin breakage and repair within the first minute, mediated by zyxin, the focal adhesion protein. In addition, this zyxin-mediated repair response is controlled by focal adhesion kinase (FAK) and leads to YAP signaling. As these factors are intimately involved in a wide range of mechanical regulation, our findings point to new roles of zyxin and YAP in anisotropic mechanotransduction, and may provide additional perspectives in cellular adaptive responses and tissue homeostasis. STATEMENT OF SIGNIFICANCE: Structure and deformation of tissues control gene expression, migration, and proliferation of the resident cells. In an effort to understand the underlying mechanisms, we find that the transcription cofactor YAP respond to mechanical stretch in a direction-dependent manner. We demonstrate that parallel stretch induces actin cytoskeleton damage, focal adhesion kinase (FAK) activation, and zyxin relocation, which are involved in the anisotropic YAP signaling. Our findings provide additional perspectives in the interactions of tissue structure and cell mechanotransduction.

Growth kinetics and power laws indicate distinct mechanisms of cell-cell interactions in the aggregation process

Cellular aggregation is a complex process orchestrated by various kinds of interactions depending on the environment. Different interactions give rise to different pathways of cellular rearrangement and the development of specialized tissues. To distinguish the underlying mechanisms, in this theoretical work, we investigate the spontaneous emergence of tissue patterns from an ensemble of single cells on a substrate following three leading pathways of cell-cell interactions, namely, direct cell adhesion contacts, matrix-mediated mechanical interaction, and chemical signaling. Our analysis shows that the growth kinetics of the aggregation process are distinctly different for each pathway and bear the signature of the specific cell-cell interactions. Interestingly, we find that the average domain size and the mass of the clusters exhibit a power law growth in time under certain interaction mechanisms hitherto unexplored. Further, as observed in experiments, the cluster size distribution can be characterized by stretched exponential functions showing distinct cellular organization processes.

Effect of micro- and nanoparticle shape on biological processes

In the drug delivery field, there is beyond doubt that the shape of micro- and nanoparticles (M&NPs) critically affects their biological fate. Herein, following an introduction describing recent technological advances for designing nonspherical M&NPs, we highlight the role of particle shape in cell capture, subcellular distribution, intracellular drug delivery, and cytotoxicity. Then, we discuss theoretical approaches for understanding the effect of particle shape on internalization by the cell membrane. Subsequently, recent advances on shape-dependent behaviors of M&NPs in the systemic circulation are detailed. In particular, the interaction of M&NPs with blood proteins, biodistribution, and circulation under flow conditions are analyzed. Finally, the hurdles and future directions for developing nonspherical M&NPs are underscored.

High Pulsatile Load Decreases Arterial Stiffness: An ex vivo Study

Measuring arterial stiffness has recently gained a lot of interest because it is a strong predictor for cardiovascular events and all-cause mortality. However, assessing blood vessel stiffness is not easy and the in vivo measurements currently used provide only limited information. Ex vivo experiments allow for a more thorough investigation of (altered) arterial biomechanical properties. Such experiments can be performed either statically or dynamically, where the latter better corresponds to physiological conditions. In a dynamic setup, arterial segments oscillate between two predefined forces, mimicking the diastolic and systolic pressures from an in vivo setting. Consequently, these oscillations result in a pulsatile load (i.e., the pulse pressure). The importance of pulse pressure on the ex vivo measurement of arterial stiffness is not completely understood. Here, we demonstrate that pulsatile load modulates the overall stiffness of the aortic tissue in an ex vivo setup. More specifically, increasing pulsatile load softens the aortic tissue. Moreover, vascular smooth muscle cell (VSMC) function was affected by pulse pressure. VSMC contraction and basal tonus showed a dependence on the amplitude of the applied pulse pressure. In addition, two distinct regions of the aorta, namely the thoracic descending aorta (TDA) and the abdominal infrarenal aorta (AIA), responded differently to changes in pulse pressure. Our data indicate that pulse pressure alters ex vivo measurements of arterial stiffness and should be considered as an important variable in future experiments. More research should be conducted in order to determine which biomechanical properties are affected due to changes in pulse pressure. The elucidation of the underlying pulse pressure-sensitive properties would improve our understanding of blood vessel biomechanics and could potentially yield new therapeutic insights.

Efficient flocking: metric versus topological interactions

Flocking is a fascinating phenomenon observed across a wide range of living organisms. We investigate, based on a simple self-propelled particle model, how the emergence of ordered motion in a collectively moving group is influenced by the local rules of interactions among the individuals, namely, metric versus topological interactions as debated in the current literature. In the case of the metric ruling, the individuals interact with the neighbours within a certain metric distance; by contrast, in the topological ruling, interaction is confined within a number of fixed nearest neighbours. Here, we explore how the range of interaction versus the number of fixed interacting neighbours affects the dynamics of flocking in an unbounded space, as observed in natural scenarios. Our study reveals the existence of a certain threshold value of the interaction radius in the case of metric ruling and a threshold number of interacting neighbours for the topological ruling to reach an ordered state. Interestingly, our analysis shows that topological interaction is more effective in bringing the order in the group, as observed in field studies. We further compare how the nature of the interactions affects the dynamics for various sizes and speeds of the flock.

Drawing Inspiration from Developmental Biology for Cardiac Tissue Engineers

A sound understanding of developmental biology is part of the foundation of effective stem cell-derived tissue engineering. Here, the key concepts of cardiac development that are successfully applied in a bioinspired approach to growing engineered cardiac tissues, are reviewed. The native cardiac milieu is studied extensively from embryonic to adult phenotypes, as it provides a resource of factors, mechanisms, and protocols to consider when working toward establishing living tissues in vitro. It begins with the various cell types that constitute the cardiac tissue. It is discussed how myocytes interact with other cell types and their microenvironment and how they change over time from the embryonic to the adult states, with a view on how such changes affect the tissue function and may be used in engineered tissue models. Key embryonic signaling pathways that have been leveraged in the design of culture media and differentiation protocols are presented. The cellular microenvironment, from extracellular matrix chemical and physical properties, to the dynamic mechanical and electrical forces that are exerted on tissues is explored. It is shown that how such microenvironmental factors can inform the design of biomaterials, scaffolds, stimulation bioreactors, and maturation readouts, and suggest considerations for ongoing biomimetic advancement of engineered cardiac tissues and regeneration strategies for the future.

Cell orientation under stretch: Stability of a linear viscoelastic model

The sensitivity of cells to alterations in the microenvironment and in particular to external mechanical stimuli is significant in many biological and physiological circumstances. In this regard, experimental assays demonstrated that, when a monolayer of cells cultured on an elastic substrate is subject to an external cyclic stretch with a sufficiently high frequency, a reorganization of actin stress fibres and focal adhesions happens in order to reach a stable equilibrium orientation, characterized by a precise angle between the cell major axis and the largest strain direction. To examine the frequency effect on the orientation dynamics, we propose a linear viscoelastic model that describes the coupled evolution of the cellular stress and the orientation angle. We find that cell orientation oscillates tending to an angle that is predicted by the minimization of a very general orthotropic elastic energy, as confirmed by a bifurcation analysis. Moreover, simulations show that the speed of convergence towards the predicted equilibrium orientation presents a changeover related to the viscous-elastic transition for viscoelastic materials. In particular, when the imposed oscillation period is lower than the characteristic turnover rate of the cytoskeleton and of adhesion molecules such as integrins, reorientation is significantly faster.

Materials for blood brain barrier modeling in vitro

Brain homeostasis relies on the selective permeability property of the blood brain barrier (BBB). The BBB is formed by a continuous endothelium that regulates exchange between the blood stream and the brain. This physiological barrier also creates a challenge for the treatment of neurological diseases as it prevents most blood circulating drugs from entering into the brain. In vitro cell models aim to reproduce BBB functionality and predict the passage of active compounds through the barrier. In such systems, brain microvascular endothelial cells (BMECs) are cultured in contact with various biomaterial substrates. However, BMEC interactions with these biomaterials and their impact on BBB functions are poorly described in the literature. Here we review the most common materials used to culture BMECs and discuss their potential impact on BBB integrity in vitro. We investigate the biophysical properties of these biomaterials including stiffness, porosity and material degradability. We highlight a range of synthetic and natural materials and present three categories of cell culture dimensions: cell monolayers covering non-degradable materials (2D), cell monolayers covering degradable materials (2.5D) and vascularized systems developing into degradable materials (3D).

Stick-slip dynamics of migrating cells on viscoelastic substrates

Stick-slip motion, a common phenomenon observed during crawling of cells, is found to be strongly sensitive to the substrate stiffness. Stick-slip behaviors have previously been investigated typically using purely elastic substrates. For a more realistic understanding of this phenomenon, we propose a theoretical model to study the dynamics on a viscoelastic substrate. Our model, based on a reaction-diffusion framework, incorporates known important interactions such as retrograde flow of actin, myosin contractility, force-dependent assembly, and disassembly of focal adhesions coupled with cell-substrate interaction. We show that consideration of a viscoelastic substrate not only captures the usually observed stick-slip jumps but also predicts the existence of an optimal substrate viscosity corresponding to maximum traction force and minimum retrograde flow which was hitherto unexplored. Moreover, our theory predicts the time evolution of individual bond force that characterizes the stick-slip patterns on soft versus stiff substrates. Our analysis also elucidates how the duration of the stick-slip cycles are affected by various cellular parameters.

Pulmonary Endothelial Mechanical Sensing and Signaling, a Story of Focal Adhesions and Integrins in Ventilator Induced Lung Injury

Patients with critical illness such as acute lung injury often undergo mechanical ventilation in the intensive care unit. Though lifesaving in many instances, mechanical ventilation often results in ventilator induced lung injury (VILI), characterized by overdistension of lung tissue leading to release of edemagenic agents, which further damage the lung and contribute to the mortality and progression of pulmonary inflammation. The endothelium is particularly sensitive, as VILI associated mechanical stress results in endothelial cytoskeletal rearrangement, stress fiber formation, and integrity loss. At the heart of these changes are integrin tethered focal adhesions (FAs) which participate in mechanosensing, structure, and signaling. Here, we present the known roles of FA proteins including c-Src, talin, FAK, paxillin, vinculin, and integrins in the sensing and response to cyclic stretch and VILI associated stress. Attention is given to how stretch is propagated from the extracellular matrix through integrins to talin and other FA proteins, as well as signaling cascades that include FA proteins, leading to stress fiber formation and other cellular responses. This unifying picture of FAs aids our understanding in an effort to prevent and treat VILI.