The role of the dynamics of focal adhesion kinase in the mechanotaxis of endothelial cells

FAK regulates tension transmission to the nucleus and endothelial transcriptome independent of kinase activity

The mechanical environment generated through the adhesive interaction of endothelial cells (ECs) with the matrix controls nuclear tension, preventing aberrant gene synthesis and the transition from restrictive to leaky endothelium, a hallmark of acute lung injury (ALI). However, the mechanisms controlling tension transmission to the nucleus and EC-restrictive fate remain elusive. Here, we demonstrate that, in a kinase-independent manner, focal adhesion kinase (FAK) safeguards tension transmission to the nucleus to maintain EC-restrictive fate. In FAK-depleted ECs, robust activation of the RhoA-Rho-kinase pathway increased EC tension and phosphorylation of the nuclear envelope protein, emerin, activating DNMT3a. Activated DNMT3a methylates the KLF2 promoter, impairing the synthesis of KLF2 and its target S1PR1 to induce the leaky EC transcriptome. Repleting FAK (wild type or kinase dead) or inhibiting RhoA-emerin-DNMT3a activities in damaged lung ECs restored KLF2 transcription of the restrictive EC transcriptome. Thus, FAK sensing and control of tension transmission to the nucleus govern restrictive endothelium to maintain lung homeostasis.

A non local model for cell migration in response to mechanical stimuli

Cell migration is one of the most studied phenomena in biology since it plays a fundamental role in many physiological and pathological processes such as morphogenesis, wound healing and tumorigenesis. In recent years, researchers have performed experiments showing that cells can migrate in response to mechanical stimuli of the substrate they adhere to. Motion towards regions of the substrate with higher stiffness is called durotaxis, while motion guided by the stress or the deformation of the substrate itself is called tensotaxis. Unlike chemotaxis (i.e. the motion in response to a chemical stimulus), these migratory processes are not yet fully understood from a biological point of view. In this respect, we present a mathematical model of single-cell migration in response to mechanical stimuli, in order to simulate these two processes. Specifically, the cell moves by changing its direction of polarization and its motility according to material properties of the substrate (e.g., stiffness) or in response to proper scalar measures of the substrate strain or stress. The equations of motion of the cell are non-local integro-differential equations, with the addition of a stochastic term to account for random Brownian motion. The mechanical stimulus to be integrated in the equations of motion is defined according to experimental measurements found in literature, in the case of durotaxis. Conversely, in the case of tensotaxis, substrate strain and stress are given by the solution of the mechanical problem, assuming that the extracellular matrix behaves as a hyperelastic Yeoh's solid. In both cases, the proposed model is validated through numerical simulations that qualitatively reproduce different experimental scenarios.

Impact of disturbed flow and arterial stiffening on mechanotransduction in endothelial cells

Disturbed flow promotes progression of atherosclerosis at particular regions of arteries where the recent studies show the arterial wall becomes stiffer. Objective of this study is to show how mechanotransduction in subcellular organelles of endothelial cells (ECs) will alter with changes in blood flow profiles applied on ECs surface and mechanical properties of arterial wall where ECs are attached to. We will examine the exposure of ECs to atherogenic flow profiles (disturbed flow) and non-atherogenic flow profiles (purely forward flow), while stiffness and viscoelasticity of arterial wall will change. A multicomponent model of endothelial cell monolayer was applied to quantify the response of subcellular organelles to the changes in their microenvironment. Our results show that arterial stiffening alters mechanotransduction in intra/inter-cellular organelles of ECs by slight increase in the transmitted stresses, particularly over central stress fibers (SFs). We also observed that degradation of glycocalyx and exposure to non-atherogenic flow profiles result in significantly higher stresses in subcellular organelles, while degradation of glycocalyx and exposure to atherogenic flow profiles result in dramatically lower stresses in the organelles. Moreover, we show that increasing the arterial wall viscoelasticity leads to slight increase in the stresses transmitted to subcellular organelles. FAs are particularly influenced with the changes in the arterial wall properties and viscoelasticity. Our study suggests that changes in viscoelasticity of arterial wall and degradation state of glycocalyx have to be considered along with arterial stiffening in designing more efficient treatment strategies for atherosclerosis. Our study provides insight into significant role of mechanotransduction in the localization of atherosclerosis by quantifying the role of ECs mechanosensors and suggests that mechanotransduction may play a key role in design of more efficient and precision therapeutics to slow down or block the progression of atherosclerosis.

The microenvironment of the atheroma expresses phenotypes of plaque instability

Data from histopathology studies of human atherosclerotic tissue specimens and from vascular imaging studies support the concept that the local arterial microenvironment of a stable atheroma promotes destabilizing conditions that result in the transition to an unstable atheroma. Destabilization is characterized by several different plaque phenotypes that cause major clinical events such as acute coronary syndrome and cerebrovascular strokes. There are several rupture-associated phenotypes causing thrombotic vascular occlusion including simple fibrous cap rupture of an atheroma, fibrous cap rupture at site of previous rupture-and-repair of an atheroma, and nodular calcification with rupture. Endothelial erosion without rupture has more recently been shown to be a common phenotype to promote thrombosis as well. Microenvironment features that are linked to these phenotypes of plaque instability are neovascularization arising from the vasa vasorum network leading to necrotic core expansion, intraplaque hemorrhage, and cap rupture; activation of adventitial and perivascular adipose tissue cells leading to secretion of cytokines, growth factors, adipokines in the outer artery wall that destabilize plaque structure; and vascular smooth muscle cell phenotypic switching through transdifferentiation and stem/progenitor cell activation resulting in the promotion of inflammation, calcification, and secretion of extracellular matrix, altering fibrous cap structure, and necrotic core growth. As the technology evolves, studies using noninvasive vascular imaging will be able to investigate the transition of stable to unstable atheromas in real time. A limitation in the field, however, is that reliable and predictable experimental models of spontaneous plaque rupture and/or erosion are not currently available to study the cell and molecular mechanisms that regulate the conversion of the stable atheroma to an unstable plaque.

Vascular mechanotransduction

This review aims to survey the current state of mechanotransduction in vascular smooth muscle cells (VSMCs) and endothelial cells (ECs), including their sensing of mechanical stimuli and transduction of mechanical signals that result in the acute functional modulation and longer-term transcriptomic and epigenetic regulation of blood vessels. The mechanosensors discussed include ion channels, plasma membrane-associated structures and receptors, and junction proteins. The mechanosignaling pathways presented include the cytoskeleton, integrins, extracellular matrix, and intracellular signaling molecules. These are followed by discussions on mechanical regulation of transcriptome and epigenetics, relevance of mechanotransduction to health and disease, and interactions between VSMCs and ECs. Throughout this review, we offer suggestions for specific topics that require further understanding. In the closing section on conclusions and perspectives, we summarize what is known and point out the need to treat the vasculature as a system, including not only VSMCs and ECs but also the extracellular matrix and other types of cells such as resident macrophages and pericytes, so that we can fully understand the physiology and pathophysiology of the blood vessel as a whole, thus enhancing the comprehension, diagnosis, treatment, and prevention of vascular diseases.

Collective Behaviors of Active Matter Learning from Natural Taxes Across Scales

Taxis orientation is common in microorganisms, and it provides feasible strategies to operate active colloids as small-scale robots. Collective taxes involve numerous units that collectively perform taxis motion, whereby the collective cooperation between individuals enables the group to perform efficiently, adaptively, and robustly. Hence, analyzing and designing collectives is crucial for developing and advancing microswarm toward practical or clinical applications. In this review, natural taxis behaviors are categorized and synthetic microrobotic collectives are discussed as bio-inspired realizations, aiming at closing the gap between taxis strategies of living creatures and those of functional active microswarms. As collective behaviors emerge within a group, the global taxis to external stimuli guides the group to conduct overall tasks, whereas the local taxis between individuals induces synchronization and global patterns. By encoding the local orientations and programming the global stimuli, various paradigms can be introduced for coordinating and controlling such collective microrobots, from the viewpoints of fundamental science and practical applications. Therefore, by discussing the key points and difficulties associated with collective taxes of different paradigms, this review potentially offers insights into mimicking natural collective behaviors and constructing intelligent microrobotic systems for on-demand control and preassigned tasks.

ZINC40099027 promotes monolayer circular defect closure by a novel pathway involving cytosolic activation of focal adhesion kinase and downstream paxillin and ERK1/2

ZINC40099027 (ZN27) is a specific focal adhesion kinase (FAK) activator that promotes murine mucosal wound closure after ischemic or NSAID-induced injury. Diverse motogenic pathways involve FAK, but the direct consequences of pure FAK activation have not been studied, and how ZN27-induced FAK activation stimulates wound closure remained unclear. We investigated signaling and focal adhesion (FA) turnover after FAK activation by ZN27 in Caco-2 cells, confirming key results in CCD841 cells. ZN27 increased Caco-2 FAK-Y-397, FAK-Y-576/7, paxillin-Y-118, and ERK 1/2 phosphorylation and decreased FAK-Y-925 phosphorylation, without altering FAK-Y-861, p38, Jnk, or Akt phosphorylation. ZN27 increased FAK-paxillin interaction while decreasing FAK-Grb2 association. ZN27 increased membrane-associated FAK-Y-397 and FAK-Y-576/7 phosphorylation and paxillin-Y-118 and ERK 1/2 phosphorylation but decreased FAK-Y-925 phosphorylation without altering Src or Grb2. Moreover, ZN27 increased the fluorescence intensity of GFP-FAK and pFAK-Y397 in FAs and increased the total number of FAs but reduced their size in GFP-FAK-transfected Caco-2 cells, consistent with increased FA turnover. In contrast, FAK-Y397F transfection prevented ZN27 effects on FAK size and number and FAK and pFAK fluorescent intensity in FAs. We confirmed the proposed FAK/paxillin/ERK pathway using PP2 and U0126 to block Src and MEK1/2 in Caco-2 and CCD841 cells. These results suggest that ZN27 promotes intestinal epithelial monolayer defect closure by stimulating autophosphorylation of FAK in the cytosol, distinct from classical models of FAK activation in the FA. Phosphorylated FAK translocates to the membrane, where its downstream substrates paxillin and ERK are phosphorylated, leading to FA turnover and human intestinal epithelial cell migration.

Mechanosensitive Channel PIEZO1 Senses Shear Force to Induce KLF2/4 Expression via CaMKII/MEKK3/ERK5 Axis in Endothelial Cells

Shear stress exerted by the blood stream modulates endothelial functions through altering gene expression. KLF2 and KLF4, the mechanosensitive transcription factors, are promoted by laminar flow to maintain endothelial homeostasis. However, how the expression of KLF2/4 is regulated by shear stress is poorly understood. Here, we showed that the activation of PIEZO1 upregulates the expression of KLF2/4 in endothelial cells. Mice with endothelial-specific deletion of Piezo1 exhibit reduced KLF2/4 expression in thoracic aorta and pulmonary vascular endothelial cells. Mechanistically, shear stress activates PIEZO1, which results in a calcium influx and subsequently activation of CaMKII. CaMKII interacts with and activates MEKK3 to promote MEKK3/MEK5/ERK5 signaling and ultimately induce the transcription of KLF2/4. Our data provide the molecular insight into how endothelial cells sense and convert mechanical stimuli into a biological response to promote KLF2/4 expression for the maintenance of endothelial function and homeostasis.

Organ-on-a-Chip: Design and Simulation of Various Microfluidic Channel Geometries for the Influence of Fluid Dynamic Parameters

Shear stress, pressure, and flow rate are fluid dynamic parameters that can lead to changes in the morphology, proliferation, function, and survival of many cell types and have a determinant impact on tissue function and viability. Microfluidic devices are promising tools to investigate these parameters and fluid behaviour within different microchannel geometries. This study discusses and analyses different designed microfluidic channel geometries regarding the influence of fluid dynamic parameters on their microenvironment at specified fluidic parameters. The results demonstrate that in the circular microchamber, the velocity and shear stress profiles assume a parabolic shape with a maximum velocity occurring in the centre of the chamber and a minimum velocity at the walls. The longitudinal microchannel shows a uniform velocity and shear stress profile throughout the microchannel. Simulation studies for the two geometries with three parallel microchannels showed that in proximity to the micropillars, the velocity and shear stress profiles decreased. Moreover, the pressure is inversely proportional to the width and directly proportional to the flow rate within the microfluidic channels. The simulations showed that the velocity and wall shear stress indicated different values at different flow rates. It was also found that the width and height of the microfluidic channels could affect both velocity and shear stress profiles, contributing to the control of shear stress. The study has demonstrated strategies to predict and control the effects of these forces and the potential as an alternative to conventional cell culture as well as to recapitulate the cell- and organ-specific microenvironment.

Single-Molecule Force Imaging Reveals That Podosome Formation Requires No Extracellular Integrin-Ligand Tensions or Interactions

Podosomes are integrin-mediated cell adhesion units involved in many cellular and physiological processes. Integrins likely transmit tensions critical for podosome functions, but such force remains poorly characterized. DNA-based tension sensors are powerful in visualizing integrin tensions but subject to degradation by podosomes which ubiquitously recruit DNase. Here, using a DNase-resistant tension sensor based on a DNA/PNA (peptide nucleic acid) duplex, we imaged podosomal integrin tensions (PIT) in the adhesion rings of podosomes on solid substrates with single molecular tension sensitivity. PIT was shown to be generated by both actomyosin contractility and actin polymerization in podosomes. Importantly, by monitoring PIT and podosome structure in parallel, we showed that extracellular integrin-ligand tensions, despite being critical for the formation of focal adhesions, are dispensable for podosome formation, as PIT reduction or elimination has an insignificant impact on structure formation and FAK (focal adhesion kinase) phosphorylation in podosomes. We further verified that even integrin-ligand interaction is dispensable for podosome formation, as macrophages form podosomes normally on passivated surfaces that block integrin-ligand interaction but support macrophage adhesion through electrostatic adsorption or Fc receptor-immunoglobin G interaction. In contrast, focal adhesions are unable to form on these passivated surfaces.

RhoA-ROCK competes with YAP to regulate amoeboid breast cancer cell migration in response to lymphatic-like flow

Lymphatic drainage generates force that induces prostate cancer cell motility via activation of Yes-associated protein (YAP), but whether this response to fluid force is conserved across cancer types is unclear. Here, we show that shear stress corresponding to fluid flow in the initial lymphatics modifies taxis in breast cancer, whereas some cell lines use rapid amoeboid migration behavior in response to fluid flow, a separate subset decrease movement. Positive responders displayed transcriptional profiles characteristic of an amoeboid cell state, which is typical of cells advancing at the edges of neoplastic tumors. Regulation of the HIPPO tumor suppressor pathway and YAP activity also differed between breast subsets and prostate cancer. Although subcellular localization of YAP to the nucleus positively correlated with overall velocity of locomotion, YAP gain- and loss-of-function demonstrates that YAP inhibits breast cancer motility but is outcompeted by other pro-taxis mediators in the context of flow. Specifically, we show that RhoA dictates response to flow. GTPase activity of RhoA, but not Rac1 or Cdc42 Rho family GTPases, is elevated in cells that positively respond to flow and is unchanged in cells that decelerate under flow. Disruption of RhoA or the RhoA effector, Rho-associated kinase (ROCK), blocked shear stress-induced motility. Collectively, these findings identify biomechanical force as a regulator amoeboid cell migration and demonstrate stratification of breast cancer subsets by flow-sensing mechanotransduction pathways.

Mechanical Loading Disrupts Focal Adhesion Kinase Activation in Mandibular Fibrochondrocytes During Murine Temporomandibular Joint Osteoarthritis

PURPOSE

Mechanical overloading is a key initiating condition for temporomandibular joint (TMJ) osteoarthritis (OA). The integrin-focal adhesion kinase (FAK) signaling axis is implicated in the mechanobiological response of cells through phosphorylation at Tyr397 (pFAK) but poorly defined in TMJ health and disease. We hypothesize that mechanical overloading disrupts TMJ homeostasis through dysregulation of FAK signaling.

MATERIALS AND METHODS

To assess if FAK and pFAK are viable clinical targets for TMJ OA, peri-articular tissues were collected from patients with TMJ OA receiving a total TMJ replacement. To compare clinical samples with preclinical in vivo studies of TMJ OA, the joints of c57/bl6 mice were surgically destabilized and treated with and without inhibitor of pFAK (iFAK). FAK signaling and TMJ OA progression was evaluated and compared using RT-PCR, western blot, immunohistochemistry, and histomorphometry. To evaluate mechanical overloading in vitro, primary murine mandibular fibrochondrocytes were seeded in a 4% agarose-collagen scaffold and loaded in a compression bioreactor with and without iFAK.

RESULTS

FAK/pFAK was mostly absent from the articular cartilage layer in the clinical sample and suppressed on the central condyle and elevated on the lateral and medial condyle in murine TMJ OA. In vitro, compressive loading lowered FAK/pFAK levels and elevated the expression of TGFβ, NG2, and MMP-13. iFAK treatment suppressed MMP13 and Col6 and elevated TGFβ, NG2, and ACAN in a load independent manner. In vivo, iFAK treatment moderately attenuated OA progression and increased collagen maturation.

CONCLUSION

These data illustrate that FAK/pFAK is implicated in the signaled dysfunction of excessive mechanical loading during TMJ OA and that iFAK treatment can moderately attenuate the progression of cartilage degeneration in the mandibular condyle.

Topological data analysis distinguishes parameter regimes in the Anderson-Chaplain model of angiogenesis

Angiogenesis is the process by which blood vessels form from pre-existing vessels. It plays a key role in many biological processes, including embryonic development and wound healing, and contributes to many diseases including cancer and rheumatoid arthritis. The structure of the resulting vessel networks determines their ability to deliver nutrients and remove waste products from biological tissues. Here we simulate the Anderson-Chaplain model of angiogenesis at different parameter values and quantify the vessel architectures of the resulting synthetic data. Specifically, we propose a topological data analysis (TDA) pipeline for systematic analysis of the model. TDA is a vibrant and relatively new field of computational mathematics for studying the shape of data. We compute topological and standard descriptors of model simulations generated by different parameter values. We show that TDA of model simulation data stratifies parameter space into regions with similar vessel morphology. The methodologies proposed here are widely applicable to other synthetic and experimental data including wound healing, development, and plant biology.

Maintenance of HDACs and H3K9me3 Prevents Arterial Flow-Induced Venous Endothelial Damage

The transition of flow microenvironments from veins to arteries in vein graft surgery induces “peel-off” of venous endothelial cells (vECs) and results in restenosis. Recently, arterial laminar shear stress (ALS) and oscillatory shear stress (OS) have been shown to affect the cell cycle and inflammation through epigenetic controls such as histone deacetylation by histone deacetylases (HDACs) and trimethylation on lysine 9 of histone 3 (H3K9me3) in arterial ECs. However, the roles of H3K9me3 and HDAC in vEC damage under ALS are not known. We hypothesized that the different responses of HDACs and H3K9me3 might cause vEC damage under the transition of venous flow to arterial flow. We found that arterial ECs showed high expression of H3K9me3 protein and were retained in the G0 phase of the cell cycle after being subjected to ALS. vECs became round under ALS with a decrease in the expression of H3K9me3, HDAC3, and HDAC5, and an increase in the expression of vascular cell adhesion molecule 1 (VCAM-1). Inhibition of HDACs activity by a specific inhibitor, phenylbutyrate, in arterial ECs caused similar ALS-induced inflammation and cell loss as observed in vECs. Activation of HDACs and H3K9me3 by ITSA-1, an HDAC activator, could prevent ALS-induced peel-off and reduced VCAM-1 expression in vECs. Moreover, shear stress modulates EC morphology by the regulation of focal adhesion kinase (FAK) expression. ITSA-1 or EGF could increase phosphorylated (p)-FAK expression in vECs under ALS. We found that perturbation of the activity of p-FAK and increase in p-FAK expression restored ALS-induced H3K9me3 expression in vECs. Hence, the abnormal mechanoresponses of H3K9me3 and HDAC in vECs after being subjected to ALS could be reversed by ITSA-1 or EGF treatment: this offers a strategy to prevent vein graft failure.

Matrix stiffening induces a pathogenic QKI-miR-7-SRSF1 signaling axis in pulmonary arterial endothelial cells

Pulmonary arterial hypertension (PAH) refers to a set of heterogeneous vascular diseases defined by elevation of pulmonary arterial pressure (PAP) and pulmonary vascular resistance (PVR), leading to right ventricular (RV) remodeling and often death. Early increases in pulmonary artery stiffness in PAH drive pathogenic alterations of pulmonary arterial endothelial cells (PAECs), leading to vascular remodeling. Dysregulation of microRNAs can drive PAEC dysfunction. However, the role of vascular stiffness in regulating pathogenic microRNAs in PAH is incompletely understood. Here, we demonstrated that extracellular matrix (ECM) stiffening downregulated miR-7 levels in PAECs. The RNA-binding protein quaking (QKI) has been implicated in the biogenesis of miR-7. Correspondingly, we found that ECM stiffness upregulated QKI, and QKI knockdown led to increased miR-7. Downstream of the QKI-miR-7 axis, the serine and arginine-rich splicing factor 1 (SRSF1) was identified as a direct target of miR-7. Correspondingly, SRSF1 was reciprocally upregulated in PAECs exposed to stiff ECM and was negatively correlated with miR-7. Decreased miR-7 and increased QKI and SRSF1 were observed in lungs from patients with PAH and PAH rats exposed to SU5416/hypoxia. Lastly, miR-7 upregulation inhibited human PAEC migration, whereas forced SRSF1 expression reversed this phenotype, proving that miR-7 depended upon SRSF1 to control migration. In aggregate, these results define the QKI-miR-7-SRSF1 axis as a mechanosensitive mechanism linking pulmonary arterial vascular stiffness to pathogenic endothelial function. These findings emphasize implications relevant to PAH and suggest the potential benefit of developing therapies that target this miRNA-dependent axis in PAH.

All Roads Lead to Directional Cell Migration

Directional cell migration normally relies on a variety of external signals, such as chemical, mechanical, or electrical, which instruct cells in which direction to move. Many of the major molecular and physical effects derived from these cues are now understood, leading to questions about whether directional cell migration is alike or distinct under these different signals, and how cells might be directed by multiple simultaneous cues, which would be expected in complex in vivo environments. In this review, we compare how different stimuli are spatially distributed, often as gradients, to direct cell movement and the mechanisms by which they steer cells. A comparison of the downstream effectors of directional cues suggests that different external signals regulate a common set of components: small GTPases and the actin cytoskeleton, which implies that the mechanisms downstream of different signals are likely to be closely related and underlies the idea that cell migration operates by a common set of physical principles, irrespective of the input.

Computational Characterization of The Dish-In-A-Dish, A High Yield Culture Platform for Endothelial Shear Stress Studies on the Orbital Shaker

Endothelial cells sense and respond to shear stress. Different in vitro model systems have been used to study the cellular responses to shear stress, but these platforms do not allow studies on high numbers of cells under uniform and controllable shear stress. The annular dish, or dish-in-a-dish (DiaD), on the orbital shaker has been proposed as an accessible system to overcome these challenges. However, the influence of the DiaD design and the experimental parameters on the shear stress patterns is not known. In this study, we characterize different designs and experimental parameters (orbit size, speed and fluid height) using computational fluid dynamics. We optimize the DiaD for an atheroprotective flow, combining high shear stress levels with a low oscillatory shear index (OSI). We find that orbit size determines the DiaD design and parameters. The shear stress levels increase with increasing rotational speed and fluid height. Based on our optimization, we experimentally compare the 134/56 DiaD with regular dishes for cellular alignment and KLF2, eNOS, CDH2 and MCP1 expression. The calculated OSI has a strong impact on alignment and gene expression, emphasizing the importance of characterizing shear profiles in orbital setups.

From tumour perfusion to drug delivery and clinical translation of in silico cancer models

In silico cancer models have demonstrated great potential as a tool to improve drug design, optimise the delivery of drugs to target sites in the host tissue and, hence, improve therapeutic efficacy and patient outcome. However, there are significant barriers to the successful translation of in silico technology from bench to bedside. More precisely, the specification of unknown model parameters, the necessity for models to adequately reflect in vivo conditions, and the limited amount of pertinent validation data to evaluate models' accuracy and assess their reliability, pose major obstacles in the path towards their clinical translation. This review aims to capture the state-of-the-art in in silico cancer modelling of vascularised solid tumour growth, and identify the important advances and barriers to success of these models in clinical oncology. Particular emphasis has been put on continuum-based models of cancer since they - amongst the class of mechanistic spatio-temporal modelling approaches - are well-established in simulating transport phenomena and the biomechanics of tissues, and have demonstrated potential for clinical translation. Three important avenues in in silico modelling are considered in this contribution: first, since systemic therapy is a major cancer treatment approach, we start with an overview of the tumour perfusion and angiogenesis in silico models. Next, we present the state-of-the-art in silico work encompassing the delivery of chemotherapeutic agents to cancer nanomedicines through the bloodstream, and then review continuum-based modelling approaches that demonstrate great promise for successful clinical translation. We conclude with a discussion of what we view to be the key challenges and opportunities for in silico modelling in personalised and precision medicine.

Good advice for endothelial cells: Get in line, relax tension, and go with the flow

Endothelial cells (ECs) are continuously subjected to fluid wall shear stress (WSS) and cyclic strain caused by pulsatile blood flow and pressure. It is well established that these hemodynamic forces each play important roles in vascular disease, but their combined effects are not well understood. ECs remodel in response to both WSS and cyclic strain to align along the vessel axis, but in areas prone to atherogenesis, such an alignment is absent. In this perspective, experimental and clinical findings will be reviewed, which have revealed the characteristics of WSS and cyclic strain, which are associated with atherosclerosis, spanning studies on whole blood vessels to individual cells to mechanosensing molecules. Examples are described regarding the use of computational modeling to elucidate the mechanisms by which EC alignment contributes to mechanical homeostasis. Finally, the need to move toward an integrated understanding of how hemodynamic forces influence EC mechanotransduction is presented, which holds the potential to move our currently fragmented understanding to a true appreciation of the role of mechanical stimuli in atherosclerosis.

A mechanobiological model to study upstream cell migration guided by tensotaxis

Cell migration is a process of crucial importance for the human body. It is responsible for important processes such as wound healing and tumor metastasis. Migration may occur in response to stimuli of chemical, physical and mechanical nature occurring in the cellular microenvironment. The interstitial flow (IF) can generate mechanical stimuli in cells that influence the cell behavior and interactions of the cells with the extracellular matrix (ECM). One of the phenomena is upstream migration, which is observed in some tumors. In this work, we present a new approach to study the adherent cell migration in a porous medium using a mechanobiological model, attempting to understand if upstream migration can be generated exclusively by mechanical factors. The influence of IF on the behavior of cells and the extracellular matrix was considered. The model is based on a system of coupled nonlinear differential equations solved by the finite element method. Several simulations were performed to study the upstream cell migration and evaluate the effects of pressure, permeability, ECM stiffness and cellular concentration variations on the cell velocity. The results indicated that upstream migration can occur in the presence of mechanical stimuli generated by IF and that the tested parameters have a direct influence on the cellular velocity, especially the pressure and the permeability.

Differential phosphorylation regulates the shear stress-induced polar activity of Rho-specific guanine nucleotide dissociation inhibitor α

The activity of Rho-specific guanine nucleotide dissociation inhibitor α (RhoGDIα) is regulated by its own phosphorylation at different amino acid sites. These phosphorylation sites may have a crucial role in local Rho GTPases activation during cell migration. This paper is designed to explore the influence of phosphorylation on shear stress-induced spatial RhoGDIα activation. Based on the fluorescence resonance energy transfer biosensor sl-RhoGDIα, which was constructed to test the RhoGDIα activity in living cells, new RhoGDIα phosphomimetic mutation (sl-S101E/S174E, sl-Y156E, sl-S101E, sl-S174E) and phosphorylation-deficient mutation (sl-S101A/S174A, sl-Y156A, sl-S101A, sl-S174A) biosensors were designed to test their effects on RhoGDIα activation upon shear stress application in human umbilical vein endothelial cells (HUVECs). The results showed lower RhoGDIα activity at the downstream of HUVECs (the region from the edge of the nucleus to the edge of the cell along with the flow). The overall decrease in RhoGDIα activity was inhibited by Y156A-mutant, whereas the polarized RhoGDIα and Rac1 activity were blocked by S101A/S174A mutant. It is concluded that the Tyr156 phosphorylation mainly mediates shear stress-induced overall RhoGDIα activity, while Ser101/Ser174 phosphorylation mediates its polarization. This study demonstrates that differential phosphorylation of RhoGDIα regulates shear stress-induced spatial RhoGDIα activation, which could be a potential target to control cell migration.

A mechanical toy model linking cell-substrate adhesion to multiple cellular migratory responses

During cell migration, forces applied to a cell from its environment influence the motion. When the cell is placed on a substrate, such a force is provided by the cell-substrate adhesion. Modulation of adhesivity, often performed by the modulation of the substrate stiffness, tends to cause common responses for cell spreading, cell speed, persistence, and random motility coefficient. Although the reasons for the response of cell spreading and cell speed have been suggested, other responses are not well understood. In this study, we develop a simple toy model for cell migration driven by the relation of two forces: the adhesive force and the plasma membrane tension. The simplicity of the model allows us to perform the calculation not only numerically but also analytically, and the analysis provides formulas directly relating the adhesivity to cell spreading, persistence, and the random motility coefficient. Accordingly, the results offer a unified picture on the causal relations between those multiple cellular responses. In addition, cellular properties that would influence the migratory behavior are suggested.

Three-dimensional distribution of wall shear stress and its gradient in red cell-resolved computational modeling of blood flow in in vivo-like microvascular networks

Using a high-fidelity, 3D computational model of blood flow in microvascular networks, we provide the full 3D distribution of wall shear stress (WSS), and its gradient (WSSG), and quantify the influence of red blood cells (RBCs) on WSS and WSSG. The deformation and flow dynamics of the individual RBCs are accurately resolved in the model, while physiologically realistic microvascular networks comprised of multiple bifurcations, convergences, and tortuous vessels are considered. A strong heterogeneity in WSS and WSSG is predicted across the networks, with the highest WSS occurring in precapillary bifurcations and capillary vessels. 3D variations of WSS and WSSG are shown to occur due to both network morphology and the influence of RBCs. The RBCs increase the WSS by as much as three times compared to that when no RBCs are present, and the highest increase is observed in venules. WSSG also increases significantly, and high WSSGs occur over wider regions in the presence of RBCs. In most vessels, the circumferential component of WSSG is observed to be greater than the axial component in the presence of RBCs, while the opposite trend is observed when RBCs are not considered. These results underscore the important role of RBCs on WSS and WSSG that cannot be predicted by widely used 1D models of network blood flow. Furthermore, the subendothelium-scale variations of WSS and WSSG predicted by the present model have implications in terms of endothelial cell functions in the microvasculature.

Effect of Compound Kushen Injection, a Natural Compound Mixture, and Its Identified Chemical Components on Migration and Invasion of Colon, Brain, and Breast Cancer Cell Lines

Traditional Chinese Medicines are promising sources of new agents for controlling cancer metastasis. Compound Kushen Injection (CKI), prepared from medicinal plants Sophora flavescens and Heterosmilax chinensis, disrupts cell cycle and induces apoptosis in breast cancer; however, effects on migration and invasion remained unknown. CKI, fractionated mixtures, and isolated components were tested in migration assays with colon (HT-29, SW-480, DLD-1), brain (U87-MG, U251-MG), and breast (MDA-MB-231) cancer cell lines. Human embryonic kidney (HEK-293) and human foreskin fibroblast (HFF) served as non-cancerous controls. Wound closure, transwell invasion, and live cell imaging showed CKI reduced motility in all eight lines. Fractionation and reconstitution of CKI demonstrated combinations of compounds were required for activity. Live cell imaging confirmed CKI strongly reduced migration of HT-29 and MDA-MB-231 cells, moderately slowed brain cancer cells, and had a small effect on HEK-293. CKI uniformly blocked invasiveness through extracellular matrix. Apoptosis was increased by CKI in breast cancer but not in non-cancerous lines. Cell viability was unaffected by CKI in all cell lines. Transcriptomic analyses of MDA-MB-231indicated down-regulation of actin cytoskeletal and focal adhesion genes with CKI treatment, consistent with observed impairment of cell migration. The pharmacological complexity of CKI is important for effective blockade of cancer migration and invasion.

Keratinocytes are mechanoresponsive to the microflow-induced shear stress

Here, we have reported that keratinocytes respond to the microflow-induced shear stress both at the collective and individual cell level. Using a microfluidic setup, we categorically showed that low shear stress of magnitude 0.06 dyne/cm² could induce morphological variation and cytoskeletal reorganization in keratinocyte, whereas higher shear stress (6 dyne/cm² ) resulted in cellular disruption. Using a series of blocker molecules specific to different mechanotransducers, we demonstrated the pivotal role of actin network in keratinocyte mechanoresponsiveness in conjugation with myosin and lipid rafts. Flow-induced shear stress also induced significant elevation in E-cadherin and Zonula occludens-1 (ZO-1) expression levels. We further showed that under the influence of shear stress, the extent of colocalization of E-cadherin and ZO-1 was more at the cell-cell junction that indicates an improvement in the epithelial phenotype. An increase in the expression of nuclear lamin was also observed in the sheared cells that suggest the transmission of mechanical signals to the nucleus. It is envisioned that this study may find its application in basic and applied organogenesis of the epidermis.

Migration-inducing gene-7 independently predicts poor prognosis of human osteosarcoma and is associated with vasculogenic mimicry

Vasculogenic mimicry (VM) is a special type of vascular channel formed by tumor cells without endothelial cell participation. Migration-inducing gene 7 (MIG-7) plays an important role in regulating VM. In this study, immunohistochemical staining was used to detect MIG-7 in tissue specimens from 141 primary osteosarcoma patients, and the relationship between MIG-7 and VM was examined. Survival analysis were performed to evaluate the prognoses. MIG-7 knockdown osteosarcoma cells were used for cell proliferation, apoptosis, migration, invasiveness and VM formation assays. A spontaneously metastasizing cell line-derived orthotopic xenograft mouse model was established to evaluate the effect of MIG-7 knockdown on tumorigenesis, VM formation and lung metastasis. MIG-7 expression was associated with VM formation. There were significant differences in overall and metastasis-free survival between the MIG-7-positive and MIG-7-negative groups. The MIG-7 expression was shown to be an independent indicator of both overall and metastasis-free survival. In vitro knockdown of MIG-7 dramatically reduced migration, invasion and VM formation in osteosarcoma cells without any significant effect on cell proliferation and apoptosis. MIG-7 knockdown also exhibited potent antitumor, antimetastasis and anti-VM effects in the orthotopic mouse model of 143B osteosarcoma. Therefore, MIG-7 serves as an independent unfavorable prognostic indicator in osteosarcoma patients and MIG-7 is an important mediator of osteosarcoma VM formation.

Geometric constraints of endothelial cell migration on electrospun fibres

Biomaterial scaffolds that can form a template for tissue growth and repair forms the basis of many tissue engineering paradigms. Cell migration and colonisation is an important, and often overlooked, first step. In this study, fibrous guidance structures were produced via electrospinning and the effect of physical features such as fibre diameter (ranging from 500 nm to 10 μm) on endothelial cell migration was assessed. Using a modified wound healing assay, fibre diameter was found to have a significant effect on the rate of wound closure and the peak migration velocity of the cells with scaffold diameter shown to influence both morphology and alignment of the migrating cells. The expression, phosphorylation and distribution of focal adhesion kinase (FAK) was disrupted on the different scaffolds with small-diameter scaffolds exhibiting increased FAK phosphorylation with the kinase present in the cytosol whereas on large-diameter scaffolds FAK was largely restricted to focal adhesions at the cell periphery. This study demonstrates that electrospun scaffolds can be used to model cell migration on fibrous substrates, and particularly for the studying effects of physical features of the substrate, and that FAK is a key mediator of cell-scaffold interactions on migrating cells.

Mechanotransmission in endothelial cells subjected to oscillatory and multi-directional shear flow

Local haemodynamics are linked to the non-uniform distribution of atherosclerosic lesions in arteries. Low and oscillatory (reversing in the axial flow direction) wall shear stress (WSS) induce inflammatory responses in endothelial cells (ECs) mediating disease localization. The objective of this study is to investigate computationally how the flow direction (reflected in WSS variation on the EC surface over time) influences the forces experienced by structural components of ECs that are believed to play important roles in mechanotransduction. A three-dimensional, multi-scale, multi-component, viscoelastic model of focally adhered ECs is developed, in which oscillatory WSS (reversing or non-reversing) parallel to the principal flow direction, or multi-directional oscillatory WSS with reversing axial and transverse components are applied over the EC surface. The computational model includes the glycocalyx layer, actin cortical layer, nucleus, cytoskeleton, focal adhesions (FAs), stress fibres and adherens junctions (ADJs). We show the distinct effects of atherogenic flow profiles (reversing unidirectional flow and reversing multi-directional flow) on subcellular structures relative to non-atherogenic flow (non-reversing flow). Reversing flow lowers stresses and strains due to viscoelastic effects, and multi-directional flow alters stress on the ADJs perpendicular to the axial flow direction. The simulations predict forces on integrins, ADJ filaments and other substructures in the range that activate mechanotransduction.

---Mechanosensitivity of aged muscle stem cells

During aging, skeletal muscle tissue progressively declines in mass, strength, and regenerative capacity. Decreased muscle stem cell (MuSC) number and impaired function might underlie the aging-related muscle wasting and impaired regenerative capacity. As yet, the search for factors that regulate MuSC fate and function has revealed several biochemical factors within the MuSC niche that may be responsible for the decline in MuSC regenerative capacity. This decline cannot be explained by environmental factors solely, as the MuSC potential to regenerate muscle tissue is not reversed by changing the biochemical MuSC niche composition. Here we discuss the likeliness that during physical exercise, MuSCs within their niche are subjected to mechanical loads, in particular pressure and shear stress, as well as associated deformations. We postulate that these physical cues are involved in the activation and differentiation of MuSCs as these cells contain several transmembrane sensor proteins that have been shown to be mechanosensitive in other cell types, that is, endothelial cells and osteoprogenitors. We will specifically address age-related changes in mechanosensing in MuSCs and their niche. Insight in the physical cues applied to the MuSCs in vivo, and how these cues affect MuSC fate and function, helps to develop new therapeutic interventions to counterbalance age-related muscle loss. This requires an approach combining two- and three-dimensional live cell imaging of MuSCs within contracting muscle tissue, mathematical finite element modeling, and cell biology. © 2017 The Authors. Journal of Orthopaedic Research® Published by Wiley Periodicals, Inc. on behalf of the Orthopaedic Research Society. J Orthop Res 36:632-641, 2018.

A minimal mechanics model for mechanosensing of substrate rigidity gradient in durotaxis

Durotaxis refers to the phenomenon in which cells can sense the spatial gradient of the substrate rigidity in the process of cell migration. A conceptual two-part theory consisting of the focal adhesion force generation and mechanotransduction has been proposed previously by Lo et al. to explain the mechanism underlying durotaxis. In the present work, we are concerned with the first part of the theory: how exactly is the larger focal adhesion force generated in the part of the cell adhering to the stiffer region of the substrate? Using a simple elasticity model and by assuming the cell adheres to the substrate continuously underneath the whole cell body, we show that the mechanics principle of static equilibrium alone is sufficient to account for the generation of the larger traction stress on the stiffer region of the substrate. We believe that our model presents a simple mechanistic understanding of mechanosensing of substrate stiffness gradient at the cellular scale, which can be incorporated in more sophisticated mechanobiochemical models to address complex problems in mechanobiology and bioengineering.

Hybrid polymer microfluidic platform to mimic varying vascular compliance and topology

Several cardiovascular pathologies and aging have been associated with alterations in the mechanical and structural properties of the vascular wall, leading to a reduction in arterial compliance and the development of constriction. In the past, rare efforts have been directed to understand the endothelial cell response to combined mechanical stimuli from fluid flow and substrate rigidity. Recent approaches using microfluidic platforms have limitations in precisely mimicking healthy and diseased vasculature conditions from altered topological and substrate compliance perspectives. To address this, we demonstrated an effective fabrication process to realize a hybrid polymer platform to test these mechanistic features of blood vessels. The salient features of the platform include circular microchannels of varying diameters, variation in substrate rigidity along the channel length, and the coexistence of microchannels with different cross sections on a single platform. The platform demonstrates the combined effects of flow-induced shear forces and substrate rigidity on the endothelial cell layer inside the circular microchannels. The experimental results indicate a pronounced cell response to flow induced shear stress via its interplay with the underlying substrate mechanics.

Stem cell mechanical behaviour modelling: substrate's curvature influence during adhesion

Recent experiments hint that adherent cells are sensitive to their substrate curvature. It is already well known that cells behaviour can be regulated by the mechanical properties of their environment. However, no mechanisms have been established regarding the influence of cell-scale curvature of the substrate. Using a numerical cell model, based on tensegrity structures theory and the non-smooth contact dynamics method, we propose to investigate the mechanical state of adherent cells on concave and convex hemispheres. Our mechanical cell model features a geometrical description of intracellular components, including the cell membrane, the focal adhesions, the cytoskeleton filament networks, the stress fibres, the microtubules, the nucleus membrane and the nucleoskeleton. The cell model has enabled us to analyse the evolution of the mechanical behaviour of intracellular components with varying curvature radii and with the removal of part of these components. We have observed the influence of the convexity of the substrate on the cell shape, the cytoskeletal force networks as well as on the nucleus strains. The more convex the substrate, the more tensed the stress fibres and the cell membrane, the more compressed the cytosol and the microtubules, leading to a stiffer cell. Furthermore, the more concave the substrate, the more stable and rounder the nucleus. These findings achieved using a verified virtual testing methodology, in particular regarding the nucleus stability, might be of significant importance with respect to the division and differentiation of mesenchymal stem cells. These results can also bring some hindsights on cell migration on curved substrates.

Flowtaxis of osteoblast migration under fluid shear and the effect of RhoA kinase silencing

Despite the important role of mechanical signals in bone remodeling, relatively little is known about how fluid shear affects osteoblastic cell migration behavior. Here we demonstrated that MC3T3-E1 osteoblast migration could be activated by physiologically-relevant levels of fluid shear in a shear stress-dependent manner. Interestingly, shear-sensitive osteoblast migration behavior was prominent only during the initial period after the onset of the steady flow (for about 30 min), exhibiting shear stress-dependent migration speed, displacement, arrest coefficient, and motility coefficient. For example, cell speed at 1 min was 0.28, 0.47, 0.51, and 0.84 μm min-1 for static, 2, 15, and 25 dyne cm-2 shear stress, respectively. Arrest coefficient (measuring how often cells are paused during migration) assessed for the first 30 min was 0.40, 0.26, 0.24, and 0.12 respectively for static, 2, 15, and 25 dyne cm-2. After this initial period, osteoblasts under steady flow showed decreased migration capacity and diminished shear stress dependency. Molecular interference of RhoA kinase (ROCK), a regulator of cytoskeletal tension signaling, was found to increase the shear-sensitive window beyond the initial period. Cells with ROCK-shRNA had increased migration in the flow direction and continued shear sensitivity, resulting in greater root mean square displacement at the end of 120 min of measurement. It is notable that the transient osteoblast migration behavior was in sharp contrast to mesenchymal stem cells that exhibited sustained shear sensitivity (as we recently reported, J. R. Soc. Interface. 2015; 12:20141351). The study of fluid shear as a driving force for cell migration, i.e., "flowtaxis", and investigation of molecular mechanosensors governing such behavior (e.g., ROCK as tested in this study) may provide new and improved insights into the fundamental understanding of cell migration-based homeostasis.

A Validated Multiscale In-Silico Model for Mechano-sensitive Tumour Angiogenesis and Growth

Vascularisation is a key feature of cancer growth, invasion and metastasis. To better understand the governing biophysical processes and their relative importance, it is instructive to develop physiologically representative mathematical models with which to compare to experimental data. Previous studies have successfully applied this approach to test the effect of various biochemical factors on tumour growth and angiogenesis. However, these models do not account for the experimentally observed dependency of angiogenic network evolution on growth-induced solid stresses. This work introduces two novel features: the effects of hapto- and mechanotaxis on vessel sprouting, and mechano-sensitive dynamic vascular remodelling. The proposed three-dimensional, multiscale, in-silico model of dynamically coupled angiogenic tumour growth is specified to in-vivo and in-vitro data, chosen, where possible, to provide a physiologically consistent description. The model is then validated against in-vivo data from murine mammary carcinomas, with particular focus placed on identifying the influence of mechanical factors. Crucially, we find that it is necessary to include hapto- and mechanotaxis to recapitulate observed time-varying spatial distributions of angiogenic vasculature.

Piezo1 Channels in Vascular Development and the Sensing of Shear Stress

A critical point in mammalian development occurs before mid-embryogenesis when the heart starts to beat, pushing blood into the nascent endothelial lattice. This pushing force is a signal, detected by endothelial cells as a frictional force (shear stress) to trigger cellular changes that underlie the essential processes of vascular remodeling and expansion required for embryonic growth. The processes are complex and multifactorial and Piezo1 became a recognized player only 2years ago, 4years after Piezo1's initial discovery as a functional membrane protein. Piezo1 is now known to be critical in murine embryonic development just at the time when the pushing force is first detected by endothelial cells. Murine Piezo1 gene disruption in endothelial cells is embryonic lethal and mutations in human PIEZO1 associate with severe disease phenotype due to abnormal lymphatic vascular development. Piezo1 proteins coassemble to form calcium-permeable nonselective cationic channels, most likely as trimers. They are large proteins with little if any resemblance to other proteins or ion channel subunits. The channels appear to sense mechanical force directly, including the force imposed on endothelial cells by physiological shear stress. Here, we review current knowledge of Piezo1 in the vascular setting and discuss hypotheses about how it might serve its vascular functions and integrate with other mechanisms. Piezo1 is a new important player for investigators in this field and promises much as a basis for better understanding of vascular physiology and pathophysiology and perhaps also discovery of new therapies.

P2Y2 receptor modulates shear stress-induced cell alignment and actin stress fibers in human umbilical vein endothelial cells

Endothelial cells release ATP in response to fluid shear stress, which activates purinergic (P2) receptor-mediated signaling molecules including endothelial nitric oxide (eNOS), a regulator of vascular tone. While P2 receptor-mediated signaling in the vasculature is well studied, the role of P2Y₂ receptors in shear stress-associated endothelial cell alignment, cytoskeletal alterations, and wound repair remains ill defined. To address these aspects, human umbilical vein endothelial cell (HUVEC) monolayers were cultured on gelatin-coated dishes and subjected to a shear stress of 1 Pa. HUVECs exposed to either P2Y₂ receptor antagonists or siRNA showed impaired fluid shear stress-induced cell alignment, and actin stress fiber formation as early as 6 h. Similarly, when compared to cells expressing the P2Y₂ Arg-Gly-Asp (RGD) wild-type receptors, HUVECs transiently expressing the P2Y₂ Arg-Gly-Glu (RGE) mutant receptors showed reduced cell alignment and actin stress fiber formation in response to shear stress as well as to P2Y₂ receptor agonists in static cultures. Additionally, we observed reduced shear stress-induced phosphorylation of focal adhesion kinase (Y397), and cofilin-1 (S3) with receptor knockdown as well as in cells expressing the P2Y₂ RGE mutant receptors. Consistent with the role of P2Y₂ receptors in vasodilation, receptor knockdown and overexpression of P2Y₂ RGE mutant receptors reduced shear stress-induced phosphorylation of AKT (S473), and eNOS (S1177). Furthermore, in a scratched wound assay, shear stress-induced cell migration was reduced by both pharmacological inhibition and receptor knockdown. Together, our results suggest a novel role for P2Y₂ receptor in shear stress-induced cytoskeletal alterations in HUVECs.

Shear-Sensitive Genes in Aortic Valve Endothelium

SIGNIFICANCE

Currently, calcific aortic valve disease (CAVD) is only treatable through surgical intervention because the specific mechanisms leading to the disease remain unclear. In this review, we explore the forces and structure of the valve, as well as the mechanosensors and downstream signaling in the valve endothelium known to contribute to inflammation and valve dysfunction.

RECENT ADVANCES

While the valvular structure enables adaptation to dynamic hemodynamic forces, these are impaired during CAVD, resulting in pathological systemic changes. Mechanosensing mechanisms-proteins, sugars, and membrane structures-at the surface of the valve endothelial cell relay mechanical signals to the nucleus. As a result, a large number of mechanosensitive genes are transcribed to alter cellular phenotype and, ultimately, induce inflammation and CAVD. Transforming growth factor-β signaling and Wnt/β-catenin have been widely studied in this context. Importantly, NADPH oxidase and reactive oxygen species/reactive nitrogen species signaling has increasingly been recognized to play a key role in the cellular response to mechanical stimuli. In addition, a number of valvular microRNAs are mechanosensitive and may regulate the progression of CAVD.

CRITICAL ISSUES

While numerous pathways have been described in the pathology of CAVD, no treatment options are available to avoid surgery for advanced stenosis and calcification of the aortic valve. More work must be focused on this issue to lead to successful therapies for the disease.

FUTURE DIRECTIONS

Ultimately, a more complete understanding of the mechanisms within the aortic valve endothelium will lead us to future therapies important for treatment of CAVD without the risks involved with valve replacement or repair. Antioxid. Redox Signal. 25, 401-414.

A Cellular Potts Model of single cell migration in presence of durotaxis

Cell migration is a fundamental biological phenomenon during which cells sense their surroundings and respond to different types of signals. In presence of durotaxis, cells preferentially crawl from soft to stiff substrates by reorganizing their cytoskeleton from an isotropic to an anisotropic distribution of actin filaments. In the present paper, we propose a Cellular Potts Model to simulate single cell migration over flat substrates with variable stiffness. We have tested five configurations: (i) a substrate including a soft and a stiff region, (ii) a soft substrate including two parallel stiff stripes, (iii) a substrate made of successive stripes with increasing stiffness to create a gradient and (iv) a stiff substrate with four embedded soft squares. For each simulation, we have evaluated the morphology of the cell, the distance covered, the spreading area and the migration speed. We have then compared the numerical results to specific experimental observations showing a consistent agreement.

Tissue constructs: platforms for basic research and drug discovery

The functions, form and mechanical properties of cells are inextricably linked to their extracellular environment. Cells from solid tissues change fundamentally when, isolated from this environment, they are cultured on rigid two-dimensional substrata. These changes limit the significance of mechanical measurements on cells in two-dimensional culture and motivate the development of constructs with cells embedded in three-dimensional matrices that mimic the natural tissue. While measurements of cell mechanics are difficult in natural tissues, they have proven effective in engineered tissue constructs, especially constructs that emphasize specific cell types and their functions, e.g. engineered heart tissues. Tissue constructs developed as models of disease also have been useful as platforms for drug discovery. Underlying the use of tissue constructs as platforms for basic research and drug discovery is integration of multiscale biomaterials measurement and computational modelling to dissect the distinguishable mechanical responses separately of cells and extracellular matrix from measurements on tissue constructs and to quantify the effects of drug treatment on these responses. These methods and their application are the main subjects of this review.

Mechanism of Focal Adhesion Kinase Mechanosensing

Mechanosensing at focal adhesions regulates vital cellular processes. Here, we present results from molecular dynamics (MD) and mechano-biochemical network simulations that suggest a direct role of Focal Adhesion Kinase (FAK) as a mechano-sensor. Tensile forces, propagating from the membrane through the PIP₂ binding site of the FERM domain and from the cytoskeleton-anchored FAT domain, activate FAK by unlocking its central phosphorylation site (Tyr576/577) from the autoinhibitory FERM domain. Varying loading rates, pulling directions, and membrane PIP₂ concentrations corroborate the specific opening of the FERM-kinase domain interface, due to its remarkably lower mechanical stability compared to the individual alpha-helical domains and the PIP₂-FERM link. Analyzing downstream signaling networks provides further evidence for an intrinsic mechano-signaling role of FAK in broadcasting force signals through Ras to the nucleus. This distinguishes FAK from hitherto identified focal adhesion mechano-responsive molecules, allowing a new interpretation of cell stretching experiments.

Focal adhesions integrate external mechanical signals into biochemical circuits allowing cellular mechanosensing. Although the zoo of mechanosensing proteins at focal adhesions is steadily growing, force-induced enzymatic mechanisms, as those uncovered for autoinhibited kinases in muscle, remain to be identified for focal adhesion downstream signaling. Here, we provide evidence that focal adhesion kinase (FAK) can act as a direct mechano-enzyme at focal adhesions, using molecular dynamics simulations and kinetic modelling. We show that anchorage of FAK to the membrane via PIP-2 is critical for this mechanical activation. Our results suggest similar mechanisms to be at play for other membrane-bound autoinhibited kinases.

Oscillatory shear stress mediates directional reorganization of actin cytoskeleton and alters differentiation propensity of mesenchymal stem cells

Shear stress stimuli differentially regulate cellular functions based on the pattern, magnitude as well as duration of the flow. Shear stress can modify intracellular kinase activities and cytoskeleton reorganization to result in changes of cell behavior. Mesenchymal stem cells (MSCs) are mechano-sensitive cells, but little is known about the effects of oscillatory shear stress (OS). In this study, we demonstrate that OS of 0.5 ± 4 dyn/cm(2) induces directional reorganization of F-actin to mediate the fate choice of MSCs through the regulation of β-catenin. We also found that intercellular junction molecules are the predominant mechanosensors of OS in MSCs to deliver the signals that result in directional rearrangement of F-actin, as well as the increase of phosphorylated β-catenin (pβ-catenin) after 30 minutes of OS stimulation. Depolymerization of F-actin and increase in pβ-catenin also lead to the upregulation of Wnt inhibitory factors sclerostin and dickkopf-1. Inhibition of β-catenin/Wnt signaling pathway is accompanied by the upregulation of sex determining region Y-box2 and NANOG to control self-renewal. In conclusion, the reorganization of actin cytoskeleton and increase in β-catenin phosphorylation triggered by OS regulate the expression of pluripotency genes via the β-catenin/Wnt signaling pathway to differentially direct fate choices of MSCs at different time points. Results from this study have provided new information regarding how MSCs respond to mechanical cues from their microenvironment in a time-dependent fashion, and such biophysical stimuli could be administered to guide the fate and differentiation of stem cells in addition to conventional biochemical approaches.

Activation of Apoptotic Signal in Endothelial Cells through Intracellular Signaling Molecules Blockade in Tumor-Induced Angiogenesis

Tumor-induced angiogenesis is the bridge between avascular and vascular tumor growth phases. In tumor-induced angiogenesis, endothelial cells start to migrate and proliferate toward the tumor and build new capillaries toward the tumor. There are two stages for sprout extension during angiogenesis. The first stage is prior to anastomosis, when single sprouts extend. The second stage is after anastomosis when closed flow pathways or loops are formed and blood flows in the closed loops. Prior to anastomosis, biochemical and biomechanical signals from extracellular matrix regulate endothelial cell phenotype; however, after anastomosis, blood flow is the main regulator of endothelial cell phenotype. In this study, the critical signaling pathways of each stage are introduced. A Boolean network model is used to map environmental and flow induced signals to endothelial cell phenotype (proliferation, migration, apoptosis, and lumen formation). Using the Boolean network model, blockade of intracellular signaling molecules of endothelial cell is investigated prior to and after anastomosis and the cell fate is obtained in each case. Activation of apoptotic signal in endothelial cell can prevent the extension of new vessels and may inhibit angiogenesis. It is shown that blockade of a few signaling molecules in endothelial cell activates apoptotic signal that are proposed as antiangiogenic strategies.

Correlation between cell migration and reactive oxygen species under electric field stimulation

Cell migration is an essential process involved in the development and maintenance of multicellular organisms. Electric fields (EFs) are one of the many physical and chemical factors known to affect cell migration, a phenomenon termed electrotaxis or galvanotaxis. In this paper, a microfluidics chip was developed to study the migration of cells under different electrical and chemical stimuli. This chip is capable of providing four different strengths of EFs in combination with two different chemicals via one simple set of agar salt bridges and Ag/AgCl electrodes. NIH 3T3 fibroblasts were seeded inside this chip to study their migration and reactive oxygen species (ROS) production in response to different EF strengths and the presence of β-lapachone. We found that both the EF and β-lapachone level increased the cell migration rate and the production of ROS in an EF-strength-dependent manner. A strong linear correlation between the cell migration rate and the amount of intracellular ROS suggests that ROS are an intermediate product by which EF and β-lapachone enhance cell migration. Moreover, an anti-oxidant, α-tocopherol, was found to quench the production of ROS, resulting in a decrease in the migration rate.

Identification and expression analysis of irak1 gene in common carp Cyprinus carpio L.: indications for a role of antibacterial and antiviral immunity

In this study, the full-length complementary (c)DNA of interleukin-1 receptor-associated kinase 1 gene (irak1) was cloned from common carp Cyprinus carpio. The complete open reading frame of irak1 contained 2109 bp encoding a protein of 702 amino acid residues that comprised a death domain, a ProST region, a serine-threonine-specific protein kinase catalytic domain and a C-terminal domain. The amino-acid sequence of C. carpio Irak1 protein shared sequence homology with grass carp Ctenopharyngodon idellus (84.5%). The phylogenetic tree of IRAKs separated the polypeptides into four clades, comprising IRAK1s, IRAK2s, IRAK3s and IRAK4s. Cyprinus carpio Irak1 fell into the cluster with previously reported IRAK1s including teleost Irak1s. The irak1 gene was highly expressed in gills, followed by brain, skin, hindgut, buccal epithelium, spleen, foregut, head kidney and liver, and was expressed at lowest levels in gonad and muscle. The irak1 messenger (m)RNA expression was up-regulated in liver, spleen, head kidney, foregut, hindgut, gills and skin after stimulation with Vibrio anguillarum and poly(I:C), and significantly high up-regulated expression was observed in liver and spleen. These results implied that irak1 might participate in antibacterial and antiviral innate immunity. These findings gave the indications that irak1 may participate in antibacterial and antiviral immunity.