Mechanical stretch stimulates growth of vascular smooth muscle cells via epidermal growth factor receptor

Piezo1 activates noncanonical EGFR endocytosis and signaling

EGFR-ERK signaling controls cell cycle progression during development, homeostasis, and disease. While EGF ligand and mechanical inputs can activate EGFR-ERK signaling, the molecules linking mechanical force to this axis have remained mysterious. We previously found that stretch promotes mitosis via the stretch-activated ion channel Piezo1 and ERK signaling. Here, we show that Piezo1 provides the missing link between mechanical signals and EGFR-ERK activation. While both EGF- and Piezo1-dependent activation trigger clathrin-mediated EGFR endocytosis and ERK activation, EGF relies on canonical tyrosine autophosphorylation, whereas Piezo1 involves Src-p38 kinase-dependent serine phosphorylation. In addition, unlike EGF, ex vivo lung slices treated with Piezo1 agonist promoted cell cycle re-entry via nuclear ERK, AP-1 (FOS and JUN), and YAP accumulation, typical of regenerative and malignant signaling. Our results suggest that mechanical activation via Piezo1, Src, and p38 may be more relevant to controlling repair, regeneration, and cancer growth than tyrosine kinase signaling via canonical EGF signaling, suggesting an alternative therapeutic approach.

V3: an enigmatic isoform of the proteoglycan versican

V3 is an isoform of the extracellular matrix (ECM) proteoglycan (PG) versican generated through alternative splicing of the versican gene such that the two major exons coding for sequences in the protein core that support chondroitin sulfate (CS) glycosaminoglycan (GAG) chain attachment are excluded. Thus, versican V3 isoform carries no GAGs. A survey of PubMed reveals only 50 publications specifically on V3 versican, so it is a very understudied member of the versican family, partly because to date there are no antibodies that can distinguish V3 from the CS-carrying isoforms of versican, that is, to facilitate functional and mechanistic studies. However, a number of in vitro and in vivo studies have identified the expression of the V3 transcript during different phases of development and in disease, and selective overexpression of V3 has shown dramatic phenotypic effects in "gain and loss of function" studies in experimental models. Thus, we thought it would be useful and instructive to discuss the discovery, characterization, and the putative biological importance of the enigmatic V3 isoform of versican.

Vibropolyfection: coupling polymer-mediated gene delivery to mechanical stimulation to enhance transfection of adherent cells

BACKGROUND

With the success of recent non-viral gene delivery-based COVID-19 vaccines, nanovectors have gained some public acceptance and come to the forefront of advanced therapies. Unfortunately, the relatively low ability of the vectors to overcome cellular barriers adversely affects their effectiveness. Scientists have thus been striving to develop ever more effective gene delivery vectors, but the results are still far from satisfactory. Therefore, developing novel strategies is probably the only way forward to bring about genuine change. Herein, we devise a brand-new gene delivery strategy to boost dramatically the transfection efficiency of two gold standard nucleic acid (NA)/polymer nanoparticles (polyplexes) in vitro.

RESULTS

We conceived a device to generate milli-to-nanoscale vibrational cues as a function of the frequency set, and deliver vertical uniaxial displacements to adherent cells in culture. A short-lived high-frequency vibrational load (t = 5 min, f = 1,000 Hz) caused abrupt and extensive plasmalemma outgrowths but was safe for cells as neither cell proliferation rate nor viability was affected. Cells took about 1 hr to revert to quasi-naïve morphology through plasma membrane remodeling. In turn, this eventually triggered the mechano-activated clathrin-mediated endocytic pathway and made cells more apt to internalize polyplexes, resulting in transfection efficiencies increased from 10-to-100-fold. Noteworthy, these results were obtained transfecting three cell lines and hard-to-transfect primary cells.

CONCLUSIONS

In this work, we focus on a new technology to enhance the intracellular delivery of NAs and improve the transfection efficiency of non-viral vectors through priming adherent cells with a short vibrational stimulation. This study paves the way for capitalizing on physical cell stimulation(s) to significantly raise the effectiveness of gene delivery vectors in vitro and ex vivo.

PDGFR-β signaling mediates HMGB1 release in mechanically stressed vascular smooth muscle cells

Mechanically stressed vascular smooth muscle cells (VSMCs) have potential roles in the development of vascular complications. However, the underlying mechanisms are unclear. Using VSMCs cultured from rat thoracic aorta explants, we investigated the effects of mechanical stretch (MS) on the cellular secretion of high mobility group box 1 (HMGB1), a major damage-associated molecular pattern that mediates vascular complications in stressed vasculature. Enzyme-linked immunosorbent assay (ELISA) demonstrated an increase in the secretion of HMGB1 in VSMCs stimulated with MS (0–3% strain, 60 cycles/min), and this secretion was markedly and time-dependently increased at 3% MS. The increased secretion of HMGB1 at 3% MS was accompanied by an increased cytosolic translocation of nuclear HMGB1; the acetylated and phosphorylated forms of this protein were significantly increased. Among various inhibitors of membrane receptors mediating mechanical signals, AG1295 (a platelet-derived growth factor receptor (PDGFR) inhibitor) attenuated MS-induced HMGB1 secretion. Inhibitors of other receptors, including epidermal growth factor, insulin-like growth factor, and fibroblast growth factor receptors, did not inhibit this secretion. Additionally, MS-induced HMGB1 secretion was markedly attenuated in PDGFR-β-deficient cells but not in cells transfected with PDGFR-α siRNA. Likewise, PDGF-DD, but not PDGF-AA, directly increased HMGB1 secretion in VSMCs, indicating a pivotal role of PDGFR-β signaling in the secretion of this protein in VSMCs. Thus, targeting PDGFR-β-mediated secretion of HMGB1 in VSMCs might be a promising therapeutic strategy for vascular complications associated with hypertension.

Vascular Smooth Muscle Cells Mechanosensitive Regulators and Vascular Remodeling

Blood vessels are subjected to mechanical loads of pressure and flow, inducing smooth muscle circumferential and endothelial shear stresses. The perception and response of vascular tissue and living cells to these stresses and the microenvironment they are exposed to are critical to their function and survival. These mechanical stimuli not only cause morphological changes in cells and vessel walls but also can interfere with biochemical homeostasis, leading to vascular remodeling and dysfunction. However, the mechanisms underlying how these stimuli affect tissue and cellular function, including mechanical stimulation-induced biochemical signaling and mechanical transduction that relies on cytoskeletal integrity, are unclear. This review focuses on signaling pathways that regulate multiple biochemical processes in vascular mesangial smooth muscle cells in response to circumferential stress and are involved in mechanosensitive regulatory molecules in response to mechanotransduction, including ion channels, membrane receptors, integrins, cytoskeletal proteins, nuclear structures, and cascades. Mechanoactivation of these signaling pathways is closely associated with vascular remodeling in physiological or pathophysiological states.

Fluid shear stress-mediated mechanotransduction in circulating leukocytes and its defect in microvascular dysfunction

Leukocytes (neutrophils, monocytes) in the active circulation exhibit multiple phenotypic indicators for a low level of cellular activity, like lack of pseudopods and minimal amounts of activated, cell-adhesive integrins on their surfaces. In contrast, before these cells enter the circulation in the bone marrow or when they recross the endothelium into extravascular tissues of peripheral organs they are fully activated. We review here a multifaceted mechanism mediated by fluid shear stress that can serve to deactivate leukocytes in the circulation. The fluid shear stress controls pseudopod formation via the FPR receptor, the same receptor responsible for pseudopod projection by localized actin polymerization. The bioactivity of macromolecular factors in the blood plasma that interfere with receptor stimulation by fluid flow, such as proteolytic cleavage in the extracellular domain of the receptor or the membrane actions of cholesterol, leads to a defective ability to respond to fluid shear stress by actin depolymerization. The cell reaction to fluid shear involves CD18 integrins, nitric oxide, cGMP and Rho GTPases, is attenuated in the presence of inflammatory mediators and modified by glucocorticoids. The mechanism is abolished in disease models (genetic hypertension and hypercholesterolemia) leading to an increased number of activated leukocytes in the circulation with enhanced microvascular resistance and cell entrapment. In addition to their role in binding to biochemical agonists/antagonists, membrane receptors appear to play a second role: to monitor local fluid shear stress levels. The fluid shear stress control of many circulating cell types such as lymphocytes, stem cells, tumor cells remains to be elucidated.

The molecular mechanism of mechanotransduction in vascular homeostasis and disease

Blood vessels are constantly exposed to mechanical stimuli such as shear stress due to flow and pulsatile stretch. The extracellular matrix maintains the structural integrity of the vessel wall and coordinates with a dynamic mechanical environment to provide cues to initiate intracellular signaling pathway(s), thereby changing cellular behaviors and functions. However, the precise role of matrix-cell interactions involved in mechanotransduction during vascular homeostasis and disease development remains to be fully determined. In this review, we introduce hemodynamics forces in blood vessels and the initial sensors of mechanical stimuli, including cell-cell junctional molecules, G-protein-coupled receptors (GPCRs), multiple ion channels, and a variety of small GTPases. We then highlight the molecular mechanotransduction events in the vessel wall triggered by laminar shear stress (LSS) and disturbed shear stress (DSS) on vascular endothelial cells (ECs), and cyclic stretch in ECs and vascular smooth muscle cells (SMCs)-both of which activate several key transcription factors. Finally, we provide a recent overview of matrix-cell interactions and mechanotransduction centered on fibronectin in ECs and thrombospondin-1 in SMCs. The results of this review suggest that abnormal mechanical cues or altered responses to mechanical stimuli in EC and SMCs serve as the molecular basis of vascular diseases such as atherosclerosis, hypertension and aortic aneurysms. Collecting evidence and advancing knowledge on the mechanotransduction in the vessel wall can lead to a new direction of therapeutic interventions for vascular diseases.

Screening as a strategy to drive regenerative medicine research

In the field of regenerative medicine, optimization of the parameters leading to a desirable outcome remains a huge challenge. Examples include protocols for the guided differentiation of pluripotent cells towards specialized and functional cell types, phenotypic maintenance of primary cells in cell culture, or engineering of materials for improved tissue interaction with medical implants. This challenge originates from the enormous design space for biomaterials, chemical and biochemical compounds, and incomplete knowledge of the guiding biological principles. To tackle this challenge, high-throughput platforms allow screening of multiple perturbations in one experimental setup. In this review, we provide an overview of screening platforms that are used in regenerative medicine. We discuss their fabrication techniques, and in silico tools to analyze the extensive data sets typically generated by these platforms.

In Silico Tissue Engineering: A Coupled Agent-Based Finite Element Approach

Over the past two decades, the increase in prevalence of cardiovascular diseases and the limited availability of autologous blood vessels and saphenous vein grafts have motivated the development of tissue-engineered vascular grafts (TEVGs). However, compliance mismatch and poor mechanical properties of the TEVGs remain as two major issues that need to be addressed. Researchers have investigated the role of various culture conditions and mechanical conditioning in deposition and orientation of collagen fibers, which are the key structural components in the vascular wall; however, the intrinsic complexity of mechanobiological interactions demands implementing new engineering approaches that allow researchers to investigate various scenarios more efficiently. In this study, we utilized a coupled agent-based finite element analysis (AB-FEA) modeling approach to study the effect of various loading modes (uniaxial, biaxial, and equibiaxial), boundary conditions, stretch magnitudes, and growth factor concentrations on growth and remodeling of smooth muscle cell-populated TEVGs, with specific focus on collagen deposition and orientation. Our simulations (12 weeks of culture) showed that biaxial cyclic loading (and not uniaxial or equibiaxial) leads to alignment of collagen fibers in the physiological directions. Moreover, axial boundary conditions of the TEVG act as determinants of fiber orientations. Decreasing the serum concentration, from 10% to 5% or 1%, significantly decreased the growth and remodeling speed, but only affected the fiber orientation in the 1% serum case. In conclusion, in silico tissue engineering has the potential to evolve the future of tissue engineering, as it will allow researchers to conceptualize various interactions and investigate numerous scenarios with great speed. In this study, we were able to predict the orientation of collagen fibers in TEVGs using a coupled AB-FEA model in less than 8 h. Impact Statement Tissue-engineered vascular grafts (TEVGs) hold potential to replace the current gold standard of vascular grafting, saphenous vein grafts. However, developing TEVGs that mimic the mechanical performance of the native tissue remains a challenging task. We developed a computational model of the grafts' remodeling processes and studied the effects of various loading mechanisms and culture conditions on collagen fiber orientation, which is a key factor in mechanical performance of the grafts. We were able to predict the fiber orientations accurately and show that biaxial loading and axial boundary conditions are important factors in collagen fiber organization.

Nanotoxicity of different sizes of graphene (G) and graphene oxide (GO) in vitro and in vivo

Graphene family nanomaterials (GFNs) have attracted significant attention due to their unique characteristics and applications in the fields of biomedicine and nanotechnology. However, previous studies highlighted the in vitro and in vivo toxicity of GFNs with size and oxidation state differences are still elusive. Therefore, we prepared graphene (G) and graphene oxide (GO) of three different sizes (S-small, M-medium, and L-large), and characterized them using multiple surface-sensitive analytical techniques. In vitro assays using HEK 293T cells revealed that the small and large sizes of G and GO significantly reduced the cell viability and increased DNA damage, accompanying with activated reactive oxygen species (ROS) generation and induced various expressions of associated critical genetic markers. Moreover, the bacterial assays highlighted that G and GO caused strong acute toxicity on Tox2 bacteria. Effects of G were higher than GO and showed size dependent effect: L > M > S, while the medium size of GO induced mild genetic toxicity on RecA bacteria. In vivo assays revealed that exposure to G and GO caused the developmental toxicity, induced ROS generation, and activated related pathways (specifically GO) in zebrafish. Taken together, G showed stronger ability to decrease the survival rate and induce the acute toxicity, while GO showed obvious toxicity in terms of DNA damages, ROS generation, and abnormal gene expressions. Our findings highlighted that G and GO differentially induced toxicity based on their varying physical characteristics, especially sizes and oxidation state, and exposure concentrations and sensitivity of the employed in vitro and in vivo models. In short, this study provided deep insights on the negative effects of GFNs exposure.

Mechanosensing and Mechanoregulation of Endothelial Cell Functions

Vascular endothelial cells (ECs) form a semiselective barrier for macromolecules and cell elements regulated by dynamic interactions between cytoskeletal elements and cell adhesion complexes. ECs also participate in many other vital processes including innate immune reactions, vascular repair, secretion, and metabolism of bioactive molecules. Moreover, vascular ECs represent a unique cell type exposed to continuous, time-dependent mechanical forces: different patterns of shear stress imposed by blood flow in macrovasculature and by rolling blood cells in the microvasculature; circumferential cyclic stretch experienced by the arterial vascular bed caused by heart propulsions; mechanical stretch of lung microvascular endothelium at different magnitudes due to spontaneous respiration or mechanical ventilation in critically ill patients. Accumulating evidence suggests that vascular ECs contain mechanosensory complexes, which rapidly react to changes in mechanical loading, process the signal, and develop context-specific adaptive responses to rebalance the cell homeostatic state. The significance of the interactions between specific mechanical forces in the EC microenvironment together with circulating bioactive molecules in the progression and resolution of vascular pathologies including vascular injury, atherosclerosis, pulmonary edema, and acute respiratory distress syndrome has been only recently recognized. This review will summarize the current understanding of EC mechanosensory mechanisms, modulation of EC responses to humoral factors by surrounding mechanical forces (particularly the cyclic stretch), and discuss recent findings of magnitude-specific regulation of EC functions by transcriptional, posttranscriptional and epigenetic mechanisms using -omics approaches. We also discuss ongoing challenges and future opportunities in developing new therapies targeting dysregulated mechanosensing mechanisms to treat vascular diseases. © 2019 American Physiological Society. Compr Physiol 9:873-904, 2019.

The cofilin phosphatase slingshot homolog 1 restrains angiotensin II-induced vascular hypertrophy and fibrosis in vivo

The dual specificity phosphatase slingshot homolog 1 (SSH1) contributes to actin remodeling by dephosphorylating and activating the actin-severing protein cofilin. The reorganization of the actin cytoskeleton has been implicated in chronic hypertension and the subsequent mechano-adaptive rearrangement of vessel wall components. Therefore, using a novel Ssh1^-/- mouse model, we investigated the potential role of SSH1 in angiotensin II (Ang II)-induced hypertension, and vascular remodeling. We found that loss of SSH1 did not produce overt phenotypic changes and that baseline blood pressures as well as heart rates were comparable between Ssh1^+/+ and Ssh1^-/- mice. Although 14 days of Ang II treatment equally increased systolic blood pressure in both genotypes, histological assessment of aortic samples indicated that medial thickening was exacerbated by the loss of SSH1. Consequently, reverse-transcription quantitative PCR analysis of the transcripts from Ang II-infused animals confirmed increased aortic expression levels of fibronectin, and osteopontin in Ssh1^-/- when compared to wild-type mice. Mechanistically, our data suggest that fibrosis in SSH1-deficient mice occurs by a process that involves aberrant responses to Ang II-induced TGFβ1. Taken together, our work indicates that Ang II-dependent fibrotic gene expression and vascular remodeling, but not the Ang II-induced pressor response, are modulated by SSH1-mediated signaling pathways and SSH1 activity is protective against Ang II-induced remodeling in the vasculature.

In vitro model reveals a role for mechanical stretch in the remodeling response of lymphatic muscle cells

OBJECTIVE

Using primary LMCs in vitro, we sought to characterize the impact of LMC remodeling on their functional and molecular response to mechanical loading and culture conditions.

METHODS

Primary "wounded leg" LMCs were derived from the hindlimb of three sheep who underwent lymphatic injury 6 weeks prior, while "control leg" LMCs were derived from the contralateral, unwounded, limb. Function of the LMCs was characterized in response to media of variable levels of serum (10% vs 0.2%) and glucose (4.5 vs 1 g/L). Functional and proteomic data were evaluated in LMCs exposed to cyclic stretch (0.1 Hz, 7.5% elongation) for 1 week.

RESULTS

LMCs were sensitive to changes in serum levels, significantly reducing overall activity and collagen synthesis under low serum conditions. LMCs from the remodeled vessel had higher baseline levels of metabolic activity but not collagen synthesis. Cyclic loading induced cellular alignment perpendicular to the axis of stretch and alterations in signaling pathways associated with metabolism. Remodeled LMCs had consistently higher levels of metabolic activity and were more resistant to strain-induced apoptosis.

CONCLUSIONS

LMCs exist on a functional spectrum, becoming more active in response to stretching and maintaining phenotypic remodeling in response to local lymphatic/tissue damage.

Where's the Leak in Vascular Barriers? A Review

Edema is typically presented as a secondary effect from injury, illness, disease, or medication, and its impact on patient wellness is nested within the underlying etiology. Therefore, it is often thought of more as an amplifier to current preexisting conditions. Edema, however, can be an independent risk factor for patient deterioration. Improper management of edema is costly not only to the patient, but also to treatment and care facilities, as mismanagement of edema results in increased lengths of hospital stay. Direct tissue trauma, disease, or inappropriate resuscitation and/or ventilation strategies result in edema formation through physical disruption and chemical messenger-based structural modifications of the microvascular barrier. Derangements in microvascular barrier function limit tissue oxygenation, nutrient flow, and cellular waste removal. Recent studies have sought to elucidate cellular signaling and structural alterations that result in vascular hyperpermeability in a variety of critical care conditions to include hemorrhage, burn trauma, and sepsis. These studies and many others have highlighted how multiple mechanisms alter paracellular and/or transcellular pathways promoting hyperpermeability. Roles for endothelial glycocalyx, extracellular matrix and basement membrane, vesiculo-vacuolar organelles, cellular junction and cytoskeletal proteins, and vascular pericytes have been described, demonstrating the complexity of microvascular barrier regulation. Understanding these basic mechanisms inside and out of microvessels aid in developing better treatment strategies to mitigate the harmful effects of excessive edema formation.

Cell Mechanosensors and the Possibilities of Using Magnetic Nanoparticles to Study Them and to Modify Cell Fate

The use of magnetic nanoparticles (MNPs) is a promising technique for future advances in biomedical applications. This idea is supported by the availability of MNPs that can target specific cell components, the variety of shapes of MNPs and the possibility of finely controlling the applied magnetic forces. To examine this opportunity, here we review the current developments in the use of MNPs to mechanically stimulate cells and, specifically, the cell mechanotransduction systems. We analyze the cell components that may act as mechanosensors and their effect on cell fate and we focus on the promising possibilities of controlling stem-cell differentiation, inducing cancer-cell death and treating nervous-system diseases.

Periodic mechanical stress activates EGFR-dependent Rac1 mitogenic signals in rat nucleus pulpous cells via ERK1/2

The mitogenic effects of periodic mechanical stress on nucleus pulpous cells have been studied extensively but the mechanisms whereby nucleus pulpous cells sense and respond to mechanical stimulation remain a matter of debate. We explored this question by performing cell culture experiments in our self-developed periodic stress field and perfusion culture system. Under periodic mechanical stress, rat nucleus pulpous cell proliferation was significantly increased (p < 0.05 for each) and was associated with increases in the phosphorylation and activation of EGFR, Rac1, and ERK1/2 (p < 0.05 for each). Pretreatment with the ERK1/2 selective inhibitor PD98059 reduced periodic mechanical stress-induced nucleus pulpous cell proliferation (p < 0.05 for each), while the activation levels of EGFR and Rac1 were not inhibited. Proliferation and phosphorylation of ERK1/2 were inhibited after pretreatment with the Rac1 inhibitor NSC23766 in nucleus pulpous cells in response to periodic mechanical stress (p < 0.05 for each), while the phosphorylation site of EGFR was not affected. Inhibition of EGFR activity with AG1478 abrogated nucleus pulpous cell proliferation (p < 0.05 for each) and attenuated Rac1 and ERK1/2 activation in nucleus pulpous cells subjected to periodic mechanical stress (p < 0.05 for each). These findings suggest that periodic mechanical stress promotes nucleus pulpous cell proliferation in part through the EGFR-Rac1-ERK1/2 signaling pathway, which links these three important signaling molecules into a mitogenic cascade.

Epidermal Growth Factor Receptor Transactivation: Mechanisms, Pathophysiology, and Potential Therapies in the Cardiovascular System

Epidermal growth factor receptor (EGFR) activation impacts the physiology and pathophysiology of the cardiovascular system, and inhibition of EGFR activity is emerging as a potential therapeutic strategy to treat diseases including hypertension, cardiac hypertrophy, renal fibrosis, and abdominal aortic aneurysm. The capacity of G protein-coupled receptor (GPCR) agonists, such as angiotensin II (AngII), to promote EGFR signaling is called transactivation and is well described, yet delineating the molecular processes and functional relevance of this crosstalk has been challenging. Moreover, these critical findings are dispersed among many different fields. The aim of our review is to highlight recent advancements in defining the signaling cascades and downstream consequences of EGFR transactivation in the cardiovascular renal system. We also focus on studies that link EGFR transactivation to animal models of the disease, and we discuss potential therapeutic applications.

Expression of V3 Versican by Rat Arterial Smooth Muscle Cells Promotes Differentiated and Anti-inflammatory Phenotypes

Arterial smooth muscle cells (ASMCs) undergo phenotypic changes during development and pathological processes in vivo and during cell culture in vitro. Our previous studies demonstrated that retrovirally mediated expression of the versican V3 splice variant (V3) by ASMCs retards cell proliferation and migration in vitro and reduces neointimal thickening and macrophage and lipid accumulation in animal models of vascular injury and atherosclerosis. However, the molecular pathways induced by V3 expression that are responsible for these changes are not yet clear. In this study, we employed a microarray approach to examine how expression of V3 induced changes in gene expression and the molecular pathways in rat ASMCs. We found that forced expression of V3 by ASMCs affected expression of 521 genes by more than 1.5-fold. Gene ontology analysis showed that components of the extracellular matrix were the most significantly affected by V3 expression. In addition, genes regulating the formation of the cytoskeleton, which also serve as markers of contractile smooth muscle cells (SMCs), were significantly up-regulated. In contrast, components of the complement system, chemokines, chemokine receptors, and transcription factors crucial for regulating inflammatory processes were among the genes most down-regulated. Consistently, we found that the level of myocardin, a key transcription factor promoting contractile SMC phenotype, was greatly increased, and the proinflammatory transcription factors NFκB1 and CCAAT/enhancer-binding protein β were significantly attenuated in V3-expressing SMCs. Overall, these findings demonstrate that V3 expression reprograms ASMCs promoting differentiated and anti-inflammatory phenotypes.

Biomechanical Origins of Muscle Stem Cell Signal Transduction

Skeletal muscle, the most abundant and widespread tissue in the human body, contracts upon receiving electrochemical signals from the nervous system to support essential functions such as thermoregulation, limb movement, blinking, swallowing and breathing. Reconstruction of adult muscle tissue relies on a pool of mononucleate, resident muscle stem cells, known as "satellite cells", expressing the paired-box transcription factor Pax7 necessary for their specification during embryonic development and long-term maintenance during adult life. Satellite cells are located around the myofibres in a niche at the interface of the basal lamina and the host fibre plasma membrane (i.e., sarcolemma), at a very low frequency. Upon damage to the myofibres, quiescent satellite cells are activated and give rise to a population of transient amplifying myogenic progenitor cells, which eventually exit the cell cycle permanently and fuse to form new myofibres and regenerate the tissue. A subpopulation of satellite cells self-renew and repopulate the niche, poised to respond to future demands. Harnessing the potential of satellite cells relies on a complete understanding of the molecular mechanisms guiding their regulation in vivo. Over the past several decades, studies revealed many signal transduction pathways responsible for satellite cell fate decisions, but the niche cues driving the activation and silencing of these pathways are less clear. Here we explore the scintillating possibility that considering the dynamic changes in the biophysical properties of the skeletal muscle, namely stiffness, and the stretch and shear forces to which a myofibre can be subjected to may provide missing information necessary to gain a full understanding of satellite cell niche regulation.

Role of epidermal growth factor receptor and endoplasmic reticulum stress in vascular remodeling induced by angiotensin II

The mechanisms by which angiotensin II (AngII) elevates blood pressure and enhances end-organ damage seem to be distinct. However, the signal transduction cascade by which AngII specifically mediates vascular remodeling such as medial hypertrophy and perivascular fibrosis remains incomplete. We have previously shown that AngII-induced epidermal growth factor receptor (EGFR) transactivation is mediated by disintegrin and metalloproteinase domain 17 (ADAM17), and that this signaling is required for vascular smooth muscle cell hypertrophy but not for contractile signaling in response to AngII. Recent studies have implicated endoplasmic reticulum (ER) stress in hypertension. Interestingly, EGFR is capable of inducing ER stress. The aim of this study was to test the hypothesis that activation of EGFR and ER stress are critical components required for vascular remodeling but not hypertension induced by AngII. Mice were infused with AngII for 2 weeks with or without treatment of EGFR inhibitor, erlotinib, or ER chaperone, 4-phenylbutyrate. AngII infusion induced vascular medial hypertrophy in the heart, kidney and aorta, and perivascular fibrosis in heart and kidney, cardiac hypertrophy, and hypertension. Treatment with erlotinib as well as 4-phenylbutyrate attenuated vascular remodeling and cardiac hypertrophy but not hypertension. In addition, AngII infusion enhanced ADAM17 expression, EGFR activation, and ER/oxidative stress in the vasculature, which were diminished in both erlotinib-treated and 4-phenylbutyrate-treated mice. ADAM17 induction and EGFR activation by AngII in vascular cells were also prevented by inhibition of EGFR or ER stress. In conclusion, AngII induces vascular remodeling by EGFR activation and ER stress via a signaling mechanism involving ADAM17 induction independent of hypertension.

Computational analysis of fluid flow within a device for applying biaxial strain to cultured cells

In vitro systems for applying mechanical strain to cultured cells are commonly used to investigate cellular mechanotransduction pathways in a variety of cell types. These systems often apply mechanical forces to a flexible membrane on which cells are cultured. A consequence of the motion of the membrane in these systems is the generation of flow and the unintended application of shear stress to the cells. We recently described a flexible system for applying mechanical strain to cultured cells, which uses a linear motor to drive a piston array to create biaxial strain within multiwell culture plates. To better understand the fluidic stresses generated by this system and other systems of this type, we created a computational fluid dynamics model to simulate the flow during the mechanical loading cycle. Alterations in the frequency or maximal strain magnitude led to a linear increase in the average fluid velocity within the well and a nonlinear increase in the shear stress at the culture surface over the ranges tested (0.5-2.0 Hz and 1-10% maximal strain). For all cases, the applied shear stresses were relatively low and on the order of millipascal with a dynamic waveform having a primary and secondary peak in the shear stress over a single mechanical strain cycle. These findings should be considered when interpreting experimental results using these devices, particularly in the case when the cell type used is sensitive to low magnitude, oscillatory shear stresses.

Barrier enhancing signals in pulmonary edema

Increased endothelial permeability and reduction of alveolar liquid clearance capacity are two leading pathogenic mechanisms of pulmonary edema, which is a major complication of acute lung injury, severe pneumonia, and acute respiratory distress syndrome, the pathologies characterized by unacceptably high rates of morbidity and mortality. Besides the success in protective ventilation strategies, no efficient pharmacological approaches exist to treat this devastating condition. Understanding of fundamental mechanisms involved in regulation of endothelial permeability is essential for development of barrier protective therapeutic strategies. Ongoing studies characterized specific barrier protective mechanisms and identified intracellular targets directly involved in regulation of endothelial permeability. Growing evidence suggests that, although each protective agonist triggers a unique pattern of signaling pathways, selected common mechanisms contributing to endothelial barrier protection may be shared by different barrier protective agents. Therefore, understanding of basic barrier protective mechanisms in pulmonary endothelium is essential for selection of optimal treatment of pulmonary edema of different etiology. This article focuses on mechanisms of lung vascular permeability, reviews major intracellular signaling cascades involved in endothelial monolayer barrier preservation and summarizes a current knowledge regarding recently identified compounds which either reduce pulmonary endothelial barrier disruption and hyperpermeability, or reverse preexisting lung vascular barrier compromise induced by pathologic insults.

Smooth muscle cell hypertrophy, proliferation, migration and apoptosis in pulmonary hypertension

Pulmonary hypertension is a multifactorial disease characterized by sustained elevation of pulmonary vascular resistance (PVR) and pulmonary arterial pressure (PAP). Central to the pathobiology of this disease is the process of vascular remodelling. This process involves structural and functional changes to the normal architecture of the walls of pulmonary arteries (PAs) that lead to increased muscularization of the muscular PAs, muscularization of the peripheral, previously nonmuscular, arteries of the respiratory acinus, formation of neointima, and formation of plexiform lesions. Underlying or contributing to the development of these lesions is hypertrophy, proliferation, migration, and resistance to apoptosis of medial cells and this article is concerned with the cellular and molecular mechanisms of these processes. In the first part of the article we focus on the concept of smooth muscle cell phenotype and the difficulties surrounding the identification and characterization of the cell/cells involved in the remodelling of the vessel media and we review the general mechanisms of cell hypertrophy, proliferation, migration and apoptosis. Then, in the larger part of the article, we review the factors identified thus far to be involved in PH intiation and/or progression and review and discuss their effects on pulmonary artery smooth muscle cells (PASMCs) the predominant cells in the tunica media of PAs.

Downregulation of miR-223 and miR-153 mediates mechanical stretch-stimulated proliferation of venous smooth muscle cells via activation of the insulin-like growth factor-1 receptor

Autologous venous grafts, used to circumvent occluded coronary arteries during coronary artery bypass, often develop thrombosis and neointimal hyperplasia. During neointimal hyperplasia, vascular smooth muscle cells (VSMCs), exposed to substantially higher pressure and hemodynamic forces, proliferate and extracellular matrix accumulate causing narrowing of the vessel lumen. Activation of insulin-like growth factor-1 receptor (IGF-1R) has been confirmed to be critically involved in mechanical stretch-stimulated VSMC proliferation. However, the comprehensive mechanisms responsible for activation of IGF-1R in VSMCs by mechanical stretch remain unclear. This study found that miR-223 and miR-153, targeted to IGF-1R, were down-regulated in VSMCs under stretch stress by miRNA microarray analysis in conjunction with Target Scan analysis. Overexpression of miR-223 or miR-153 down-regulated IGF-1R expression and activity in VSMCs under stretch stress. Specifically, overexpression of miR-223 and miR-153 inhibited stretch stress-enhanced VSMC proliferation and the activity of PI3K-AKT signaling. In conclusion, our study indicates that miR-153 and miR-223 are reduced in VSMCs by stretch stress, contributing to IGF-1R activation and resultant VSMC proliferation. Thus, miR-153 and miR-223 may be viable therapeutic targets for mechanical stretch-induced neointimal hyperplasia in vein grafts.

The roles of integrins in mediating the effects of mechanical force and growth factors on blood vessels in hypertension

Hypertension is characterized by a sustained increase in vasoconstriction and attenuated vasodilation in the face of elevated mechanical stress in the blood vessel wall. To adapt to the increased stress, the vascular smooth muscle cell and its surrounding environment undergo structural and functional changes known as vascular remodeling. Multiple mechanisms underlie the remodeling process, including increased expression of humoral factors and their receptors as well as adhesion molecules and their receptors, all of which appear to collaborate and interact in the response to pressure elevation. In this review, we focus on the interactions between integrin signaling pathways and the activation of growth factor receptors in the response to the increased mechanical stress experienced by blood vessels in hypertension.

Aldosterone up-regulates 12- and 15-lipoxygenase expression and LDL oxidation in human vascular smooth muscle cells

Several lines of evidence suggest that aldosterone excess may have detrimental effects in the cardiovascular system, independent of its interaction with the renal epithelial cells. Here we examined the possibility that aldosterone modulates 12- and/or 15-lipoxygenase (LO) expression/activity in human vascular smooth muscle cells (VSMC), in vitro, thereby potentially contributing to both vascular reactivity and atherogenesis. Following 24 h treatment of VSMC with aldosterone (1 nmol/L), there was a approximately 2-fold increase in the generation rate of 12 hydroxyeicosatetraenoic acid (12-HETE), 70% increase in platelet type 12-LO mRNA expression (P < 0.001) along with a approximately 3-fold increase in 12-LO protein expression, which were blocked by the mineralocorticoid receptor (MR) antagonists spironolactone (100 nmol/L) and eplerelone (100 nmol/ml). Additionally, aldosterone (1 nmol/L; 24 h) increased the production of 15-HETE (50%; P < 0.001) and the expression of 15-LO type 2 mRNA (50%; P < 0.05) (in VSMC). Aldosterone also increased the 12- and 15-LO type 2 mRNA expression in a line of human aortic smooth muscle cells (T/G HA-VSMC) (60% and 50%, respectively). Aldosterone-induced 12- and 15-LO type 2 mRNA expressions were blocked by the EGF-receptor antagonist AG 1478 and by the MAPK-kinase inhibitor UO126. Aldosterone-treated VSMC also showed increased LDL oxidation, (approximately 2-fold; P < 0.001), which was blocked by spironolactone. In conclusion, aldosterone increased 12- and 15-LO expression in human VSMC, in association with increased 12- and 15-HETE generation and enhanced LDL oxidation and may directly augment VSMC contractility, hypertrophy, and migration through 12-HETE and promote LDL oxidation via the pro-oxidative properties of these enzymes.

Linking cell structure to gene regulation: signaling events and expression controls on the model genes PAI-1 and CTGF

The microtubule and microfilament cytoskeletal systems as well as cell-to-cell contacts and cell-matrix interactions are critical regulators of cell structure and function. Alterations in cell shape profoundly influence signaling events and gene expression programs that impact a spectrum of biological responses including cell growth, migration and apoptosis. These same pathways also contribute to the progression of several important pathologic conditions (e.g., arteriosclerosis, vascular fibrosis, and endothelial dysfunction). Indeed, hemodynamic forces in the vascular compartment are established modifiers of endothelial and smooth muscle cell cytoarchitecture and orchestrate complex genetic and biological responses in concert with contributions from the extracellular matrix (ECM), growth factors (e.g., EGF, and TGF-beta) and cell adhesion receptors (e.g., integrins, and cadherins). The profibrotic matricellular proteins plasminogen activator inhibitor-1 (PAI-1) and connective tissue growth factor (CTGF) are prominent members of a subset of genes the expression of which is highly responsive to cell shape-altering stimuli (i.e., disruption of the actin-based and microtubule networks, shear strain and cyclic stretch). Since both PAI-1 and CTGF are major mediators of cardiovascular fibrotic disease, understanding cell structure-linked signaling cascades provides potential avenues for focused therapy. It is increasingly evident that growth factor receptors (EGFR) are activated by changes in cytoarchitecture and that the "repressive state" of certain signaling proteins (e.g., SMAD, and Rho-GEFs) is maintained by sequestration on cell structural networks. Functional repression can be relieved by cytoskeletal perturbations (e.g., in response to treatment with network-specific drugs) resulting in activation of signaling cascades (e.g., Rho, and MAPK) with associated changes in gene reprogramming. Recent studies document a complex network of both similar and unique signaling control elements leading to the induction of PAI-1 and CTGF in response to modifications in cell shape. The purpose of this review is to highlight our current understanding of "cell deformation"-responsive signaling cascades focusing on the potential value of targeting such pathways, and their model response genes (e.g., PAI-1, and CTGF), as a therapeutic option for the treatment of fibrotic diseases.

Pyk2 activation triggers epidermal growth factor receptor signaling and cell motility after wounding sheets of epithelial cells

Activation of the epidermal growth factor receptor (EGFR) is a key signaling event that promotes cells to move and cover wounds in many epithelia. We have previously shown that wounding activates the EGFR through activation of the Src family kinases (SFKs), which induce proteolytic shedding of epidermal growth factor-like ligands from the cell surface. A major goal in wound healing research is to identify early signals that promote motility, and here we examined the hypothesis that members of the focal adhesion kinase family are upstream activators of the SFKs after wounding. We found that focal adhesion kinase is not activated by wounding but that a different family member, Pyk2 (PTK2B/RAFTK/CAKbeta), is activated rapidly and potently. Pyk2 interaction with c-Src is increased after wounding, as determined by co-immunoprecipitation experiments. Disruption of Pyk2 signaling either by small interfering RNA or by expression of a dominant negative mutant led to inhibition of wound-induced activation of the SFKs and the EGFR, and conversely, overexpression of wild-type Pyk2 stimulated SFK and EGFR kinase activities in cells. In wound healing studies, Pyk2 small interfering RNA or dominant negative inhibited cell migration. These results show that activation of Pyk2 is an early signal that promotes wound healing by stimulating the SFK/EGFR signaling pathway.

Mechanical stretch-induced RhoA activation is mediated by the RhoGEF Vav2 in mesangial cells

Increased intraglomerular pressure is an important hemodynamic determinant of glomerulosclerosis, and can be modelled in vitro by exposing mesangial cells (MC) to cyclic mechanical stretch. We have previously shown that the GTPase RhoA mediates stretch-induced fibronectin production. Here we investigate the role of the RhoGEF Vav2 in the activation of RhoA by stretch. Primary rat MC were exposed to 1 Hz cyclic stretch, previously shown to induce maximal RhoA activation at 1 min. Total Vav2 tyrosine phosphorylation and specific phosphorylation on Y172, required for activation, were increased by 1 min of stretch. Overexpression of dominant-negative Vav2 Y172/159F in COS-1 cells or downregulation of Vav2 by siRNA in MC prevented stretch-induced RhoA activation. Vav2 is known to be activated in response to growth factors, and we have previously shown the epidermal growth factor receptor (EGFR) to be transactivated by stretch in MC. Both Vav2 Y172 phosphorylation and RhoA activation were blocked by the EGFR inhibitor AG1478 and prevented in MC overexpressing kinase inactive EGFR. Stretch led to physical association between the EGFR and Vav2, and this was dependent on EGFR activation. EGFR Y992 phosphorylation, required for growth factor-induced Vav2 phosphorylation, was also induced by stretch. Activation of both Src and PI3K were necessary upstream mediators of stretch-induced Vav2 Y172 phosphorylation and RhoA activation. In summary, stretch-induced RhoA activation is dependent on transactivation of the EGFR and activation of the RhoGEF Vav2. Src and PI3K are both required upstream of Vav2 and RhoA activation.

Effects of cyclic strain and growth factors on vascular smooth muscle cell responses

Under physiological and pathological conditions, vascular smooth muscle cells (SMC) are exposed to different biochemical factors and biomechanical forces. Previous studies pertaining to SMC responses have not investigated the effects of both factors on SMCs. Thus, in our research we investigated the combined effects of growth factors like Bfgf (basic fibroblast growth factor), TGF-β (transforming growth factor β) and PDGF (platelet-derived growth factor) along with physiological cyclic strain on SMC responses. Physiological cyclic strain (10% strain) significantly reduced SMC proliferation compared to static controls while addition of growth factors bFGF, TGF-β or PDGF-AB had a positive influence on SMC growth compared to strain alone. Microarray analysis of SMCs exposed to these growth factors and cyclic strain showed that several bioactive genes (vascular endothelial growth factor, epidermal growth factor receptor, etc.) were altered upon exposure. Further work involving biochemical and pathological cyclic strain stimulation will help us better understand the role of cyclic strain and growth factors in vascular functions and development of vascular disorders.

Stretch-induced mitogen-activated protein kinase activation in lung fibroblasts is independent of receptor tyrosine kinases

Lung growth and remodeling are modulated by mechanical stress, with fibroblasts thought to play a leading role. Little mechanistic information is available about how lung fibroblasts respond to mechanical stress. We exposed cultured lung fibroblasts to tonic stretch and measured changes in phosphorylation status of mitogen-activated protein kinases (MAPKs), selected receptor tyrosine kinases (RTKs), and phospholipase Cgamma1 (PLCgamma1) and activation of the small G-protein Ras. Human lung fibroblasts (LFs) were seeded on matrix-coated silicone membranes and exposed to equibiaxial 10 to 40% static stretch or 20% contraction. LFs were stimulated with EGF, FGF2, or PDGF-BB or exposed to stretch in the presence of inhibitors of EGFR (AG1478), FGFR (PD173074), and PDGFR (AG1296). Phospho-MAPK, phospho-RTK, and phospho-PLCgamma1 levels were measured by Western blotting. Active GTP-Ras was quantified by immunoblotting after pull-down with a glutathione S-transferase-Raf-RBD construct. Normalized p-ERK1/2, p-JNK, and p-p38 levels increased after stretch but not contraction. Ligands to RTKs broadly stimulated MAPKs, with the responses to EGF and PDGF most similar to stretch in terms of magnitude and rank order of MAPK responses. Stretching cells failed to elicit measurable activation of EGFR, FGFR (FRS2alpha phosphorylation), or PDGFR. Potent inhibitors of the kinase activity of each receptor failed to attenuate stretch-induced MAPK activation. PLCgamma1 and Ras, prominent effectors downstream of RTKs, were not activated by stretch. Our findings demonstrate that MAPKs are potently activated by stretch in lung fibroblasts, but, in contrast to stress responses observed in other cell types, RTKs are not necessary for stretch-induced MAPK activation in LFs.

The involvement of aldosterone in cyclic stretch-mediated activation of NADPH oxidase in vascular smooth muscle cells

Increasing evidence suggests that aldosterone is implicated in the pathogenesis of cardiovascular diseases. We examined whether aldosterone contributes to the cyclic stretch (CS)-induced reactive oxygen species (ROS) generation in rat aortic smooth muscle cells (RASMCs). RASMCs were exposed to uniaxial CS and thereafter collected to evaluate the expressions of mRNA or protein relating aldosterone synthesis and the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase activity. CS strength-dependently enhanced NADPH oxidase activity. CS induced cytochrome P450 aldosterone synthase (CYP11B2) and increased aldosterone synthesis but did not influence the levels of 11beta-hydroxysteroid dehydrogenase 2 and mineralocorticoid receptor (MR). This CYP11B2 induction was almost completely suppressed by treatment with an extracellular signal-regulated kinase (ERK) inhibitor, U0126, whereas olmesartan, an angiotensin II (Ang II) receptor blocker (ARB), only partially suppressed CS-induced CYP11B2 expression and ERK phosphorylation. A selective MR antagonist, eplerenone (10 micromol l(-1)), significantly attenuated the CS-induced NADPH oxidase activation even in the presence of ARBs. In conclusion, aldosterone synthesis, which is partially independent of Ang II, may have an important role in CS-stimulated ROS generation in cultured RASMCs. We also suggest the potential benefit of eplerenone in the treatment of cardiovascular diseases.

Long-term cyclic stretch controls pulmonary endothelial permeability at translational and post-translational levels

We have previously described differential effects of physiologic (5%) and pathologic (18%) cyclic stretch (CS) on agonist-induced pulmonary endothelial permeability. This study examined acute and chronic effects of CS on agonist-induced intracellular signaling and cell morphology in the human lung macro- and microvascular endothelial cell (EC) monolayers. Endothelial permeability was assessed by analysis of morphological changes, parameters of cell contraction and measurements of transendothelial electrical resistance. Exposure of both microvascular and macrovascular EC to 18% CS for 2-96 h increased thrombin-induced permeability and monolayer disruption. Interestingly, the ability to promote thrombin responses was present in EC cultures exposed to 48-96 h of CS even after replating onto non-elastic substrates. In turn, physiologic CS preconditioning (72 h) attenuated thrombin-induced paracellular gap formation and MLC phosphorylation in replated EC cultures. Long-term preconditioning at 18% CS (72 h) increased the content of signaling and contractile proteins including Rho GTPase, MLC, MLC kinase, ZIP kinase, PAR1, caldesmon and HSP27 in the pulmonary microvascular and macrovascular cells. We conclude that short term CS regulates EC permeability via modulation of agonist-induced signaling, whereas long-term CS controls endothelial barrier at both post-translational level and via magnitude-dependent regulation of pulmonary EC phenotype, signaling and contractile protein expression.

A finite element model predicts the mechanotransduction response of tendon cells to cyclic tensile loading

The importance of fluid-flow-induced shear stress and matrix-induced cell deformation in transmitting the global tendon load into a cellular mechanotransduction response is yet to be determined. A multiscale computational tendon model composed of both matrix and fluid phases was created to examine how global tendon loading may affect fluid-flow-induced shear stresses and membrane strains at the cellular level. The model was then used to develop a quantitative experiment to help understand the roles of membrane strains and fluid-induced shear stresses on the biological response of individual cells. The model was able to predict the global response of tendon to applied strain (stress, fluid exudation), as well as the associated cellular response of increased fluid-flow-induced shear stress with strain rate and matrix-induced cell deformation with strain amplitude. The model analysis, combined with the experimental results, demonstrated that both strain rate and strain amplitude are able to independently alter rat interstitial collagenase gene expression through increases in fluid-flow-induced shear stress and matrix-induced cell deformation, respectively.

Apical epidermal growth factor receptor signaling: regulation of stretch-dependent exocytosis in bladder umbrella cells

The apical surface of polarized epithelial cells receives input from mediators, growth factors, and mechanical stimuli. How these stimuli are coordinated to regulate complex cellular functions such as polarized membrane traffic is not understood. We analyzed the requirement for growth factor signaling and mechanical stimuli in umbrella cells, which line the mucosal surface of the bladder and dynamically insert and remove apical membrane in response to stretch. We observed that stretch-stimulated exocytosis required apical epidermal growth factor (EGF) receptor activation and that activation occurred in an autocrine manner downstream of heparin-binding EGF-like growth factor precursor cleavage. Long-term changes in apical exocytosis depended on protein synthesis, which occurred upon EGF receptor-dependent activation of mitogen-activated protein kinase signaling. Our results indicate a novel physiological role for the EGF receptor that couples upstream mechanical stimuli to downstream apical EGF receptor activation that may regulate apical surface area changes during bladder filling.

Down-regulation of ERK but not MEK phosphorylation in cultured endothelial cells by repeated changes in cyclic stretch

OBJECTIVE

Effects of cyclic stretch on endothelial cells are studied usually by exposing cells cultured under stretch-free conditions to some levels of cyclic stretch, but in vivo these cells experience both increase and decrease in stretch. Experiments were designed to study how endothelial cells maintained under certain levels of cyclic stretch responded to shifts in stretch frequencies and amplitudes.

METHODS

Confluent endothelial cells cultured on flexible silicone membranes with or without pre-stretching for 2-12 h were exposed to various levels of stretch amplitude or frequency and assayed for extracellular signal-regulated kinase 1/2 (ERK) phosphorylation.

RESULTS

When endothelial cells without pre-stretching were cyclically stretched, ERK phosphorylation increased, peaking approximately 15 min and slowly decreased. In contrast, when pre-stretched cells were exposed to either higher or lower stretch condition, ERK phosphorylation transiently decreased within 5 min, indicating that some mechanism which down-regulated ERK phosphorylation was activated. Because phosphorylation of ERK kinase (MEK) was not inhibited in these cells, this mechanism targeted ERK directly, not the upstream kinases of the Ras-Raf-MEK-ERK cascade. Furthermore, this ERK down-regulation in pre-stretched cells was not induced by agonists, was inhibited by Na(3)VO(4) but not okadaic acid, and was detected in the cytosolic fraction. Repeated shifts in stretch conditions induced continuous down-regulation of ERK but not MEK phosphorylation.

CONCLUSIONS

Endothelial cells are capable of down-regulating ERK phosphorylation in a cyclic stretch- and tyrosine phosphatase-dependent manner. Frequent changes in stretch conditions constitutively activated this ability, which could play some role in regulating ERK activity in endothelial cells in vivo.

Inhibition of EGFR signaling abrogates smooth muscle proliferation resulting from sustained distension of the urinary bladder

Urinary bladder outlet obstruction results in sustained stretch of the detrusor muscle and can lead to pathological smooth muscle hyperplasia and hypertrophy. The epidermal growth factor receptor (EGFR) is a cognate receptor for mitogens implicated in bladder hyperplasia/hypertrophy. Here, we investigated the potential for modulation of this pathway by pharmacologic targeting with a clinically available EGFR antagonist using an organ culture model of bladder stretch injury as a test system. Urinary bladders from adult female rats were distended in vivo with medium containing the EGFR inhibitor ZD1839 (gefitinib, Iressa). The bladders were excised and incubated in ex vivo organ culture for 4-24 h. EGFR phosphorylation, DNA proliferation, and the extent of apoptosis in the cultured tissues were assessed. To verify that the smooth muscle cells (SMC) are a target of the EGFR inhibitor, primary culture human and rat bladder SMC were subjected to cyclic mechanical stretch in vitro in the presence of ZD1839. Levels of phosphorylated EGFR were significantly increased in the detrusor muscle with 12 h of stretch in the organ cultures. This activation coincided with a subsequent 23-fold increase in DNA synthesis and a 30-fold decrease in apoptosis in the muscle compartment at 24 h. In the presence of ZD1839, DNA synthesis was reduced to basal levels without an increase in the rate of apoptosis under ex vivo conditions. Mechanical stretch of bladder SMC in vitro resulted in a significant increase in DNA synthesis, which was completely abrogated by treatment with ZD1839 but not by AG825, an inhibitor of the related receptor, ErbB2. Our results indicate that the EGFR pathway is a physiologically relevant signaling mechanism in hypertrophic bladder disease resulting from mechanical distension and may be amenable to pharmacologic intervention.

Abnormalities of the bronchial arteries in asthma

STUDY OBJECTIVES

The bronchial arteries supply systemic blood to the airways, tracheobronchial lymph nodes, and nerves. Their structure has not been studied in patients with asthma.

DESIGN

Case-control study of pathologic changes of bronchial arteries in asthma.

PARTICIPANTS AND METHODS

Postmortem lungs were examined from three case groups: (1) fatal asthma (n = 12), death due to asthma; (2) nonfatal asthma (n = 12), asthmatic and death due to nonrespiratory causes; and (3) nonasthmatic control subjects (n = 12), no history of asthma and death due to nonrespiratory causes. In bronchial arteries with outer diameters of 0.1 to 1.0 mm, the areas of lumen, intima, and media were measured and compared between case groups.

RESULTS

There were no significant differences in artery size (outer diameter) or in medial area between the three groups. In the two asthma groups, the intimal area was increased (p < 0.05), with a corresponding decrease in luminal area compared with the control group. There was a significant effect of gender, age, and smoking on intimal area. In the asthma cases, the area of bronchial artery intima was related to duration of asthma (p < 0.05), and this increase was associated with smooth muscle proliferation, reduplication, and calcification of the elastica, but not with inflammatory cell infiltration.

CONCLUSIONS

While the pathophysiologic significance of these changes is uncertain, the relation to duration of asthma, age, and smoking suggests a secondary response to chronic airway disease.

Activations of mitogen-activated protein kinase and nuclear factor-kappaB by mechanical stretch result in ventilation-induced lung injury

Mechanical ventilation is an important therapeutic technique for patients with respiratory failure. Nonetheless, it may cause or worsen lung injury. The specific triggers for cytokine release and the cellular origins of the inflammatory mediators in ventilation-induced lung injury (VILI) have yet to be defined. With the development of cytomechanics, we can study the lung cell response to mechanical strain. The initial step is mechanosensation, including stretch-activated ionchannels and the ECM-integrin-cytoskeleton pathway. Several intracellular signaling pathways then are activated and eventually result in increased transcription of specific genes. Mitogen-activated protein kinase cascade, nuclear factor(NF)-kappaB, PKC are all activated by mechanical stretch. But the mechanisms regulating lung stretch-induced cytokine production are still unclear. I hypotheses mechanical stretch initiate specific genes transcription, then the cytokines stimulate the cell again. This formed a positive feed back loop, which caused VILI. These studies may lead to the identification of new targets for therapeutic interventions and help to develop less aggressive ventilation strategies for patients with acute respiratory failure.

EGFR-activated signaling and actin remodeling regulate cyclic stretch-induced NRF2-ARE activation

Cyclic stretch (CS) associated with mechanical ventilation (MV) can cause excessive alveolar and endothelial distention, resulting in lung injury and inflammation. Antioxidant enzymes (AOEs) play a major role in suppressing these effects. The transcription factor Nrf2, via the antioxidant response element (ARE), alleviates pulmonary toxicant- and oxidant-induced oxidative stress by up-regulating the expression of several AOEs. Although gene expression profiling has revealed the induction of AOEs in the lungs of rodents exposed to MV, the mechanisms by which mechanical forces, such as CS, regulate the activation of Nrf2-dependent ARE-transcriptional responses are poorly understood. To mimic mechanical stress associated with MV, we have cultured pulmonary alveolar epithelial and endothelial cells on collagen I-coated BioFlex plates and subjected them to CS. CS exposure stimulated ARE-driven transcriptional responses and subsequent AOE expression. Ectopic expression of a dominant-negative Nrf2 suppressed the CS-stimulated ARE-driven responses. Our findings suggest that actin remodeling is necessary but not sufficient for high-level CS-induced ARE activation in both epithelial and endothelial cells. We also found that inhibition of EGFR activity by a pharmacologic agent ablated the CS-induced ARE transcriptional response in both cell types. Additional studies revealed that amphiregulin, an EGFR ligand, regulates this process. We further demonstrated that the PI3K-Akt pathway acts as the downstream effector of EGFR and regulates CS-induced ARE-activation in an oxidative stress-dependent manner. Collectively, these novel findings suggest that EGFR-activated signaling and actin remodeling act in concert to regulate the CS-induced Nrf2-ARE transcriptional response and subsequent AOE expression.

Activation of Akt by mechanical stretching in human epidermal keratinocytes

Mechanical stretching represents an important part of the signaling in skin. We have previously demonstrated that mechanical stretching induced proliferative phenotypes in human keratinocytes, as shown in increased 5-bromo-2'-deoxyuridine (BrdU) incorporation, ERK1/2 activation, and keratin K6 induction. Here we have further investigated the antiapoptotic signals in human keratinocytes provoked by mechanical stretching in vitro. Keratinocytes were plated on flexible silicone supports to transmit mechanical stretching to keratinocytes, involving continuous stretching by +20%. Stretching of these cells for 15-30 min caused the phosphorylation and activation of Akt. Inhibition of mitogen and extracellular signal-regulated kinase (MEK1/2) with U0126 and phosphoinositide 3-OH kinase (PI 3-K) with Wortmannin attenuated Akt activation. The epidermal growth factor (EGF) receptor kinase inhibitor, AG1478, and calcium channel inhibitor, gadolinium (Gd3+), also inhibited Akt activation. In summary, our results demonstrate the activation of the Akt signaling pathway by mechanical stretching, generating not only proliferative but also antiapoptotic signals in human keratinocytes.

Mechanical stretch induces clustering of ?1-integrins and facilitates adhesion

Human epithelial cells are permanently stimulated by external mechanical forces. The present in vitro study suggests that keratinocytes respond to mechanical strain by a coordinated spatial and functional utilization of beta1-integrins and the epidermal growth factor receptor (EGFR) with impact to the adhesion properties. It was found that a single mechanical stretch applied to HaCaT keratinocytes elevates the substrate adhesion, in particular to fibronectin and collagen type IV but not to laminin indicating the relevance of beta1-integrins in this process. This was confirmed using a functional blocking antibody directed against beta1-integrins which reversed the stretch-induced adhesion. Furthermore, mechanical stretch gives rise to a rapid redistribution of beta1-integrins in clusters on the basal cell membrane, without changing the overall amount of this particular integrin subset. Concomitantly, the EGFR co-localizes with beta1-integrin suggesting a functional cooperation of both membrane proteins in mechano-signaling. This is corroborated by data showing that stretch-induced activation of the EGFR and the downstream element extracellular regulated kinase 1/2 (ERK1/2) is reversed by preincubation with beta1-integrin antibodies. Vice versa, blocking the EGFR using a specific inhibitor abrogates stretch-induced ERK1/2 activation. In summary, these results show a functional cooperation of beta1-integrins and EGFR in the adhesion complex supporting the transmission of stretch-induced signals.

Molecular mechanisms of the vascular responses to haemodynamic forces

Blood vessels are permanently subjected to mechanical forces in the form of stretch, encompassing cyclic mechanical strain due to the pulsatile nature of blood flow and shear stress. Significant variations in mechanical forces, of physiological or physiopathological nature, occur in vivo. These are accompanied by phenotypical modulation of smooth muscle cells and endothelial cells, producing structural modifications of the arterial wall. In all the cases, vascular remodelling can be allotted to a modification of the tensional strain or shear, and underlie a trend to reestablish baseline mechanical conditions. Vascular cells are equipped with numerous receptors that allow them to detect and respond to the mechanical forces generated by pressure and shear stress. The cytoskeleton and other structural components have an established role in mechanotransduction, being able to transmit and modulate tension within the cell via focal adhesion sites, integrins, cellular junctions and the extracellular matrix. Mechanical forces also initiate complex signal transduction cascades, including nuclear factor-kappaB and mitogen-activated protein kinase pathways, leading to functional changes within the cell.

An introductory review of cell mechanobiology

Mechanical loads induce changes in the structure, composition, and function of living tissues. Cells in tissues are responsible for these changes, which cause physiological or pathological alterations in the extracellular matrix (ECM). This article provides an introductory review of the mechanobiology of load-sensitive cells in vivo, which include fibroblasts, chondrocytes, osteoblasts, endothelial cells, and smooth muscle cells. Many studies have shown that mechanical loads affect diverse cellular functions, such as cell proliferation, ECM gene and protein expression, and the production of soluble factors. Major cellular components involved in the mechanotransduction mechanisms include the cytoskeleton, integrins, G proteins, receptor tyrosine kinases, mitogen-activated protein kinases, and stretch-activated ion channels. Future research in the area of cell mechanobiology will require novel experimental and theoretical methodologies to determine the type and magnitude of the forces experienced at the cellular and sub-cellular levels and to identify the force sensors/receptors that initiate the cascade of cellular and molecular events.

Arterial hemodynamics and mechanical properties after circulatory intervention in the chick embryo

Altered blood pressure and flow impact cardiac function during morphogenesis. How the arterial system supports cardiac morphogenesis after circulatory disruptions is not well characterized. We manipulated arterial flow via left atrial ligation (LAL) or arterial load via right vitelline artery ligation (VAL) in Hamburger-Hamilton (HH) stage 21 chick embryos. Embryos were reincubated for 1 h (HH21), 14 h (HH24) or 30 h (HH27). At each stage we measured simultaneous dorsal aortic blood pressure and flow, and calculated arterial compliance, impedance and hydraulic power. LAL acutely reduced stroke volume (Vs), cardiac output (Q) and hydraulic power. Arterial pressure was preserved by a compensatory increase in characteristic impedance and decrease in compliance. Impedance parameters and compliance normalized by HH24 and all parameters normalized by HH27. VAL acutely increased arterial resistance. Embryos maintained arterial pressure by decreasing Vs and Q. These parameters remained altered through HH27. In summary, despite the intervention, compensatory alterations in Vs and arterial resistance maintained arterial pressure and fraction of oscillatory power within a narrow range. These results suggest that the maintenance of arterial pressure and circulatory energy efficiency, but not arterial flow, is critical to embryogenesis.