Molecular basis of the effects of mechanical stretch on vascular smooth muscle cells

A structural finite element model for lamellar unit of aortic media indicates heterogeneous stress field after collagen recruitment

Incorporation of collagen structural information into the study of biomechanical behavior of ascending thoracic aortic (ATA) wall tissue should provide better insight into the pathophysiology of ATA. Structurally motivated constitutive models that include fiber dispersion and recruitment can successfully capture overall mechanical response of the arterial wall tissue. However, these models cannot examine local microarchitectural features of the collagen network, such as the effect of fiber disruptions and interaction between fibrous and non-fibrous components, which may influence emergent biomechanical properties of the tissue. Motivated by this need, we developed a finite element based three-dimensional structural model of the lamellar units of the ATA media that directly incorporates the collagen fiber microarchitecture. The fiber architecture was computer generated utilizing network features, namely fiber orientation distribution, intersection density and areal concentration, obtained from image analysis of multiphoton microscopy images taken from human aneurysmal ascending thoracic aortic media specimens with bicuspid aortic valve (BAV) phenotype. Our model reproduces the typical J-shaped constitutive response of the aortic wall tissue. We found that the stress state in the non-fibrous matrix was homogeneous until the collagen fibers were recruited, but became highly heterogeneous after that event. The degree of heterogeneity was dependent upon local network architecture with high stresses observed near disrupted fibers. The magnitude of non-fibrous matrix stress at higher stretch levels was negatively correlated with local fiber density. The localized stress concentrations, elucidated by this model, may be a factor in the degenerative changes in aneurysmal ATA tissue.

Shear stress increases cytotoxicity and reduces transfection efficiency of liposomal gene delivery to CHO-S cells

Animal cells in suspension experience shear stress in different situations such as in vivo due to hemodynamics, or in vitro due to agitation in large-scale bioreactors. Shear stress is known to affect cell physiology, including binding and uptake of extracellular cargo. In adherent cells the effects of exposure to shear stress on particle binding kinetics and uptake have been studied. There are however no reports on the effect of shear stress on extracellular cargo delivery to suspension cells. In this study, we have evaluated the effect of shear stress on transfection of CHO-S cells using Lipofectamine 2000 in a simple flow apparatus. Our results show decreased cell growth and transfection efficiency upon lipoplex assisted transfection of CHO-S while being subjected to shear stress. This effect is not seen to the same extent when cells are exposed to shear stress in absence of the lipoplex complex and subsequently transfected, or if the lipoplex is subjected to shear stress and subsequently used to transfect the cells. It is also not seen to the same extent when cells are exposed to shear stress in presence of liposome alone, suggesting that the observed effect is dependent on interaction of the lipoplex with cells in the presence of shear stress. These results suggest that studies involving liposomal DNA delivery in presence of shear stress such as large scale transient protein expression should account for the effect of shear during lipoplex assisted DNA delivery.

Nuclear envelope proteins modulate proliferation of vascular smooth muscle cells during cyclic stretch application

Cyclic stretch is an important inducer of vascular smooth muscle cell (VSMC) proliferation, which is crucial in vascular remodeling during hypertension. However, the molecular mechanism remains unclear. We studied the effects of emerin and lamin A/C, two important nuclear envelope proteins, on VSMC proliferation in hypertension and the underlying mechano-mechanisms. In common carotid artery of hypertensive rats in vivo and in cultured cells subjected to high (15%) cyclic stretch in vitro, VSMC proliferation was increased significantly, and the expression of emerin and lamin A/C was repressed compared with normotensive or normal (5%) cyclic stretch controls. Using targeted siRNA to mimic the repressed expression of emerin or lamin A/C induced by 15% stretch, we found that VSMC proliferation was enhanced under static and 5%-stretch conditions. Overexpression of emerin or lamin A/C reversed VSMC proliferation induced by 15% stretch. Hence, emerin and lamin A/C play critical roles in suppressing VSMC hyperproliferation induced by hyperstretch. ChIP-on-chip and MOTIF analyses showed that the DNAs binding with emerin contain three transcription factor motifs: CCNGGA, CCMGCC, and ABTTCCG; DNAs binding with lamin A/C contain the motifs CVGGAA, GCCGCYGC, and DAAGAAA. Protein/DNA array proved that altered emerin or lamin A/C expression modulated the activation of various transcription factors. Furthermore, accelerating local expression of emerin or lamin A/C reversed cell proliferation in the carotid artery of hypertensive rats in vivo. Our findings establish the pathogenetic role of emerin and lamin A/C repression in stretch-induced VSMC proliferation and suggest mechanobiological mechanism underlying this process that involves the sequence-specific binding of emerin and lamin A/C to specific transcription factor motifs.

Unraveling the role of mechanical stimulation on smooth muscle cells: A comparative study between 2D and 3D models

A thorough understanding of cell response to combined culture configuration and mechanical cues is of paramount importance in vascular tissue engineering applications. Herein, we investigated and compared the response of vascular smooth muscle cells (vSMCs) cultured in different culture environments (2D cell monolayers and 3D cellularized collagen-based gels) in combination with mechanical stimulation (7% uniaxial cyclic strain, 1 Hz) for 2 and 5 days. When cyclic strain was applied, two different responses, in terms of cell orientation and expression of contractile-phenotype proteins, were observed in 2D and 3D models. Specifically, in 2D configuration, cyclic strain caused ∼50% of cell population to align nearly perpendicular (80-90 degrees) to the strain direction, while not influencing the contractile-phenotype protein expression, as compared to the 2D static controls. Conversely, the application of uniaxial strain to 3D constructs induced a ∼60% cell alignment almost parallel (0-10 degrees) to the strain direction. Moreover, 3D mechanical stimulation applied for 5 days induced a twofold increase of SM α-actin level and a 14-fold increase of calponin expression as compared to 3D static controls. Altogether these findings provide a new insight into the potential to drive cell behavior by modulating the extracellular matrix and the biomechanical environment. Biotechnol. Bioeng. 2016;113: 2254-2263. © 2016 Wiley Periodicals, Inc.

An approach to quantifying 3D responses of cells to extreme strain

The tissues of hollow organs can routinely stretch up to 2.5 times their length. Although significant pathology can arise if relatively large stretches are sustained, the responses of cells are not known at these levels of sustained strain. A key challenge is presenting cells with a realistic and well-defined three-dimensional (3D) culture environment that can sustain such strains. Here, we describe an in vitro system called microscale, magnetically-actuated synthetic tissues (micro-MASTs) to quantify these responses for cells within a 3D hydrogel matrix. Cellular strain-threshold and saturation behaviors were observed in hydrogel matrix, including strain-dependent proliferation, spreading, polarization, and differentiation, and matrix adhesion retained at strains sufficient for apoptosis. More broadly, the system shows promise for defining and controlling the effects of mechanical environment upon a broad range of cells.

A New Fluidized Bed Bioreactor Based on Diversion-Type Microcapsule Suspension for Bioartificial Liver Systems

A fluidized bed bioreactor containing encapsulated hepatocytes may be a valuable alternative to a hollow fiber bioreactor for achieving the improved mass transfer and scale-up potential necessary for clinical use. However, a conventional fluidized bed bioreactor (FBB) operating under high perfusion velocity is incapable of providing the desired performance due to the resulting damage to cell-containing microcapsules and large void volume. In this study, we developed a novel diversion-type microcapsule-suspension fluidized bed bioreactor (DMFBB). The void volume in the bioreactor and stability of alginate/chitosan microcapsules were investigated under different flow rates. Cell viability, synthesis and metabolism functions, and expression of metabolizing enzymes at transcriptional levels in an encapsulated hepatocyte line (C3A cells) were determined. The void volume was significantly less in the novel bioreactor than in the conventional FBB. In addition, the microcapsules were less damaged in the DMFBB during the fluidization process as reflected by the results for microcapsule retention rates, swelling, and breakage. Encapsulated C3A cells exhibited greater viability and CYP1A2 and CYP3A4 activity in the DMFBB than in the FBB, although the increases in albumin and urea synthesis were less prominent. The transcription levels of several CYP450-related genes and an albumin-related gene were dramatically greater in cells in the DMFBB than in those in the FBB. Taken together, our results suggest that the DMFBB is a promising alternative for the design of a bioartificial liver system based on a fluidized bed bioreactor with encapsulated hepatocytes for treating patients with acute hepatic failure or other severe liver diseases.

Prevention of Venous Neointimal Hyperplasia by a Multitarget Receptor Tyrosine Kinase Inhibitor

BACKGROUND/AIMS

Venous neointimal hyperplasia (NH) is the predominant cause of stenosis in hemodialysis arteriovenous grafts (AVG), but there is currently no clinically used therapy to prevent NH.

METHODS

A porcine AVG model was used to identify potential pharmacological targets to prevent NH. Sunitinib, a broad-spectrum tyrosine kinase inhibitor, was examined as a potential anti-NH drug utilizing in vitro and ex vivo models.

RESULTS

In an in vivo porcine model, PDGF, VEGF and their receptors PDGFR-α and VEGFR-2 were upregulated at the venous anastomosis within 2 weeks after AVG placement, with NH development by 4 weeks. Sunitinib inhibited PDGF-stimulated proliferation, migration, phosphorylation of MAPK and PI3K/Akt proteins and changes in the expression of cell-cycle regulatory proteins in vascular smooth-muscle cells as well as VEGF-stimulated endothelial cell proliferation in vitro. In an ex vivo model, significant NH was observed in porcine vein segments perfused for 12 days under pathological shear stress. Sunitinib (100 nM) inhibited NH formation, with the intima-to-lumen area ratio decreasing from 0.45 ± 0.25 to 0.04 ± 0.02 (p < 0.05) with treatment.

CONCLUSION

These findings demonstrate sunitinib to be a potential NH-preventive drug as well as the utility of an ex vivo model to investigate pharmacotherapies under pathophysiological flow conditions.

Mechanical stretch-induced vascular hypertrophy occurs through modulation of leptin synthesis-mediated ROS formation and GATA-4 nuclear translocation

Background: Obesity and hypertension are associated with increased leptin production contributing to cardiovascular remodeling. Mechanisms involving mechanical stretch-induced leptin production and the cross talk between signaling pathways leading to vascular remodeling have not been fully elucidated.

Methods and Results: Rat portal vein (RPV) organ culture was used to investigate the effect of mechanical stretch on leptin protein expression in vascular smooth muscle cells (VSMCs). Moreover, the involvement of reactive oxygen species (ROS), the RhoA/ROCK pathway, actin cytoskeleton dynamics and the transcriptional factor GATA-4 activation in mechanical stretch-induced vascular remodeling were investigated. Stretching the RPV for 1 or 24 h significantly increased leptin protein level and ROS formation in VSMCs, which was prevented by 1 h pretreatment with the ROCK inhibitor Y-27632 and the actin cytoskeleton depolymerization agent cytochalasin D. Moreover, Western blotting and immunohistochemistry revealed that mechanical stretch or treatment with 3.1 nmol/L leptin for 24 h significantly increased actin polymerization, as reflected by an increase in the F-actin to G-actin ratio. Increases in blood vessels’ wet weight and [³H]-leucine incorporation following a 24 h treatment with conditioned media from cultured stretched RPVs indicated RPV hypertrophy. This effect was prevented by 1 h pretreatment with anti-leptin antibody, indicating leptin’s crucial role in promoting VSMC hypertrophy. As an index of GATA-4 activation, GATA-4 nuclear translocation was assessed by immunohistochemistry method. Pretreating VSMC with leptin for 1 h significantly activated GATA-4 nuclear translocation, which was potently attenuated by the NADPH oxidase inhibitor apocynin, Y-27632, and cytochalasin D.

Conclusion: Our results demonstrate that ROS formation, RhoA/ROCK pathway, and GATA-4 activation play a pivotal role in mechanical stretch-induced leptin synthesis leading to VSMC remodeling.

Arterial Expression of the Calcium-Sensing Receptor Is Maintained by Physiological Pulsation and Protects against Calcification

Vascular calcification (VC) is common in chronic kidney disease (CKD) and contributes to cardiovascular mortality. The calcium-sensing receptor (CaSR) is present in human artery, senses extracellular calcium and may directly modulate VC. Objective: to investigate the association between arterial cyclic strain, CaSR expression and VC. Methods and Results: human aortic smooth muscle cells (HAoSMC) were cultured under static or strained conditions, with exposure to CaSR agonists, the calcimimetic R568, and after CaSR silencing and over-expression. High extracellular calcium reduced CaSR expression and promoted osteochondrogenic transformation and calcium deposition. This was partially prevented by cyclic strain and exposure to R568. CaSR silencing enhanced calcification and osteochondrogenic transformation, whereas CaSR over-expression attenuated this procalcific response, demonstrating a central role for the CaSR in the response to cyclic strain and regulation of VC. In arterial explants from CKD patients (n = 11) and controls (n = 9), exposure to R568 did not significantly alter calcium deposition, osteochondrogenic markers or total artery calcium content. Conclusions: physiological mechanical strain is important for arterial homeostasis and may protect arteries from VC. The beneficial effects of cyclic strain may be mediated via the CaSR.

Variability in vascular smooth muscle cell stretch-induced responses in 2D culture

The pulsatile nature of blood flow exposes vascular smooth muscle cells (VSMCs) in the vessel wall to mechanical stress, in the form of circumferential and longitudinal stretch. Cyclic stretch evokes VSMC proliferation, apoptosis, phenotypic switching, migration, alignment, and vascular remodeling. Given that these responses have been observed in many cardiovascular diseases, a defined understanding of their underlying mechanisms may provide critical insight into the pathophysiology of cardiovascular derangements. Cyclic stretch-triggered VSMC responses and their effector mechanisms have been studied in vitro using tension systems that apply either uniaxial or equibiaxial stretch to cells grown on an elastomer-bottomed culture plate and ex vivo by stretching whole vein segments with small weights. This review will focus mainly on VSMC responses to the in vitro application of mechanical stress, outlining the inconsistencies in acquired data, and comparing them to in vivo or ex vivo findings. Major discrepancies in data have been seen in mechanical stress-induced proliferation, apoptosis, and phenotypic switching responses, depending on the stretch conditions. These discrepancies stem from variations in stretch conditions such as degree, axis, duration, and frequency of stretch, wave function, membrane coating, cell type, cell passage number, culture media content, and choice of in vitro model. Further knowledge into the variables that cause these incongruities will allow for improvement of the in vitro application of cyclic stretch.

The role of septal perforators and "myocardial bridging effect" in atherosclerotic plaque distribution in the coronary artery disease

The distribution of atherosclerotic plaque burden in the human coronary arteries is not uniform. Plaques are located mostly in the left anterior descending artery (LAD), then in the right coronary artery (RCA), circumflex branch (LCx) and the left main coronary artery (LM) in a decreasing order of frequency. In the LAD and LCx, plaques tend to cluster within the proximal segment, while in the RCA their distribution is more uniform. Several factors have been involved in this phenomenon, particularly flow patterns in the left and right coronary artery. Nevertheless, it does not explain the difference in lesion frequency between the LAD and the LCx as these are both parts of the left coronary artery. Branching points are considered to be the risk points of atherosclerosis. In the LCx, the number of side branches is lower than in the LAD or RCA and there are no septal perforators with intramuscular courses like in the proximal third of the LAD and the posterior descending artery (PDA). We hypothesized that septal branches generate disturbed flow in the LAD and PDA in a similar fashion to the myocardial bridge (myocardial bridging effect). This coronary architecture determines the non-uniform plaque distribution in coronary arteries and LAD predisposition to plaque formation.

Device-Based In Vitro Techniques for Mechanical Stimulation of Vascular Cells: A Review

The most common cause of death in the developed world is cardiovascular disease. For decades, this has provided a powerful motivation to study the effects of mechanical forces on vascular cells in a controlled setting, since these cells have been implicated in the development of disease. Early efforts in the 1970 s included the first use of a parallel-plate flow system to apply shear stress to endothelial cells (ECs) and the development of uniaxial substrate stretching techniques (Krueger et al., 1971, “An in Vitro Study of Flow Response by Cells,” J. Biomech., 4(1), pp. 31–36 and Meikle et al., 1979, “Rabbit Cranial Sutures in Vitro: A New Experimental Model for Studying the Response of Fibrous Joints to Mechanical Stress,” Calcif. Tissue Int., 28(2), pp. 13–144). Since then, a multitude of in vitro devices have been designed and developed for mechanical stimulation of vascular cells and tissues in an effort to better understand their response to in vivo physiologic mechanical conditions. This article reviews the functional attributes of mechanical bioreactors developed in the 21st century, including their major advantages and disadvantages. Each of these systems has been categorized in terms of their primary loading modality: fluid shear stress (FSS), substrate distention, combined distention and fluid shear, or other applied forces. The goal of this article is to provide researchers with a survey of useful methodologies that can be adapted to studies in this area, and to clarify future possibilities for improved research methods.

Mechanical stretch promotes matrix metalloproteinase-2 and prolyl-4-hydroxylase α1 production in human aortic smooth muscle cells via Akt-p38 MAPK-JNK signaling

Hypertension can increase mechanical stretch on the vessel wall, an important stimulus that induces collagen remodeling. Prolyl-4-hydroxylaseα1 (P4Hα1) and matrix metalloproteinases (MMPs) are essential for collagen synthesis and degradation. However, the effect of mechanical strain and collagen synthesis remains largely unknown. This study aimed to identify the effect of stretch on MMPs and P4Hα1 and the involved signaling pathways. Human aortic smooth muscle cells (HASMCs) were stimulated with mechanical stretch (0, 10% and 18% strain), and production of P4Hα1 as well as production and gelatinolytic activity of MMP-2 was force-dependently increased. Mechanical stretch at 18% also increased the expression of type I and III collagen and the phosphorylation of Akt, p38 mitogen-activated protein kinase (MAPK) and c-Jun N-terminal kinase (JNK). MMP-2 production and activity enhanced by 18% stretch were inhibited by the PI3K/Akt inhibitor LY294002. Blockade of p38 MAPK or JNK inhibited the promoting effect of stretch on P4Hα1. The in vivo model of aortic banding showed increased protein levels of MMP-2, P4Hα1 and collagen I and III in the aorta. Thus, mechanical stretch increased MMP-2 and P4Hα1 expression in HASMCs via AKT-P38 MAPK-JNK signaling, thereby inducing vascular remodeling.

Pulmonary hypertension caused by pulmonary venous hypertension

The effect of pulmonary venous hypertension (PVH) on the pulmonary circulation is extraordinarily variable, ranging from no impact on pulmonary vascular resistance (PVR) to a marked increase. The reasons for this are unknown. Both acutely reversible pulmonary vasoconstriction and pathological remodeling (especially medial hypertrophy and intimal hyperplasia) account for increased PVR when present. The mechanisms involved in vasoconstriction and remodeling are not clearly defined, but increased wall stress, especially in small pulmonary arteries, presumably plays an important role. Myogenic contraction may account for increased vascular tone and also indirectly stimulate remodeling of the vessel wall. Increased wall stress may also directly cause smooth muscle growth, migration, and intimal hyperplasia. Even long-standing and severe pulmonary hypertension (PH) usually abates with elimination of PVH, but PVH-PH is an important clinical problem, especially because PVH due to left ventricular noncompliance lacks definitive therapy. The role of targeted PH therapy in patients with PVH-PH is unclear at this time. Most prospective studies indicate that these medications are not helpful or worse, but there is ample reason to think that a subset of patients with PVH-PH may benefit from phosphodiesterase inhibitors or other agents. A different approach to evaluating possible pharmacologic therapy for PVH-PH may be required to better define its possible utility.

Cyclic mechanical stretching promotes migration but inhibits invasion of rat bone marrow stromal cells

Bone marrow stromal cells (BMSCs, also broadly known as bone marrow-derived mesenchymal stem cells) are multipotent stem cells that have a self-renewal capacity and multilineage differentiation potential. Mechanical stretching plays a vital role in regulating the proliferation and differentiation of BMSCs. However, little is known about the effects of cyclic stretching on BMSC migration and invasion. In this study, using a custom-made cell-stretching device, we studied the effects of cyclic mechanical stretching on rat BMSC migration and invasion using a Transwell Boyden Chamber. The protein secretion of matrix metalloproteinase-2 (MMP-2) and matrix metalloproteinase-9 (MMP-9) was detected by gelatin zymography, and the activation of focal adhesion kinase (FAK) and extracellular signal regulated kinase1/2 (ERK1/2) was measured by western blot. We found that cyclic mechanical stretching with 10% amplitude at 1Hz frequency for 8h promotes BMSC migration, but reduces BMSC invasion. FAK and ERK1/2 signals were activated in BMSCs after exposure to cyclic stretching. In the presence of the FAK phosphorylation blocker PF573228 or the ERK1/2 phosphorylation blocker PD98059, the cyclic-stretch-promoted migration of BMSCs was completely suppressed. On the other hand, cyclic mechanical stretching reduced the secretion of MMP-2 and MMP-9 in BMSCs, and PF573228 suppressed the cyclic-stretch-reduced secretion of MMP-2 and MMP-9. The decrease of BMSC invasion induced by mechanical stretching is partially restored by PF573228 but remained unaffected by PD98059. Taken together, these data show that cyclic mechanical stretching promotes BMSC migration via the FAK-ERK1/2 signalling pathway, but reduces BMSC invasion by decreasing secretion of MMP-2 and MMP-9 via FAK, independent of the ERK1/2 signal.

Mechanical stretch upregulates SDF-1α in skin tissue and induces migration of circulating bone marrow-derived stem cells into the expanded skin

BACKGROUND

Skin and soft tissue expansion is a procedure that stimulates skin regeneration by applying continuous mechanical stretching of normal donor skin for reconstruction purposes. We have reported that topical transplantation of bone marrow-derived mesenchymal stem cells (MSCs) can accelerate mechanical stretch induced skin regeneration. However, it is unclear how circulating MSCs respond to mechanical stretch in skin tissue.

METHODS

MSCs from luciferase-Tg Lewis rats were transplanted into a rat tissue expansion model and tracked in vivo by luminescence imaging. Expression levels of chemokines including macrophage inflammatory protein-1α, thymus and activation-regulated chemokine, secondary lymphoid tissue chemokine, cutaneous T-cell attracting chemokine, and stromal-derived factor-1α (SDF-1α) were elevated in mechanically stretched tissues, as were their related chemokine receptors in MSCs. Chemotactic assays were conducted in vitro and in vivo to assess the impact of chemokine expression on MSC migration.

RESULTS

MSC migration was observed in mechanically stretched skin. Mechanical stretching induced temporal upregulation of chemokine expression. Among all the tested chemokines, SDF-1α showed the most significant increase in stretched skin, suggesting a strong connection to migration of MSCs. The in vitro chemotactic assay showed that conditioned medium from mechanically stretched cells induced MSC migration, which could be blocked with the CXCR4 antagonist AMD3100, as effectively as medium containing 50 ng/ml rat recombinant SDF-1α. Results from in vivo study also showed that MSC migration to mechanically stretched skin was significantly blocked by AMD3100. Moreover, migrating MSCs expressed differentiation markers, suggesting a contribution of MSCs to skin regeneration through differentiation.

CONCLUSION

Mechanical stretching can upregulate SDF-1α in skin and recruit circulating MSCs through the SDF-1α/CXCR4 pathway.

Estrogen receptor beta and truncated variants enhance the expression of transfected MMP-1 promoter constructs in response to specific mechanical loading

Background

Joint diseases such as osteoarthritis (OA) predominantly afflict post-menopausal women, suggesting a pertinent role for female hormones. Estrogen receptor beta (ER-β) has been detected in connective tissues of the knee joint suggesting that these tissues are responsive to the hormone estrogen. Matrix metalloproteinase-1 (MMP-1) activity contributes to cartilage degradation, a key factor leading to OA development in synovial joints. Two polymorphic forms of MMP-1 exist due to a deletion/insertion of the guanine residue in the promoter, and the 2G allelic variant of MMP-1 exhibits more activity than the 1G allele. Previous studies have demonstrated that the polymorphic forms of the human MMP-1 are influenced by the modulating effects of estrogen receptor isoforms. In addition to hormonal influences, physiological factors such as altered mechanical loading are also contributory features of OA. In the present study, the combined influence of biomechanical and hormonal variables on the activity of MMP-1 isoforms was evaluated. We hypothesized that the combined effects of ER-β and sheer stress will differentially activate the two allelic forms of MMP-1 in a hormone-independent manner.

Methods

HIG-82 synoviocytes were transiently transfected with 1G or 2G alleles (±) ER-β and subjected to either shear or equibiaxial stress. Next, 1G/2G promoter activity was measured to determine the combined influence of physiological stimuli. Truncated ER-β constructs were used to determine the importance of different domains of ER-β on 1G/2G activation.

Results

The 2G allele exhibited a constitutively higher activity than the 1G allele, which was further increased when the transfected cells were subject to shear stress, but not equibiaxial stress. Moreover, the combination of ER-β and shear stress further increased the activity levels of the 1G/2G allelic variants. Additionally, select AF-2 truncated ER-β variants led to increased activity levels for the 2G allele, indicating the AF-1 domain was likely involved in the response to mechanical stimulation.

Conclusions

These results suggest that the 1G/2G alleles of MMP-1 are influenced by specific mechanical stimuli like shear stress, as well as the ER-β receptor. These findings contribute to the potential allelic involvement in connective tissue diseases such as OA in females compared to males.

Multiple approaches to predicting oxygen and glucose consumptions by HepG2 cells on porous scaffolds in an axial-flow bioreactor

In this study, the distribution of oxygen and glucose was evaluated along with consumption by hepatocytes using three different approaches. The methods include (i) Computational Fluid Dynamics (CFD) simulation, (ii) residence time distribution (RTD) analysis using a step-input coupled with segregation model or dispersion model, and (iii) experimentally determined consumption by HepG2 cells in an open-loop. Chitosan-gelatin (CG) scaffolds prepared by freeze-drying and polycaprolactone (PCL) scaffolds prepared by salt leaching technique were utilized for RTD analyses. The scaffold characteristics were used in CFD simulations i.e. Brinkman's equation for flow through porous medium, structural mechanics for fluid induced scaffold deformation, and advection-diffusion equation coupled with Michaelis-Menten rate equations for nutrient consumption. With the assumption that each hepatocyte behaves like a micro-batch reactor within the scaffold, segregation model was combined with RTD to determine exit concentration. A flow rate of 1 mL/min was used in the bioreactor seeded with 0.6 × 10(6) HepG2 cells/cm(3) on CG scaffolds and oxygen consumption was measured using two flow-through electrodes located at the inlet and outlet. Glucose in the spent growth medium was also analyzed. RTD results showed distribution of nutrients to depend on the surface characteristics of scaffolds. Comparisons of outlet oxygen concentrations between the simulation results, and experimental results showed good agreement with the dispersion model. Outlet oxygen concentrations from segregation model predictions were lower. Doubling the cell density showed a need for increasing the flow rate in CFD simulations. This integrated approach provide a useful strategy in designing bioreactors and monitoring tissue regeneration.

Actin and microtubules play distinct roles in governing the anisotropic deformation of cell nuclei in response to substrate strain

Physical forces arising in the cellular microenvironment have been hypothesized to play a major role in governing cell function. Moreover, it is thought that gene regulation may be sensitive to nuclear deformations taking place in response to extracellular forces over short and long timescales. Although nuclear responses to mechanical stimuli over long timescales are relatively well studied, the short-term responses are poorly understood. Therefore, to characterize the short-term instantaneous deformation of the nucleus in a mechanically dynamic environment, we exposed MDCK epithelial monolayers to varying mechanical strain fields. The results reveal that nuclei deform anisotropically in response to substrate strain, specifically, the minor nuclear axis is significantly more deformable than the major axis. We show that upon microtubule depolymerization, nuclear deformation anisotropy completely disappears. Moreover, the removal of actin causes a significant increase in nuclear deformation along the minor axis and a corresponding increase in mechanical anisotropy. The results demonstrate that the nucleus deforms in a manner that is very much dependent on the direction of strain and the characteristics of the strain field. Actin and microtubules also appear to play distinct roles in controlling the anisotropic deformation of the nucleus in response to mechanical forces that arise in the cellular microenvironment.

Biomechanical factors in atherosclerosis: mechanisms and clinical implications

Blood vessels are exposed to multiple mechanical forces that are exerted on the vessel wall (radial, circumferential and longitudinal forces) or on the endothelial surface (shear stress). The stresses and strains experienced by arteries influence the initiation of atherosclerotic lesions, which develop at regions of arteries that are exposed to complex blood flow. In addition, plaque progression and eventually plaque rupture is influenced by a complex interaction between biological and mechanical factors-mechanical forces regulate the cellular and molecular composition of plaques and, conversely, the composition of plaques determines their ability to withstand mechanical load. A deeper understanding of these interactions is essential for designing new therapeutic strategies to prevent lesion development and promote plaque stabilization. Moreover, integrating clinical imaging techniques with finite element modelling techniques allows for detailed examination of local morphological and biomechanical characteristics of atherosclerotic lesions that may be of help in prediction of future events. In this ESC Position Paper on biomechanical factors in atherosclerosis, we summarize the current 'state of the art' on the interface between mechanical forces and atherosclerotic plaque biology and identify potential clinical applications and key questions for future research.

Systemic and renal oxidative stress in the pathogenesis of hypertension: modulation of long-term control of arterial blood pressure by resveratrol

Hypertension affects over 25% of the global population and is associated with grave and often fatal complications that affect many organ systems. Although great advancements have been made in the clinical assessment and treatment of hypertension, the cause of hypertension in over 90% of these patients is unknown, which hampers the development of targeted and more effective treatment. The etiology of hypertension involves multiple pathological processes and organ systems, however one unifying feature of all of these contributing factors is oxidative stress. Once the body's natural anti-oxidant defense mechanisms are overwhelmed, reactive oxygen species (ROS) begin to accumulate in the tissues. ROS play important roles in normal regulation of many physiological processes, however in excess they are detrimental and cause widespread cell and tissue damage as well as derangements in many physiological processes. Thus, control of oxidative stress has become an attractive target for pharmacotherapy to prevent and manage hypertension. Resveratrol (trans-3,5,4'-Trihydroxystilbene) is a naturally occurring polyphenol which has anti-oxidant effects in vivo. Many studies have shown anti-hypertensive effects of resveratrol in different pre-clinical models of hypertension, via a multitude of mechanisms that include its function as an anti-oxidant. However, results have been mixed and in some cases resveratrol has no effect on blood pressure. This may be due to the heavy emphasis on peripheral vasodilator effects of resveratrol and virtually no investigation of its potential renal effects. This is particularly troubling in the arena of hypertension, where it is well known and accepted that the kidney plays an essential role in the long term regulation of arterial pressure and a vital role in the initiation, development and maintenance of chronic hypertension. It is thus the focus of this review to discuss the potential of resveratrol as an anti-hypertensive treatment via amelioration of oxidative stress within the framework of the fundamental physiological principles of long term regulation of arterial blood pressure.

Role of biomechanical forces in stem cell vascular lineage differentiation

Mechanical forces have long been known to play a role in the maintenance of vascular homeostasis in the mature animal and in developmental regulation in the fetus. More recently, it has been shown that stem cells play a role in vascular repair and remodeling in response to biomechanical stress. Laminar shear stress can directly activate growth factor receptors on stem/progenitor cells, initiating signaling pathways leading toward endothelial cell differentiation. Cyclic strain can stimulate stem cell differentiation toward smooth muscle lineages through different mechanisms. In vivo, blood flow in the coronary artery is significantly altered after stenting, leading to changes in biomechanical forces on the vessel wall. This disruption may activate stem cell differentiation into a variety of cells and cause delayed re-endothelialization. Based on progress in the research field, the present review aims to explore the role of mechanical forces in stem cell differentiation both in vivo and in vitro and to examine what this means for the application of stem cells in the clinic, in tissue engineering, and for the management of aberrant stem cell contribution to disease.

A novel stretching platform for applications in cell and tissue mechanobiology

Tools that allow the application of mechanical forces to cells and tissues or that can quantify the mechanical properties of biological tissues have contributed dramatically to the understanding of basic mechanobiology. These techniques have been extensively used to demonstrate how the onset and progression of various diseases are heavily influenced by mechanical cues. This article presents a multi-functional biaxial stretching (BAXS) platform that can either mechanically stimulate single cells or quantify the mechanical stiffness of tissues. The BAXS platform consists of four voice coil motors that can be controlled independently. Single cells can be cultured on a flexible substrate that can be attached to the motors allowing one to expose the cells to complex, dynamic, and spatially varying strain fields. Conversely, by incorporating a force load cell, one can also quantify the mechanical properties of primary tissues as they are exposed to deformation cycles. In both cases, a proper set of clamps must be designed and mounted to the BAXS platform motors in order to firmly hold the flexible substrate or the tissue of interest. The BAXS platform can be mounted on an inverted microscope to perform simultaneous transmitted light and/or fluorescence imaging to examine the structural or biochemical response of the sample during stretching experiments. This article provides experimental details of the design and usage of the BAXS platform and presents results for single cell and whole tissue studies. The BAXS platform was used to measure the deformation of nuclei in single mouse myoblast cells in response to substrate strain and to measure the stiffness of isolated mouse aortas. The BAXS platform is a versatile tool that can be combined with various optical microscopies in order to provide novel mechanobiological insights at the sub-cellular, cellular and whole tissue levels.

Computational modeling of hypertensive growth in the human carotid artery

Arterial hypertension is a chronic medical condition associated with an elevated blood pressure. Chronic arterial hypertension initiates a series of events, which are known to collectively initiate arterial wall thickening. However, the correlation between macrostructural mechanical loading, microstructural cellular changes, and macrostructural adaptation remains unclear. Here, we present a microstructurally motivated computational model for chronic arterial hypertension through smooth muscle cell growth. To model growth, we adopt a classical concept based on the multiplicative decomposition of the deformation gradient into an elastic part and a growth part. Motivated by clinical observations, we assume that the driving force for growth is the stretch sensed by the smooth muscle cells. We embed our model into a finite element framework, where growth is stored locally as an internal variable. First, to demonstrate the features of our model, we investigate the effects of hypertensive growth in a real human carotid artery. Our results agree nicely with experimental data reported in the literature both qualitatively and quantitatively.

Mechanical stretch suppresses microRNA-145 expression by activating extracellular signal-regulated kinase 1/2 and upregulating angiotensin-converting enzyme to alter vascular smooth muscle cell phenotype

Phenotype modulation of vascular smooth muscle cells (VSMCs) plays an important role in the pathogenesis of various vascular diseases, including hypertension and atherosclerosis. Several microRNAs (miRNAs) were found involved in regulating the VSMC phenotype with platelet-derived growth factor (PDGF) treatment, but the role of miRNAs in the mechanical stretch-altered VSMC phenotype is not clear. Here, we identified miR-145 as a major miRNA contributing to stretch-altered VSMC phenotype by miRNA array, quantitative RT-PCR and gain- and loss-of-function methods. Our data demonstrated that 16% stretch suppressed miR-145 expression, with reduced expression of contractile markers of VSMCs cultured on collagenI; overexpression of miR-145 could partially recover the expression in stretched cells. Serum response factor (SRF), myocardin, and Kruppel-like factor 4 (KLF4) are major regulators of the VSMC phenotype. The effect of stretch on myocardin and KLF4 protein expression was altered by miR-145 mimics, but SRF expression was not affected. In addition, stretch-activated extracellular signal-regulated kinase 1/2 (ERK1/2) and up-regulated angiotensin-converting enzyme (ACE) were confirmed to be responsible for the inhibition of miR-145 expression. Mechanical stretch inhibits miR-145 expression by activating the ERK1/2 signaling pathway and promoting ACE expression, thus modulating the VSMC phenotype.

Pressure stress reduces inducible NO synthase expression by interleukin-1β stimulation in cultured rat vascular smooth muscle cells

Elevated mechanical stress applied to vascular walls is well known to modulate vascular remodeling. We investigated the effect of pulsatile pressure stress on nitric oxide (NO) production and inducible NO synthase (iNOS) expression by interleukin-1β (IL-1β) stimulation in rat vascular smooth muscle cells (VSMCs). VSMCs were enzymatically isolated from aortic media of Wistar rats. Pulsatile pressure applied to VSMCs was repeatedly given between 80 and 160 mm Hg at a frequency of 4 cycles per min using an original apparatus. Protein expression and activation were evaluated by Western blot analysis. mRNA expression was evaluated by real-time reverse transcription-polymerase chain reaction. The pulsatile pressure reduced IL-1β-induced NO production, iNOS protein, and mRNA expression. The pressure also reduced GTP cyclohydrolase I mRNA expression. Furthermore, the pressure reduced phosphorylation of IL-1β-induced extracellular signal-regulated kinase (ERK), nuclear factor-κB (NF-κB) p65, and I-κBα. The pressure had no effect on I-κBβ degradation by IL-1β stimulation. The present study shows for the first time that pressure stress reduces IL-1β-induced iNOS expression via a mechanism involving the ERK-NF-κB signaling pathway.

Thoracic Aortic Rupture and Aortopulmonary Fistulation in the Friesian Horse

Aortic rupture in horses is a rare condition. Although it is relatively common in the Friesian breed, only limited histopathologic information is available. Twenty Friesian horses (1–10 years old) were diagnosed with aortic rupture by postmortem examination. Ruptured aortic walls were analyzed with histology and immunohistochemistry. Based on the histologic and immunohistochemical findings, these cases were divided into 3 groups: acute ( n = 4, 20%), subacute ( n = 8, 40%), and chronic ( n = 8, 40%). Features common to samples from horses in all groups included accumulation of mucoid material; disorganization and fragmentation of the elastic laminae; aortic medial smooth muscle hypertrophy; and medial necrosis of varying degrees, ranging from mild and patchy in the acute cases to severe midzonal necrosis in the chronic cases. Inflammation, most likely secondary to medial necrosis, varied from predominantly neutrophilic infiltrates in the media and periadventitial tissue in the acute group to the presence of mainly hemosiderophages in the periadventitial tissue in the chronic group. Medial fibrosis with aberrant collagen morphology was seen in the subacute group and, more commonly, in the chronic group. Only minimal changes were seen in the aortic vasa vasorum. Smooth muscle hypertrophy and accumulation of mucoid material were not related to the age of the lesions. The findings of this study suggest that a connective tissue disorder affecting elastin or collagen in the aortic media is potentially the underlying cause of aortic rupture in Friesian horses.

Angiogenic microenvironment augments impaired endothelial responses under diabetic conditions

Diabetes-induced cardiomyopathy is characterized by cardiac remodeling, fibrosis, and endothelial dysfunction, with no treatment options currently available. Hyperglycemic memory by endothelial cells may play the key role in microvascular complications in diabetes, providing a potential target for therapeutic approaches. This study tested the hypothesis that a proangiogenic environment can augment diabetes-induced deficiencies in endothelial cell angiogenic and biomechanical responses. Endothelial responses were quantified for two models of diabetic conditions: 1) an in vitro acute and chronic hyperglycemia where normal cardiac endothelial cells were exposed to high-glucose media, and 2) an in vivo chronic diabetes model where the cells were isolated from rats with type I streptozotocin-induced diabetes. Capillary morphogenesis, VEGF and nitric oxide expression, cell morphology, orientation, proliferation, and apoptosis were determined for cells cultured on Matrigel or proangiogenic nanofiber hydrogel. The effects of biomechanical stimulation were assessed following cell exposure to uniaxial strain. The results demonstrate that diabetes alters cardiac endothelium angiogenic response, with differential effects of acute and chronic exposure to high-glucose conditions, consistent with the concept that endothelial cells may have a long-term "hyperglycemic memory" of the physiological environment in the body. Furthermore, endothelial cell exposure to strain significantly diminishes their angiogenic potential following strain application. Both diabetes and strain-associated deficiencies can be augmented in the proangiogenic nanofiber microenvironment. These findings may contribute to the development of novel approaches to reverse hyperglycemic memory of endothelium and enhance vascularization of the diabetic heart, where improved angiogenic and biomechanical responses can be the key factor to successful therapy.

Magnetoactive sponges for dynamic control of microfluidic flow patterns in microphysiological systems

We developed a microfluidic flow-control system capable of dynamically generating various flow patterns on demand. The flow-control system is based on novel magnetoactive sponges embedded in microfluidic flow channels. Applying a non-uniform magnetic field compresses the magnetoactive sponge, significantly reducing porosity and hydraulic conductivity. Tuning the applied magnetic field can dynamically vary the flow rate in the microfluidic channel. Pulsatile and physiological flow patterns with frequency between 1 and 3 Hz, flow rates between 0.5 and 10 μL min(-1) and duration over 3 weeks have been achieved. Smooth muscle cells in engineered blood vessels perfused for 7 days aligned perpendicular to the flow direction under pulsatile but not steady flow, similar to the in vivo orientation. Owing to its various advantages over traditional flow-control methods, the new system potentially has important applications in microfluidic-based microphysiological systems to simulate the physiological nature of blood flow.

Impact of cyclic stretch on induced elastogenesis within collagenous conduits

In vitro tissue engineering of vascular conduits requires a synergy between several external factors, including biochemical supplementation and mechanotranductive stimulation. The goal of this study was to improve adult human vascular smooth muscle cell orientation and elastic matrix synthesis within 3D tubular collagen gel constructs. We used a combination of elastogenic factors (EFs) previously tested in our lab, along with cyclic circumferential strains at low amplitude (2.5%) delivered at a range of frequencies (0.5, 1.5, and 3 Hz). After 21 days of culture, the constructs were analyzed for elastic matrix outcomes, activity of matrix metalloproteinases (MMPs)-2 and -9, cell densities and phenotype, and mechanical properties of constructs. While cell densities remained unaffected by the addition of stretch, contractile phenotypic markers were elevated in all stretched constructs relative to control. Constructs cultured with EFs stretched at 1.5 Hz exhibited the maximum elastin mRNA expression and total matrix elastin (over sixfold vs. the static EFs control). MMP-2 content was comparable in all treatment conditions, but MMP-9 levels were elevated at the higher frequencies (1.5 and 3 Hz). Minimal circumferential orientation was achieved and the mechanical properties remained comparable among the treatment conditions. Overall, constructs treated with EFs and stretched at 1.5 Hz exhibited the most elastogenic outcomes.

The role of SIRT6 in the differentiation of vascular smooth muscle cells in response to cyclic strain

Vascular smooth muscle cells (VSMCs) may switch their phenotype between a quiescent contractile phenotype and a synthetic phenotype in response to cyclic strain, and this switch may contribute to hypertension, atherosclerosis, and restenosis. SIRT 6 is a member of the sirtuin family, and plays an important role in different cell processes, including differentiation. We hypothesized that cyclic strain modulates the differentiation of VSMCs via a transforming growth factor-β1 (TGF-β1)-Smad-SIRT6 pathway. VSMCs were subjected to cyclic strain using a Flexercell strain unit. It was demonstrated that the strain stimulated the secretion of TGF-β1 into the supernatant of VSMCs. After exposed to the strain, the expressions of contractile phenotype markers, including smooth muscle protein 22 alpha, alpha-actin, and calponin, and phosphorylated Smad2, phosphorylated Smad5, SIRT6 and c-fos were up-regulated in VSMCs by western blot and immunofluorescence. And the expression of intercellular-adhesion molecule-1 (ICAM-1) was also increased detected by flow cytometry. The strained-induced up-regulation of SIRT6 was blocked by a TGF-β1 neutralizing antibody. Furthermore, the effects of strain on VSMCs were abrogated by SIRT6-specific siRNA transfection via the suppression c-fos and ICAM-1. These results suggest that SIRT6 may play a critical role in the regulation of VSMC differentiation in response to the cyclic strain.

Estimated in vivo postnatal surface growth patterns of the ovine main pulmonary artery and ascending aorta

Delineating the normal postnatal development of the pulmonary artery (PA) and ascending aorta (AA) can inform our understanding of congenital abnormalities, as well as pulmonary and systolic hypertension. We thus conducted the following study to delineate the PA and AA postnatal growth deformation characteristics in an ovine model. MR images were obtained from endoluminal surfaces of 11 animals whose ages ranged from 1.5 months/15.3 kg mass (very young) to 12 months/56.6 kg mass (adult). A bicubic Hermite finite element surface representation was developed for the each artery from each animal. Under the assumption that the relative locations of surface points were retained during growth, the individual animal surface fits were subsequently used to develop a method to estimate the time-evolving local effective surface growth (relative to the youngest measured animal) in the end-diastolic state. Results indicated that the spatial and temporal surface growth deformation patterns of both arteries, especially in the circumferential direction, were heterogeneous, leading to an increase in taper and increase in cross-sectional ellipticity of the PA. The longitudinal PA growth stretch of a large segment on the posterior wall reached 2.57 ± 0.078 (mean ± SD) at the adult stage. In contrast, the longitudinal growth of the AA was smaller and more uniform (1.80 ± 0.047). Interestingly, a region of the medial wall of both arteries where both arteries are in contact showed smaller circumferential growth stretches-specifically 1.12 ± 0.012 in the PA and 1.43 ± 0.071 in the AA at the adult stage. Overall, our results indicated that contact between the PA and AA resulted in increasing spatial heterogeneity in postnatal growth, with the PA demonstrating the greatest changes. Parametric studies using simplified geometric models of curved arteries during growth suggest that heterogeneous effective surface growth deformations must occur to account for the changes in measured arterial shapes during the postnatal growth period. This result suggests that these first results are a reasonable first-approximation to the actual effective growth patterns. Moreover, this study clearly underscores how functional growth of the PA and AA during postnatal maturation involves complex, local adaptations in tissue formation. Moreover, the present results will help to lay the basis for functional replacement by defining critical geometric metrics.

Fluid Mechanics, Arterial Disease, and Gene Expression

This review places modern research developments in vascular mechanobiology in the context of hemodynamic phenomena in the cardiovascular system and the discrete localization of vascular disease. The modern origins of this field are traced, beginning in the 1960s when associations between flow characteristics, particularly blood flow-induced wall shear stress, and the localization of atherosclerotic plaques were uncovered, and continuing to fluid shear stress effects on the vascular lining endothelial) cells (ECs), including their effects on EC morphology, biochemical production, and gene expression. The earliest single-gene studies and genome-wide analyses are considered. The final section moves from the ECs lining the vessel wall to the smooth muscle cells and fibroblasts within the wall that are fluid me chanically activated by interstitial flow that imposes shear stresses on their surfaces comparable with those of flowing blood on EC surfaces. Interstitial flow stimulates biochemical production and gene expression, much like blood flow on ECs.

Nonphysiologic blood flow triggers endothelial and arterial remodeling in vivo: implications for novel left ventricular assist devices with a peripheral anastomosis

OBJECTIVES

Less invasive circulatory support devices have been developed that require anastomosis to a peripheral artery. The Symphony Heart Assist System (Abiomed, Inc, Danvers, Mass) is a volume-displacement pump sewn to the subclavian artery to provide partial circulatory support. The surgical configuration produces nonphysiologic blood pressure and bidirectional flow in the subclavian artery. Our objective was to identify effects of altered hemodynamics on arterial structure and function.

METHODS

In calves (n = 23; 80-100 kg), the Symphony pump was sewn end-to-side to the carotid artery. Acutely, carotid blood pressure and flow were recorded to evaluate hemodynamic changes. After medium-term support (1-4 weeks), carotid artery was studied. Histologic and molecular assays evaluated architectural changes. Quantitative real-time polymerase chain reaction evaluated gene expression of matrix metalloproteinase (MMP)-2, MMP-9, and connective tissue growth factor. In vitro carotid arterial-ring studies evaluated physiologic responses.

RESULTS

During Symphony support, carotid arterial pressure was 200/15 mm Hg. Antegrade flow increased significantly (P < .05) from 1.40 ± 0.32 to 4.29 ± 0.33 L/min. Flow during native cardiac diastole reversed completely from 0.25 ± 0.05 to -4.15 ± 0.38 L/min in carotid artery proximal to the anastomosis. After medium-term support, the carotid artery was significantly dilated with significantly thinner tunica media and thicker tunica adventitia than in control carotid arteries. MMP-9 gene expression decreased significantly, connective tissue growth factor gene expression increased significantly, and collagen, elastin, and total extracellular matrix increased significantly. Endothelial cells were significantly hypertrophied and produced significantly more von Willebrand factor. Endothelial apoptosis increased significantly. Platelet-endothelial interactions decreased significantly. Endothelial-independent contraction decreased significantly, whereas endothelial-dependent relaxation increased modestly.

CONCLUSIONS

Assisted circulation with a left ventricular assist device triggered arterial remodeling that allowed a peripheral artery to accommodate the altered hemodynamics of a novel partial-support pump. Further delineation of remodeling pathways may be of significance for the emerging field of partial circulatory support.

A novel system for studying mechanical strain waveform-dependent responses in vascular smooth muscle cells

While many studies have examined the effects mechanical forces on vSMCs, there is a limited understanding of how the different arterial strain waveforms that occur in disease and different vascular beds alter vSMC mechanotransduction and phenotype. Here, we present a novel system for applying complex, time-varying strain waveforms to cultured cells and use this system to understand how these waveforms can alter vSMC phenotype and signaling. We have developed a highly adaptable cell culture system that allows the application of mechanical strain to cells in culture and can reproduce the complex dynamic mechanical environment experienced by arterial cells in the body. Using this system, we examined whether the type of applied strain waveform altered phenotypic modulation of vSMCs by mechanical forces. Cells exposed to the brachial waveform had increased phosphorylation of AKT, EGR-1, c-Fos expression and cytoskeletal remodeling in comparison to cells treated with the aortic waveform. In addition, vSMCs exposed to physiological waveforms had adopted a more differentiated phenotype in comparison to those treated with static or sinusoidal cyclic strain, with increased expression of vSMC markers desmin, calponin and SM-22 as well as increased expression of regulatory miRNAs including miR-143, -145 and -221. Taken together, our studies demonstrate the development of a novel system for applying complex, time-varying mechanical forces to cells in culture. In addition, we have shown that physiological strain waveforms have powerful effects on vSMC phenotype.

Uniaxial cell stretching device for live-cell imaging of mechanosensitive cellular functions

External mechanical stretch plays an important role in regulating cellular behaviors through intracellular mechanosensitive and mechanotransductive machineries such as the F-actin cytoskeleton (CSK) structures and focal adhesions (FAs) anchoring the F-actin CSK to the extracellular environment. Studying the mechanoresponsive behaviors of the F-actin CSK and FAs in response to cell stretch has great importance for further understanding mechanotransduction and mechanobiology. In this work, we developed a novel cell stretching device combining dynamic directional cell stretch with in situ subcellular live-cell imaging. Using a cam and follower mechanism and applying a standard mathematical model for cam design, we generated different dynamic stretch outputs. By examining stretch-mediated FA dynamics under step-function static stretch and the realignment of cell morphology and the F-actin CSK under cyclic stretch, we demonstrated successful applications of our cell stretching device for mechanobiology studies where external stretch plays an important role in regulating subcellular molecular dynamics and cellular phenotypes.

Biomechanical regulation of vascular smooth muscle cell functions: from in vitro to in vivo understanding

Vascular smooth muscle cells (VSMCs) have critical functions in vascular diseases. Haemodynamic factors are important regulators of VSMC functions in vascular pathophysiology. VSMCs are physiologically active in the three-dimensional matrix and interact with the shear stress sensor of endothelial cells (ECs). The purpose of this review is to illustrate how haemodynamic factors regulate VSMC functions under two-dimensional conditions in vitro or three-dimensional co-culture conditions in vivo. Recent advances show that high shear stress induces VSMC apoptosis through endothelial-released nitric oxide and low shear stress upregulates VSMC proliferation and migration through platelet-derived growth factor released by ECs. This differential regulation emphasizes the need to construct more actual environments for future research on vascular diseases (such as atherosclerosis and hypertension) and cardiovascular tissue engineering.

Mechanical stretching induces matrix metalloproteinase-2 expression in rat retinal glial (Müller) cells

Pathological myopia, as one of the leading causes of blindness, is characterized by excessive and progressive elongation of the eyeball with concomitant degenerative changes in the posterior segment of the eye. During the progressive distension of the posterior pole, the retina, choroid, and sclera are subjected to constant mechanical force, as a result of which, tissue remodeling occurs. Active remodeling of the sclera in myopia has been studied intensively. By comparison, retinal remodeling under mechanical stretching has attracted little attention, and further research is therefore required. In this study, we showed that constant mechanical stretching of rat retinal Müller cells for 24 h led to a significant increase in the intracellular matrix metalloproteinase-2 mRNA and protein levels. The extracellular secretory matrix metalloproteinase-2 protein levels and activity were also enhanced. These results suggest a possible novel molecular mechanism that would account for retinal remodeling in many ocular diseases in which the retina is often overstretched, such as pathological myopia and proliferative vitreoretinopathy.

Mechanical stretch increases MMP-2 production in vascular smooth muscle cells via activation of PDGFR-β/Akt signaling pathway

Increased blood pressure, leading to mechanical stress on vascular smooth muscle cells (VSMC), is a known risk factor for vascular remodeling via increased activity of matrix metalloproteinase (MMP) within the vascular wall. This study aimed to identify cell surface mechanoreceptors and intracellular signaling pathways that influence VSMC to produce MMP in response to mechanical stretch (MS). When VSMC was stimulated with MS (0–10% strain, 60 cycles/min), both production and gelatinolytic activity of MMP-2, but not MMP-9, were increased in a force-dependent manner. MS-enhanced MMP-2 expression and activity were inhibited by molecular inhibition of Akt using Akt siRNA as well as by PI3K/Akt inhibitors, LY293002 and AI, but not by MAPK inhibitors such as PD98059, SP600125 and SB203580. MS also increased Akt phosphorylation in VSMC, which was attenuated by AG1295, a PDGF receptor (PDGFR) inhibitor, but not by inhibitors for other receptor tyrosine kinase including EGF, IGF, and FGF receptors. Although MS activated PDGFR-α as well as PDGFR-β in VSMC, MS-induced Akt phosphorylation was inhibited by molecular deletion of PDGFR-β using siRNA, but not by inhibition of PDGFR-α. Collectively, our data indicate that MS induces MMP-2 production in VSMC via activation of Akt pathway, that is mediated by activation of PDGFR-β signaling pathways.

Effects of cyclic stretch on the molecular regulation of myocardin in rat aortic vascular smooth muscle cells

BACKGROUND

The expression of myocardin, a cardiac-restricted gene, increases during environmental stress. How mechanical stretch affects the regulation of myocardin in vascular smooth muscle cells (VSMCs) is not fully understood. We identify the mechanisms and pathways through which mechanical stretch induces myocardin expression in VSMCs.

RESULTS

Rat VSMCs grown on a flexible membrane base were stretched to 20% of maximum elongation, at 60 cycles per min. An in vivo model of aorta-caval shunt in adult rats was also used to investigate myocardin expression. Cyclic stretch significantly increased myocardin and angiotensin II (AngII) expression after 18 and 6 h of stretch. Addition of extracellular signal-regulated kinases (ERK) pathway inhibitor (PD98059), ERK small interfering RNA (siRNA), and AngII receptor blocker (ARB; losartan) before stretch inhibited the expression of myocardin protein. Gel shift assay showed that myocardin-DNA binding activity increased after stretch. PD98059, ERK siRNA and ARB abolished the binding activity induced by stretch. Stretch increased while myocardin-mutant plasmid, PD98059, and ARB abolished the promoter activity. Protein synthesis by measuring [3H]proline incorporation into the cells increased after cyclic stretch, which represented hypertrophic change of VSMCs. An in vivo model of aorta-caval shunt also demonstrated increased myocardin protein expression in the aorta. Confocal microscopy showed increased VSMC size 24 h after cyclic stretch and VSMC hypertrophy after creation of aorta-caval shunt for 3 days.

CONCLUSIONS

Cyclic stretch enhanced myocardin expression mediated by AngII through the ERK pathway in cultured rat VSMCs. These findings suggest that myocardin plays a role in stretch-induced VSMC hypertrophy.

Gene expression changes under cyclic mechanical stretching in rat retinal glial (Müller) cells

Objective

The retina is subjected to tractional forces in various conditions. As the predominant glial element in the retina, Müller cells are active players in all forms of retinal injury and disease. In this study, we aim to identify patterns of gene expression changes induced by cyclic mechanical stretching in Müller cells.

Methods

Rat Müller cells were seeded onto flexible bottom culture plates and subjected to a cyclic stretching regimen of 15% equibiaxial stretching for 1 and 24 h. RNA was extracted and amplified, labeled, and hybridized to rat genome microarrays. The expression profiles were analyzed using GeneSpring software, and gene ontology analysis and the Kyoto Encyclopedia of Genes and Genomes (KEGG) were used to select, annotate, and visualize genes by function and pathway. The selected genes of interest were further validated by Quantitative Real-time PCR (qPCR).

Results

Microarray data analysis showed that at 1 and 24 h, the expression of 532 and 991 genes in the Müller cells significantly (t-test, p<0.05) differed between the mechanically stretched and unstretched groups. Of these genes, 56 genes at 1 h and 62 genes at 24 h showed more than a twofold change in expression. Several genes related to response to stimulus (e.g., Egr2, IL6), cell proliferation (e.g., Areg, Atf3), tissue remodeling (e.g., PVR, Loxl2), and vasculogenesis (e.g., Epha2, Nrn1) were selected and validated by qPCR. KEGG pathway analysis showed significant changes in MAPK signaling at both time points.

Conclusions

Cyclic mechanical strain induces extensive changes in the gene expression in Müller cells through multiple molecular pathways. These results indicate the complex mechanoresponsive nature of Müller cells, and they provide novel insights into possible molecular mechanisms that would account for many retinal diseases in which the retina is often subjected to mechanical forces, such as pathological myopia and proliferative vitreoretinopathy.

Biological responses in stented arteries

Vascular walls change their dimension and mechanical properties in response to injury such as balloon angioplasty and endovascular stent implantation. Placement of bare metal stents induces neointimal proliferation/restenosis which progresses through different phases of repair with time involving a cascade of cellular reactions. These phases just like wound healing comprise distinct steps consisting of thrombosis, inflammation, proliferation, and migration followed by remodelling. It is noteworthy that animals show a rapid progression of healing after stent deployment compared with man. During stenting, endothelial cells are partially to completely destroyed or crushed along with medial wall injury and stretching promoting activation of platelets, and thrombus formation accompanied by inflammatory reaction. Macrophages and platelets play a central role through the release of cytokines and growth factors that induce vascular smooth muscle cell accumulation within the intima. Smooth muscle cells undergo complex phenotypic changes including migration and proliferation from the media towards the intima, and transition from a contractile to a synthetic phenotype; the molecular mechanisms responsible for this change are highlighted in this review. Since studies in animals and man show that smooth muscle cells play a dominant role in restenosis, drugs like rapamycin and paclitaxel have been coated on stent with polymers to allow local slow release of drugs, which have resulted in dramatic reduction of restenosis that was once the Achilles' heel of interventional cardiologists.