Morphological response of human endothelial cells subjected to cyclic strain in vitro

Microfluidic endothelial cell culture model to replicate disturbed flow conditions seen in atherosclerosis susceptible regions

Atherosclerotic lesions occur non-randomly at vascular niches in bends and bifurcations where fluid flow can be characterized as "disturbed" (low shear stress with both forward and retrograde flow). Endothelial cells (ECs) at these locations experience significantly lower average shear stress without change in the levels of pressure or strain, which affects the local balance in mechanical stresses. Common in vitro models of atherosclerosis focus primarily on shear stress without accounting for pressure and strain loading. To overcome this limitation, we used our microfluidic endothelial cell culture model (ECCM) to achieve accurate replication of pressure, strain, and shear stress waveforms associated with both normal flow seen in straight sections of arteries and disturbed flow seen in the abdominal aorta in the infrarenal segment at the wall distal to the inferior mesenteric artery (IMA), which is associated with high incidence of atherosclerotic lesion formation. Human aortic endothelial cells (HAECs) were cultured within the ECCM under both normal and disturbed flow and evaluated for cell shape, cytoskeletal alignment, endothelial barrier function, and inflammation using immunofluorescence microscopy and flow cytometry. Results clearly demonstrate quantifiable differences between cells cultured under disturbed flow conditions, which are cuboidal with short and randomly oriented actin microfilaments and show intermittent expression of β-Catenin and cells cultured under normal flow. However, in the absence of pro-inflammatory stimulation, the levels of expression of activation markers: intra cellular adhesion molecule-1 (ICAM-1), vascular cell adhesion molecule-1 (VCAM-1), platelet endothelial cell adhesion molecule-1 (PECAM-1), and vascular endothelial cell growth factor - receptor 2 (VEGF-R2) known to be involved in the initiation of plaque formation were only slightly higher in HAECs cultured under disturbed flow in comparison to cells cultured under normal flow.

Loss of mechanical strain impairs abdominal wall fibroblast proliferation, orientation, and collagen contraction function

BACKGROUND

Laparotomy wound load forces are reduced when dehiscence and incisional hernia formation occur. The purpose of this study was to determine the effects of strain loss on abdominal fascial fibroblast proliferation, orientation, and collagen compaction function.

METHODS

Cultured rat linea alba fibroblasts were subjected to continuous cyclic strain (CS), CS interrupted at 24 or 48 hours followed by culture at rest (IS-24 and IS-48) or were cultured without mechanical strain (NS). Cell number was measured and images analyzed for cell orientation. Fibroblasts from these groups were seeded onto the surface of (FPCL-S) or mixed into (FPCL-M) a collagen gel matrix and gel area was measured over time.

RESULTS

Continuous strain stimulated proliferation when compared with the nonstrained cells. The loss of strain (IS) delayed proliferation compared with CS throughout (P < .05). CS fibroblasts aligned perpendicular to the direction of strain within 12 hours. Within 12 hours of strain loss, IS-48 fibroblasts became significantly less aligned (P < .0001), and seemed similar to the randomly organized NS fibroblasts 48 hours after strain removal. The CS and IS-24 groups demonstrated faster and greater overall FPCL-M compaction than both the IS-48 and NS groups (P < .0002). The CS group contracted the gel faster than the NS group in FPCL-S (P = .029).

CONCLUSION

Mechanical strain rapidly induces a proliferative, morphologic, and functional response in abdominal wall fibroblasts that is dependent on the continued presence of the strain signal and quickly lost when the load force is removed. The loss of wound edge tension that occurs during laparotomy wound separation and hernia formation may contribute to impaired wound healing through loss of a key stimulatory mechanical signal with important implications for abdominal wall reconstruction.

A zyxin-mediated mechanism for actin stress fiber maintenance and repair

To maintain mechanical homeostasis, cells must recognize and respond to changes in cytoskeletal integrity. By imaging live cells expressing fluorescently tagged cytoskeletal proteins, we observed that actin stress fibers undergo local, acute, force-induced elongation and thinning events that compromise their stress transmission function, followed by stress fiber repair that restores this capability. The LIM protein zyxin rapidly accumulates at sites of strain-induced stress fiber damage and is essential for stress fiber repair and generation of traction force. Zyxin promotes recruitment of the actin regulatory proteins α-actinin and VASP to compromised stress fiber zones. α-Actinin plays a critical role in restoration of actin integrity at sites of local stress fiber damage, whereas both α-actinin and VASP independently contribute to limiting stress fiber elongation at strain sites, thus promoting stabilization of the stress fiber. Our findings demonstrate a mechanism for rapid repair and maintenance of the structural integrity of the actin cytoskeleton.

Reconstituting organ-level lung functions on a chip

Here, we describe a biomimetic microsystem that reconstitutes the critical functional alveolar-capillary interface of the human lung. This bioinspired microdevice reproduces complex integrated organ-level responses to bacteria and inflammatory cytokines introduced into the alveolar space. In nanotoxicology studies, this lung mimic revealed that cyclic mechanical strain accentuates toxic and inflammatory responses of the lung to silica nanoparticles. Mechanical strain also enhances epithelial and endothelial uptake of nanoparticulates and stimulates their transport into the underlying microvascular channel. Similar effects of physiological breathing on nanoparticle absorption are observed in whole mouse lung. Mechanically active "organ-on-a-chip" microdevices that reconstitute tissue-tissue interfaces critical to organ function may therefore expand the capabilities of cell culture models and provide low-cost alternatives to animal and clinical studies for drug screening and toxicology applications.

Stretch magnitude and frequency-dependent actin cytoskeleton remodeling in alveolar epithelia

Alveolar epithelial cells (AEC) maintain integrity of the blood-gas barrier with gasket-like intercellular tight junctions (TJ) that are anchored internally to the actin cytoskeleton. We hypothesize that stretch rapidly reorganizes actin (<10 min) into a perijunctional actin ring (PJAR) in a manner that is dependent on magnitude and frequency of the stretch, accompanied by spontaneous movement of actin-anchored receptors at the plasma membrane. Primary AEC monolayers were stretched biaxially to create a change in surface area (DeltaSA) of 12%, 25%, or 37% in a cyclic manner at 0.25 Hz for up to 60 min, or held tonic at 25% DeltaSA for up to 60 min, or left unstretched. By 10 min of stretch PJARs were evident in 25% and 37% DeltaSA at 0.25 Hz, but not for 12% DeltaSA at 0.25 Hz, or at tonic 25% DeltaSA, or with no stretch. Treatment with 1 muM jasplakinolide abolished stretch-induced PJAR formation, however. As a rough index of remodeling rate, we measured spontaneous motions of 5-mum microbeads bound to actin focal adhesion complexes on the apical membrane surfaces; within 1 min of exposure to DeltaSA of 25% and 37%, these motions increased substantially, increased with increasing stretch frequency, and were consistent with our mechanistic hypothesis. With a tonic stretch, however, the spontaneous motion of microbeads attenuated back to unstretched levels, whereas PJAR remained unchanged. Stretch did not increase spontaneous microbead motion in human alveolar epithelial adenocarcinoma A549 monolayers, confirming that this actin remodeling response to stretch was a cell-type specific response. In summary, stretch of primary rat AEC monolayers forms PJARs and rapidly reorganized actin binding sites at the plasma membrane in a manner dependent on stretch magnitude and frequency.

Optimal matrix rigidity for stress fiber polarization in stem cells

The shape and differentiation of human mesenchymal stem cells is especially sensitive to the rigidity of their environment; the physical mechanisms involved are unknown. A theoretical model and experiments demonstrate here that the polarization/alignment of stress-fibers within stem cells is a non-monotonic function of matrix rigidity. We treat the cell as an active elastic inclusion in a surrounding matrix whose polarizability, unlike dead matter, depends on the feedback of cellular forces that develop in response to matrix stresses. The theory correctly predicts the monotonic increase of the cellular forces with the matrix rigidity and the alignment of stress-fibers parallel to the long axis of cells. We show that the anisotropy of this alignment depends non-monotonically on matrix rigidity and demonstrate it experimentally by quantifying the orientational distribution of stress-fibers in stem cells. These findings offer a first physical insight for the dependence of stem cell differentiation on tissue elasticity.

Theoretical concepts and models of cellular mechanosensing

Recent discoveries have established that mechanical properties of the cellular environment such as its rigidity, geometry, and external stresses play an important role in determining the cellular function and fate. Mechanical properties have been shown to influence cell shape and orientation, regulate cell proliferation and differentiation, and even govern the development and organization of tissues. In recent years, many theoretical and experimental investigations have been carried out to elucidate the mechanisms and consequences of the mechanosensitivity of cells. In this review, we discuss recent theoretical concepts and approaches that explain and predict cell mechanosensitivity. We focus on the interplay of active and passive processes that govern cell-cell and cell-matrix interactions and discuss the role of this interplay in the processes of cell adhesion, regulation of cytoskeleton mechanics and the response of cells to applied mechanical stresses.

Mechanical Stability Determines Stress Fiber and Focal Adhesion Orientation

It is well documented in a variety of adherent cell types that in response to anisotropic signals from the microenvironment cells alter their cytoskeletal organization. Previous theoretical studies of these phenomena were focused primarily on the elasticity of cytoskeletal actin stress fibers (SFs) and of the substrate while the contribution of focal adhesions (FAs) through which the cytoskeleton is linked to the external environment has not been considered. Here we propose a mathematical model comprised of a single linearly elastic SF and two identical linearly elastic FAs of a finite size at the endpoints of the SF to investigate cytoskeletal realignment in response to uniaxial stretching of the substrate. The model also includes the contribution of the chemical potential energies of the SF and the FAs to the total potential energy of the SF-FA assembly. Using the global (Maxwell's) stability criterion, we predict stable configurations of the SF-FA assembly in response to substrate stretching. Model predictions obtained for physiologically feasible values of model parameters are consistent with experimental data from the literature. The model shows that elasticity of SFs alone can not predict their realignment during substrate stretching and that geometrical and elastic properties of SFs and FAs need to be included.

Effect of focal adhesion proteins on endothelial cell adhesion, motility and orientation response to cyclic strain

Focal adhesion proteins link cell surface integrins and intracellular actin stress fibers and therefore play an important role in mechanotransduction and cell motility. When endothelial cells are subjected to cyclic mechanical strain, time-lapse imaging revealed that cells underwent significant morphological changes with their resultant long axes aligned away from the strain direction. To explore how this response is regulated by focal adhesion-associated proteins the expression levels of paxillin, focal adhesion kinase (FAK), and zyxin were knocked down using gene silencing techniques. In addition, rescue of endogenous and two mutant zyxins were used to investigate the specific role of zyxin interactions. Cells with decreased zyxin expression levels and rescue with the mutant lacking zyxin/alpha-actinin binding exhibited lower orientation angles after comparable times of stretching as compared to normal and control cells. However, knockdown of the expression levels of paxillin and FAK and rescue with the mutant lacking zyxin/VASP (vasodilator-stimulated phosphoprotein) binding did not significantly affect the degree of cell orientation. In addition, wound closure speed and cell-substratum adhesive strength were observed to be significantly reduced only for cells with zyxin depletion and the mutation lacking zyxin/alpha-actinin binding. These results suggest that zyxin and its interaction with alpha-actinin are important in the regulation of endothelial cell adhesive strength, motility and orientation response to mechanical stretching.

Quantification of the temporal evolution of collagen orientation in mechanically conditioned engineered cardiovascular tissues

Load-bearing soft tissues predominantly consist of collagen and exhibit anisotropic, non-linear visco-elastic behavior, coupled to the organization of the collagen fibers. Mimicking native mechanical behavior forms a major goal in cardiovascular tissue engineering. Engineered tissues often lack properly organized collagen and consequently do not meet in vivo mechanical demands. To improve collagen architecture and mechanical properties, mechanical stimulation of the tissue during in vitro tissue growth is crucial. This study describes the evolution of collagen fiber orientation with culture time in engineered tissue constructs in response to mechanical loading. To achieve this, a novel technique for the quantification of collagen fiber orientation is used, based on 3D vital imaging using multiphoton microscopy combined with image analysis. The engineered tissue constructs consisted of cell-seeded biodegradable rectangular scaffolds, which were either constrained or intermittently strained in longitudinal direction. Collagen fiber orientation analyses revealed that mechanical loading induced collagen alignment. The alignment shifted from oblique at the surface of the construct towards parallel to the straining direction in deeper tissue layers. Most importantly, intermittent straining improved and accelerated the alignment of the collagen fibers, as compared to constraining the constructs. Both the method and the results are relevant to create and monitor load-bearing tissues with an organized anisotropic collagen network.

Stretch-activated force shedding, force recovery, and cytoskeletal remodeling in contractile fibroblasts

The stress fiber network within contractile fibroblasts structurally reinforces and provides tension, or "tone", to tissues such as those found in healing wounds. Stress fibers have previously been observed to polymerize in response to mechanical forces. We observed that, when stretched sufficiently, contractile fibroblasts diminished the mechanical tractions they exert on their environment through depolymerization of actin filaments then restored tissue tension and rebuilt actin stress fibers through staged Ca(++)-dependent processes. These staged Ca(++)-modulated contractions consisted of a rapid phase that ended less than a minute after stretching, a plateau of inactivity, and a final gradual phase that required several minutes to complete. Active contractile forces during recovery scaled with the degree of rebuilding of the actin cytoskeleton. This complementary action demonstrates a programmed regulatory mechanism that protects cells from excessive stretch through choreographed active mechanical and biochemical healing responses.

Tumor-derived endothelial cells exhibit aberrant Rho-mediated mechanosensing and abnormal angiogenesis in vitro

Tumor blood vessels exhibit abnormal structure and function that cause disturbed blood flow and high interstitial pressure, which impair delivery of anti-cancer agents. Past efforts to normalize the tumor vasculature have focused on inhibition of soluble angiogenic factors, such as VEGF; however, capillary endothelial (CE) cell growth and differentiation during angiogenesis are also influenced by mechanical forces conveyed by the extracellular matrix (ECM). Here, we explored the possibility that tumor CE cells form abnormal vessels because they lose their ability to sense and respond to these physical cues. These studies reveal that, in contrast to normal CE cells, tumor-derived CE cells fail to reorient their actin cytoskeleton when exposed to uniaxial cyclic strain, exhibit distinct shape sensitivity to variations in ECM elasticity, exert greater traction force, and display an enhanced ability to retract flexible ECM substrates and reorganize into tubular networks in vitro. These behaviors correlate with a constitutively high level of baseline activity of the small GTPase Rho and its downstream effector, Rho-associated kinase (ROCK). Moreover, decreasing Rho-mediated tension by using the ROCK inhibitor, Y27632, can reprogram the tumor CE cells so that they normalize their reorientation response to uniaxial cyclic strain and their ability to form tubular networks on ECM gels. Abnormal Rho-mediated sensing of mechanical cues in the tumor microenvironment may therefore contribute to the aberrant behaviors of tumor CE cells that result in the development of structural abnormalities in the cancer microvasculature.

Two characteristic regimes in frequency-dependent dynamic reorientation of fibroblasts on cyclically stretched substrates

Cells adherent on a cyclically stretched substrate with a periodically varying uniaxial strain are known to dynamically reorient nearly perpendicular to the strain direction. We investigate the dynamic reorientation of rat embryonic and human fibroblast cells over a range of stretching frequency from 0.0001 to 20 s(-1) and strain amplitude from 1% to 15%. We report quantitative measurements that show that the mean cell orientation changes exponentially with a frequency-dependent characteristic time from 1 to 5 h. At subconfluent cell densities, this characteristic time for reorientation shows two characteristic regimes as a function of frequency. For frequencies below 1 s(-1), the characteristic time decreases with a power law as the frequency increases. For frequencies above 1 s(-1), it saturates at a constant value. In addition, a minimum threshold frequency is found below that no significant cell reorientation occurs. Our results are consistent for the two different fibroblast types and indicate a saturation of molecular mechanisms of mechanotransduction or response machinery for subconfluent cells within the frequency regime under investigation. For confluent cell layers, we observe similar behaviors of reorientation under cyclic stretch but no saturation in the characteristic time with frequency, suggesting that cell-cell contacts can play an important role in the response machinery of cells under mechanical strain.

Analysis and Interpretation of Stress Fiber Organization in Cells Subject to Cyclic Stretch

Numerical simulations that incorporate a biochemomechanical model for the contractility of the cytoskeleton have been used to rationalize the following observations. Uniaxial cyclic stretching of cells causes stress fibers to align perpendicular to the stretch direction, with degree of alignment dependent on the stretch strain magnitude, as well as the frequency and the transverse contraction of the substrate. Conversely, equibiaxial cyclic stretching induces a uniform distribution of stress fiber orientations. Demonstrations that the model successfully predicts the alignments experimentally found are followed by a parameter study to investigate the influence of a range of key variables including the stretch magnitude, the intrinsic rate sensitivity of the stress fibers, the straining frequency, and the transverse contraction of the substrate. The primary predictions are as follows. The rate sensitivity has a strong influence on alignment, equivalent to that attained by a few percent of additional stretch. The fiber alignment increases with increasing cycling frequency. Transverse contraction of the substrate causes the stress fibers to organize into two symmetrical orientations with respect to the primary stretch direction.

Effect of mechanical boundary conditions on orientation of angiogenic microvessels

AIM

Mechanical forces are important regulators of cell and tissue phenotype. We hypothesized that mechanical loading and boundary conditions would influence neovessel activity during angiogenesis.

METHODS AND RESULTS

Using an in vitro model of angiogenesis sprouting and a mechanical loading system, we evaluated the effects of boundary conditions and applied loading. The model consisted of rat microvessel fragments cultured in a 3D collagen gel, previously shown to recapitulate angiogenic sprouting observed in vivo. We examined changes in neovascular growth in response to four different mechanical conditions. Neovessel density, diameter, length and orientation were measured from volumetric confocal images of cultures exposed to no external load (free-floating shape control), intrinsic loads (fixed ends, no stretch), static external load (static stretch), or cyclic external load (cyclic stretch). Neovessels sprouted and grew by the third day of culture and continued to do so during the next 3 days of loading. The numbers of neovessels and branch points were significantly increased in the static stretch group when compared with the free-floating shape control group. In all mechanically loaded cultures, neovessel diameter and length distributions were heterogeneous, whereas they were homogeneous in shape control cultures. Neovessels were significantly more oriented along the direction of mechanical loading than those in the shape controls. Interestingly, collagen fibrils were organized parallel and adjacent to growing neovessels.

CONCLUSION

Externally applied boundary conditions regulate neovessel sprouting and elongation during angiogenesis, affecting both neovessel growth characteristics and network morphometry. Furthermore, neovessels align parallel to the direction of stress/strain or internally generated traction, and this may be because of collagen fibril alignment induced by the growing neovessels themselves.

Effect of different frequencies of tensile strain on human dermal fibroblast proliferation and survival

The aim of this study is to compare the effect of a high-frequency repetitive (HF) stretch or an intermittent (I) stretch on the cell proliferation and survival of human dermal fibroblasts and to determine the activation of any relevant signal pathways. Cultured human dermal fibroblasts were exposed to either HF or I stretch. Cell number was measured by counting, while DNA synthesis was assessed by 5-bromo-2'-deoxyuridine (BrdU) staining and apoptosis by terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling staining. To investigate the potential mechanisms of repetitive strain on the proliferation and survival of fibroblasts, the activation of relevant transduction pathways, such as p38 mitogen-activated protein kinase (MAPK), extracellular signal-regulated kinase (ERK)1/2, AKT, and BAD, was assessed by Western blot. In addition, the effect of inhibition of these pathways on the fibroblast response was also studied. After either HF or I stretch for 7 days, fibroblast number was significantly decreased and there were less BrdU-positive cells. The numbers of apoptotic and/or necrotic fibroblasts were not affected. p38 MAPK and ERK1/2 were significantly activated after HF stretch, but AKT and BAD were significantly activated after I stretch. The inhibitors of p38 MAPK and MAPK/ERK kinase as well as dominant-negative AKT reduced cell number after both HF and I stretch but these pathways were not critical for the stretch-induced decrease in cell number.

Effect of strain magnitude on the tissue properties of engineered cardiovascular constructs

Mechanical loading is a powerful regulator of tissue properties in engineered cardiovascular tissues. To ultimately regulate the biochemical processes, it is essential to quantify the effect of mechanical loading on the properties of engineered cardiovascular constructs. In this study the Flexercell FX-4000T (Flexcell Int. Corp., USA) straining system was modified to simultaneously apply various strain magnitudes to individual samples during one experiment. In addition, porous polyglycolic acid (PGA) scaffolds, coated with poly-4-hydroxybutyrate (P4HB), were partially embedded in a silicone layer to allow long-term uniaxial cyclic mechanical straining of cardiovascular engineered constructs. The constructs were subjected to two different strain magnitudes and showed differences in biochemical properties, mechanical properties and organization of the microstructure compared to the unstrained constructs. The results suggest that when the tissues are exposed to prolonged mechanical stimulation, the production of collagen with a higher fraction of crosslinks is induced. However, straining with a large strain magnitude resulted in a negative effect on the mechanical properties of the tissue. In addition, dynamic straining induced a different alignment of cells and collagen in the superficial layers compared to the deeper layers of the construct. The presented model system can be used to systematically optimize culture protocols for engineered cardiovascular tissues.

Dynamic fibroblast cultures: response to mechanical stretching

Mechanical forces play an important role in the organization, growth and function of tissues. Dynamic extracellular environment affects cellular behavior modifying their orientation and their cytoskeleton. In this work, human fibroblasts have been subjected for three hours to increasing substrate deformations (1-25%) applied as cyclic uniaxial stretching at different frequencies (from 0.25 Hz to 3 Hz). Our objective was to identify whether and in which ranges the different deformations magnitude and rate were the factors responsible of the cell alignment and if actin cytoskeleton modification was involved in these responses. After three hours of cyclically stretched substrate, results evidenced that fibroblasts aligned perpendicularly to the stretch direction at 1% substrate deformation and reached statistically higher orientation at 2% substrate deformation with unmodified values at 5-20%, while 25% substrate deformation induced cellular death. It was also shown that a percentage of cells oriented perpendicularly to the deformation were not influenced by increased frequency of cyclical three hours deformations (0.25%3 Hz). Cyclic substrate deformation was shown also to involve actin fibers which orient perpendicularly to the stress direction as well. Thus, we argue that a substrate deformation induces a dynamic change in cytoskeleton able to modify the entire morphology of the cells.

Role of heat shock protein 70 in induction of stress fiber formation in rat arterial endothelial cells in response to stretch stress

We investigated the mechanism by which endothelial cells (ECs) resist various forms of physical stress using an experimental system consisting of rat arterial EC sheets. Formation of actin stress fibers (SFs) and expression of endothelial heat-shock stress proteins (HSPs) in response to mechanical stretch stress were assessed by immunofluorescence microscopy. Stretch stimulation increased expression of HSPs 25 and 70, but not that of HSP 90. Treatment with SB203580, a p38 MAP kinase inhibitor that acts upstream of the HSP 25 activation cascade, or with geldanamycin, an inhibitor of HSP 90, had no effect on the SF formation response to mechanical stretch stress. In contrast, treatment with quercetin, an HSP 70 inhibitor, inhibited both upregulation of endothelial HSP 70 and formation of SFs in response to tensile stress. In addition, treatment of stretched ECs with cytochalasin D, which disrupts SF formation, did not adversely affect stretch-induced upregulation of endothelial HSP 70. Our data suggest that endothelial HSP 70 plays an important role in inducing SF formation in response to tensile stress.

Actin structure in the outflow tract of normal and glaucomatous eyes

To characterize the in situ distribution of actin in Schlemm's canal endothelium (SCE) and juxtacanalicular tissue (JCT) cells in glaucomatous human eyes, and compare to the distribution in normal eyes. Fresh human eye bank eyes were perfused and fixed at pressure (n=27 normal eyes and 22 confirmed glaucomatous eyes). Schlemm's canal was opened by microdissection and outflow tissues were labelled for confocal microscopy to visualize F-actin, nuclei, laminin and/or CD31. Images were acquired in Z-series from the inner wall of Schlemm's canal, juxtacanalicular tissue and outer corneoscleral meshwork. In normal eyes, inner wall Schlemm's canal endothelial (SCE) cells showed a dense peripheral F-actin band, as previously described. JCT cells showed a more random and amorphous F-actin distribution. In glaucoma eyes, peripheral F-actin bands were less common in inner wall SCE cells; instead, F-actin was more centrally located within the cell and appeared "tangled". These actin tangles were also prominent in JCT cells of glaucoma eyes. Glaucoma eyes also demonstrated structures with features of cross-linked actin networks (CLANs), and more frequent occurrence of punctuate actin concentrations. There was a significant degree of heterogeneity, with some regions from glaucomatous eyes appearing normal and vice versa. F-actin architecture in human outflow pathway cells in situ differs between normal and glaucoma eyes, with glaucomatous tissue showing a more "disordered" actin architecture overall. Some of these changes are likely due to effects secondary to administration of anti-glaucoma medications. Most of the changes that we observed could potentially affect the biomechanical properties of the outflow pathway tissues in glaucoma, but their role in the pathogenesis of ocular hypertension remains unclear.

p38 Mitogen-activated protein kinase activation in endothelial cell is implicated in cell alignment and elongation induced by fluid shear stress

Fluid shear stress is thought to be important in maintaining the phenotype of endothelial cells (ECs) in vivo. The purpose of the study was to determine the effect of varying levels of laminar shear stress on EC elongation and alignment and the role of p38 mitogen-activated protein kinase (MAPK) on the morphologic change induced by shear stress. Cultured bovine aortic ECs were subjected to 1, 4, 7, 14, or 20 dyne/cm(2) laminar steady shear stress. On morphometric analysis of static ECs, the average orientation angle was 41 degrees , whereas after 24 h shear stress at 1, 4, 7, 14, and 20 dyne/cm(2) the angles were 34 degrees, 33 degrees, 16 degrees, 11 degrees, and 10 degrees, respectively. The shape index of static ECs was 0.76, whereas the indexes of ECs exposed to shear stress were 0.72, 0.72, 0.65, 0.50, and 0.47, respectively. The time and the magnitude of activation of p38 MAPK were dependent on the level of shear stress. The results indicate that a minimum shear stress of 7 to 14 dynes/cm(2) is necessary for cell alignment and elongation and this correlates with activity of p38 MAPK. ECs exposed to shear stress in the presence of the p38 MAPK inhibitor SB-203580 did not orient in any manner and the shape index was similar to the static cells.

Cyclic strain-mediated matrix metalloproteinase regulation within the vascular endothelium: a force to be reckoned with

The vascular endothelium is a dynamic cellular interface between the vessel wall and the bloodstream, where it regulates the physiological effects of humoral and biomechanical stimuli on vessel tone and remodeling. With respect to the latter hemodynamic stimulus, the endothelium is chronically exposed to mechanical forces in the form of cyclic circumferential strain, resulting from the pulsatile nature of blood flow, and shear stress. Both forces can profoundly modulate endothelial cell (EC) metabolism and function and, under normal physiological conditions, impart an atheroprotective effect that disfavors pathological remodeling of the vessel wall. Moreover, disruption of normal hemodynamic loading can be either causative of or contributory to vascular diseases such as atherosclerosis. EC-matrix interactions are a critical determinant of how the vascular endothelium responds to these forces and unquestionably utilizes matrix metalloproteinases (MMPs), enzymes capable of degrading basement membrane and interstitial matrix molecules, to facilitate force-mediated changes in vascular cell fate. In view of the growing importance of blood flow patterns and mechanotransduction to vascular health and pathophysiology, and considering the potential value of MMPs as therapeutic targets, a timely review of our collective understanding of MMP mechanoregulation and its impact on the vascular endothelium is warranted. More specifically, this review primarily summarizes our current knowledge of how cyclic strain regulates MMP expression and activation within the vascular endothelium and subsequently endeavors to address the direct and indirect consequences of this on vascular EC fate. Possible relevance of these phenomena to vascular endothelial dysfunction and pathological remodeling are also addressed.

Cytoskeletal response of microvessel endothelial cells to an applied stress force at the submicrometer scale studied by atomic force microscopy

Cytoskeleton fibers form an intricate three-dimensional network to provide structure and function to microvessel endothelial cells. During accommodation to blood flowing, stress fiber bundles become more prominent and align with the direction of blood flow. This network either mechanically resists the applied shear stress (lateral force) or, if deformed, is dynamically remodeled back to a preferred architecture. However, the detailed response of these stress fiber bundles to applied lateral force at submicrometer scales are as yet poorly understood. In our in vitro study, the tip, topography probe in lateral force microscopy of atomic force microscopy, acted as a tool for exerting quantitative vertical and lateral force on the filaments of the cytoskeleton. Moreover, the authors developed a formula to calculate the value of lateral force exerted on every point of the filaments. The results show that cytoskeleton fibers of healthy tight junctions in rat cerebral microvessel endothelial cells formed a cross-type network, and were reinforced and elongated in the direction of scanning under lateral force of 15-42 nN. Under peroxidation (H(2)O(2) of 300 micromol/L), the cytoskeleton remodeled at intercellular junctions, and changed over the meshwork structures into a dense bundle, that redistributed the stress. Once mechanical forces were exerted on an area, the cells shrank and lost morphologic tight junctions. It would be useful in our understanding of certain pathological processes, such as cerebral ischemia/reperfusion injury, which maybe caused by biomechanical forces and which are overlooked in current disease models.

Cyclic strain modulates tubulogenesis of endothelial cells in a 3D tissue culture model

Angiogenesis is the formation of new blood vessels from preexisting capillaries or venules. It occurs in a mechanically dynamic environment due to blood flow, but the role of hemodynamic forces in angiogenesis remains poorly understood. We have developed a unique in vitro system for the investigation of angiogenesis under cyclic strain. In this system, tubulogenesis of vascular endothelial cells in 3D collagen gels occurs under well-defined cyclic strain, which mimics blood-pressure-induced stretch. Using this system, we demonstrate that cyclic strain results in alignment of endothelial-cord-like structures perpendicular to the principal axis of stretch. Such preferential orientation was the most evident in deep and long cord-like structures. This in vitro system, along with the novel findings of strain-modulated endothelial tube morphology, enables the formation of an experimental basis for understanding the role of cyclic strain in the regulation of angiogenesis.

Actin structure in the outflow tract of normal and glaucomatous eyes

PURPOSE

To characterize the in situ distribution of actin in Schlemm's canal endothelium (SCE) and juxtacanalicular tissue (JCT) cells in glaucomatous human eyes, and compare to the distribution in normal eyes.

METHODS

Fresh human eye bank eyes were perfused and fixed at pressure (n=27 normal eyes and 22 confirmed glaucomatous eyes). Schlemm's canal was opened by microdissection and outflow tissues were labelled for confocal microscopy to visualize F-actin, nuclei, laminin and/or CD31. Images were acquired in Z-series from the inner wall of Schlemm's canal, juxtacanalicular tissue and outer corneoscleral meshwork.

RESULTS

In normal eyes, inner wall Schlemm's canal endothelial (SCE) cells showed a dense peripheral F-actin band, as previously described. JCT cells showed a more random and amorphous F-actin distribution. In glaucoma eyes, peripheral F-actin bands were less common in inner wall SCE cells; instead, F-actin was more centrally located within the cell and appeared 'tangled'. These actin tangles were also prominent in JCT cells of glaucoma eyes. Glaucoma eyes also demonstrated structures with features of cross-linked actin networks (CLANs), and more frequent occurrence of punctuate actin concentrations. There was a significant degree of heterogeneity, with some regions from glaucomatous eyes appearing normal and vice versa.

CONCLUSION

F-actin architecture in human outflow pathway cells in situ differs between normal and glaucoma eyes, with glaucomatous tissue showing a more 'disordered' actin architecture overall. Some of these changes are likely due to effects secondary to administration of anti-glaucoma medications. Most of the changes that we observed could potentially affect the biomechanical properties of the outflow pathway tissues in glaucoma, but their role in the pathogenesis of ocular hypertension remains unclear.

Influence of Elastic and Non-elastic External Dacron Mesh Support on Para-anastomotic Hypercompliance in End-to-End Anastomoses

OBJECTIVE

To quantify the influence of elastic and non-elastic external mesh support on para-anastomotic hypercompliance in end-to-end anastomoses (ex vivo).

MATERIALS

Six end-to-end anastomoses prepared from ovine carotid arteries without mesh support and with external elastic and non-elastic dacron mesh support.

METHODS

Compliance profiles of the anastomised arterial segments were measured successively, in the same anastomotic configuration without mesh support, with external elastic dacron mesh support and with external non-elastic dacron mesh support (randomized order). A pulsatile ex vivo perfusion system using a laser scan micrometer to monitor outer systolic and diastolic diameter was employed.

RESULTS

Median pre-anastomotic and post-anastomotic hypercompliance without external mesh support were 1.45 and 1.19%/100 mmHg, respectively, above reference compliance. Use of the elastic mesh support significantly reduced the median hypercompliance to 0.68%/100 mmHg (pre-anastomotic) and to 0.34%/100 mmHg (post-anastomotic) above reference compliance. The non-elastic mesh support caused approximately the same significantly reduced median hypercompliance to 0.53%/100 mmHg (pre-anastomotic) and 0.43%/100 mmHg (post-anastomotic).

CONCLUSIONS

Both elastic and non-elastic external mesh support significantly reduced pre- and post-anastomotic hypercompliance.

Nicotine induces mitogen-activated protein kinase dependent vascular smooth muscle cell migration

Cigarette smoke, specifically the nicotine contained within, has been shown to cause ultrastructural changes in vascular endothelium resulting in the development of atherosclerosis. Our study examines the effects of nicotine on vascular smooth muscle cell (VSMC) migration and attempts to eludicidate the cellular mechanisms governing those effects. Bovine aortic VSMC were cultured in 10% fetal bovine serum (FBS) growth media and exposed to 10(-8) nicotine for varying periods of time. Boyden chamber chemotaxis assays and a scrape injury model using confluent cells were used to assess cell motility. Activation of the mitogen-activated protein kinases (MAPK), p38 and p44/42, was assessed using Western blotting methods. Nicotine, itself, did not cause significant VSMC migration. However, augmented migration was seen in nicotine-treated VSMCs (16.6+/-3-fold) and media (17.0+/-4-fold) with 10% FBS as chemoattractant. Inhibitors of p38 and p44/42 diminished this migration by 48.5+/-6% and 29.4+/-2%, respectively. Immunoblotting verified p38 and p44/42 activation with nicotine and inhibition with inhibitors of p38 and p44/42. Nicotine-treated endothelial cell (EC) conditioned media (CM) was shown to increase migration 20.3+/-l.l-fold. This chemotactic effect was diminished both with heat treatment and serial dilution. In conclusion, nicotine enhances the chemoatactiveness of VSMC. This migration is mediated via the MAPKs p38 and p44/42. Nicotine causes EC production of a chemoattractant molecule that enhances VSMC migration.

Comparison of the effects of cyclic stretching and compression on endothelial cell morphological responses

Recent results demonstrate the exquisite sensitivity of cell orientation responses to the pattern of imposed deformation. Cells undergoing pure in-plane uniaxial stretching orient differently than cells that are simply elongated--likely because the latter stimulus produces simultaneous compression in the unstretched direction. It is not known, however, if cells respond differently to pure stretching than to pure compression. This study was performed to address this issue. Human aortic endothelial cells were seeded on deformable silicone membranes and subjected to various magnitudes and rates of pure stretching or compression. The cell orientation and cytoskeletal stress fiber organization responses were examined. Both stretching and compression resulted in magnitude-dependent but not rate-dependent orientation responses away from the deforming direction. Compression produced a slower temporal response than stretching. However, stress fiber reorganization responses-early disruption followed by reassembly into parallel arrays along the cells' long axes were similar between the two stimuli. Moreover, the cell orientation and stress fiber responses appeared to be uncoupled since disruption of stress fibers was not required for the cell orientation. Moreover, parallel actin stress fibers were observed at oblique angles to the deforming direction indicating that stress fibers can reassemble when undergoing deformation.

Effect of Combined Cyclic Stretch and Fluid Shear Stress on Endothelial Cell Morphological Responses

Endothelial cells in vivo are normally subjected to multiple mechanical stimuli such as stretch and fluid shear stress (FSS) but because each stimulus induces magnitude-dependent morphologic responses, the relative importance of each stimulus in producing the normal in vivo state is not clear. Using cultured human aortic endothelial cells, this study first determined equipotent levels of cyclic stretch, steady FSS, and oscillatory FSS with respect to the time course of cell orientation. We then tested whether these levels of stimuli were equipotent in combination with each other by imposing simultaneous cyclic stretch and steady FSS or cyclic stretch and oscillatory FSS so as to reinforce or counteract the cells’ orientation responses. Equipotent levels of the three stimuli were 2% cyclic stretch at 2%∕s, 80dynes∕cm2 steady FSS and 20±10dynes∕cm2 oscillatory FSS at 20dyne∕cm2-s. When applied in reinforcing fashion, cyclic stretch and oscillatory, but not steady, FSS were additive. Both pairs of stimuli canceled when applied in counteracting fashion. These results indicate that this level of cyclic stretch and oscillatory FSS sum algebraically so that they are indeed equipotent. In addition, oscillatory FSS is a stronger stimulus than steady FSS for inducing cell orientation. Moreover, arterial endothelial cells in vivo are likely receiving a stronger stretch than FSS stimulus.

The influence of hemodynamics and wall biomechanics on the thrombogenicity of vein segments perfused in vitro1

This study addresses the hypothesis that exposure to peripheral arterial (ART) or coronary (COR) hemodynamics and wall biomechanics affect platelet deposition on vein segments. Intact human saphenous vein (HSV) and porcine internal jugular vein (PIJV) segments were studied under venous (VEN), ART, and COR environments using in vitro perfusion systems. Wall shear stress (tau) and circumferential wall stress (sigma(theta)) were calculated for PIJV segments. Platelet deposition was measured using a radioactive assay. PIJV ART segments exhibited a 14% increase in inner diameter over time (P < 0.05). tau, acting on PIJV ART specimens, was less at 6 h compared with time 0 (P < 0.05). sigma(theta) was lower in the VEN specimens compared with ART and COR groups (P < 0.01). Platelet deposition decreased by 40% on PIJV ART segments (P < 0.05) but increased 3.2-fold on PIJV COR segments (P < 0.05) versus VEN control segments. Platelet deposition was increased 1.75-fold in COR HSV cases versus VEN segments. These data indicate that short-term exposure to COR conditions lead to enhanced platelet deposition, whereas ART conditions decrease platelet deposition.

Quantification of stretch-induced cytoskeletal remodeling in vascular endothelial cells by image processing

BACKGROUND

Reorientation of the cell axis induced by cyclic stretching is an early response to mechanical forces in vitro. However, quantitative assay for this phenomenon has been difficult due to lack of robust methods. We hypothesized that cell orientation may be redefined by the orientation of actin fibers. We developed image processing methods to quantitate the orientation and density of actin fibers.

METHODS

A convolution filter using Sobel kernels was adapted to determine the orientation and density of actin fibers in human endothelial cells. Unidirectional stretching (10%, 0.5 Hz) was applied to induce cytoskeletal remodeling by varying the duration of stimulation (control, 0.5, 1, 2, 5, 10, and 20 h). Actin fibers were visualized by fluorescent phalloidin. The image processing method was compared with the manual method for reproducibility. Both confluent and subconfluent cells were tested to assess the efficacy of the methods.

RESULTS

Cyclic stretch-induced dense and uninterrupted actin cabling formed across the cell body and, later, the actin fibers became aligned perpendicular to the stretch direction. The variance of actin fiber orientation became smaller after 2 h of stretch (F < 0.01). The actin fiber density index, a derived parameter related to the density of actin fibers, increased as early as 30 min of stretching (P < 0.05) and decreased after 10 h of stretching. Reproducibility of our method was extremely good. Applicability of the method was not compromised by cell density.

CONCLUSIONS

Our method is reliable for quantifying cytoskeletal remodeling induced by mechanical force.

Involvement of S6 kinase and p38 mitogen activated protein kinase pathways in strain-induced alignment and proliferation of bovine aortic smooth muscle cells

Bovine aortic smooth muscle cell (SMC) phenotype can be altered by physical forces. This has been demonstrated by cyclic strain-induced changes in proliferation and alignment. However, the intracellular coupling pathways remain ill defined. In the present study, we examined whether the p38 and S6 kinase pathway were involved in the mitogenic and morphological changes seen in SMCs exposed to cyclic strain. We seeded bovine aortic SMCs on silastic membranes that were deformed with 150-mmHg vacuum. Cyclic strain induced both alignment and proliferation of SMCs. SB202190, a specific inhibitor of p38, hindered SMC alignment, but not proliferation. Rapamycin, a specific inhibitor of the mTOR-S6 kinase pathway, attenuated strain-induced proliferation, but not alignment. Peak activation of p38 and S6 kinase was 351 +/- 76.9% at 5 min and 363 +/- 56.2% at 60 min compared with static control, respectively (P < 0.05). The results suggest that strain-induced SMC alignment is dependent on activation of p38, but not S6 kinase. Strain induced SMC proliferation is S6 kinase, but not p38 activation, dependent.

Fibroblast orientation to stretch begins within three hours

Most connective tissue cells align in response to stretch. Previous studies have shown these responses occur within 12-14 h of initiation of stretch, but do not identify the time at which this orientation occurs, nor whether the orientation continues after cessation of stretch. To ascertain the earliest times at which fibroblast orientation occurs, we cultured primary human fibroblasts on deformable culture dishes and stretched (1 Hz, 8% uniaxial strain) them for up to 24 h. We photographed the cells at 0.5, 1-6, 8, 10, 12, 14, 16, and 24 h. Similarly cells were photographed at 1-3, or 4 h after cessation of stretch for stretch durations of 1, 2, and 3 h. Orientation of cells were ascertained by an interactive computer program. The fibroblasts began to orient by 2-3 h and orientation appeared nearly complete by 24 h. Cultures stretched for 2 or 3 h continued to exhibit greater degrees of orientation (compared to controls) for 2 or 3 h respectively after cessation of stretch. We conclude fibroblasts begin to orient within 3 h of initiation of stretch, and that they continue to orient for several hours after cessation of stretch.

Specificity of endothelial cell reorientation in response to cyclic mechanical stretching

Evidence suggests that cellular responses to mechanical stimuli depend specifically on the type of stimuli imposed. For example, when subjected to fluid shear stress, endothelial cells align along the flow direction. In contrast, in response to cyclic stretching, cells align away from the stretching direction. However, a few aspects of this cell alignment response remain to be clarified: (1) Is the cell alignment due to actual cell reorientation or selective cell detachment? (2) Does the resulting cell alignment represent a response of the cells to elongation or shortening, or both? (3) Does the cell alignment depend on the stretching magnitude or rate, or both? Finally, the role of the actin cytoskeleton and microtubules in the cell alignment response remains unclear. To address these questions, we grew human aortic endothelial cells on deformable silicone membranes and subjected them to three types of cyclic stretching: simple elongation, pure uniaxial stretching and equi-biaxial stretching. Examination of the same cells before and after stretching revealed that they reoriented. Cells subjected to either simple elongation or pure uniaxial stretching reoriented specifically toward the direction of minimal substrate deformation, even though the directions for the two types of stretching differed by only about 20 degrees. At comparable stretching durations, the extent of cell reorientation was more closely related to the stretching magnitude than the stretching rate. The actin cytoskeleton of the endothelial cell subjected to either type of stretching was reorganized into parallel arrays of actin filaments (i.e., stress fibers) aligned in the direction of the minimal substrate deformation. Furthermore, in response to equi-biaxial stretching, the actin cytoskeleton was remodeled into a "tent-like" structure oriented out of the membrane plane-again towards the direction of the minimal substrate deformation. Finally, abolishing microtubules prevented neither the formation of stress fibers nor cell reorientation. Thus, endothelial cells respond very specifically to the type of deformation imposed upon them.

Invited review: focal adhesion and small heat shock proteins in the regulation of actin remodeling and contractility in smooth muscle

Smooth muscle cells are able to adapt rapidly to chemical and mechanical signals impinging on the cell surface. It has been suggested that dynamic changes in the actin cytoskeleton contribute to the processes of contractile activation and mechanical adaptation in smooth muscle. In this review, evidence for functionally important changes in actin polymerization during smooth muscle contraction is summarized. The functions and regulation of proteins associated with "focal adhesion complexes" (membrane-associated dense plaques) in differentiated smooth muscle, including integrins, focal adhesion kinase (FAK), c-Src, paxillin, and the 27-kDa small heat shock protein (HSP27) are described. Integrins in smooth muscles are key elements of mechanotransduction pathways that communicate with and are regulated by focal adhesion proteins that include FAK, c-Src, and paxillin as well as proteins known to mediate cytoskeletal remodeling. Evidence that functions of FAK and c-Src protein kinases are closely intertwined is discussed as well as evidence that focal adhesion proteins mediate key signal transduction events that regulate actin remodeling and contraction. HSP27 is reviewed as a potentially significant effector protein that may regulate actin dynamics and cross-bridge function in response to activation of p21-activated kinase and the p38 mitogen-activated protein kinase signaling pathway by signaling pathways linked to integrin proteins. These signaling pathways are only part of a large number of yet to be defined pathways that mediate acute adaptive responses of the cytoskeleton in smooth muscle to environmental stimuli.

Dynamic reorientation of cultured cells and stress fibers under mechanical stress from periodic stretching

Cell lines derived from rat aorta and frog kidney were cultured on elastic membrane, and mechanical stress was given to the cells by stretching the membrane periodically. Cell reorientation oblique to the direction of stretching occurred as a result of the rapid withdrawal of cell periphery located along the direction of stretching and gradual extension of the cell membrane toward the direction oblique to the direction of stretching. Dynamic reorganization of stress fibers in living cells was visualized by labeling stress fibers with TRITC(3)-actin or EGFP-tagged moesin fragments with actin-binding ability. Stress fibers aligned in the direction of stretching disappeared soon after the start of stretching and then obliquely reoriented stress fibers appeared. The stretch-induced reorientation of cultured cells was suppressed by an inhibitor of stretch-activated (SA) cation channels and by a Ca(2+) chelator. However, the rearrangement of stress fibers was not affected by these agents. From these results, we suggest that Ca(2+) influx via SA channels is involved in stretch-induced cell reorientation but stress fiber rearrangement is independent of SA channels. Therefore, cell reorientation does not simply depend on the arrangement of stress fibers but may be controlled by some additional mechanism(s) which is regulated by calcium signaling.

Podocytes respond to mechanical stress in vitro

Glomerular capillary pressure is thought to affect the structure and function of glomerular cells. However, it is unknown whether podocytes are intrinsically sensitive to mechanical forces. In the present study, differentiated mouse podocytes were cultured on flexible silicone membranes. Biaxial cyclic stress (0.5 Hz and 5% linear strain) was applied to the membranes for up to 3 d. Mechanical stress reduced the size of podocyte cell bodies, and processes became thin and elongated. Podocytes did not align in the inhomogeneous force field. Whereas the network of microtubules and that of the intermediate filament vimentin exhibited no major changes, mechanical stress induced a reversible reorganization of the actin cytoskeleton: transversal stress fibers (SF) disappeared and radial SF that were connected to an actin-rich center (ARC) formed. Epithelial and fibroblast cell lines did not exhibit a comparable stress-induced reorganization of the F-actin. Confocal and electron microscopy revealed an ellipsoidal and dense filamentous structure of the ARC. Myosin II, alpha-actinin, and the podocyte-specific protein synaptopodin were present in radial SF, but, opposite to F-actin, they were not enriched in the ARC. The formation of the ARC and of radial SF in response to mechanical stress was inhibited by nonspecific blockade of Ca(2+) influx with Ni(2+) (1 mM), by Rho kinase inhibition with Y-27632 (10 microM), but not by inhibition of stretch-activated cation channels with Gd(3+) (50 microM). In summary, mechanical stress induces a unique reorganization of the actin cytoskeleton in podocytes, featuring radial SF and an ARC, which differ in protein composition. The F-actin reorganization in response to mechanical stress depends on Ca(2+) influx and Rho kinase. The present study provides the first direct evidence that podocytes are mechanosensitive.

Cell alignment is induced by cyclic changes in cell length: studies of cells grown in cyclically stretched substrates

Many types of cells, when grown on the surface of a cyclically stretched substrate, align away from the stretch direction. Although cell alignment has been described as an avoidance response to stretch, the specific deformation signal that causes a cell population to become aligned has not been identified. Planar surface deformation is characterized by three strains: two normal strains describe the length changes of two initially perpendicular lines and one shear strain describes the change in the angle between the two lines. The present study was designed to determine which, if any, of the three strains was the signal for cell alignment. Human fibroblasts and osteoblasts were grown in deformable, rectangular, silicone culture dishes coated with ProNectin, a biosynthetic polymer containing the RGD ligand of fibronectin. 24 h after plating the cells, the dishes were cyclically stretched at 1 Hz to peak dish stretches of 0% (control), 4%, 8%, and 12%. After 24 h of stretching, the cells were fixed, stained, and their orientations measured. The cell orientation distribution was determined by calculating the percent of cells whose orientation was within each of eighteen 5 degrees angular intervals. We found that the alignment response was primarily driven by the substrate strain which tended to lengthen the cell (axial strain). We also found that for each cell type there was an axial strain limit above which few cells were found. The axial strain limit for fibroblasts, 4.2 +/- 0.4%, (mean +/- 95% confidence), was lower than for osteoblasts, 6.4 +/- 0.6%. We suggest that the fibroblasts are more responsive to stretch because of their more highly developed actin cytoskeleton.

Cyclic tensile stretch inhibition of nitric oxide release from osteoblast-like cells is both G protein and actin-dependent

Recent reports indicate the alteration of nitric oxide (NO) synthesis with mechanical stress loaded on the osteoblast and NO is considered to have a significant role in mechanotransduction. We found the involvement of guanine-nucleotide-binding regulatory proteins (G proteins), especially Gi, in stress-inhibited NO release of osteoblast-like cells (JOR:17;593-597, 1999). To determine further the mechanism involved in this process, we measured c-Jun N-terminal kinase/stress-activated protein kinase (JNK/SAPK) activity under cyclic tensile stretch loaded on osteoblast-like cells. Cyclic stretch significantly enhanced JNK/SAPK activity and pertussis toxin clearly reversed stress-enhanced JNK/SAPK activity. Cytochalasin D, actin microfilament disrupting reagent, also abolished the stress activation of JNK/SAPK. We propose a model for signaling events induced by cyclic tensile stretch, namely a transmembrane mechanosensor which couples Gi-protein, actin cytoskeleton and finally activates JNK/SAPK activity of osteoblasts.

In vitro endothelialization of bioprosthetic heart valves provides a cell monolayer with proliferative capacities and resistance to pulsatile flow

OBJECTIVES

Degeneration of bioprosthetic heart valves has been suggested to be at least partly an immunogenic reaction toward the xenogeneic tissue. An autologous endothelial lining has been proposed to overcome this problem. We examined in vitro endothelialization of such tissue and retention of endothelial cells after exposure to flow resembling the in vivo situation.

METHODS

Cultured human saphenous vein endothelial cells were used to in vitro endothelialize photo-oxidized bioprosthetic heart valves. The endothelialized valves were mounted in a specially designed flow device, creating a pulsatile flow through the valve. Maintenance of a confluent cell layer and deposition of basement membrane markers were determined with immunohistochemical labeling.

RESULTS

Labeling of the main components of the basement membrane, laminin and collagen type IV, was verified within 6 hours after in vitro endothelialization. Under static conditions, 4-mm wide denudations were completely re-endothelialized in 4 days, which was similar to the growth rate on gelatin-coated cell culture plastic, which served as a control material. After exposure of endothelialized valves to pulsatile flows for 24 hours (80 beats/min, 3.4 L/min), there were minimal cell losses from the bioprostheses. The cell layer adapted to the pulsatile flow, as verified by rearrangement of morphology and intracellular stress fibers.

CONCLUSIONS

This study shows the feasibility of in vitro endothelialization of photo-oxidized bioprosthetic heart valves. The cells are able to withstand a pulsatile flow in vitro, to develop basement membrane-like structures, and to re-endothelialize denuded areas. This technology may be used to enhance the performance of bioprosthetic heart valve prostheses.

Role of mitogen-activated protein kinases in pulmonary endothelial cells exposed to cyclic strain

The aim of this study was to examine the role of mitogen-activated protein kinases (MAPKs) activation in bovine pulmonary arterial endothelial cells (EC) exposed to cyclic strain. EC were subjected to 10% average strain at 60 cycles/min. Cyclic strain induced activation of extracellular signal-regulated kinase (ERK; 1.5-fold), c-Jun NH(2)-terminal protein kinase (JNK; 1.9-fold), and p38 (1. 5-fold) with a peak at 30 min. To investigate the functional role of the activated MAPKs, we analyzed cells after treatment with PD-98059, a specific ERK kinase inhibitor, or SB-203580, a catalytic inhibitor for p38, and after transient transfection with JNK(K-R), and MEKK(K-M) the respective catalytically inactive mutants of JNK1 and MAPK kinase kinase-1. Cyclic strain increased activator protein-1 (AP-1) binding activity, which was blocked by PD-98059 and SB-203580. Activity of AP-1-dependent luciferase reporter driven by 12-O-tetradecanoyl-phorbol-13-acetate-responsive element (TRE) was induced by cyclic strain, and this was attenuated by PD-98059, MEKK(K-M), JNK(K-R), and SB-203580. PD-98059 and SB-203850 did not inhibit cell alignment and migration induced by cyclic strain. MEKK(K-M) and JNK(K-R) transfection did not block cyclic strain-induced cell alignment. In conclusion, cyclic strain activates ERK, JNK, and p38, and their activation plays a role in transcriptional activation of AP-1/TRE but not in cell alignment and migration changes in bovine pulmonary arterial EC.

Theoretical study of intracellular stress fiber orientation under cyclic deformation

We studied stress fiber orientation under a wide range of uniaxial cyclic deformations. We devised and validated a hypothesis consisting of two parts, as follows: (1) a stress fiber aligns to avoid a mechanical stimulus in the fiber direction under cyclic deformation. This means that, among all allowable directions, a stress fiber aligns in the direction which minimizes the stimulus, i. e., the summation of the changes in length of the stress fiber over one stretch cycle; and (2) there is a limit in the sensitivity of the cellular response to the mechanical stimulus. Due to this sensing limit, the orientation angle in stress fibers is distributed around the angle corresponding to the minimum stimulus. To validate this hypothesis, we approximated an anisotropic deformation of the membrane on which cells were to be cultured. We then obtained the relationships between the stretch range and the fiber angle in the undeformed state which minimize the mechanical stimuli, assuming that the membrane on which stress fibers and cells adhered was homogeneous and incompressible. Numerical simulation results showed that the proposed hypothesis described our previous experimental results well and was consistent with the experimental results in the literature. The simulation results, taking account of the second part of the hypothesis with a small value for the limit in sensitivity to the mechanical stimulus, could explain why cell orientation is distributed so widely with cyclic stretch ranges of <10%. The proposed hypothesis can be applied to various types of deformation because the mechanical stimulus is always sensed and accumulates under cyclic deformation without the necessity of a reference state to measure the stimulus.