Structure-function relationships in the stem cell's mechanical world A: seeding protocols as a means to control shape and fate of live stem cells

Shape and fate are intrinsic manifestations of form and function at the cell scale. Here we hypothesize that seeding density and protocol affect the form and function of live embryonic murine mesenchymal stem cells (MSCs) and their nuclei. First, the imperative for study of live cells was demonstrated in studies showing changes in cell nucleus shape that were attributable to fixation per se. Hence, we compared live cell and nuclear volume and shape between groups of a model MSC line (C3H10T1/2) seeded at, or proliferated from 5,000 cells/cm2 to one of three target densities to achieve targeted development contexts. Cell volume was shown to be dependent on initial seeding density whereas nucleus shape was shown to depend on developmental context but not seeding density. Both smaller cell volumes and flatter nuclei were found to correlate with increased expression of markers for mesenchymal condensation as well as chondrogenic and osteogenic differentiation but a decreased expression of pre-condensation and adipogenic markers. Considering the data presented here, both seeding density and protocol significantly alter the morphology of mesenchymal stem cells even at very early stages of cell culture. Thus, these design parameters may play a critical role in the success of tissue engineering strategies seeking to recreate condensation events. However, a better understanding of how these changes in cell volume and nucleus shape relate to the differentiation of MSCs is important for prescribing precise seeding conditions necessary for the development of the desired tissue type. In a companion study (Part B, following), we address the effect of concomitant volume and shape changing stresses on spatiotemporal distribution of the cytoskeletal proteins actin and tubulin. Taken together, these studies bring us one step closer to our ultimate goal of elucidating the dynamics of nucleus and cell shape change as tissue templates grow (cell proliferation) and specialize (cell differentiation).     

In Situ Spatiotemporal Mapping of Flow Fields around Seeded Stem Cells at the Subcellular Length Scale

A major hurdle to understanding and exploiting interactions between the stem cell and its environment is the lack of a tool for precise delivery of mechanical cues concomitant to observing sub-cellular adaptation of structure. These studies demonstrate the use of microscale particle image velocimetry (μ-PIV) for in situ spatiotemporal mapping of flow fields around mesenchymal stem cells, i.e. murine embryonic multipotent cell line C3H10T1/2, at the subcellular length scale, providing a tool for real time observation and analysis of stem cell adaptation to the prevailing mechanical milieu. In the absence of cells, computational fluid dynamics (CFD) predicts flow regimes within 12% of μ-PIV measures, achieving the technical specifications of the chamber and the flow rates necessary to deliver target shear stresses at a particular height from the base of the flow chamber. However, our μ-PIV studies show that the presence of cells per se as well as the density at which cells are seeded significantly influences local flow fields. Furthermore, for any given cell or cell seeding density, flow regimes vary significantly along the vertical profile of the cell. Hence, the mechanical milieu of the stem cell exposed to shape changing shear stresses, induced by fluid drag, varies with respect to proximity of surrounding cells as well as with respect to apical height. The current study addresses a previously unmet need to predict and observe both flow regimes as well as mechanoadaptation of cells in flow chambers designed to deliver precisely controlled mechanical signals to live cells. An understanding of interactions and adaptation in response to forces at the interface between the surface of the cell and its immediate local environment may be key for de novo engineering of functional tissues from stem cell templates as well as for unraveling the mechanisms underlying multiscale development, growth and adaptation of organisms.     

Inhibition of RhoA but not ROCK induces chondrogenesis of chick limb mesenchymal cells

Cell shape change and cytoskeletal reorganization are known to be involved in the chondrogenesis. Negative role of RhoA, a cytoskeleton-regulating protein, and its downstream target, Rho-associated protein kinase (ROCK) in the chondrogenesis has been studied in many different culture systems including primary chondrocytes, chondrogenic cell lines, dedifferentiated chondrocytes, and micromass culture of mesenchymal cells. To further investigate the role of RhoA and ROCK in the chondrogenesis, we examined the RhoA-ROCK-myosin light chains (MLC) pathway in low density culture of chick limb bud mesenchymal cells. We observed for the first time that inhibition of RhoA by C3 cell-permeable transferase, CT04, induced chondrogenesis of undifferentiated mesenchymal single cells following dissolution of actin stress fibers. Inhibition of RhoA activity by CT04 was confirmed by pull down assay using the Rho-GTP binding domain of Rhotekin. CT04 also inhibited ROCK activity. In contrast, inhibition of ROCK by Y27632 neither altered the actin stress fibers nor induced chondrogenesis. In addition, inhibition of RhoA or ROCK did not affect the phosphorylation of MLC. Inhibition of myosin light chain kinase (MLCK) by ML-7 or inhibition of myosin ATPase with blebbistatin dissolved actin stress fibers and induced chondrogenesis. ML-7 reduced the MLC phosphorylation. Taken together, our current study suggests that RhoA uses other pathway than ROCK/MLC in the modulation of actin stress fibers and chondrogenesis. Our data also imply that, irrespective of mechanisms, dissolution of actin stress fibers is crucial for chondrogenesis.     

Fluid-Flow Induced Wall Shear Stress and Epithelial Ovarian Cancer Peritoneal Spreading

Epithelial ovarian cancer (EOC) is usually discovered after extensive metastasis have developed in the peritoneal cavity. The ovarian surface is exposed to peritoneal fluid pressures and shear forces due to the continuous peristaltic motions of the gastro-intestinal system, creating a mechanical micro-environment for the cells. An in vitro experimental model was developed to expose EOC cells to steady fluid flow induced wall shear stresses (WSS). The EOC cells were cultured from OVCAR-3 cell line on denuded amniotic membranes in special wells. Wall shear stresses of 0.5, 1.0 and 1.5 dyne/cm² were applied on the surface of the cells under conditions that mimic the physiological environment, followed by fluorescent stains of actin and β-tubulin fibers. The cytoskeleton response to WSS included cell elongation, stress fibers formation and generation of microtubules. More cytoskeletal components were produced by the cells and arranged in a denser and more organized structure within the cytoplasm. This suggests that WSS may have a significant role in the mechanical regulation of EOC peritoneal spreading.     

In vitro aging of articular chondrocytes identified by analysis of DNA and tubulin content and relationship to cell size and protein content

In vitro senescence of chondrocytes, characterized by a decline in the proliferation rate during late passages, resulted from a rapid growth rate in early subcultures to a complete loss of division after seven to nine passages. One senescent-associated phenotypic change was the apparent increase in the density of cytoplasmic cytoskeletal proteins. We examined the relationship between tubulin content and growth (measured by DNA and total protein contents and cell volume), using flow cytometry, in the assessment of cytoskeleton analysis during in vitro aging. In contrast with previous microscopic observations of tubulin organization, flow cytometry revealed a tubulin content that was modulated as a function of protein content and/or cell volume.     

Quantitative analysis of cytoskeletal proteins throughout the cell cycle of the MRC-5 fibroblastic cell line

Fluorescent dyes were used to stain actin, vimentin, tubulin and DNA in the same MRC-5 fibroblastic cells. Cytofluorometry and image analysis were then used to quantitatively evaluate the F actin, vimentin and tubulin content throughout the cell cycle. The results showed that different cells can have the same DNA content while their cytoskeletal protein content is variable. The data also showed that cytoskeletal protein content variations exist throughout the cell cycle of the fibroblastic cell line. The F actin content increased during the cell cycle from G1 to G2 phases and decreased in M phase. The amount of tubulin in the G2 was about twice as much as that in the G1 phase, before decreasing in the M phase; there was a threshold of tubulin content for G2 cells entering S phase.     

Interplay between Cytoskeletal Stresses and Cell Adaptation under Chronic Flow

Using stress sensitive FRET sensors we have measured cytoskeletal stresses in α-actinin and the associated reorganization of the actin cytoskeleton in cells subjected to chronic shear stress. We show that long-term shear stress reduces the average actinin stress and this effect is reversible with removal of flow. The flow-induced changes in cytoskeletal stresses are found to be dynamic, involving a transient decrease in stress (phase-I), a short-term increase (3–6 min) (Phase-II), followed by a longer-term decrease that reaches a minimum in ∼20 min (Phase-III), before saturating. These changes are accompanied by reorganization of the actin cytoskeleton from parallel F-actin bundles to peripheral bundles. Blocking mechanosensitive ion channels (MSCs) with Gd³⁺ and GsMTx4 (a specific inhibitor) eliminated the changes in cytoskeletal stress and the corresponding actin reorganization, indicating that Ca²⁺ permeable MSCs participate in the signaling cascades. This study shows that shear stress induced cell adaptation is mediated via MSCs.     

The cytoskeletal system of nucleated erythrocytes. I. Composition and function of major elements

We have studied the dogfish erythrocyte cytoskeletal system, which consists of a marginal band of microtubules (MB) and trans-marginal band material (TBM). The TBM appeared in whole mounts as a rough irregular network and in thin sections as a surface-delimiting layer completely enclosing nucleus and MB. In cells incubated at 0 degrees C for 30 min or more, the MB disappeared but the TBM remained. MB reassembly occurred with rewarming, and was inhibited by colchicine. Flattened elliptical erythrocyte morphology was retained even when MBs were absent. Total solubilization of MB and TBM at low pH, or dissolution of whole anucleate cytoskeletons, yielded components comigrating with actin, spectrin, and tubulin standards during gel electrophoresis. Mass-isolated MBs, exhibiting ribbonlike construction apparently maintained by cross-bridges, contained four polypeptides in the tubulin region of the gel. Only these four bands were noticeably increased in the soluble phase obtained from cells with 0 degrees C- disassembled MBs. The best isolated MB preparations contained tubulin but no components comigrating with high molecular weight microtubule- associated proteins, spectrin, or actin. Actin and spectrin therefore appear to be major TBM constituents, with tubulin localized in the MB. The results are interpreted in terms of an actin- and spectrin- containing subsurface cytoskeletal layer (TBM), related to that of mammalian erythrocytes, which maintains cell shape in the absence of MBs. Observations on abnormal pointed erythrocytes containing similarly pointed MBs indicate further that the MB can deform the TBM from within so as to alter cell shape. MBs may function in this manner during normal cellular morphogenesis and during blood flow in vivo.     

Distinct effects of calcium- and cyclic AMP-enhancing factors on cytoskeletal synthesis and assembly in mouse osteoblastic cells.

Previous studies have indicated that the effects of parathyroid hormone (PTH) on osteoblastic function involve alteration of cytoskeletal assembly. We have reported that after a transitory cell retraction, PTH induces respreading with stimulation of actin, vimentin and tubulins synthesis in mouse bone cells and that this effect is not mediated by cAMP. In order to further elucidate the role of intracellular cAMP and calcium on PTH action on bone cell shape and cytoskeleton we have compared the effects of calcium- and cAMP-enhancing factors on actin, tubulin and vimentin synthesis in relation with mouse bone cell morphology, DNA synthesis and alkaline phosphatase activity as a marker of differentiation. Confluent mouse osteoblastic cells were treated with 0.1 mM isobutylmethylxanthine (IBMX) for 24 h. This treatment caused an increase in the levels of cytoskeletal subunits associated with an elevation of cAMP. Under these conditions, PTH (20 nM) and forskolin (0.1 microM) produced persistent cytoplasmic retraction. PTH and forskolin treatment in presence of IBMX (24 h) induced inhibitory effects on actin and tubulin synthesis evaluated by [35S]methionine incorporation into cytoskeletal proteins identified on two-dimensional gel electrophoresis. Under these culture conditions PTH and forskolin also caused disassembly of microfilament and microtubules as shown by the marked reduction in Triton X soluble-actin and alpha- and beta-tubulins. In contrast, incubation of mouse bone cells with 1 microM calcium ionophore A23187 (24 h) resulted in increased monomeric and polymeric forms of actin and tubulin while not affecting intracellular cAMP. Alkaline phosphatase activity was increased in all conditions while DNA synthesis evaluated by [3H]thymidine incorporation into DNA was stimulated by PTH combined with forskolin and inhibited by the calcium ionophore. These data indicate that persistent elevation of cAMP levels induced by PTH and forskolin with IBMX cause cell retraction with actin and tubulin disassembly whereas rising cell calcium induces cytoskeletal protein assembly and synthesis in mouse osteoblasts. The results point to a distinct involvement of calcium and cAMP in both cytoskeletal assembly and DNA synthesis in mouse bone cells.     

Cytoskeletal involvement during hypo-osmotic swelling and volume regulation in cultured chick cardiac myocytes

The membrane skeleton in spherical cardiac myocytes subjected to hypo-osmotic challenge was examined by laser scanning confocal microscopy. A distinct cortical layer intimately localized under the plasmalemma was revealed for spectrin and actin (including filamentous actin and alpha-sarcomeric actin). Desmin filaments were abundant and in close contact with the plasmalemma. During swelling and subsequent regulatory volume decrease (RVD) the structural integrity of these cytoskeletal elements remained intact, and the close association between actin and plasmalemma persisted as confirmed by double immunolabeling. Subplasmalemmal beta-tubulin labeling was sparse. Hypo-osmotic conditions disrupted the microtubules and depolymerized tubulin. Neither pretreatment with taxol nor with colchicine, resulted in any effect on cell volume regulation. The present results show that actin, desmin, and spectrin contribute to a subplasmalemmal cytoskeletal network in spherical cardiac myocytes, and that this membrane skeleton remains structurally intact during swelling and RVD. It is suggested that the integrity of this membrane skeleton is important for stabilization of the plasmalemma and the membrane-integrated proteins during hypo-osmotic challenge, and that it may participate in the regulation of the cell volume.     

Expression of simple epithelial cytokeratins in bovine pulmonary microvascular endothelial cells.

Polypeptides of bovine aortic, pulmonary artery, and pulmonary microvascular endothelial cells, as well as vascular smooth muscle cells and retinal pericytes were evaluated by two-dimensional gel electrophoresis. The principal cytoskeletal proteins in all of these cell types were actin, vimentin, tropomyosin, and tubulin. Cultured pulmonary microvascular endothelial cells also expressed 12 unique polypeptides including a 41 kd acidic type I and two isoforms of a 52 kd basic type II simple epithelial cytokeratin microvascular endothelial cell expression of the simple epithelial cytokeratins was maintained in cultured in the presence or absence of retinal-derived growth factor, and regardless of whether cells were cultured on gelatin, fibronectin, collagen I, collagen IV, laminin, basement membrane proteins, or plastic. Cytokeratin expression was maintained through at least 50 population doublings in culture. The expression of cytokeratins was found to be regulated by cell density. Pulmonary microvascular endothelial cells seeded at 2.5 X 10(5) cell/cm2 (confluent seeding) expressed 3.5 times more cytokeratins than cells seeded at 1.25 X 10(4) cells/cm2 (sparse seeding). Vimentin expression was not altered by cell density. By indirect immunofluorescence microscopy it was determined that the cytokeratins were distributed cytoplasmically at subconfluent cell densities but that cytokeratin 19 sometimes localized at regions of cell-cell contact after cells reached confluence. Vimentin had a cytoplasmic distribution regardless of cell density. These results suggest that pulmonary microvascular endothelial cell have a distinctive cytoskeleton that may provide them with functionally unique properties when compared with endothelial cells derived from the macrovasculature. In conjunction with conventional endothelial cell markers, the presence of simple epithelial cytokeratins may be an important biochemical criterion for identifying pulmonary microvascular endothelial cells.     

Tubulin polymerization modulates interleukin-2 receptor signal transduction in human T cells

Few data exist on the modulation of cytokine receptor signaling by the actin or tubulin cytoskeleton. Therefore, we studied interleukin-2 receptor (IL-2R) signaling in phytohemagglutinine (PHA)-pretreated human T cells in the context of alterations in the cytoskeletal system induced by cytochalasin D (CyD), jasplaklinolide (Jas), taxol (Tax), or colchicine (Col). We found that changes in cytoskeletal tubulin polymerization altered the strength of several IL-2-triggered signals. Moreover, Tax-induced tubulin hyperpolymerization augmented the surface expression of the IL-2R ss -chain and enhanced the association of the IL-2R beta -chain with cytoskeletal tubulin. The IL-2R beta-chain, in turn, was constitutively associated with tubulin and, more weakly, actin. To exclude the possibility that these associations are artifacts caused by PHA, we confirmed them in T cells from TCR-transgenic DO 11.10 mice stimulated with their nominal antigen. We conclude that altered polymerization of cytoskeletal components, especially tubulin, is accompanied by modulation of IL-2 signaling at the receptor level.     

Tubulin Polymerization Modulates Interleukin-2 Receptor Signal Transduction in Human T Cells

Few data exist on the modulation of cytokine receptor signaling by the actin or tubulin cytoskeleton. Therefore, we studied interleukin-2 receptor (IL-2R) signaling in phytohemagglutinine (PHA)-pretreated human T cells in the context of alterations in the cytoskeletal system induced by cytochalasin D (CyD), jasplaklinolide (Jas), taxol (Tax), or colchicine (Col). We found that changes in cytoskeletal tubulin polymerization altered the strength of several IL-2-triggered signals. Moreover, Tax-induced tubulin hyperpolymerization augmented the surface expression of the IL-2R β -chain and enhanced the association of the IL-2R γ -chain with cytoskeletal tubulin. The IL-2R β -chain, in turn, was constitutively associated with tubulin and, more weakly, actin. To exclude the possibility that these associations are artifacts caused by PHA, we confirmed them in T cells from TCR-transgenic DO11.10 mice stimulated with their nominal antigen. We conclude that altered polymerization of cytoskeletal components, especially tubulin, is accompanied by modulation of IL-2 signaling at the receptor level.     

Cytoarchitecture of Kirsten sarcoma virus-transformed rat kidney fibroblasts: butyrate-induced reorganization within the actin microfilament network

Murine sarcoma virus-transformed rat fibroblasts (KNRK cells) undergo marked cytoarchitectural reorganization during in vitro exposure to sodium-n-butyrate (NaB) resulting in restoration of (1) a more typical fibroblastoid morphology, (2) proper cell-to-cell orientation, and (3) substratum adherence. Augmented cell spreading, involving greater than 90% of the population, was a function of culture density and time of exposure to NaB (2 mM final concentration). Induced cell spreading reflected a 2.5- to 3.0-fold increase in both total cellular actin content and deposition of actin into the detergent-resistant cytoskeleton. Cytoskeletal actin deposition in response to NaB was accompanied by the formation of occasionally dense, parallel alignments of F-actin-containing microfilaments and by a dramatic increase in the size and incidence of actin-enriched membrane ruffles. Long-term NaB-treated cells exhibited parallel orientations of microfilaments similar to those found in untransformed fibroblasts. Increased cytoskeletal actin occurred within 24 hr of NaB exposure, correlating with the initial reorganization of actin-containing microfilaments detected microscopically, and reflected concomitant 3-fold increases in cellular alpha-actinin and fibronectin content. In contrast, the amount of vimentin, tropomyosin, and tubulin in NaB-treated cells was significantly decreased. NaB-induced morphologic restructuring of sarcoma virus-transformed fibroblasts, thus, impacts on all three basic cytoskeletal systems. Selective increases, however, were evident in particular cytoskeletal proteins (actin, alpha-actinin, fibronectin) implicated in microfilament networking and cell spreading.     

Partial specific volume and adiabatic compressibility of G-actin depend on the bound nucleotide

We determined the partial specific volume and partial specific adiabatic compressibility of either ATP- or ADP-bound monomeric actin in the presence of Ca(2+) by measuring the density of and sound velocity in a monomeric actin solution at 18 degrees C. The partial specific volume of ATP-bound monomeric actin, equal to 0.744 cm(3)/g, which is exceptionally high among globular proteins, was reduced to 0.727 cm(3)/g when the tightly bound ATP was replaced with ADP. Associated with this, the adiabatic compressibility of ATP-bound monomeric actin, equal to 8.8 x 10(-12) cm(2)/dyne, decreased to 5.8 x 10(-12) cm(2)/dyne, which is a common value for globular proteins. These results suggested that an extraordinarily soft global conformation of ATP-bound monomeric actin is packed into a compact mass associated with the hydrolysis of bound ATP. When monomeric actin was limitedly proteolyzed at subdomain 2 with subtilisin, the nucleotide-dependent flexibility of the global conformation of monomeric actin was lost.     

Cell density-dependent decrease in cytoskeletal actin and myosin in cultured osteoblastic cells: correlation with cyclic AMP changes.

During bone development, osteoblasts form a contiguous layer along recently deposited osteoid and their morphology changes from fibroblast-like to cuboidal. In culture, similar changes occur with increased cell density. We examined the possible role of cyclic AMP in this process since cyclic AMP was reported to increase in fibroblasts with increased cell density and similar shape changes were seen in response to parathyroid hormone, which also increases cellular cyclic AMP in osteoblastic cells. Osteoblast-enriched rat calvaria cells were seeded at increasing density. The distribution between Triton X-100 extractable and nonextractable actin and myosin was estimated by polyacrylamide gel electrophoresis. Intracellular cyclic AMP was estimated by prelabeling the cellular ATP pool with 3H-adenine, followed by extraction and separation of 3H-cAMP by high-performance liquid chromatography. We found that osteoblastic cells contain about 40 pg actin and 5.3 pg myosin per cell. Around 60% of the actin and 70% of the myosin were in the nonextractable (crosslinked) form at cell densities of 10,000 to 50,000 cells per cm2. Above 50,000 cells/cm2, there was a cell density-dependent reduction in crosslinked actin and myosin and a concomitant increase in cellular cyclic AMP. A comparable rise in cyclic AMP, produced by incubation with phosphodiesterase inhibitors, and treatment with other agents that increase cyclic AMP produced a similar decrease in the level of cytoskeletal actin and myosin. Cytochalasin B treatment, through its effect on actin polymerization, produced similar changes in cell shape and cytoskeletal actin. The findings suggest that an elevation in intracellular cyclic AMP may play a role in the density-dependent changes in cell shape and microfilament organization observed in osteoblasts.     

Bacterial lipopolysaccharide induces cytoskeletal rearrangement in small intestinal lamina propria fibroblasts: actin assembly is essential for lipopolysaccharide signaling

Cytoskeletal proteins are major components of the cell backbone and regulate cell shape and function. The purpose of this study was to investigate the effect of lipopolysaccharide (LPS) on the dynamics and organization of the cytoskeletal proteins, actin, vimentin, tubulin and vinculin in human small intestinal lamina propria fibroblasts (HSILPF). A noticeable change in the actin architecture was observed after 30 min incubation with LPS with the formation of orthogonal fibers and further accumulation of actin filament at the cell periphery by 2 h. Reorganization of the vimentin network into vimentin bundling was conspicuous at 2 h. With further increase in the time period of LPS exposure, diffused staining of vimentin along with vimentin bundling was observed. Vinculin plaques distributed in the cell body and cell periphery in the control cells rearrange to cell periphery in LPS-treated cells by 30 min of LPS exposure. However, there was no change in the tubulin architecture in HSILPF in response to LPS. LPS increased the F-actin pool in HSILPF in a concentration-dependent manner with no difference in the level of G-actin. A time-dependent study depicted an increase in the G-actin pool at 10 and 20 min of LPS exposure followed by a decrease at further time intervals. The F-actin pool in LPS-treated cells was lower than the control levels at 10 and 20 min of LPS exposure followed by a sharp increase until 120 min and finally returning to the basal level at 140 and 160 min. Further (35)S-methionine incorporation studies suggested a new pool of actin synthesis, whereas the synthesis of other cytoskeletal filaments was not altered. Cytochalasin B, an actin-disrupting agent, severely affected the LPS induced increased percentage of 'S' phase cells and IL-6 synthesis in HSILPF. We conclude that dynamic and orchestrated organization of the cytoskeletal filaments and actin assembly in response to LPS may be a prime requirement for the LPS induced increase in percentage of 'S' phase cells and IL-6 synthesis     

Flow-induced focal adhesion remodeling mediated by local cytoskeletal stresses and reorganization

Cells respond to fluid shear stress through dynamic processes involving changes in actomyosin and other cytoskeletal stresses, remodeling of cell adhesions, and cytoskeleton reorganization. In this study we simultaneously measured focal adhesion dynamics and cytoskeletal stress and reorganization in MDCK cells under fluid shear stress. The measurements used co-expression of fluorescently labeled paxillin and force sensitive FRET probes of α-actinin. A shear stress of 0.74 dyn/cm² for 3 hours caused redistribution of cytoskeletal tension and significant focal adhesion remodeling. The fate of focal adhesions is determined by the stress state and stability of the linked actin stress fibers. In the interior of the cell, the mature focal adhesions disassembled within 35-40 min under flow and stress fibers disintegrated. Near the cell periphery, the focal adhesions anchoring the stress fibers perpendicular to the cell periphery disassembled, while focal adhesions associated with peripheral fibers sustained. The diminishing focal adhesions are coupled with local cytoskeletal stress release and actin stress fiber disassembly whereas sustaining peripheral focal adhesions are coupled with an increase in stress and enhancement of actin bundles. The results show that flow induced formation of peripheral actin bundles provides a favorable environment for focal adhesion remodeling along the cell periphery. Under such condition, new FAs were observed along the cell edge under flow. Our results suggest that the remodeling of FAs in epithelial cells under flow is orchestrated by actin cytoskeletal stress redistribution and structural reorganization.     

Flow-induced focal adhesion remodeling mediated by local cytoskeletal stresses and reorganization

Cells respond to fluid shear stress through dynamic processes involving changes in actomyosin and other cytoskeletal stresses, remodeling of cell adhesions, and cytoskeleton reorganization. In this study we simultaneously measured focal adhesion dynamics and cytoskeletal stress and reorganization in MDCK cells under fluid shear stress. The measurements used co-expression of fluorescently labeled paxillin and force sensitive FRET probes of α-actinin. A shear stress of 0.74 dyn/cm² for 3 hours caused redistribution of cytoskeletal tension and significant focal adhesion remodeling. The fate of focal adhesions is determined by the stress state and stability of the linked actin stress fibers. In the interior of the cell, the mature focal adhesions disassembled within 35-40 min under flow and stress fibers disintegrated. Near the cell periphery, the focal adhesions anchoring the stress fibers perpendicular to the cell periphery disassembled, while focal adhesions associated with peripheral fibers sustained. The diminishing focal adhesions are coupled with local cytoskeletal stress release and actin stress fiber disassembly whereas sustaining peripheral focal adhesions are coupled with an increase in stress and enhancement of actin bundles. The results show that flow induced formation of peripheral actin bundles provides a favorable environment for focal adhesion remodeling along the cell periphery. Under such condition, new FAs were observed along the cell edge under flow. Our results suggest that the remodeling of FAs in epithelial cells under flow is orchestrated by actin cytoskeletal stress redistribution and structural reorganization.