Cell shape, cytoskeletal mechanics, and cell cycle control in angiogenesis

A dual-color fluorescence imaging-based system for the dissection of antiangiogenic and chemotherapeutic activity of molecules

We have developed a simple yet sensitive dual color fluorescence-based technique for dissecting the tumor-neovascularization relationship and evaluated the susceptibility of each component to therapeutic interventions. Green fluorescent protein (GFP)-expressing melanoma cells were cocultured with endothelial cells on different three-dimensional (3-D) matrices and exposed to multiple growth factors and molecules with established anti-angiogenic or anticancer activities. Cells were fixed and stained with propidium iodide, imaged using a confocal microscope, and stereologically analyzed. Three-dimensionality of the system was tested by depth-coding and pseudocolor 3-D reconstruction in the z-axis. Selective ablation of the tumor cells was affected by the anthracycline antibiotic doxorubicin. Treatment with vascular endothelial growth factor (VEGF) and hepatocyte growth factor (HGF) promoted the neovascular responses on matrigel and collagen-1 matrices. VEGF-induced angiogenesis was inhibited after treatment with combretastatin and thalidomide. In contrast, HGF exerted a protective effect against these anti-angiogenics in a matrigel matrix. However, this effect was lost when the matrix was substituted with collagen, suggesting that the extracellular matrix impinges on cellular function, possibly through an Akt-mediated mechanism. The VEGF-receptor antagonist PTK787 also selectively ablated the VEGF-induced angiogenic effect without inhibiting the HGF-induced response, demonstrating the sensitivity of the system to detect modulation of distinct signal cascades. The current model encompasses the possibility of studying tumor-angiogenesis-matrix interaction on the same platform, expanding the rapid screening of novel molecules in a simulated clinicopathological setting.

Three-dimensional model of angiogenesis: coculture of human retinal cells with bovine aortic endothelial cells in the NASA bioreactor

Ocular angiogenesis is the leading cause of blindness and is associated with diabetic retinopathy and age-related macular degeneration. We describe, in this report, our preliminary studies using a horizontally rotating bioreactor (HRB), developed by the National Aeronautics and Space Administration (NASA), to explore growth and differentiation-associated events in the early phase of ocular angiogenesis. Human retinal (HRet) cells and bovine endothelial cells (ECs) were cocultured on laminin-coated Cytodex-3 microcarrier beads in an HRB for 1-36 days. Endothelial cells grown alone in the HRB remained cuboidal and were well differentiated. However, when HRet cells were cocultured with ECs, cordlike structures formed as early as 18-36 h and were positive for von Willebrand factor. In addition to the formation of cords and capillary-like structures, ECs showed the beginning of sprouts. The HRB seems not only to promote accelerated capillary formation, but also to enhance differentiation of retinal precursor cells. This leads to the formation of rosette-like structures (which may be aggregates of photoreceptors that were positive for rhodopsin). Upregulation of vascular endothelial growth factor and basic fibroblast growth factor was seen in retinal cells grown in the HRB as compared with monolayers and could be one of the factors responsible for accelerated capillary formation. Hence, the HRB promotes three-dimensional assembly and differentiation, possibly through promoting cell-to-cell interaction and/or secretion of growth and differentiation factors.

Actin microdomains on endothelial cells: association with CD44, ERM proteins, and signaling molecules during quiescence and wound healing

During studies of the actin cytoskeleton in cultured endothelial cells we have observed that the luminal side of many cells contains F-actin microdomains that are rich in the hyaluronan receptor CD44 and in ezrin-radixin-moesin (ERM) proteins. A small subpopulation of the domains are also enriched in tyrosine phosphorylated proteins and signaling molecules. Confocal microscopy of rat aortic endothelial cells in situ demonstrated that similar microdomains occur in vivo. During healing of endothelial wounds, characteristic alterations of the actin cytoskeleton occurred. Thus, in many cells close to the wound, focal F-actin branching points appeared. The branching points were similar to the microdomains in that they colocalized with CD44 and ERM proteins, but, in addition, they formed centers for actin filament branching and were associated with phosphorylated protein kinase C alpha/betaII. These colocalization data are consonant with the view that activated PKC is responsible for activating ERM-mediated crosslinking between CD44 and the actin cytoskeleton. Importantly, inhibition of PKC activity decreased staining for phosphorylated ERM proteins, decreased the frequency of F-actin branching points, and inhibited monolayer wound healing. Together, our data show that endothelial cells contain a novel actin cytoskeletal structure, the F-actin microdomain, and suggest that during wound healing such structures become associated with activated signaling molecules and thereby enhance actin cytoskeletal remodeling.

Role of RhoA, mDia, and ROCK in cell shape-dependent control of the Skp2-p27kip1 pathway and the G1/S transition

Cell shape-dependent control of cell-cycle progression underlies the spatial differentials of growth that drive tissue morphogenesis, yet little is known about how cell distortion impacts the biochemical signaling machinery that is responsible for growth control. Here we show that the Rho family GTPase, RhoA, conveys the "cell shape signal" to the cell-cycle machinery in human capillary endothelial cells. Cells accumulating p27(kip1) and arrested in mid G(1) phase when spreading were inhibited by restricted extracellular matrix adhesion, whereas constitutively active RhoA increased expression of the F-box protein Skp2 required for ubiquitination-dependent degradation of p27(kip1) and restored G(1) progression in these cells. Studies with dominant-negative and constitutively active forms of mDia1, a downstream effector of RhoA, and with a pharmacological inhibitor of ROCK, another RhoA target, revealed that RhoA promoted G(1) progression by altering the balance of activities between these two downstream effectors. These data indicate that signaling proteins such as mDia1 and ROCK, which are thought to be involved primarily in cytoskeletal remodeling, also mediate cell growth regulation by coupling cell shape to the cell-cycle machinery at the level of signal transduction.

Variation in cell–substratum adhesion in relation to cell cycle phases

The quantification of focal adhesion sites offers an assessable method of measuring cell-substrate adhesion. Such measurement can be hindered by intra-sample variation that may be cell cycle derived. A combination of autoradiography and immunolabelling techniques, for scanning electron microscopy (SEM), were utilised simultaneously to identify both S-phase cells and their focal adhesion sites. Electron-energy 'sectioning' of the sample, by varying the accelerating voltage of the electron beam, combined with backscattered electron (BSE) imaging, allowed for S-phase cell identification in one energy 'plane' image and quantitation of immunogold label in another. As a result, it was possible simultaneously to identify S-phase cells and their immunogold-labelled focal adhesions sites on the same cell. The focal adhesion densities were calculated both for identified S-phase cells and the remaining non-S-phase cells present. The results indicated that the cell cycle phase was a significant factor in determining the density of focal adhesions, with non-S-phase cells showing a larger adhesion density than S-phase cells. Focal adhesion morphology was also seen to correspond to cell cycle phase; with 'dot' adhesions being more prevalent on smaller non-S-phase and the mature 'dash' type on larger S-phase cells. This study demonstrated that when quantitation of focal adhesion sites is required, it is necessary to consider the influence of cell cycle phases on any data collected.

Mechanisms governing the visco-elastic responses of living cells assessed by foam and tensegrity models

The visco-elastic properties of living cells, measured to date by various authors, vary considerably, depending on the experimental methods and/or on the theoretical models used. In the present study, two mechanisms thought to be involved in cellular visco-elastic responses were analysed, based on the idea that the cytoskeleton plays a fundamental role in cellular mechanical responses. For this purpose, the predictions of an open unit-cell model and a 30-element visco-elastic tensegrity model were tested, taking into consideration similar properties of the constitutive F-actin. The quantitative predictions of the time constant and viscosity modulus obtained by both models were compared with previously published experimental data obtained from living cells. The small viscosity modulus values (10(0)-10(3) Pa x s) predicted by the tensegrity model may reflect the combined contributions of the spatially rearranged constitutive filaments and the internal tension to the overall cytoskeleton response to external loading. In contrast, the high viscosity modulus values (10(3)-10(5) Pa x s) predicted by the unit-cell model may rather reflect the mechanical response of the cytoskeleton to the bending of the constitutive filaments and/or to the deformation of internal components. The present results suggest the existence of a close link between the overall visco-elastic response of micromanipulated cells and the underlying architecture.

Angiogenesis in collagen I requires alpha2beta1 ligation of a GFP*GER sequence and possibly p38 MAPK activation and focal adhesion disassembly

Angiogenesis depends on proper collagen biosynthesis and cross-linking, and type I collagen is an ideal angiogenic scaffold, although its mechanism is unknown. We examined angiogenesis using an assay wherein confluent monolayers of human umbilical vein endothelial cells were overlain with collagen in a serum-free defined medium. Small spaces formed in the cell layer by 2 h, and cells formed net-like arrays by 6-8 h and capillary-like lumens by 24 h. Blocking of alpha2beta1, but not alpha1 or alpha(v)beta3 integrin function halted morphogenesis. We found that a triple-helical, homotrimeric peptide mimetic of a putative alpha2beta1 binding site: alpha1(I)496-507 GARGERGFP*GER (where single-letter amino acid nomenclature is used, P* = hydroxyproline) inhibited tube formation, whereas a peptide carrying another putative site: alpha1(I)127-138 GLP*GERGRP*GAP* or control peptides did not. A chemical inhibitor of p38 mitogen-activated protein kinase (p38 MAPK), SB202190, blocked tube formation, and p38 MAPK activity was increased in collagen-treated cultures, whereas targeting MAPK kinase (MEK), focal adhesion kinase (FAK), or phosphatidylinositol 3-kinase (PI3K) had little effect. Collagen-treated cells had fewer focal adhesions and 3- to 5-fold less activated FAK. Thus capillary morphogenesis requires endothelial alpha2beta1 integrin engagement of a single type I collagen integrin-binding site, possibly signaling via p38 MAPK and focal adhesion disassembly/FAK inactivation.

Modulation of smooth muscle phenotype in vitro by homologous cell substrate

We have developed a novel cell culture system that supports the shortening of smooth muscle cells. Primary rat airway smooth muscle cells were plated on an ethanol-fixed, confluent monolayer of homologous smooth muscle cells (homologous cell substrate, HCS). Cells grown on HCS exhibited morphological and functional characteristics consistent with a differentiated phenotype. Cells on HCS were spindle shaped with a well-defined long axis, whereas cells grown on glass were larger and irregularly shaped. Smooth muscle-specific alpha-actin immunostained diffusely in cells on HCS, whereas it appeared as stress fibers in cells on glass. Agonists recruited a greater fraction of HCS cells to contract, resulting in greater changes in cell area or length on average, but the maximal capacity of shortening of individual cells was similar between the groups. Unlike cells on glass, cells on HCS shortened to methacholine. HCS was reversible and persisted over several passages. Agonists stimulated intracellular Ca(2+) oscillations in cells on HCS, whereas they elicited biphasic peak and plateau transients in cells on glass. HCS modulates smooth muscle cell phenotype in vitro.

Cell spreading controls endoplasmic and nuclear calcium: a physical gene regulation pathway from the cell surface to the nucleus

Cells attaching to an extracellular matrix through integrins flatten out (spread) on the matrix, eliciting cellular signals needed for survival. We show that the shape of the nucleus changes and the nuclear calcium level increases in spreading cells. Moreover, cell spreading and osmotic stretching of isolated nuclei cause release of perinuclear Ca(2+), and patch clamping of nuclei reveals stretch-activated Ca(2+) permeable channels. Gene expression assays with myocyte enhancer factor 2, which is activated by calmodulin-dependent kinase IV, indicate that the elevation in nuclear Ca(2+) is functionally significant. We propose a mechano-transduction pathway in which spreading-induced nuclear stretching releases Ca(2+) from the perinuclear space, and the resulting Ca(2+) elevation in the nucleus provokes changes in gene expression.

Tensegrity I. Cell structure and hierarchical systems biology

In 1993, a Commentary in this journal described how a simple mechanical model of cell structure based on tensegrity architecture can help to explain how cell shape, movement and cytoskeletal mechanics are controlled, as well as how cells sense and respond to mechanical forces (J. Cell Sci. 104, 613-627). The cellular tensegrity model can now be revisited and placed in context of new advances in our understanding of cell structure, biological networks and mechanoregulation that have been made over the past decade. Recent work provides strong evidence to support the use of tensegrity by cells, and mathematical formulations of the model predict many aspects of cell behavior. In addition, development of the tensegrity theory and its translation into mathematical terms are beginning to allow us to define the relationship between mechanics and biochemistry at the molecular level and to attack the larger problem of biological complexity. Part I of this two-part article covers the evidence for cellular tensegrity at the molecular level and describes how this building system may provide a structural basis for the hierarchical organization of living systems--from molecule to organism. Part II, which focuses on how these structural networks influence information processing networks, appears in the next issue.

Function of the cytoskeleton in gravisensing during spaceflight

Since astronauts and cosmonauts have significant bone loss in microgravity we hypothesized that there would be physiological changes in cellular bone growth and cytoskeleton in the absence of gravity. Investigators from around the world have studied a multitude of bone cells in microgravity including Ros 17/2.8, Mc3T3-E1, MG-63, hFOB and primary chicken calvaria. Changes in cytoskeleton and extracellular matrix (ECM) have been noted in many of these studies. Investigators have noted changes in shape of cells exposed to as little as 20 seconds of microgravity in parabolic flight. Our laboratory reported that quiescent osteoblasts activated by sera under microgravity conditions had a significant 60% reduction in growth (p<0.001) but a paradoxical 2-fold increase in release of the osteoblast autocrine factor PGE2 when compared to ground controls. In addition, a collapse of the osteoblast actin cytoskeleton and loss of focal adhesions has been noted after 4 days in microgravity. Later studies in Biorack on STS-76, 81 and 84 confirmed the increased release of PGE2 and collapse of the actin cytoskeleton in cells grown in microgravity conditions, however flown cells under 1 g conditions maintained normal actin cytoskeleton and fibronectin matrix. The changes seen in the cytoskeleton are probably not due to alterations in fibronectin message or protein synthesis since no differences have been noted in microgravity. Multiple investigators have observed actin and microtubule cytoskeletal modifications in microgravity, suggesting a common root cause for the change in cell architecture. The inability of the 0 g grown osteoblast to respond to sera activation suggests that there is a major alteration in anabolic signal transduction under microgravity conditions, most probably through the growth factor receptors and/or the associated kinase pathways that are connected to the cytoskeleton. Cell cycle is dependent on the cytoskeleton. Alterations in cytoskeletal structure can block cell growth either in G1 (F-actin microfilament collapse), or in G2/M (inhibition of microtubule polymerization during G2/M-phase). We therefore hypothesize that microgravity would inhibit growth in either G1, or G2/M.

A cellular tensegrity model to analyse the structural viscoelasticity of the cytoskeleton

This study describes the viscoelastic properties of a refined cellular-tensegrity model composed of six rigid bars connected to a continuous network of 24 viscoelastic pre-stretched cables (Voigt bodies) in order to analyse the role of the cytoskeleton spatial rearrangement on the viscoelastic response of living adherent cells. This structural contribution was determined from the relationships between the global viscoelastic properties of the tensegrity model, i.e., normalized viscosity modulus (eta(*)), normalized elasticity modulus (E(*)), and the physical properties of the constitutive elements, i.e., their normalized length (L(*)) and normalized initial internal tension (T(*)). We used a numerical method to simulate the deformation of the structure in response to different types of loading, while varying by several orders of magnitude L(*) and T(*). The numerical results obtained reveal that eta(*) remains almost independent of changes in T(*) (eta(*) proportional, variant T(*+0.1)), whereas E(*) increases with approximately the square root of the internal tension T(*) (from E(*) proportional, variant T(*+0.3) to E(*) proportional, variant T(*+0.7)). Moreover, structural viscosity eta(*) and elasticity E(*) are both inversely proportional to the square of the size of the structure (eta(*) proportional, variant L(*-2) and E(*) proportional, variant L(*-2)). These structural properties appear consistent with cytoskeleton (CSK) mechanical properties measured experimentally by various methods which are specific to the CSK micromanipulation in living adherent cells. Present results suggest, for the first time, that the effect of structural rearrangement of CSK elements on global CSK behavior is characterized by a faster cellular mechanical response relatively to the CSK element response, which thus contributes to the solidification process observed in adherent cells. In extending to the viscoelastic properties the analysis of the mechanical response of the cellular 30-element tensegrity model, the present study contributes to the understanding of recent results on the cellular-dynamic response and allows to reunify the scattered data reported for the viscoelastic properties of living adherent cells.

The antiangiogenic activity of cleaved high molecular weight kininogen is mediated through binding to endothelial cell tropomyosin

Conformationally altered proteins and protein fragments derived from the extracellular matrix and hemostatic system may function as naturally occurring angiogenesis inhibitors. One example of such a protein is cleaved high molecular weight kininogen (HKa). HKa inhibits angiogenesis by inducing apoptosis of proliferating endothelial cells, effects mediated largely by HKa domain 5. However, the mechanisms underlying the antiangiogenic activity of HKa have not been characterized, and its binding site on proliferating endothelial cells has not been defined. Here, we report that the induction of endothelial cell apoptosis by HKa, as well as the antiangiogenic activity of HKa in the chick chorioallantoic membrane, was inhibited completely by antitropomyosin monoclonal antibody TM-311. TM-311 also blocked the high-affinity Zn2+-dependent binding of HKa to both purified tropomyosin and proliferating endothelial cells. Confocal microscopic analysis of endothelial cells stained with monoclonal antibody TM-311, as well as biotin labeling of cell surface proteins on intact endothelial cells, revealed that tropomyosin exposure was enhanced on the surface of proliferating cells. These studies demonstrate that the antiangiogenic effects of HKa depend on high-affinity binding to endothelial cell tropomyosin.

Nanoscale modifications of PET polymer surfaces via oxygen-plasma discharge yield minimal changes in attachment and growth of mammalian epithelial and mesenchymal cells in vitro

Surface topography is believed to be a factor affecting cellular morphology, proliferation, and differentiation. The effect of surface roughness in the micron to supramicron range has been investigated previously. In the current study, the influence of nanoscale surface roughness was examined in terms of its effects on morphology, cytoskeleton expression, proliferation, differentiation, and apoptosis of three model cell types. Polyethylene terephthalate (PET) disks were etched using oxygen plasma to produce uniform and decidedly nanoscale levels of surface roughness. Three distinct types of cell lines-mouse 3T3-L1 preadipocytes, human JEG-3 choriocarcinoma cells, and human MCF-7 breast adenocarcinoma cells-were cultured on the plasma-treated disks. Untreated PET disks were used as a control. Cytoskeletal proteins (F-actin and cytokeratin) exhibited similar patterns of expression. Cell morphology also was similar on both surfaces. Cell growth kinetics for the three types of cells and hormone secretion from the JEG-3 cells were not significantly different from that of the controls (p > 0.05). However, the differentiation of preadipocyte 3T3-L1 cells into lipid-laden fat cells was modestly affected by nanoscale surface topography. In addition, 15-deoxy-Delta(12,14)-prostaglandin J(2) (15dPGJ(2))-induced apoptosis of the JEG-3 and MCF-7 cells revealed differences between the two surfaces. Plasma-treated surfaces showed more differentiated and apoptotic cells, respectively, compared to the controls. These results indicate that nanoscale roughness contributes in only moderate ways to cellular adhesion, proliferation, and differentiation in the cell lines tested.

Spontaneously formed tumorigenic hybrids of Meth A sarcoma and macrophages grow faster and are better vascularized than the parental tumor

Macrophages and Meth A sarcoma cells spontaneously fuse and give rise to tumorigenic hybrid cell lines with a mixed phenotype. We report here that the hybrid tumors grow faster and have a strikingly better developed vasculature than the parent sarcoma. Thus, electron microscopy and immunohistochemical analysis revealed that in the most active areas of neovascularization, the tumors that emerged from inocula of monoclonal hybrid cell populations had a microvessel density nearly twice that of Meth A tumors after 1 week of growth. Moreover, the proportion of vessels associated with pericytes, detected by staining for smooth muscle alpha-actin, was 3 times higher in the hybrid tumors, attesting to the more advanced differentiation of their vasculature. The collagenous stroma component was also more extensive in the hybrid tumors. Concentration of the angiogenic proteins vascular endothelial growth factor (VEGF) and transforming growth factor-beta (TGF-beta) were significantly higher in supernatants of hybrid cell cultures compared with Meth A cultures. These observations indicate that the growth advantage of the hybrid tumors over the parental sarcoma is due to a higher angiogenic capacity. Because the malignant features of many tumors correlate with angiogenesis and because macrophages are known to be major producers of angiogenic factors, our data open the possibility that the intense neovascularization of highly aggressive cancers in some cases reflects the acquisition of macrophage traits by heterotypic cell fusion.

Assessment of mechanical properties of adherent living cells by bead micromanipulation: comparison of magnetic twisting cytometry vs optical tweezers

We compare the measurements of viscoelastic properties of adherent alveolar epithelial cells by two micromanipulation techniques: (i) magnetic twisting cytometry and (ii) optical tweezers, using microbeads of same size and similarly attached to F-actin. The values of equivalent Young modulus E, derived from linear viscoelasticity theory, become consistent when the degree of bead immersion in the cell is taken into account. E-values are smaller in (i) than in (ii): approximately 34-58 Pa vs approximately 29-258 Pa, probably because higher stress in (i) reinforces nonlinearity and cellular plasticity. Otherwise, similar relaxation time constants, around 2 s, suggest similar dissipative mechanisms.

Impact of sterilization on the porous design and cell behavior in collagen sponges prepared for tissue engineering

This study investigates the impact of different sterilization processes on structural integrity and stability of collagen sponges designed for tissue engineering. Collagen sponges with uniform pore size (20 microm) were sterilized either with ethylene oxide (EO) or gamma irradiation (2.5 Mrad). Gamma-sterilized sponges showed a dramatic decrease of resistance against enzyme degradation and severe shrinkage after cell seeding. Collapsed porosity inhibited fibroblasts and barred completely the human umbilical vein endothelial cell ingrowth into the sponges. On the contrary, the porous structure and stability of EO-sterilized sponges remained almost unaltered. Fibroblasts and endothelial cells exhibited favorable proliferation and migration within sponges with normal morphology. Tubular formation by seeded endothelial cells occurred early in the first week. Therefore, we emphasize that the impact of sterilization of biomaterials is substantial and any new procedure has to be evaluated by correlating the impact of the procedure on the porous structure with cell proliferation behavior.

A discrete cell cycle checkpoint in late G(1) that is cytoskeleton-dependent and MAP kinase (Erk)-independent

Cell spreading on extracellular matrix and associated changes in the actin cytoskeleton (CSK) are necessary for progression through G(1) and entry into S phase of the cell cycle. Pharmacological disruption of CSK integrity inhibits early mitogenic signaling to the extracellular signal-regulated kinase (Erk) subfamily of the mitogen-activated protein kinases (MAPKs) and arrests the cell cycle in G(1). Here we show that this block of G(1) progression is not simply a consequence of inhibition of the MAPK/Erk pathway but instead it reveals the existence of a discrete CSK-sensitive checkpoint. Use of PD98059 to inhibit MAPK/Erk and cytochalasin D (Cyto D) to disrupt the actin CSK at progressive time points in G(1) revealed that the requirement for MAPK/Erk activation lasts only to mid-G(1), while the actin CSK must remain intact up to late G(1) restriction point, R, in order for capillary endothelial cells to enter S phase. Additional analysis using Cyto D pulses defined a narrow time window of 3 h just prior to R in which CSK integrity was shown to be critical for the G(1)/S transition. Cyto D treatment led to down-regulation of cyclin D1 protein and accumulation of the cdk inhibitor, p27(Kip1), independent of cell cycle phase, suggesting that these changes resulted directly from CSK disruption rather than from a general cell cycle block. Together, these data indicate the existence of a distinct time window in late G(1) in which signals elicited by the CSK act independently of early MAPK/Erk signals to drive the cell cycle machinery through the G(1)/S boundary and, hence, promote cell growth.

Cell density and N-cadherin interactions regulate cell proliferation in the sensory epithelia of the inner ear

Sensory hair cells in the inner ears of nonmammalian vertebrates can regenerate after injury. In many species, replacement hair cells are produced by the proliferation of epithelial supporting cells. Thus, the ability of supporting cells to undergo renewed proliferation is a key determinant of regenerative ability. The present study used cultures of isolated inner ear sensory epithelia to identify cellular signals that regulate supporting cell proliferation. Small pieces of sensory epithelia from the chicken utricle were cultured in glass microwells. Under those conditions, cell proliferation was inversely related to local cell density. The signaling molecules N-cadherin, beta-catenin, and focal adhesion kinase were immunolocalized in the cultured epithelial cells, and high levels of phosphotyrosine immunoreactivity were present at cell-cell junctions and focal contacts of proliferating cells. Binding of microbeads coated with a function-blocking antibody to N-cadherin inhibited ongoing proliferation. The growth of epithelial cells was also affected by the density of extracellular matrix molecules. The results suggest that cell density, cell-cell contact, and the composition of the extracellular matrix may be critical influences on the regulation of sensory regeneration in the inner ear.

Capillary plexus development in the day five to day six chick chorioallantoic membrane is inhibited by cytochalasin D and suramin

The chick chorioallantoic membrane (CAM) is a valuable model for evaluating angiogenesis and vasculogenesis. Our purpose was to characterize the formation of the CAM vasculature, in particular the capillary plexus, between days five and six after fertilization and to examine the mode of action of cytochalasin D and suramin on vascular development during this interval. The CAM increased 20-fold in size between days five and six, during which time the capillary plexus forms by both migration of mesodermal blood vessels toward the ectoderm and by the formation of new vessels from angioblasts near the ectoderm. Between days five and six, the CAM becomes thinner, and the density of the mesodermal cells decreases. To determine the mode of action of anti-angiogenic drugs on the day five to day six CAM, various concentrations of cytochalasin D or suramin were added directly to day five CAMs, and their effects were evaluated on day six. Both drugs significantly inhibited CAM growth, altered branching patterns of the major vessels, decreased area of the major vessels, and inhibited the formation of the capillary plexus by inhibiting both vasculogenesis and the migration of mesodermal blood vessels to the ectoderm. Cytochalasin D also inhibited compartmentalization of the plexus. Cytochalasin D and suramin were inhibitory at similar doses. This study provides new information on early CAM development, establishes the mode of action and dose dependency of cytochalasin D and suramin on day five to day six CAMs, and demonstrates that the day five to day six CAM provides a useful assay to examine the effect of anti-angiogenic drugs on blood vessel development, including capillary plexus formation.

Microfilament disruption in a noncycling organized tissue, the corneal endothelium, initiates mitosis

The adult corneal endothelium represents a noncycling cell population that resides as a monolayer on its basement membrane, Descemet's membrane. Evidence is presented for the first time, showing that mitotic regulation in this organized tissue, residing on its natural basement membrane, is coupled to microfilament integrity. When mitotically quiescent rat corneal endothelia are organ cultured in medium containing serum and cytochalasin B, low levels of mitosis are initiated. Supplementing the culture medium with either insulin or IGF-2 augments this response and results in increased cell density within the tissue monolayer. Fluorescence microscopy of actin using TRITC-conjugated phalloidin revealed that cellular circumferential microfilament bundles appear unaffected by cytochalasin B treatment, whereas the cytoplasmic microfilaments appear to be completely disrupted. These results suggest the possibility that the actin cytoskeleton is involved with the regulation of cell growth in the corneal endothelium.

Cell mechanics: mechanical response, cell adhesion, and molecular deformation

As the basic unit of life, the cell is a biologically complex system, the understanding of which requires a combination of various approaches including biomechanics. With recent progress in cell and molecular biology, the field of cell mechanics has grown rapidly over the last few years. This review synthesizes some of these recent developments to foster new concepts and approaches, and it emphasizes molecular-level understanding. The focuses are on the common themes and interconnections in three related areas: (a) the responses of cells to mechanical forces, (b) the mechanics and kinetics of cell adhesion, and (c) the deformation of biomolecules. Specific examples are also given to illustrate the quantitative modeling used in analyzing biological processes and physiological functions.

In situ cell nucleus deformation in tendons under tensile load; a morphological analysis using confocal laser microscopy

Cell and cell nucleus deformations have been implicated in the mechanotransduction of mechanical loads acting on tissues. While in situ cell nucleus deformation in response to increasing tissue strains has been examined in articular cartilage this phenomenon has not been investigated in tendons. To examine in situ cell nuclei deformation in tendons undergoing tensile strain rat tail tendons were harvested from adult Sprague-Dawley rats and stained with acridine orange to highlight the cell nuclei. The tendons were mounted on a custom-designed, low-load, tensile testing device affixed to the mechanical stage of a confocal laser microscope. Cells within the tendons were isolated for analysis. Images of individual cells were captured at 0% strain as well as sequentially at 2%, 4% and 6% grip-to-grip tendon strain. Digital images of the cell nuclei were then measured in the x (length) and y (height) axis and deformation expressed as a percentage of cell nuclei strain. In addition, centroid-to-centroid distances of adjacent cell nuclei within each image were measured and used to calculate local tissue strain. There was a weak (r2 = 0.34) but significant (p < 0.01) correlation between local tissue strain and cell nucleus strain in the x axis. The results of this study support the hypothesis that in situ cell nucleus deformation does occur during tensile loading of tendons. This deformation may play a significant role in the mechanical signal transduction pathway of this tissue.

Substratum nanotopography and the adhesion of biological cells. Are symmetry or regularity of nanotopography important?

Animal cells live in environments where many of the features that surround them are on the nanoscale, for example detail on collagen molecules. Do cells react to objects of this size and if so, what features of the molecules are they responding to? Here we show, by fabricating nanometric features in silica and by casting reverse features in polycaprolactone and culturing vertebrate cells in culture upon them, that cells react in their adhesion to the features. With cliffs, adhesion is enhanced at the cliff edge, while pits or pillars in ordered arrays diminish adhesion. The results implicate ordered topography and possibly symmetry effects in the adhesion of cells. Parallel results were obtained in the adhesion of carboxylate-surfaced 2-microm-diameter particles to these surfaces. These results are in agreement with recent predictions from non-biological nanometric systems.

Cooperative control of Akt phosphorylation, bcl-2 expression, and apoptosis by cytoskeletal microfilaments and microtubules in capillary endothelial cells

Capillary endothelial cells can be switched between growth and apoptosis by modulating their shape with the use of micropatterned adhesive islands. The present study was carried out to examine whether cytoskeletal filaments contribute to this response. Disruption of microfilaments or microtubules with the use of cytochalasin D or nocodazole, respectively, led to levels of apoptosis in capillary cells equivalent to that previously demonstrated by inducing cell rounding with the use of micropatterned culture surfaces containing small (<20 microm in diameter) circular adhesive islands coated with fibronectin. Simultaneous disruption of microfilaments and microtubules led to more pronounced cell rounding and to enhanced levels of apoptosis approaching that observed during anoikis in fully detached (suspended) cells, indicating that these two cytoskeletal filament systems can cooperate to promote cell survival. Western blot analysis revealed that the protein kinase Akt, which is known to be critical for control of cell survival became dephosphorylated during cell rounding induced by disruption of the cytoskeleton, and that this was accompanied by a decrease in bcl-2 expression as well as a subsequent increase in caspase activation. This ability of the cytoskeleton to control capillary endothelial cell survival may be important for understanding the relationship among extracellular matrix turnover, cell shape changes, and apoptosis during angiogenesis inhibition.

Fibronectin, integrins, and growth control

Cell proliferation is controlled not only by soluble mitogens but also by components of the extracellular matrix (ECM) such as fibronectin, to which cells adhere via the integrin family of transmembrane receptors. Input from both growth factor receptors and integrins is required to stimulate progression through the G1 phase of the cell cycle, via induction of G1 cyclins and suppression of inhibitors of the G1 cyclin-dependent kinases. Extensive crosstalk takes place between integrin and growth factor receptor signaling pathways, and mitogenic signaling is weak and transient in the absence of integrin-mediated cell adhesion. In normal untransformed cells, all of the important mitogenic signal transduction cascades, namely those downstream of the Ras and Rho family small GTPases and the phosphoinositide 3-OH kinase-PKB/Akt pathway, are regulated by integrin-mediated cell adhesion. As a result, these cells are anchorage-dependent for growth. In contrast, constitutive activity of each of these pathways has been reported in cancer cells, which not only reduces their mitogen dependence but also allows these cells to grow in an anchorage-independent fashion.

Three-dimensional cell-scaffold constructs promote efficient gene transfection: implications for cell-based gene therapy

To date, introduction of gene-modified cells in vivo is still a critical limitation for cell-based gene therapy. In this study, based on tissue engineering techniques, we developed a three-dimensional (3-D) transfection system to be cell-based gene delivery vehicle. Human trophoblast-like ED(27) and fibroblastic NIH3T3 cells were used as model cell lines. Cells were seeded onto PET fibrous matrices and plated on polyethylene terephathalate (PET) films as 2-D transfection control. The cell-matrices and cell-films were transfected with pCMV-betagal and pEGFP (green fluorescent protein) reporter gene vectors using LipofectAmine reagent. Gene expression on 3-D versus 2-D growth surface were investigated. The effects of seeding method, seeding density, porosity of the PET matrix, and culturing time of the cell-matrix complex on cDNA transfection and expression in the 3-D cell-matrix complex were also investigated. The beta-gal assay and GFP detection showed that 3-D transfection promoted a higher gene expression level and longer expression time as compared to 2-D transfection. There existed an optimal initial cell seeding density for gene transfection of 3-D cell-matrix complex. Cells seeded on PET matrices with a lower porosity ( approximately 87%) had higher gene expression activities than cells in the matrices with a higher porosity ( approximately 90%). Also, Higher gene expression levels of beta-gal were obtained for the more uniformly seeded matrices that were seeded with a depth-filtration method. The results from this study demonstrate the potential utility of cells seeded onto 3-D fibrous matrices as cell-based gene delivery vehicle for in vitro study of gene expression or in vivo gene therapy.

Measurement of cell microrheology by magnetic twisting cytometry with frequency domain demodulation

Magnetic twisting cytometry (MTC) (Wang N, Butler JP, and Ingber DE, Science 260: 1124-1127, 1993) is a useful technique for probing cell micromechanics. The technique is based on twisting ligand-coated magnetic microbeads bound to membrane receptors and measuring the resulting bead rotation with a magnetometer. Owing to the low signal-to-noise ratio, however, the magnetic signal must be modulated, which is accomplished by spinning the sample at approximately 10 Hz. Present demodulation approaches limit the MTC range to frequencies <0.5 Hz. We propose a novel demodulation algorithm to expand the frequency range of MTC measurements to higher frequencies. The algorithm is based on coherent demodulation in the frequency domain, and its frequency range is limited only by the dynamic response of the magnetometer. Using the new algorithm, we measured the complex modulus of elasticity (G*) of cultured human bronchial epithelial cells (BEAS-2B) from 0.03 to 16 Hz. Cells were cultured in supplemented RPMI medium, and ferromagnetic beads (approximately 5 microm) coated with an RGD peptide were bound to the cell membrane. Both the storage (G', real part of G*) and loss (G", imaginary part of G*) moduli increased with frequency as omega(alpha) (2 pi x frequency) with alpha approximately equal to 1/4. The ratio G"/G' was approximately 0.5 and varied little with frequency. Thus the cells exhibited a predominantly elastic behavior with a weak power law of frequency and a nearly constant proportion of elastic vs. frictional stresses, implying that the mechanical behavior conformed to the so-called structural damping (or constant-phase) law (Maksym GN, Fabry B, Butler JP, Navajas D, Tschumperlin DJ, LaPorte JD, and Fredberg JJ, J Appl Physiol 89: 1619-1632, 2000). We conclude that frequency domain demodulation dramatically increases the frequency range that can be probed with MTC and reveals that the mechanics of these cells conforms to constant-phase behavior over a range of frequencies approaching three decades.

Differential phosphorylation of paxillin in response to surface-bound serum proteins during early osteoblast adhesion

An early signaling event during the adhesion and spreading of cells is integrin-mediated tyrosine phosphorylation of the cytoskeletal adaptor protein paxillin and the non-receptor tyrosine kinase pp125(FAK) at focal contacts. To determine the influence of surface-charge and -adsorbed adhesion proteins on this signaling pathway, paxillin phosphorylation was examined during attachment of MC3T3-E1 osteoblast-like cell onto charged and uncharged polystyrene, and on adsorbed layers of serum proteins, fibronectin (Fn), vitronectin (Vn), a mixture of Fn and Vn, and albumin. Paxillin phosphorylation was induced 2.4-fold (P < 0.05) on charged vs uncharged polystyrene only in the presence of serum proteins. Activation of paxillin via Fn or Vn alone, or in combination, resulted in significantly lower phosphorylation signals compared to whole serum (41 +/- 6.9%, P < 0.05, 45 +/- 5.9%, P < 0.05, and 76 +/- 9.8%, P < 0.075, respectively). Confocal laser microscopy confirmed increased co-localization of phosphotyrosine and paxillin at protruding lamellopodia of spreading osteoblasts on charged vs uncharged serum-pretreated polystyrene. Taken together, these data suggest that subtle differences in surface characteristics mediate effects on adhering cells via adsorbed serum proteins involving the cytoskeletal adaptor protein paxillin.

The effect of static in vivo bending on the murine intervertebral disc

BACKGROUND CONTEXT

Intervertebral disc cell function in vitro has been linked to features of the local environment that can be related to deformation of the extracellular matrix. Epidemiologic data suggest that certain regimens of spinal loading accelerate disc degeneration in vivo. Yet, the direct association between disc cell function, spinal loading and ultimately tissue degeneration is poorly characterized.

PURPOSE

To examine the relationships between tensile and compressive matrix strains, cell activity and annular degradation.

STUDY DESIGN/SETTING

An in vivo study of the biologic, morphologic and biomechanical consequences of static bending applied to the murine intervertebral disc. SUBJECT SAMPLE: Twenty-five skeletally mature Swiss Webster mice (12-week-old males) were used in this study.

OUTCOME MEASURES

Bending neutral zone, bending stiffness, yield point in bending, number of apoptotic cells, annular matrix organization, cell shape, aggrecan gene expression, and collagen II gene expression.

METHODS

Mouse tail discs were loaded for 1 week in vivo with an external device that applied bending stresses. Mid-sagittal sections of the discs were analyzed for cell death, collagen II and aggrecan gene expression, and tissue organization. Biomechanical testing was also performed to measure the bending stiffness and strength.

RESULTS

Forceful disc bending induced increased cell death, decreased aggrecan gene expression and decreased tissue organization preferentially on the concave side. By contrast, collagen II gene expression was symmetrically reduced. Asymmetric loading did not alter bending mechanical behavior of the discs.

CONCLUSIONS

In this model, annular cell death was related to excessive matrix compression (as opposed to tension). Collagen II gene expression was most negatively influenced by the static nature of the loading (immobilization), rather than the specific state of stress (tension or compression).

Theoretical analysis of shape transformation of V-79 cells after treatment with cytochalasin B

We observed that after treatment of V-79 fibroblasts with cytochalasin B the area of cell contact with the substrate is essentially reduced, the microtubules are organized into rodlike structures and the actin filaments are disintegrated. Remnants of the actin cortex become concentrated in the form of discrete patches under the plasma membrane. The described changes in the organization of the cytoskeleton and of the cortical shell are accompanied by the formation of a cell shape resembling the Greek letter phi. We calculated that the phi shape corresponds to the minimum of the stretching energy of the cortical shell at relevant geometrical constraints. In line with this result, if cytochalasin B treatment was followed by colchicine application which disrupted the microtubular rod, the characteristic phi shape completely disappeared. This study suggests that the effect of the microtubular rod on the cell shape can be theoretically well described by taking into account some basic conditions for the mechanical equilibrium of the cell cortical shell and the appropriate geometrical constraints.

In vivo interaction between dynamitin and MacMARCKS detected by the fluorescent resonance energy transfer method

Dynamitin is a subunit of the dynactin complex regulating microtubule-dependent motor functions, and MacMARCKS (Macrophage-enriched myristoylated alanine-rich protein kinase C substrate) is a major protein kinase C substrate regulating integrin activation. The interaction between dynamitin and MacMARCKS has been implicated in integrin-dependent cell spreading. However, the in vivo interaction of these two proteins in living cells has not been demonstrated. Spatial and temporal information about the interaction is also lacking. In this study, we used the fluorescent resonance energy transfer method to demonstrate in vivo interaction between MacMARCKS and dynamitin with cyan fluorescent protein (CFP)-conjugated dynamitin as the donor fluorophore and yellow fluorescent protein (YFP)-conjugated MacMARCKS as the acceptor fluorophore. The interaction of these two fusion proteins was studied both in vitro and in vivo, and typical fluorescent resonance energy transfer was observed; the CFP emission peak increased while the YFP emission peak decreased when protein interaction was abolished. Spatial and temporal information was obtained in RAW macrophage cells. In resting macrophage cells, dynamitin-MacMARCKS interaction is concentrated at the cell periphery, although the majority of dynamitin is distributed at the perinuclear region of the cells. When cells were treated with phorbol 12-myristate 13-acetate, both proteins concentrated to perinuclear regions of the cells, and yet the interaction disappeared as the cell spread. Similar events were also observed in 293 cells. Thus, we conclude that dynamitin and MacMARCKS indeed interact in living cells.

Induced membrane hypo/hyper-mechanosensitivity: a limitation of patch-clamp recording

Practical limitations of the patch-clamp technique when recording mechanogated membrane ion channels are considered. Mechanical overstimulation of the patch or the cell from excessive suction/pressure protocols induces morphological and functional changes. In particular, the plasma membrane becomes decoupled from the underlying cytoskeleton to form either membrane blebs (cell-attached) or ghosts (whole cell). As a consequence, a membrane ion channel may show either a decrease or an increase in its native mechanosensitivity or even acquire mechanosensitivity. The effect varies with ion channel and cell type and presumably arises because of a disruption of membrane-cytoskeleton interactions. We consider that such disruptions are a pathological consequence of excessive mechanical stress, either during or after seal formation, rather than an immutable consequence of patch-clamp recording. By careful attention to the suction/pressure protocols during sealing and throughout recording, such artifacts can be avoided.

Endocytosis of ultrafine particles by A549 cells

Alveolar epithelium's capacity to ingest inhaled ultrafine particles is not well characterized. The objectives of this study were to use an in vitro model of type II lung epithelium and evaluate the cells' ability to take up ultrafine particles (titanium dioxide [TiO(2)], 50 nm diameter). The human epithelial cell line A549 was grown on aclar substrates and exposed to 40 microg/ml TiO(2) particles for 3, 6, and 24 h before imaging with energy-filtering transmission electron microscopy. Elemental mapping and electron energy loss spectroscopy were used to colocalize Ti/O with electron-dense particles. Particle endocytosis was compared in A549 cells with and without pretreatment with cytochalasin D (cyto D) (2 microg/ml). After 3 h of TiO(2) exposure, cells internalized aggregates of the ultrafine particles which were observed in cytosolic, membrane-bound vacuoles. After 24 h of exposure there were considerably more intracellular aggregates of membrane-bound particles, and aggregated particles were also enmeshed in loosely and tightly packed lamellar bodies. Throughout 24 h of exposure a preponderance of particles remained associated with the free surface of the cells and were not internalized. The majority of membrane-bound vacuoles contained aggregates of particles and only occasionally did they contain as few as two or three particles, despite the use of several different approaches to assure the possibility for individual particles to be ingested and detected. There was morphologic evidence of microfilament disturbance, but no evidence of a decrease in internalized particles in cells pretreated with cyto D. Thus, this model of type II epithelium is able to internalize aggregates of ultrafine particles.

Organization of junctional proteins in proliferating cat corneal endothelium during wound healing

PURPOSE

To evaluate for the first time cell junctional protein organization in proliferating corneal endothelial cells during in vivo wound healing.

METHODS

A total of 16 cats (32 eyes) were used in this study. A single 3-mm diameter (n = 24) or 1- to 2-mm diameter (n = 8) scrape injury was created in the central corneal endothelium of each eye. Twenty-four, 48, 72 hours or 5 days after scrape injury, eyes were collected for in situ double- or triple-labeling with phalloidin, anti-ZO-1, alpha-catenin, beta-catenin, and MIB-1 (monoclonal antibody to Ki67, a marker for actively cycling cells) and were imaged using confocal laser microscopy.

RESULTS

In 3-mm diameter injuries, endothelial cells completely resurfaced the wound 48 to 72 hours after scrape injury; smaller wounds resurfaced by 48 hours. Ki67 staining was negative 24 hours after scrape injury in all cases. Ki67-positive cells were observed in the central region of the wounds after 48 and 72 hours, and mitotic figures and pairs of postmitotic cells were observed. On day 5, Ki67-positive cells were rarely detected, and no mitotic figures were observed. In the wound area, a significant increase in cell area and a reduction in hexagonality were observed in cycling cells after 48 and 72 hours. Normal apical, pericellular staining of f-actin, ZO-1, alpha-catenin, and beta-catenin was partially maintained at all times during wound healing of small and large wounds. Double-labeling confirmed that these proteins were also present along the apical cell border in Ki67-positive cells.

CONCLUSIONS

After in vivo scrape injury, proliferation is limited temporally and spatially to spreading endothelial cells within the wound. Cell junctional connections appear to be maintained in actively cycling cells during healing.

IgG2 anti-Galalpha1-3Gal does not induce porcine aortic endothelial cell accommodation in vitro

BACKGROUND

Xenografts that have been protected from hyperacute rejection (HAR) are termed accommodated if they are not then rejected despite the presence of xenoantibody. It has been proposed that IgG may confer resistance to complement dependent cytotoxicity (CDC), a conventional in vitro marker of accommodation. We hypothesized that noncytotoxic IgG2 anti-Galalpha1-3Gal was responsible for this effect.

METHODS AND RESULTS

We purified IgG anti-Galalpha1-3Gal from pooled human normal immunoglobulin and three sera, by elution from protein G and Galalpha1-3Gal-R immunoadsorbents. The eluates were IgM free and > or =95% IgG2. They bound to Galalpha1-3Gal, porcine aortic endothelial cells (PAEC) and lymphocytes. It was not possible to block IgM binding to PAEC or lymphocytes using IgG anti-Galalpha1-3Gal (200 microg/ml). The eluates were noncytotoxic in micro-CDC assays. To investigate accommodation, PAEC were cultured with subsaturating doses of the four IgG eluates for up to 144 hr. Resistance of nontrypsinized PAEC to CDC by human serum was measured in a cell viability assay. PAEC were not rendered resistant to CDC in any of the experiments. To investigate the possibility that accommodation might be induced by non-Galalpha1-3Gal IgG, the experiments were repeated using HNIg, again with no protection demonstrated.

CONCLUSIONS

Using primary PAEC monolayers, we were unable to induce resistance to CDC with human normal immunoglobulin and its IgG2 anti-Gabeta1-3Gal subset. This contradicts previous experiments using trypsinized, immortalized cells. Although resistance to CDC is not an ideal marker of accommodation, the detrimental effects of IgG make it unlikely that it will become a useful clinical means of inducing accommodation.

Disruption of the actin cytoskeleton leads to inhibition of mitogen-induced cyclin E expression, Cdk2 phosphorylation, and nuclear accumulation of the retinoblastoma protein-related p107 protein

The actin cytoskeleton has been found to be required for mitogen-stimulated cells to passage through the cell cycle checkpoint. Here we show that selective disruption of the actin cytoskeleton by dihydrocytochalasin B (H(2)CB) blocked the mitogenic effect in normal Swiss 3T3 cells, leading to cell cycle arrest at mid to late G(1) phase. Cells treated with H(2)CB remain tightly attached to the substratum and respond to mitogen-induced MAP kinase activation. Upon cytoskeleton disruption, however, growth factors fail to induce hyperphosphorylation of the retinoblastoma protein (pRb) and the pRb-related p107. While cyclin D1 induction and cdk4-associated kinase activity are not affected, induction of cyclin E expression and activation of cyclin E-cdk2 complexes are greatly inhibited in growth-stimulated cells treated with H(2)CB. The inhibition of cyclin E expression appears to be mediated at least in part at the RNA level and the inhibition of cdk2 kinase activity is also attributed to the decrease in cdk2 phosphorylation and proper subcellular localization. The expression patterns of cdk inhibitors p21 and p27 are similar in both untreated and H(2)CB-treated cells upon serum stimulation. In addition, the changes in subcellular localization of pRb and p107 appear to be linked to their phosphorylation states and disruption of normal actin structure affects nuclear migration of p107 during G(1)-to-S progression. Taken together, our results suggest that the actin cytoskeleton-dependent G(1) arrest is linked to the cyclin-cdk pathway. We hypothesize that normal actin structure may be important for proper localization of certain G(1) regulators, consequently modulating specific cyclin and kinase expression.

In vitro characterization of mesenchymal stem cell-seeded collagen scaffolds for tendon repair: effects of initial seeding density on contraction kinetics

Mesenchymal stem cells (MSCs) were isolated from bone marrow, culture-expanded, and then seeded at 1, 4, and 8 million cells/mL onto collagen gel constructs designed to augment tendon repair in vivo. To investigate the effects of seeding density on the contraction kinetics and cellular morphology, the contraction of the cell/collagen constructs was monitored over time up to 72 h in culture conditions. Constructs seeded at 4 and 8 million cells/mL showed no significant differences in their gross appearance and dimensions throughout the contraction process. By contrast, constructs seeded at 1 million cells/mL initially contracted more slowly and their diameters at 72 h were 62 to 73% larger than those seeded at higher densities. During contraction, MSCs reoriented and elongated significantly with time. Implants prepared at higher seeding densities showed more well aligned and elongated cell nuclei after 72 h of contraction. Changes in nuclear morphology of the MSCs in response to physical constraints provided by the contracted collagen fibrils may trigger differentiation pathways toward the fibroblastic lineage and influence the cell synthetic activity. Controlling the contraction and organization of the cells and matrix will be critical for successfully creating tissue engineered grafts.

The tensegrity model applied to the lens: a hypothesis for the presence of the fiber cell ball and sockets

The concept of tensegrity refers to the tensional integrity of a system. Such a system mechanically stabilizes itself by balancing the opposing forces of tension and compression. One category of this model involves a 'pre-stressed' condition in which the structural elements are in a state of tensional compression. The other category includes rigid struts that can sustain the opposing forces. The lens of the eye contains epithelial cells that differentiate at the lens equator to fiber cells. With time, these fiber cells are packed into the center of the lens or the lens nucleus. Part of the process of differentiation is the loss of the cellular nucleus and organelles. During maturation of the lens fiber cells, much of the cytoskeleton is lost as a result of proteolysis. As the lens cells mature, ball and socket joints and interlocking ridges appear on the cellular surface. Applying the tensegrity model to the lens, it appears that the lens cells go from prestressed state of tensional compression to the other state that is defined by rigid connections between cells.

Cell-anchorage, cell cytoskeleton, and Rho-GTPase family in regulation of cell cycle progression

It has been well known that cell-anchorage and the cell cytoskeleton are deeply involved in the regulation of cell proliferation and cell cycle. However, the precise molecular mechanism involved in cell-anchorage and the cell cytoskeleton have remained be to elucidated. The recent great volume of information regarding cell cycle regulators such as cyclin, cyclin-dependent kinases (CDKs) and CDK inhibitors (CKI) has facilitated the understanding of the cell cycle in mammalian cells. In this review, we will focus on these cell cycle regulators to discuss the regulation of cell proliferation controlled by cell-anchorage and the cytoskeleton, and especially the roles of Rho family GTPases.

Inhibition of farnesyltransferase with A-176120, a novel and potent farnesyl pyrophosphate analogue

Farnesylation of Ras is required for its transforming activity in human cancer and the reaction is catalysed by the enzyme farnesyltransferase. Recently, we discovered a novel chemical series of potent farnesyl pyrophosphate (FPP) analogues which selectively inhibited farnesyltransferase. Our most potent compound to date in this series, A-176120, selectively inhibited farnesyltransferase activity (IC(50) 1.2+/-0.3 nM) over the closely related enzymes geranylgeranyltransferase I (GGTaseI) (IC(50) 423+/-1.8 nM), geranylgeranyltransferase II (GGTaseII) (IC(50) 3000 nM) and squalene synthase (SSase) (IC(50)>10000 nM). A-176120 inhibited ras processing in H-ras-transformed NIH3T3 cells and HCT116 K-ras-mutated cells (ED(50) 1.6 and 0.5 microM, respectively). The anti-angiogenic potential of A-176120 was demonstrated by a decrease in Ras processing, cell proliferation and capillary structure formation of human umbilical vein endothelial cells (HUVEC), and a decrease in the secretion of vascular endothelial growth factor (VEGF) from HCT116 cells. In vivo, A-176120 reduced H-ras NIH3T3 tumour growth and extended the lifespan of nude mice inoculated with H- or K-ras-transformed NIH3T3 cells. A-176120 also had an additive effect in combination with cyclophosphamide in nude mice inoculated with K-ras NIH3T3 transformed cells. Overall, our results demonstrate that A-176120 is a potent FPP mimetic with both antitumour and anti-angiogenic properties.

A newly designed tensile tester for cells and its application to fibroblasts

A tensile test system for cells has been designed and applied to fibroblasts from the rabbit patellar tendon. It consists of a thermostatic test chamber, an inverted fluorescence microscope, micromanipulators, a direct drive linear actuator, a cantilever-type load cell, and a video dimension analyzer (VDA). The test chamber and the microscope are mounted on a vibration isolator. A cell floated in Hanks' balanced salt solution of 37 degrees C is gripped with a pair of micropipettes which have very fine tips (outer diameter = 20 approximately 30 microm, inner diameter = 3 approximately 5 microm) and are coated with a cell adhesive, Cell-Tak, at their ends. One of the micropipettes is fixed to the load cell; the other one is attached to the linear actuator which is used to stretch the cell. Load applied to the cell is measured with the load cell, while elongation of the cell is determined with the VDA using the images of the ends of the micropipettes as markers. The measurement accuracy of the load cell was +/-0.05 microN. All the fibroblasts tested were firmly attached to the micropipettes during tensile testing, and showed local non-uniform deformation. The maximum load and elongation to failure of the cells were 0.9+/-0.2 microN and 86+/-24 microm, respectively.

Mechanical signalling and angiogenesis. The integration of cell-extracellular matrix couplings

In vitro angiogenesis assays have shown that the couplings between fibrin gel and cell traction forces trigger biogel pre-patterning, consisting, in the formation of lacunae which evolve toward capillary-like structures (CLS) networks. Depending on the experimental conditions (number of seeded cells, gel elasticity,...), this pre-patterning can be enhanced or inhibited. A theoretical model based on a description of the cell-biogel biochemical and mechanical interactions is proposed as a basis for understanding how integrating these interactions can lead to the pre-patterning of the biogel. We showed that the critical parameter values corresponding to the bifurcation of the model solutions correspond to threshold values of the experimental variables. Furthermore, simulations of the mechanocellular model give rise to dynamic remodelling patterns of the biogel which are in good agreement both with the lacunae morphologies and with the time and space scales derived from the in vitro angiogenesis assays. Special attention has been paid in the simulations to cell proteolytic activity and to the amplitude of cell traction forces. We finally discussed how modelling guided experiments can be inferred from these results.

Disruption of cadherin-related junctions triggers autocrine expression of vascular endothelial growth factor in bovine aortic endothelial cells : effects on cell proliferation and death resistance

The mechanisms involved in the blockade of proliferation in confluent endothelial cells are insufficiently understood. In this regard, the continuity of intercellular junctions appears to be critical to the regulation of endothelial monolayer cell growth. The present study examined the hypothesis that the disruption of the intercellular adherens junctions will trigger both endothelial cell proliferation and autocrine production of growth factors. With this purpose, we assessed the changes in growth, death resistance, and expression of vascular endothelial growth factor (VEGF) under conditions of disruption of the intercellular junctions between endothelial cells. Disruption of cell junctions was produced by means of a specific anti-vascular endothelial cadherin monoclonal antibody, EGTA, or cytochalasin D. Our results disclosed that these maneuvers induce an increase in VEGF mRNA production, with transcription of the 121-, 165-, and 189-amino acid isoforms of VEGF. Further evidence of the relationship between endothelial cells monolayer continuity and VEGF protein expression was obtained by the demonstration of an increase in VEGF protein, as determined by Western blot, induced by the aforementioned maneuvers, as well as by immunocytochemical detection of increased VEGF staining in the areas surrounding a mechanical endothelial injury and in endothelial cells at subconfluence. In functional terms, the autocrine expression of VEGF was associated with growth-promoting and cytoprotective effects, as assessed by [(3)H]thymidine uptake, (51)Cr release, and flow cytometry. In conclusion, our results reveal that disruption of homophilic interendothelial junctions induces VEGF expression. Under these conditions, autocrine VEGF appears to have a relevant role in death inhibition and proliferation of endothelial cells.

Phosphorylated human keratinocyte ornithine decarboxylase is preferentially associated with insoluble cellular proteins

Ornithine decarboxylase (ODC), the first enzyme in polyamine biosynthesis, is highly regulated by many trophic stimuli, and changes in its levels and organization correlate with cytoskeletal changes in normal human epidermal keratinocytes (NHEK). NHEK ODC exhibits a filamentous perinuclear/nuclear localization that becomes more diffuse under conditions that alter actin architecture. We have thus asked whether ODC colocalizes with a component of the NHEK cytoskeleton. Confocal immunofluorescence showed that ODC distribution in NHEK was primarily perinuclear; upon disruption of the actin cytoskeleton with cytochalasin D, ODC distribution was diffuse. The ODC distribution in untreated NHEK overlapped with that of keratin in the perinuclear but not cytoplasmic area; after treatment with cytochalasin D, overlap between staining for ODC and for keratin was extensive. No significant overlap with actin and minimal overlap with tubulin filament systems were observed. Subcellular fractionation by sequential homogenizations and centrifugations of NHEK lysates or detergent and salt extractions of NHEK in situ revealed that ODC protein and activity were detectable in both soluble and insoluble fractions, with mechanical disruption causing additional solubilization of ODC activity (three- to sevenfold above controls). Fractionation and ODC immunoprecipitation from [(32)P]orthophosphate-labeled NHEK lysates showed that a phosphorylated form of ODC was present in the insoluble fractions. Taken together, these data suggest that two pools of ODC exist in NHEK. The first is the previously described soluble pool, and the second is enriched in phospho-ODC and associated with insoluble cellular material that by immunohistochemistry appears to be organized in conjunction with the keratin cytoskeleton.

Differential responses of adult cardiac fibroblasts to in vitro biaxial strain patterns

Different patterns of extracellular matrix (ECM) remodeling in the heart are thought to be dependent on altered mechanical and chemical conditions and can contribute to cardiac dysfunction. Cardiac fibroblasts are the primary regulators of the ECM and may respond to mechanical factors in vitro. We hypothesized that different types of in vitro strains, e.g. tensile or compressive, can stimulate different functional responses in cultured adult rat cardiac fibroblasts. In this study, we first showed that a single step in strain applied by a uniaxial stretch system stimulated collagen III and fibronectin mRNA levels and transforming growth factor-beta(1) (TGF-beta(1)) activity in the adult phenotype of rat cardiac fibroblasts. Two-dimensional deformations were measured by tracking fluorescent microspheres attached to the substrate and cultured cells. For 10% uniaxial strain, mean principal strains were 0. 104 +/- 0.018 in the direction of stretch and -0.042 +/- 0.013 in the perpendicular direction, verifying that the fibroblasts were simultaneously subjected to tensile (positive) and compressive (negative) strains. Furthermore, these cells were also subjected to area change and to shear. In order to examine the distinct effects of different types of deformation on cardiac fibroblasts, an equibiaxial stretch system was used to apply either pure tensile or compressive area strains, in the absence of shear. Magnitudes of equibiaxial strain were selected to apply local cell area changes identical to those applied in the uniaxial system. Results showed that pure tensile and compressive area strains induced divergent responses in ECM mRNA levels. TGF-beta(1) activity was dependent on the magnitude of applied area strain regardless of the mode of deformation. These findings demonstrate that adult cardiac fibroblasts may respond differently to varied types of mechanical loading, suggesting that ECM remodeling may be locally regulated by specific mechanical stimuli in the heart.