Cytostructural dynamics of spreading and translocating cells

Nanoscopic Approach to Study the Early Stages of Epithelial to Mesenchymal Transition (EMT) of Human Retinal Pigment Epithelial (RPE) Cells In Vitro

The maintenance of visual function is supported by the proper functioning of the retinal pigment epithelium (RPE), representing a mosaic of polarized cuboidal postmitotic cells. Damage factors such as inflammation, aging, or injury can initiate the migration and proliferation of RPE cells, whereas they undergo a pseudo-metastatic transformation or an epithelial to mesenchymal transition (EMT) from cuboidal epithelioid into fibroblast-like or macrophage-like cells. This process is recognized as a key feature in several severe ocular pathologies, and is mimicked by placing RPE cells in culture, which provides a reasonable and well-characterized in vitro model for a type 2 EMT. The most obvious characteristic of EMT is the cell phenotype switching, accompanied by the cytoskeletal reorganization with changes in size, shape, and geometry. Atomic force microscopy (AFM) has the salient ability to label-free explore these characteristics. Based on our AFM results supported by the genetic analysis of specific RPE differentiation markers, we elucidate a scheme for gradual transformation from the cobblestone to fibroblast-like phenotype. Structural changes in the actin cytoskeletal reorganization at the early stages of EMT lead to the development of characteristic geodomes, a finding that may reflect an increased propensity of RPE cells to undergo further EMT and thus become of diagnostic significance.

Characterization and Analysis of Collective Cellular Behaviors in 3D Dextran Hydrogels with Homogenous and Clustered RGD Compositions

The interactions between substrate materials and cells usually play an important role in the hydrogel-based 3D cell cultures. However, the hydrogels that are usually used could not be parametrically regulated, especially for quantitatively regulating the spatial distribution of the adhesion sites for cells in 3D. Here, we employed the semisynthetic hydrogel consisting of maleimide-dextran, Arg-Gly-Asp (RGD) peptides, and cell degradable crosslinkers to biochemically characterize the evolutionary behaviors of NIH-3T3 fibroblasts and C2C12 cells in 3D. Moreover, by comparing the cell-adhesive efficacy of 3D dextran hydrogels with four different RGD clustering rates, we explored the underlying regulation law of C2C12 connections and 3T3 aggregations. The results showed that mal-dextran hydrogel could promise cells stable viability and continuous proliferation, and induce more self-organized multicellular structures relative to 2D culture. More importantly, we found that RGD-clustered mal-dextran hydrogel has the advantage of enhancing C2C12 cell elongation and the breadthwise-aggregated connection, and promoting the 3T3 cell aggregating degree compared to that with homogenous RGD. Further, the advantages of RGD clustering hydrogel could be amplified by appropriately reducing RGD concentration. Such RGD-composition controllable mal-dextran hydrogel can function as a regulator of the collective cellular behaviors, which provides useful information for quantitatively designing the tailored hydrogel system and exploiting advanced biomaterials.

The tension mounts: stress fibers as force-generating mechanotransducers

Stress fibers (SFs) are often the most prominent cytoskeletal structures in cells growing in tissue culture. Composed of actin filaments, myosin II, and many other proteins, SFs are force-generating and tension-bearing structures that respond to the surrounding physical environment. New work is shedding light on the mechanosensitive properties of SFs, including that these structures can respond to mechanical tension by rapid reinforcement and that there are mechanisms to repair strain-induced damage. Although SFs are superficially similar in organization to the sarcomeres of striated muscle, there are intriguing differences in their organization and behavior, indicating that much still needs to be learned about these structures.

Investigating circular dorsal ruffles through varying substrate stiffness and mathematical modeling

Circular dorsal ruffles (CDRs) are transient actin-rich ringlike structures that form on the dorsal surface of growth-factor stimulated cells. However, the dynamics and mechanism of formation of CDRs are still unknown. It has been observed that CDR formation leads to stress fibers disappearing near the CDRs. Because stress fiber formation can be modified by substrate stiffness, we examined the effect of substrate stiffness on CDR formation by seeding NIH 3T3 fibroblasts on glass and polydimethylsiloxane substrates of varying stiffnesses from 20 kPa to 1800 kPa. We found that increasing substrate stiffness increased the lifetime of the CDRs. We developed a mathematical model of the signaling pathways involved in CDR formation to provide insight into this lifetime and size dependence that is linked to substrate stiffness via Rac-Rho antagonism. From the model, increasing stiffness raised mDia1-nucleated stress fiber formation due to Rho activation. The increased stress fibers present increased replenishment of the G-actin pool, therefore prolonging Arp2/3-nucleated CDR formation due to Rac activation. Negative feedback by WAVE-related RacGAP on Rac explained how CDR actin propagates as an excitable wave, much like wave propagation in other excitable medium, e.g., nerve signal transmission.

Contribution of Filopodia to Cell Migration: A Mechanical Link between Protrusion and Contraction

Numerous F-actin containing structures are involved in regulating protrusion of membrane at the leading edge of motile cells. We have investigated the structure and dynamics of filopodia as they relate to events at the leading edge and the function of the trailing actin networks. We have found that although filopodia contain parallel bundles of actin, they contain a surprisingly nonuniform spatial and temporal distribution of actin binding proteins. Along the length of the actin filaments in a single filopodium, the most distal portion contains primarily T-plastin, while the proximal portion is primarily bound by α-actinin and coronin. Some filopodia are stationary, but lateral filopodia move with respect to the leading edge. They appear to form a mechanical link between the actin polymerization network at the front of the cell and the myosin motor activity in the cell body. The direction of lateral filopodial movement is associated with the direction of cell migration. When lateral filopodia initiate from and move toward only one side of a cell, the cell will turn opposite to the direction of filopodial flow. Therefore, this filopodia-myosin II system allows actin polymerization driven protrusion forces and myosin II mediated contractile force to be mechanically coordinated.

Polygonal arrays of microfilaments in epithelial cells of the intact lens

Polygonal arrays of microfilaments have been discovered to line the inner apical plasma membrane of anterior epithelial cells of the intact rabbit lens. When tangential sections are studied with the electron microscope, the polygonal arrays are seen to consist of central vertices interconnected by rays of filaments. The rays near the cell periphery insert into the lateral plasma membrane. The vertices are spaced about 1 micron apart and appear to be attached to the apical plasma membrane. The polygonal arrays have little depth as judged by stereo-pairs and are incorporated within the dense band of microfilaments seen in cross-section at the epithelio-fiber junction. The diameter of the filaments and their similarity to actin-containing polygonal arrays described by other investigators in cultured cells suggest that these structures contain actin in lens epithelial cells. The function of the polygonal arrays in relation to maintenance of lens shape or to changes in lens shape in accommodation is discussed.

Cytoskeletal role in differential adhesion patterns of normal fibroblasts and breast cancer cells inside silicon microenvironments

In this paper we studied differential adhesion of normal human fibroblast cells and human breast cancer cells to three dimensional (3-D) isotropic silicon microstructures and investigated whether cell cytoskeleton in healthy and diseased state results in differential adhesion. The 3-D silicon microstructures were formed by a single-mask single-isotropic-etch process. The interaction of these two cell lines with the presented microstructures was studied under static cell culture conditions. The results show that there is not a significant elongation of both cell types attached inside etched microstructures compared to flat surfaces. With respect to adhesion, the cancer cells adopt the curved shape of 3-D microenvironments while fibroblasts stretch to avoid the curved sidewalls. Treatment of fibroblast cells with cytochalasin D changed their adhesion, spreading and morphology and caused them act similar to cancer cells inside the 3-D microstructures. Statistical analysis confirmed that there is a significant alteration (P < 0.001) in fibroblast cell morphology and adhesion property after adding cytochalasin D. Adding cytochalasin D to cancer cells made these cells more rounded while there was not a significant alteration in their adhesion properties. The distinct geometry-dependent cell-surface interactions of fibroblasts and breast cancer cells are attributed to their different cytoskeletal structure; fibroblasts have an organized cytoskeletal structure and less deformable while cancer cells deform easily due to their impaired cytoskeleton. These 3-D silicon microstructures can be used as a tool to investigate cellular activities in a 3-D architecture and compare cytoskeletal properties of various cell lines.

The role of actomyosin contractility in the formation and dynamics of actin bundles during fibroblast spreading

We studied the process of formation of stress fibres and involvement of phosphorylation of myosin-II during spreading of Swiss 3T3 fibroblasts. In cells that were allowed to spread for 1 h on a glass surface, circular bundles of actin and myosin-II filament were present. At 2-3 h after the plating, cells showed a polygonal and polarized shape. The proportion of the cells having circular bundles was decreased, whereas that of the cells with straight bundles of actin filaments was increased. At 4 h after the plating, cells were completely polarized and stress fibres were present at the periphery and the dorsal and ventral surfaces of the cells. Thus, spreading cells possessed different forms of actomyosin bundles corresponding to the cell shape. In circular bundles and stress fibres, myosin regulatory light chains were diphosphorylated. Formation of circular bundles and stress fibres was suppressed after the treatment of the cells with Y-27632, a Rho-kinase inhibitor, or blebbistatin, a myosin-II inhibitor. In digitonin-extracted cells, circular bundles as well as stress fibres contracted following the addition of Mg-ATP. These results suggest that circular bundles are contractile structures containing actin and phosphorylated myosin-II filaments, and the formation of circular bundles is regulated by Rho-kinase.

BMP4 induces EMT and Rho GTPase activation in human ovarian cancer cells

We identified previously an autocrine bone morphogenetic protein-4 (BMP4) signalling pathway in primary human normal ovarian surface epithelial (OSE) and epithelial ovarian cancer (OvCa) cells. Herein we show that treatment of OvCa cells with BMP4 produced morphological alterations and increased cellular adhesion, motility and invasion. The BMP4 inhibitor noggin blocked the BMP4-induced phenotype, and decreased autocrine BMP4-mediated OvCa cell motility and adherence. In response to exogenous BMP4, the epithelial-mesenchymal transition (EMT) markers Snail and Slug mRNA and protein were up-regulated, E-cadherin mRNA and protein were down-regulated and the network of alpha smooth muscle actin changed to resemble a mesenchymal cell. We also observed changes in the level of activated Rho GTPases in OvCa cells treated with BMP4, strongly suggesting that the changes in morphology, adhesion, motility and invasion are probably mediated through the activation of these molecules. Strikingly, treatment of normal OSE cells with BMP4 or noggin failed to alter cell motility, providing evidence that OSE and OvCa cells possess a distinct capability to respond to BMP4. Overall, our studies suggest a link between autocrine BMP signalling mediated through the Rho GTPase family and Snail- and Slug-induced EMT that may collectively contribute to aggressive OvCa behaviour.

Non-Hertzian Approach to Analyzing Mechanical Properties of Endothelial Cells Probed by Atomic Force Microscopy

Detailed measurements of cell material properties are required for understanding how cells respond to their mechanical environment. Atomic force microscopy (AFM) is an increasingly popular measurement technique that uniquely combines subcellular mechanical testing with high-resolution imaging. However, the standard method of analyzing AFM indentation data is based on a simplified “Hertz” theory that requires unrealistic assumptions about cell indentation experiments. The objective of this study was to utilize an alternative “pointwise modulus” approach, that relaxes several of these assumptions, to examine subcellular mechanics of cultured human aortic endothelial cells (HAECs). Data from indentations in 2‐to5‐μm square regions of cytoplasm reveal at least two mechanically distinct populations of cellular material. Indentations colocalized with prominent linear structures in AFM images exhibited depth-dependent variation of the apparent pointwise elastic modulus that was not observed at adjacent locations devoid of such structures. The average pointwise modulus at an arbitrary indentation depth of 200nm was 5.6±3.5kPa and 1.5±0.76kPa (mean±SD, n=7) for these two material populations, respectively. The linear structures in AFM images were identified by fluorescence microscopy as bundles of f-actin, or stress fibers. After treatment with 4μM cytochalasin B, HAECs behaved like a homogeneous linear elastic material with an apparent modulus of 0.89±0.46kPa. These findings reveal complex mechanical behavior specifically associated with actin stress fibers that is not accurately described using the standard Hertz analysis, and may impact how HAECs interact with their mechanical environment.

Foot and mouth: podosomes, invadopodia and circular dorsal ruffles

The plasma membrane of many motile cells undergoes highly regulated protrusions and invaginations that support the formation of podosomes, invadopodia and circular dorsal ruffles. Although they are similar in appearance and in their formation--which is mediated by a highly conserved actin-membrane apparatus--these transient surface membrane distortions are distinct. Their function is to help the cell as it migrates, attaches and invades.

The flightless I protein colocalizes with actin- and microtubule-based structures in motile Swiss 3T3 fibroblasts: evidence for the involvement of PI 3-kinase and Ras-related small GTPases

The flightless I protein contains an actin-binding domain with homology to the gelsolin family and is likely to be involved in actin cytoskeletal rearrangements. It has been suggested that this protein is involved in linking the cytoskeletal network with signal transduction pathways. We have developed antibodies directed toward the leucine rich repeat and gelsolin-like domains of the human and mouse homologues of flightless I that specifically recognize expressed and endogenous forms of the protein. We have also constructed a flightless I-enhanced green fluorescent fusion vector and used this to examine the localization of the expressed protein in Swiss 3T3 fibroblasts. The flightless I protein localizes predominantly to the nucleus and translocates to the cytoplasm following serum stimulation. In cells stimulated to migrate, the flightless I protein colocalizes with beta-tubulin- and actin-based structures. Members of the small GTPase family, also implicated in cytoskeletal control, were found to colocalize with flightless I in migrating Swiss 3T3 fibroblasts. LY294002, a specific inhibitor of PI 3-kinase, inhibits the translocation of flightless I to actin-based structures. Our results suggest that PI 3-kinase and the small GTPases, Ras, RhoA and Cdc42 may be part of a common functional pathway involved in Fliih-mediated cytoskeletal regulation. Functionally, we suggest that flightless I may act to prepare actin filaments or provide factors required for cytoskeletal rearrangements necessary for cell migration and/or adhesion.

Activation of endogenous thrombin receptors causes clustering and sensitization of epidermal growth factor receptors of swiss 3T3 cells without transactivation

The G protein-coupled thrombin receptor can induce cellular responses in some systems by transactivating the epidermal growth factor (EGF) receptor. This is in part due to the stimulation of ectoproteases that generate EGF receptor ligands. We show here that this cannot account for the stimulation of proliferation or migration by thrombin of Swiss 3T3 cells. Thrombin has no direct effect on the activation state of the EGF receptor or of its downstream effectors. However, thrombin induces the subcellular clustering of the EGF receptor at filamentous actin-containing structures at the leading edge and actin arcs of migrating cells in association with other signaling molecules, including Shc and phospholipase Cgamma1. In these thrombin-primed cells, the subsequent migratory response to EGF is potentiated. Thrombin did not potentiate the EGF-stimulated EGF receptor phosphorylation. Thus, in Swiss 3T3 cells the G protein-coupled thrombin receptor can potentiate the EGF tyrosine kinase receptor response when activated by EGF, and this appears to be due to the subcellular concentration of the receptor with downstream effectors and not to the overall ability of EGF to induce receptor transphosphorylation. Thus, the EGF receptor subcellular localization which is altered by thrombin appears to be an important determinant of the efficacy of downstream EGF receptor signaling in cell migration.

Mechanism of lateral movement of filopodia and radial actin bundles across neuronal growth cones

We investigated the motion of filopodia and actin bundles in lamellipodia of motile cells, using time-lapse sequences of polarized light images. We measured the velocity of retrograde flow of the actin network and the lateral motion of filopodia and actin bundles of the lamellipodium. Upon noting that laterally moving filopodia and actin bundles are always tilted with respect to the direction of retrograde flow, we propose a simple geometric model for the mechanism of lateral motion. The model establishes a relationship between the speed of lateral motion of actin bundles, their tilt angle with respect to the direction of retrograde flow, and the speed of retrograde flow in the lamellipodium. Our experimental results verify the quantitative predictions of the model. Furthermore, our observations support the hypothesis that lateral movement of filopodia is caused by retrograde flow of tilted actin bundles and by their growth through actin polymerization at the tip of the bundles inside the filopodia. Therefore we conclude that the lateral motion of tilted filopodia and actin bundles does not require a separate motile mechanism but is the result of retrograde flow and the assembly of actin filaments and bundles near the leading edge of the lamellipodium.

Birefringence imaging directly reveals architectural dynamics of filamentous actin in living growth cones

We have investigated the dynamic behavior of cytoskeletal fine structure in the lamellipodium of nerve growth cones using a new type of polarized light microscope (the Pol-Scope). Pol-Scope images display with exquisite resolution and definition birefringent fine structures, such as filaments and membranes, without having to treat the cell with exogenous dyes or fluorescent labels. Furthermore, the measured birefringence of protein fibers in the thin lamellipodial region can be interpreted in terms of the number of filaments in the bundles. We confirmed that birefringent fibers are actin-based using conventional fluorescence-labeling methods. By recording movies of time-lapsed Pol-Scope images, we analyzed the creation and dynamic composition of radial fibers, filopodia, and intrapodia in advancing growth cones. The strictly quantitative information available in time-lapsed Pol-Scope images confirms previously deduced behavior and provides new insight into the architectural dynamics of filamentous actin.

The integrin alpha6beta4 functions in carcinoma cell migration on laminin-1 by mediating the formation and stabilization of actin-containing motility structures

Functional studies on the alpha6beta4 integrin have focused primarily on its role in the organization of hemidesmosomes, stable adhesive structures that associate with the intermediate filament cytoskeleton. In this study, we examined the function of the alpha6beta4 integrin in clone A cells, a colon carcinoma cell line that expresses alpha6beta4 but no alpha6beta1 integrin and exhibits dynamic adhesion and motility on laminin-1. Time-lapse videomicroscopy of clone A cells on laminin-1 revealed that their migration is characterized by filopodial extension and stabilization followed by lamellae that extend in the direction of stabilized filopodia. A function-blocking mAb specific for the alpha6beta4 integrin inhibited clone A migration on laminin-1. This mAb also inhibited filopodial formation and stabilization and lamella formation. Indirect immunofluorescence microscopy revealed that the alpha6beta4 integrin is localized as discrete clusters in filopodia, lamellae, and retraction fibers. Although beta1 integrins were also localized in the same structures, a spatial separation of these two integrin populations was evident. In filopodia and lamellae, a striking colocalization of the alpha6beta4 integrin and F-actin was seen. An association between alpha6beta4 and F-actin is supported by the fact that alpha6beta4 integrin and actin were released from clone A cells by treatment with the F-actin- severing protein gelsolin and that alpha6beta4 immunostaining at the marginal edges of clone A cells on laminin-1 was resistant to solubilization with Triton X-100. Cytokeratins were not observed in filopodia and lamellipodia. Moreover, alpha6beta4 was extracted from these marginal edges with a Tween-40/deoxycholate buffer that solubilizes the actin cytoskeleton but not cytokeratins. Three other carcinoma cell lines (MIP-101, CCL-228, and MDA-MB-231) exhibited alpha6beta4 colocalized with actin in filopodia and lamellae. Formation of lamellae in these cells was inhibited with an alpha6-specific antibody. Together, these results indicate that the alpha6beta4 integrin functions in carcinoma migration on laminin-1 through its ability to promote the formation and stabilization of actin-containing motility structures.

Influence of adhesion and cytoskeletal integrity on fibroblast traction

Cellular contractility plays an important role in cell morphogenesis and tissue pattern formation. In this context, we examined how the expression of cell traction depends on cell-to-substrate contacts and cytoskeletal organization. Qualitative observation of chick fibroblasts cultured on an elastic film of polydimethylsiloxane indicated a strong spatial relationship between wrinkle pattern and distribution of actin stress fibers and focal contacts. In order to further characterize cell contractility, the projected area of Triton-permeabilized fibroblasts upon ATP-induced retraction was measured in various conditions of substrate adhesivity, cytoskeletal perturbation, and temperature. In all conditions, the relationship between degree of cell retraction and ATP concentration was well described by the laws of enzyme kinetics. Culturing cells on a gelatin-coated substrate, decreasing the temperature, using phosphate ribonucleotides other than ATP, and treating cells with cytochalasin D all diminished the rate of cell retraction, indicating that fibroblast traction is generated by a temperature- and ATP-dependent actin/myosin stress fiber sliding mechanism, transmitted to the substrate through focal adhesions. Treatment of cells with either nocodazole or taxol did not affect retraction of permeabilized fibroblasts upon stimulation with ATP, suggesting that microtubules do not directly resist cell traction. Treatment of cells with vanadate increased cell retraction, suggesting that intermediate filaments help transmit tension.

Structure and activation dynamics of RBL-2H3 cells observed with scanning force microscopy

Surface and subsurface dynamics of Rat Basophilic Leukemia cells, a model system of stimulated secretion, were imaged using Scanning Force Microscopy (SFM) at a rate of 50-60 s/image. Cytoskeletal elements and organelles were tracked within quiescent cells and those activated after IgE receptor crosslinking. In addition, surface waves were observed moving within the plasma membrane. The structures seen in quiescent and activated cells can be correlated with those seen in electron micrographs and topographic SFM images of fixed detergent-extracted cells. Furthermore, images of the detergent-extracted nuclei reveal the presence of numerous nuclear pore complexes. High-magnification images of the nuclear pore complexes show evidence of subunit structure and exhibit dimensions consistent with those reported previously using electron microscopy. The behavior and overall change in morphology of cells observed during activation was consistent with that observed under similar conditions with Differential Interference Contrast microscopy. This study demonstrates that SFM, unlike other techniques, can be used to provide high-resolution information in both fixed and living cells.

Dominant negative mutation in cell surface beta 1,4-galactosyltransferase inhibits cell-cell and cell-matrix interactions

In addition to its traditional location within the Golgi complex, beta 1,4-galactosyltransferase (GalTase) is also present on the cell surface, where it is thought to function as a cell adhesion molecule by binding to extracellular oligosaccharide ligands. Recent studies suggest that cells contain two forms of GalTase with distinct cytoplasmic domains. The longer form of GalTase contains a 13-amino acid cytoplasmic extension and is preferentially targeted to the plasma membrane, relative to the shorter GalTase protein that is confined primarily to the Golgi compartment. In this study, we created a dominant negative mutation that interferes with the function of cell surface GalTase by transfecting into cells cDNAs encoding truncated versions of the long form of GalTase containing the complete cytoplasmic and transmembrane domains, but devoid of the catalytic domain. In both F9 embryonal carcinoma cells and Swiss 3T3 fibroblasts, overexpressing the truncated long GalTase (TLGT) protein displaced the endogenous cell surface GalTase from its association with the cytoskeleton, resulting in a loss of intercellular adhesion and cell spreading specifically on matrices that use GalTase as a cell surface receptor. In contrast, overexpressing the analogous truncated short GalTase (TSGT) protein did not affect cell morphology or GalTase activity. In control assays, inducing the TLGT protein had no effect on cell interactions with fibronectin (which is independent of GalTase), or on the cytoskeleton attachment of another matrix receptor (beta 1 integrin), or on overall glycoprotein synthesis, thus eliminating nonspecific effects of the TLGT protein on cellular adhesion and metabolism. These results represent the first molecular manipulation of cell surface GalTase expression and confirm its function as a cell adhesion molecule. These studies further suggest that the cytoskeleton contains a defined, saturable number of binding sites for GalTase, which enables it to function as an adhesion molecule.

Effects of cytochalasin and colcemid on cortical flow in coelomocytes

Sea urchin coelomyocytes naturally flatten on a substratum into a discoid morphology and display striking, centripetally directed cortical flow along the radii of the cell when viewed with time lapse, video enhanced microscopy. The rate of cortical flow averaged 4.5 microns/min in the peripheral most 10 microns of cytoplasm but slows considerably in the perinuclear region. Cytochalasin B causes: (1) the flow to stop, (2) the buildup of an actin filament-rich peripheral ridge of cytoskeletal material, (3) the centrifugal dissolution of a portion of the actin cytoskeleton, and (4) the contraction of other portions of the cytoskeleton into foci. Cytochalasin D (CD), on the other hand, causes the flowing actin meshwork to become severed from the edge of the cell and allows it to be drawn at least part way in towards the nucleus. A smaller peripheral ridge of actin filament buildup is also seen with CD. Colcemid induces another striking change in the cytoskeleton. The centripetal progression of the actin is not stopped by colcemid, but shortly after leaving the periphery of the cell, the linear elements within the flow become reoriented into arcs. The long axis of the arcs is roughly parallel with the cell's edge. The effects of all three drugs are reversible. The results are discussed in light of other systems and potential mechanisms for cortical flow.

Comparison of actin and cell surface dynamics in motile fibroblasts

We have investigated the dynamic behavior of actin in fibroblast lamellipodia using photoactivation of fluorescence. Activated regions of caged resorufin (CR)-labeled actin in lamellipodia of IMR 90 and MC7 3T3 fibroblasts were observed to move centripetally over time. Thus in these cells, actin filaments move centripetally relative to the substrate. Rates were characteristic for each cell type; 0.66 +/- 0.27 microns/min in IMR 90 and 0.36 +/- 0.16 microns/min in MC7 3T3 cells. In neither case was there any correlation between the rate of actin movement and the rate of lamellipodial protrusion. The half-life of the activated CR-actin filaments was approximately 1 min in IMR 90 lamellipodia, and approximately 3 min in MC7 3T3 lamellipodia. Thus continuous filament turnover accompanies centripetal movement. In both cell types, the length of time required for a section of the actin meshwork to traverse the lamellipodium was several times longer than the filament half-life. The dynamic behavior of the dorsal surface of the cell was also observed by tracking lectin-coated beads on the surface and phase-dense features within lamellipodia of MC7 3T3 cells. The movement of these dorsal features occurred at rates approximately three times faster than the rate of movement of the underlying bulk actin cytoskeleton, even when measured in the same individual cells. Thus the transport of these dorsal features must occur by some mechanism other than simple attachment to the moving bulk actin cytoskeleton.

Cell surface beta-1,4-galactosyltransferase is associated with the detergent-insoluble cytoskeleton on migrating mesenchymal cells

Cell surface beta-1,4-galactosyltransferase (GalTase) partially mediates a variety of cell interactions with laminin-containing matrices, including mesenchymal cell spreading and migration and neurite initiation, by binding to N-linked oligosaccharides within the E8 domain of laminin. Previous studies using indirect immunofluorescence have suggested that some surface GalTase colocalizes with actin-containing microfilaments in migrating cells. In this study, we present more direct biochemical evidence showing that surface GalTase is associated with the detergent-insoluble cytoskeleton and that this association is dependent upon the integrity of the cytoskeleton, valency of the anti-GalTase antibody, and migratory status of the cell. Two-thirds of the surface GalTase was associated with the detergent-insoluble cytoskeleton when assayed either by monovalent anti-GalTase Fab fragments or by extracting any detergent-soluble GalTase prior to labeling with intact anti-GalTase IgG. However, 80-100% of the surface GalTase could be induced to associate with the cytoskeleton when cross-linked with anti-GalTase IgG prior to detergent extraction. Destabilizing cytoskeleton-protein interactions with high levels of KCl, elevated pH, or cytochalasin B reduced the amount of surface GalTase retained in the detergent-insoluble cytoskeleton fraction. Finally, we have shown previously that laminin induces the expression of GalTase onto lamellipodia of migrating cells, and in this study, we show that the laminin-induced increase in surface GalTase is cytoskeletally associated. Collectively, these data suggest that cell surface GalTase participates in cell spreading and migration on laminin by virtue of its association with the cytoskeleton.

In vitro bone resorption by isolated multinucleated giant cells from giant cell tumour of bone: light and electron microscopic study

The behaviour of multinucleated giant cells (GCs), obtained from a giant cell tumour of the tibia and cultured on glass coverslips or on devitalized bone slices, was studied using light and electron microscopy. Monitoring the GCs on bone slices by phase-contrast microscopy revealed that they had removed calcified bone matrix resulting in excavation of lacunae, with subsequent lateral extension and perforation of the bone slices. Electron microscopy demonstrated for the first time that the GCs responsible for exavating lacunae had two specific membrane modifications, ruffled border and clear zone, and showed basically similar cytoplasmic fine structures to those of osteoclasts. Fluorescence images of the GCs on glass and on bone after rhodamine-conjugated phalloidin staining revealed that most of the GCs had an intensely fluorescent peripheral band composed of a number of F-actin dots called podosomes. Some GCs showed unusual arrangements of podosomes suggesting abortive attempts at GC formation. We have demonstrated that the band structure of the GCs cultured on bone is intimately involved in bone resorption. Two stromal cell types could be recognized. The predominant type, which seemed to be the only neoplastic element because of its proliferative capability, showed quite different fine structural and cytoskeletal features from the GCs. The other type, which was much less frequent and seemed not to proliferate, had morphological similarities to the GCs, and seemed to be their precursor. Importantly GCs cultured on bone and the osteoclasts share common structures for adhesion to and resorption of bone, strongly supporting the view that the GCs of the giant cell tumour of bone are potentially active bone resorbers and can be regarded as osteoclasts.

Centripetal flow and directed reassembly of the major sperm protein (MSP) cytoskeleton in the amoeboid sperm of the nematode, Ascaris suum

The cytoskeleton of the amoeboid spermatozoa of Ascaris suum consists of major sperm protein (MSP) filaments arranged into long, branched fiber complexes that span the length of the pseudopod and treadmill rearward continuously due to assembly and disassembly at opposite ends of the complexes (Sepsenwol et al., Journal of Cell Biology 108:55-66, (1989)). Examination by video-enhanced microscopy showed that this cytoskeletal flow is tightly coupled to sperm locomotion. The fiber complexes treadmilled rearward at the same rate (10-50 microns/min) as the cell crawled forward. Only fiber complexes with their plasmalemmal ends within a limited sector along the leading edge of the pseudopod underwent continuous assembly. Thus, the location of this sector, which occupies about 50% of the pseudopod perimeter, determined the direction of sperm locomotion. Treatment of sperm with agents that lower intracellular pH, such as weak acids and protonophores, caused the fiber complexes to disassemble completely in 4-5 sec. Removal of these compounds resulted in reassembly of the cytoskeleton in a pattern that mimicked treadmilling in intact sperm. The fiber complexes were reconstructed by assembly at their plasmalemmal ends so that within 30-60 sec the entire filament system reformed and the cell resumed locomotion. Both cytoskeletal reassembly and treadmilling required exogenous HCO3-. These results suggest that variation in intracellular pH may help regulate cytoskeletal treadmilling and thereby play a significant role in sperm locomotion.

Cell locomotion: new research tests old ideas on membrane and cytoskeletal flow

Recent studies on the mobility of membrane markers on crawling cells indicate that there is no long-range centripetal flow of membrane proteins or lipids during cell locomotion. In this article we reflect on the history of ideas about membrane flow in cells, and we discuss how these new findings will shift the focus of research in cell locomotion away from the cell surface to the molecular interactions and dynamics of the actin cytoskeleton.

Stress fibers in the mesenteric mesothelial cells of the large intestine of the bullfrog, Rana catesbeiana

Actin-containing cytoplasmic fibers were visualized in the mesenteric mesothelial cells of the large intestine of bullfrog tadpoles by rhodamine-phalloidin staining of en face preparations of mesothelial cells. These fibers were concurrently stained by immunofluorescence using antibodies to myosin or alpha-actinin. Electron microscopy showed the presence of bundles of microfilaments in the basal cytoplasm of the cells. Such fibers in the mesothelial cells may be comparable to the stress fibers present in cultured cells. The mesothelial cells initially formed axially oriented stress fibers when they changed from a rhombic to a slender spindle-like shape. On the other hand, stress fibers disappeared as cells transformed from elongated to polygonal shapes during the period of metamorphic climax. Expression of stress fibers in these cells appears to be related to the degree of tension loaded on the mesentery, which may be generated by mesenteric winding. These stress fibers in the mesothelial cells may serve to regulate cellular transformation. They may also help to maintain cellular integrity by strengthening the cellular attachment to subepithelial tissue against tensile stress exerted on the mesentery.

Effects of a tumour promoter, 12-0-tetradecanoyl-phorbol-13-acetate (TPA), on expression of differentiated phenotype in the chick retinal pigmented epithelial cells and on their interactions with the native basement membrane and with artificial substrata

Chick retinal pigmented epithelial (RPE) cells grown in vitro on basement membrane matrices from the Engelbreth-Holm-Swarm tumour (BM-matrigel) do not spread, and they maintain their differentiated phenotype, most notably the heavy pigmentation. Maintenance of the differentiated phenotype by RPE cells on BM-matrigel is promoted not only by the biochemical composition of the gel but also by its mechanical properties, i.e., its low rigidity prevents cell spreading. In this report, RPE cells on BM-matrigel were treated with 12-O-tetradecanoyl-phorbol-13-acetate (TPA) to promote the transformed phenotype and diminish cell traction. In contrast to most cell types TPA treatment induced RPE cells to increase their spread area. TPA promoted RPE cell spreading on BM-matrigel and changed the spatial organization of actin and actin-associated proteins in the cytoskeleton-ECM linkage complexes, uncoupling actin from its extracellular counterpart. TPA did not affect other components of the cytoskeleton in RPE cells. TPA also affected labile adhesions i.e., focal contacts and adherens junctions in statu nascendi, but preformed, stable adherens junctions were resistant to TPA. TPA enhanced proliferation, blocked melanogenesis and thus inhibited differentiation of RPE cells grown on either artificial substrata or their natural basement membrane.

Cytoskeletal organization of migrating retinal pigment epithelial cells during wound healing in organ culture

Retinal pigment epithelial (RPE) cells maintained in organ culture on Bruch's membrane and the associated choroid spread and migrate into a linear wound along the exposed basal lamina. Changes in cell shape, in the organization of microfilaments, and in cell-cell and cell-substratum interactions during this time were examined by epifluorescence and transmission electron microscopy. In contrast to cuboidal stationary cells distant from the wound edge, which display well-developed apical circumferential microfilament bundles (CMBs) associated with zonulae adhaerentes junctions, the migrating RPE cells near the wound edge instead are flat, and, in addition to microfilament bundles near junctions between adjacent cells, display prominent stress fibers. Furthermore, monoclonal antibodies to vinculin labeled regions at the terminal ends of these stress fibers indicating that the RPE cells form focal contacts with the basal lamina at these sites. Electron microscopy of these regions of cell-substratum interaction confirmed the presence of microfilament bundles that terminate on the cell membrane. Folds present in the basal lamina near these sites suggest that tension is being generated by the microfilaments in the stress fibers as the migrating cells pull on the underlying basal lamina through these adhesion points.

Correlated distribution of actin, myosin, and microtubules at the leading edge of migrating Swiss 3T3 fibroblasts

The formation of lamellipodia in migrating cells involves dynamic processes that occur in a cyclic manner as the leading edge of a cell slowly advances. We used video-enhanced contrast microscopy (VEC) to monitor the motile behavior of cells to classify protrusions into the temporal stages of initial and established protrusions (Fisher et al.: Cell Motility and the Cytoskeleton 11:235-247, 1988), and to monitor the fixation of cells. Multiple parameter fluorescence imaging methods (DeBiasio et al.: Journal of Cell Biology 105:1613-1622, 1987; Waggoner et al.: Methods in Cell Biology, Vol. 30, Part B, pp. 449-478, 1989) were then used to determine and to map accurately the distributions of actin, myosin and microtubules in specific types of protrusions. Initial protrusions exhibited no substructure as evidenced by VEC and actin was diffusely arranged, while myosin and microtubules were absent. Newly established protrusions contained diffuse actin as well as actin in microspikes. There was a delay in the appearance of myosin into established protrusions relative to the presence of actin. Microtubules were found in established protrusions after myosin was detected, and they were oriented parallel to the direction of migration. Actin and myosin were also localized in fibers transverse to the direction of migration at the base of initial and established protrusions. Image analysis was used to quantify the orientation of actin fibers relative to the leading edge of motile cells. The combined use of VEC, multiple parameter immunofluorescence, and image analysis should have a major impact on defining complex relationships within cells.

The dynamic distribution of fluorescent analogues of actin and myosin in protrusions at the leading edge of migrating Swiss 3T3 fibroblasts

The formation of protrusions at the leading edge of the cell is an essential step in fibroblast locomotion. Using fluorescent analogue cytochemistry, ratio imaging, multiple parameter analysis, and fluorescence photobleaching recovery, the distribution of actin and myosin was examined in the same protrusions at the leading edge of live, locomoting cells during wound-healing in vitro. We have previously defined two temporal stages of the formation of protrusions: (a) initial protrusion and (b) established protrusion (Fisher et al., 1988). Actin was slightly concentrated in initial protrusions, while myosin was either totally absent or present at extremely low levels at the base of the initial protrusions. In contrast, established protrusions contained diffuse actin and actin microspikes, as well as myosin in both diffuse and structured forms. Actin and myosin were also localized along concave transverse fibers near the base of initial and established protrusions. The dynamics of myosin penetration into a relatively stable, established protrusion was demonstrated by recording sequential images over time. Myosin was shown to be absent from an initial protrusion, but diffuse and punctate myosin was detected in the same protrusion within 1-2 min. Fluorescence photobleaching recovery indicated that myosin was 100% immobile in the region behind the leading edge containing transverse fibers, in comparison to the 21% immobile fraction detected in the perinuclear region. Possible explanations of the delayed penetration of myosin into established protrusions and the implications on the mechanism of protrusion are discussed.

Actin filament patterns in mouse lens epithelium: a study of the effects of aging, injury, and genetics

Using mainly fluorescence microscopy after rhodamine-phalloidin staining, the F-actin distribution in the mouse lens epithelium was studied with regard to the effects of age, genetic strain, and mechanical injury. These studies have revealed that aside from its association with the plasma membrane the structural organization of F-actin in the mouse lens epithelium in situ is characterized by two major configurations: (1) a filamentous arrangement in such patterns as stress fibers, polygonal arrays (PAs), and meshworks, and (2) a highly concentrated structure called a sequestered actin bundle (SAB). The aging study indicated that the SAB is a consistent character in C57BL/6 mice from the age of 5 wk on, but not in CF1 mice. The size and shape of the SAB change gradually with age as inferred from two-dimensional measurements. The genetic study on the SAB character using hybrids and congenic strains showed that it is inherited as a Mendelian dominant, probably multigenic mode. Finally, the injury study revealed a structural modification in cells around the wound, including flattening of cells at the edge and extension of processes into the wound space. In the rest of the epithelium, injury amplified membrane infolding and fluorescence of polygonal arrays but diminished the size and fluorescence intensity of SABs. These changes are thought to be correlated with wound repair involving cell division and migration. These studies illustrate the variability in F-actin expression in situ in lens epithelial cells that can be induced by intrinsic and extrinsic factors.

Centripetal transport of cytoplasm, actin, and the cell surface in lamellipodia of fibroblasts

Wound healing in Swiss 3T3 cultures was investigated with video-enhanced contrast (VEC) microscopy. The formation of protrusions at the leading edge of cells along wound was investigated in detail during the spreading stage, which usually lasted from 1 to 4 hr postwounding. Lamellipodia exhibited a continuous rearward, or centripetal, transport of a variety of cellular constituents at rates of approximately 0.26 microns/sec from the leading edge. The lamellipodia were also the sites of lateral migration as well as extension and retraction of actin microspikes. Actin fibers oriented transversely to the direction of movement were also observed to transport centripetally at similar rates. These fibers may in part give rise to large actin fibers forming at the interface between the base of the lamellipodia and the lamellae. Beads 0.5 microns in diameter attached to the dorsal surfaces of lamellipodia also transported centripetally at rates of approximately 0.21 microns/sec. Thus there is an apparent correlation between transport of a variety of structures within lamellipodia and with surface movements of lamellipodia.

Stress fiber reformation after ATP depletion

Fluorescently labeled heavy meromyosin, alpha-actinin, and vinculin were used to localize actin, alpha-actinin, and vinculin, respectively, in permeabilized and living cells during the process of stress fiber reassembly, which occurred when cells were removed from ATP-depleting medium (20 mM sodium azide and 10 mM 2-deoxyglucose). In 80% of the cells recovering from ATP depletion, small, scattered plaques containing actin, alpha-actinin, and vinculin were replaced by long, thin, periodic fibers within 5 minutes of removal of the inhibitors. These nascent stress fibers grew broader as recovery progressed, until they attained the thickness of stress fibers in control cells. In the other 20% of the cells, the scattered plaques aggregated within 5 minutes of reversal, and almost all the actin, alpha-actinin, and vinculin in the cells became localized in one perinuclear aggregate, with a diameter of approximately 15-25 micron. As recovery progressed, all aggregates resembled rings, with diameters that increased at about 0.5 micron/minute and grew to as large as 70 micron in some giant cells. As the size of the rings increased, fibers radiated outward from them and sometimes spanned the diameter of the rings. The shape of the cells did not change during this time. By 1 hour after reversal, the rings were no longer present and all cells had networks of stress fibers. Indirect immunofluorescence techniques used to localize tubulin and vimentin indicated that microtubules and intermediate filaments were not constituents of the rings, and the rings were not closely apposed to the substrate, judging from reflection contrast optics. The rapid rearrangement of attachment plaques into a perinuclear aggregate that spreads radially in the cytoplasm occurs at the same speed as fibroblast and chromosomal movement, but is unlike other types of intracytoplasmic motility.

Possible translocation of actin and alpha-actinin along stress fibers

We have employed fluorescent analogue cytochemistry and fluorescence photobleaching to study the mobility of actin and alpha-actin along stress fibers. Rhodamine-labeled actin or alpha-actinin microinjected into embryonic chick cardiac fibroblasts soon became incorporated into stress fibers. A pulse of a laser microbeam was used to photobleach small spots on the fluorescent stress fibers. Images of the bleached fiber were recorded with an intensified image processing system at 2-3 min intervals. The distance between the bleached spot and the terminus of the stress fiber, which remained stationary throughout the experiment, was then measured in the successive images. Movement of bleached spots was detected along stress fibers located in the apparently trailing processes of polygonal fibroblasts, and only occurred in one direction: away from the distal tip of the stress fiber. The rate of movement calculated for alpha-actinin-injected cells was 0.24 +/- 0.12 micron/min, for actin-injected cells, 0.29 +/- 0.11 micron/min. The rate did not seem to be affected by the location of the spot relative to the distal end of the stress fiber unless the spot was located within the most distal 5 microns of the stress fiber. Anti-myosin antibody staining indicated that stress fibers which demonstrated translocation were relatively depleted of myosin. The apparent translocation of proteins along stress fibers, possibly generated by stretching, may be related to the retraction of cell processes during locomotion.

Behavior and ultrastructure of in vitro ageing mouse embryo fibroblasts grown in collagen gels

We have compared the behaviour and the ultrastructure of embryonic mouse fibroblasts embedded into collagen gels at early, middle and late population doubling levels (PDL). Late mouse fibroblasts were able to induce gel contraction with a greater efficiency than young cells. We did not find any differences in the organization of these gels. Electron microscope observations on gels containing fibroblasts of different PDL showed that the collagen lattice induced new specific and distinct phenotypes. The well-known ultrastructural differences between young and late fibroblasts grown on plastic substrates were less prominent when these cells were embedded into collagen gels. The late fibroblasts grown into gels kept their large size and their lobulated nuclei and resembled fibroblasts grown on plastic surfaces. However, dramatic changes were observed in their pattern of microfilaments, in the dispersion of their chromatin and in their ergastoplasmic structure; these characteristics observed in late fibroblasts grown into gels were close to those of young cells. The new phenotypes of young, middle-aged and late fibroblasts in the collagen gels seemed to be stable and did not display the characteristics of an older phenotype on continued incubation. When the fibroblasts left the gel, they returned to their initial phenotype.

Endothelial cell monolayer integrity. I. Characterization of dense peripheral band of microfilaments

Although the endothelial cell (EC) cytoskeleton has been studied both in vitro and in vivo, little is known about its role in endothelial integrity. We have previously suggested that specific EC microfilament (MF) structure, which we have termed the "dense peripheral band (DPB)," may play a major role in this process. We have extended our studies to characterize this structure in pig aortic ECs in vitro. During the growth of EC cultures, the DPB appears only when the cultures have attained confluency. Using double fluorescent labeling, we found that alpha-actinin, myosin, and tropomyosin colocalized with the F-actin making up the DPB. Occasional microtubules were present in this region, although there was no preferred association between microtubules and the DPB. Colocalization studies revealed vinculin plaques at the cell-cell interface. Thin MFs extended from the DPB into the cytoplasmic side of these plaques. The DPB was completely disrupted by low dose cytochalasin B within 30 minutes, whereas many central MF bundles were still present at 24 hours. The results of this study suggest that the DPB is a distinct structure in the confluent EC monolayer and is closely associated with the ability of ECs to form and maintain the EC monolayer. The disruption of the DPB as an important initial event in the pathogenesis of vascular diseases such as atherosclerosis is discussed.

Decoration of microtubules by fluorescently labeled microtubule-associated protein 2 (MAP2) does not interfere with their spatial organization and progress through mitosis in living fibroblasts

Microtubule-associated protein 2 (MAP2) derivatized with iodoacetamidotetramethylrhodamine or with iodoacetamidofluorescein binds to microtubules after injection into living interphase cells [Scherson et al, 1984]. The binding of derivatized MAP2 stabilized microtubules in vitro; it was therefore important to check if the binding of MAP2 in vivo perturbed the dynamics and organization of the microtubule network. We have addressed these questions by studying the effect of the injection of derivatized MAP2 on mitosis in PtK 1 cells and on the recovery of the microtubule network from low temperature incubation in interphase cells. We found that the presence of derivatized MAP2 did not change the duration of any mitotic stage and that the injected cell normally completed mitosis. We subsequently showed that the injected MAP2 bound to the microtubules within 5 minutes after injection and remained bound throughout the course of mitosis. The reorganization of the microtubule network upon cooling and rewarming was studied in the cytoplasm of human foreskin fibroblasts (356 cells). During the recovery, the distribution of the fluorescent MAP2 in living cells was identical with the microtubule pattern visualized by immunofluorescence in lysed and fixed cells. In these experiments, the fluorescent MAP2 bound to microtubules can be considered as a nonperturbing reporter of the microtubule network. This result is discussed in terms of the role of MAPs in the dynamics and organization of microtubules in living cells.

Fibronectin localization and endocytosis in early and late mouse embryonic fibroblasts in primary culture: a study by light and electron microscopic immunocytochemistry

In spreading fibroblasts, strong endocytosis of exogenous fluorescent fibronectin (FN) was observed from the beginning of their attachment to the substratum. In early fibroblasts, the internalized FN was localized both in the peripheral ruffles and in the perinuclear cytoplasm; in late fibroblasts, whose spreading was slower, FN uptake was not detected in the ruffles. In growing cultures, supracellular FN fibres, detected by direct fluorescence microscopy or by the indirect peroxidase-anti-peroxidase (PAP) complex technique, were scarce on early cells, but very numerous on the upper face and on the filopodia of late cells. At the ultrastructural level, FN, localized with the immuno-gold staining method, was found associated with fibres of the extracellular matrix and the upper face of the cells. FN was endocytosed via smooth vesicles and we suggest that the internalization process is slower in the late cells. In confluent early cultures, an extended network of pericellular FN was observed as usual. The pericellular FN of late grouped cells was present as a few coarse fibres connected with some of the cell surface threads.

A re-evaluation of cytoplasmic gelsolin localization

Gelsolin is a 90,000-mol-wt Ca2+-binding, actin-associated protein that can nucleate actin filament growth, sever filaments, and cap barbed filament ends. Brevin is a closely related 92,000-mol-wt plasma protein with similar properties. Gelsolin has been reported to be localized on actin filaments in stress fibers, in cardiac and skeletal muscle I-bands, and in cellular regions where actin filaments are known to be concentrated. Previous localization studies have used sera or antibody preparations that contain brevin. Using purified brevin-free IgG and IgA monoclonal antibodies or affinity-purified polyclonal antibodies for gelsolin and brevin, we find no preferential stress fiber staining in cultured human fibroblasts or I-band staining in isolated rabbit skeletal muscle sarcomeres. Cardiac muscle frozen sections show no pronounced I-band staining, except in local areas where brevin may have penetrated from adjacent blood vessels. Spreading platelets show endogenous gelsolin localized at the cell periphery, in the central cytoplasmic mass and on thin fibers that radiate from the central cytoplasm. Addition of 3-30 micrograms/ml of brevin to the antibodies restores intense stress fiber and I-band staining. We see no evidence for large-scale severing and removal of filaments in stress fibers in formaldehyde-fixed, acetone-permeabilized cells even at brevin concentrations of 30 micrograms/ml. The added brevin or brevin antibody complex binds to actin filaments and is detected by the fluorescently tagged secondary antibody. Brevin binding occurs in either Ca2+ or EGTA, but is slightly more intense in EGTA suggesting some severing and filament removal may occur in Ca2+. The I-band staining is limited to the region where actin and myosin do not overlap. In addition, brevin does not appear to bind at the Z-line. A comparison of cells double-labeled with fluorescein-phallotoxin, exogenous brevin, and a monoclonal antibody, detected with a rhodamine-labeled secondary antibody, shows almost complete co-localization of F-actin with the brevin-gelsolin-binding sites. A major exception is in the area of the adhesion plaque. A quantitative comparison of the fluorescein-rhodamine fluorescence intensities along a stress fiber and into the adhesion plaque shows that the fluorescein signal, associated with F-actin, increases while the rhodamine signal decreases. We infer that exogenous brevin or endogenous gelsolin can bind to and potentially sever most actin filaments, but that actin-associated proteins in the adhesion plaque can prevent binding and severing.(ABSTRACT TRUNCATED AT 400 WORDS)