What structures, besides adhesions, prevent spread cells from rounding up?

Distinct roles of nonmuscle myosin II isoforms for establishing tension and elasticity during cell morphodynamics

Nonmuscle myosin II (NM II) is an integral part of essential cellular processes, including adhesion and migration. Mammalian cells express up to three isoforms termed NM IIA, B, and C. We used U2OS cells to create CRISPR/Cas9-based knockouts of all three isoforms and analyzed the phenotypes on homogenously coated surfaces, in collagen gels, and on micropatterned substrates. In contrast to homogenously coated surfaces, a structured environment supports a cellular phenotype with invaginated actin arcs even in the absence of NM IIA-induced contractility. A quantitative shape analysis of cells on micropatterns combined with a scale-bridging mathematical model reveals that NM IIA is essential to build up cellular tension during initial stages of force generation, while NM IIB is necessary to elastically stabilize NM IIA-generated tension. A dynamic cell stretch/release experiment in a three-dimensional scaffold confirms these conclusions and in addition reveals a novel role for NM IIC, namely the ability to establish tensional homeostasis.

The Research Advance of Cell Bridges in vitro

The microenvironment in which cells reside in vivo dictates their biological and mechanical functioning is associated with morphogenetic and regenerative processes and may find implications in regenerative medicine and tissue engineering. The development of nano- and micro-fabricated technologies, three-dimensional (3D) printing technique, and biomimetic medical materials have enabled researchers to prepare novel advanced substrates mimicking the in vivo microenvironment. Most of the novel morphologies and behaviors of cells, including contact guidance and cell bridges which are observed in vivo but are not perceived in the traditional two-dimensional (2D) culture system, emerged on those novel substrates. Using cell bridges, cell can span over the surface of substrates to maintain mechanical stability and integrity of tissue, as observed in physiological processes, such as wound healing, regeneration and development. Compared to contact guidance, which has received increased attention and is investigated extensively, studies on cell bridges remain scarce. Therefore, in this mini-review, we have comprehensively summarized and classified different kinds of cell bridges formed on various substrates and highlighted possible biophysical mechanisms underlying cell bridge formation for their possible implication in the fields of tissue engineering and regenerative medicine.

Tension and Elasticity Contribute to Fibroblast Cell Shape in Three Dimensions

The shape of animal cells is an important regulator for many essential processes such as cell migration or division. It is strongly determined by the organization of the actin cytoskeleton, which is also the main regulator of cell forces. Quantitative analysis of cell shape helps to reveal the physical processes underlying cell shape and forces, but it is notoriously difficult to conduct it in three dimensions. Here we use direct laser writing to create 3D open scaffolds for adhesion of connective tissue cells through well-defined adhesion platforms. Due to actomyosin contractility in the cell contour, characteristic invaginations lined by actin bundles form between adjacent adhesion sites. Using quantitative image processing and mathematical modeling, we demonstrate that the resulting shapes are determined not only by contractility, but also by elastic stress in the peripheral actin bundles. In this way, cells can generate higher forces than through contractility alone.

Modeling cell shape and dynamics on micropatterns

Adhesive micropatterns have become a standard tool to study cells under defined conditions. Applications range from controlling the differentiation and fate of single cells to guiding the collective migration of cell sheets. In long-term experiments, single cell normalization is challenged by cell division. For all of these setups, mathematical models predicting cell shape and dynamics can guide pattern design. Here we review recent advances in predicting and explaining cell shape, traction forces and dynamics on micropatterns. Starting with contour models as the simplest approach to explain concave cell shapes, we move on to network and continuum descriptions as examples for static models. To describe dynamic processes, cellular Potts, vertex and phase field models can be used. Different types of model are appropriate to address different biological questions and together, they provide a versatile tool box to predict cell behavior on micropatterns.

Inverted formin 2 in focal adhesions promotes dorsal stress fiber and fibrillar adhesion formation to drive extracellular matrix assembly

Actin filaments and integrin-based focal adhesions (FAs) form integrated systems that mediate dynamic cell interactions with their environment or other cells during migration, the immune response, and tissue morphogenesis. How adhesion-associated actin structures obtain their functional specificity is unclear. Here we show that the formin-family actin nucleator, inverted formin 2 (INF2), localizes specifically to FAs and dorsal stress fibers (SFs) in fibroblasts. High-resolution fluorescence microscopy and manipulation of INF2 levels in cells indicate that INF2 plays a critical role at the SF-FA junction by promoting actin polymerization via free barbed end generation and centripetal elongation of an FA-associated actin bundle to form dorsal SF. INF2 assembles into FAs during maturation rather than during their initial generation, and once there, acts to promote rapid FA elongation and maturation into tensin-containing fibrillar FAs in the cell center. We show that INF2 is required for fibroblasts to organize fibronectin into matrix fibers and ultimately 3D matrices. Collectively our results indicate an important role for the formin INF2 in specifying the function of fibrillar FAs through its ability to generate dorsal SFs. Thus, dorsal SFs and fibrillar FAs form a specific class of integrated adhesion-associated actin structure in fibroblasts that mediates generation and remodeling of ECM.

Dynamics of cell shape and forces on micropatterned substrates predicted by a cellular Potts model

Micropatterned substrates are often used to standardize cell experiments and to quantitatively study the relation between cell shape and function. Moreover, they are increasingly used in combination with traction force microscopy on soft elastic substrates. To predict the dynamics and steady states of cell shape and forces without any a priori knowledge of how the cell will spread on a given micropattern, here we extend earlier formulations of the two-dimensional cellular Potts model. The third dimension is treated as an area reservoir for spreading. To account for local contour reinforcement by peripheral bundles, we augment the cellular Potts model by elements of the tension-elasticity model. We first parameterize our model and show that it accounts for momentum conservation. We then demonstrate that it is in good agreement with experimental data for shape, spreading dynamics, and traction force patterns of cells on micropatterned substrates. We finally predict shapes and forces for micropatterns that have not yet been experimentally studied.

United we stand: integrating the actin cytoskeleton and cell-matrix adhesions in cellular mechanotransduction

Many essential cellular functions in health and disease are closely linked to the ability of cells to respond to mechanical forces. In the context of cell adhesion to the extracellular matrix, the forces that are generated within the actin cytoskeleton and transmitted through integrin-based focal adhesions are essential for the cellular response to environmental clues, such as the spatial distribution of adhesive ligands or matrix stiffness. Whereas substantial progress has been made in identifying mechanosensitive molecules that can transduce mechanical force into biochemical signals, much less is known about the nature of cytoskeletal force generation and transmission that regulates the magnitude, duration and spatial distribution of forces imposed on these mechanosensitive complexes. By focusing on cell-matrix adhesion to flat elastic substrates, on which traction forces can be measured with high temporal and spatial resolution, we discuss our current understanding of the physical mechanisms that integrate a large range of molecular mechanotransduction events on cellular scales. Physical limits of stability emerge as one important element of the cellular response that complements the structural changes affected by regulatory systems in response to mechanical processes.

Contractile network models for adherent cells

Cells sense the geometry and stiffness of their adhesive environment by active contractility. For strong adhesion to flat substrates, two-dimensional contractile network models can be used to understand how force is distributed throughout the cell. Here we compare the shape and force distribution for different variants of such network models. In contrast to Hookean networks, cable networks reflect the asymmetric response of biopolymers to tension versus compression. For passive networks, contractility is modeled by a reduced resting length of the mechanical links. In actively contracting networks, a constant force couple is introduced into each link in order to model contraction by molecular motors. If combined with fixed adhesion sites, all network models lead to invaginated cell shapes, but only actively contracting cable networks lead to the circular arc morphology typical for strongly adhering cells. In this case, shape and force distribution are determined by local rather than global determinants and thus are suited to endow the cell with a robust sense of its environment. We also discuss nonlinear and adaptive linker mechanics as well as the relation to tissue shape.

Substrate, focal adhesions, and actin filaments: a mechanical unit with a weak spot for mechanosensitive proteins

Mechanosensing is a vital prerequisite for dynamic remodeling of focal adhesions and cytoskeletal structures upon substrate deformation. For example, tissue formation, directed cell orientation or cell differentiation are regulated by such mechanosensing processes. Focal adhesions and the actin cytoskeleton are believed to be involved in these processes, but where mechanosensing molecules are located and how elastic substrate, focal adhesions and the cytoskeleton couple with each other upon substrate deformation still remains obscure. To approach these questions we have developed a sensitive method to apply defined spatially decaying deformation fields to cells cultivated on ultrasoft elastic substrates and to accurately quantify the resulting displacements of the actin cytoskeleton, focal adhesions, as well as the substrate. Displacement fields were recorded in live cell microscopy by tracking either signals from fluorescent proteins or marker particles in the substrate. As model cell type we used myofibroblasts. These cells are characterized by highly stable adhesion and force generating structures but are still able to detect mechanical signals with high sensitivity. We found a rigid connection between substrate and focal adhesions. Furthermore, stress fibers were found to be barely extendable almost over their whole lengths. Plastic deformation took place only at the very ends of actin filaments close to focal adhesions. As a result, this area became elongated without extension of existing actin filaments by polymerization. Both ends of the stress fibers were mechanically coupled with detectable plastic deformations on either site. Interestingly, traction force dependent substrate deformation fields remained mostly unaffected even when stress fiber elongations were released. These data argue for a location of mechanosensing proteins at the ends of actin stress fibers and describe, except for these domains, the whole system to be relatively rigid for tensile strain with a mechanical coupling between the front and rear end of a cell.

Force generated by actomyosin contraction builds bridges between adhesive contacts

Extracellular matrices in vivo are heterogeneous structures containing gaps that cells bridge with an actomyosin network. To understand the basis of bridging, we plated cells on surfaces patterned with fibronectin (FN)-coated stripes separated by non-adhesive regions. Bridges developed large tensions where concave cell edges were anchored to FN by adhesion sites. Actomyosin complexes assembled near those sites (both actin and myosin filaments) and moved towards the centre of the non-adhesive regions in a treadmilling network. Inhibition of myosin-II (MII) or Rho-kinase collapsed bridges, whereas extension continued over adhesive areas. Inhibition of actin polymerization (latrunculin-A, jasplakinolide) also collapsed the actomyosin network. We suggest that MII has distinct functions at different bridge regions: (1) at the concave edges of bridges, MIIA force stimulates actin filament assembly at adhesions and (2) in the body of bridges, myosin cross-links actin filaments and stimulates actomyosin network healing when breaks occur. Both activities ensure turnover of actin networks needed to maintain stable bridges from one adhesive region to another.

Effect of adhesion geometry and rigidity on cellular force distributions

The behavior and fate of tissue cells are controlled by the rigidity and geometry of their adhesive environment, possibly through forces localized to sites of adhesion. We introduce a mechanical model that predicts cellular force distributions for cells adhering to adhesive patterns with different geometries and rigidities. For continuous adhesion along a closed contour, forces are predicted to be localized to the corners. For discrete sites of adhesion, the model predicts the forces to be mainly determined by the lateral pull of the cell contour. With increasing distance between two neighboring sites of adhesion, the adhesion force increases because the cell shape results in steeper pulling directions. Softer substrates result in smaller forces. Our predictions agree well with experimental force patterns measured on pillar assays.

Rho GTPase signaling and PTH 3-34, but not PTH 1-34, maintain the actin cytoskeleton and antagonize bisphosphonate effects in mouse osteoblastic MC3T3-E1 cells

Cytoskeletal elements are critical for cell morphology and signal transduction, and are involved in many cellular processes including motility, intracellular transport, and differentiation. Small GTP-binding proteins (G proteins) of the Ras family, such as RhoA, influence various elements of the cytoskeleton. RhoA stabilizes the actin cytoskeleton and promotes formation of focal adhesions. We found previously that RhoA is expressed in osteoblastic cells and is translocated to the plasma membrane and activated by PTH 1-34 as well as by Nleu(8,18) Tyr(34) PTH 3-34 amide, a PTH analog that does not increase cAMP. We therefore investigated effects of manipulating RhoA on the actin cytoskeleton of osteoblastic MC3T3-E1 cells. Three inhibitors were used: 1) GGTI-2166, a geranylgeranyl transferase I inhibitor that prevents the isoprenylation and membrane translocation of RhoA, 2) Y-27632, a Rho kinase inhibitor, and 3) alendronate, a nitrogen (N)-containing bisphosphonate that reduces intracellular geranylgeranylpyrophosphate through inhibiting farnesyl pyrophosphate synthase. To increase RhoA activity, we used the geranylgeranyl group donor geranylgeraniol (GGOH), and a constitutively active RhoA. The F-actin cytoskeleton and focal adhesions (FA) were visualized with rhodamine-phalloidin and fluorescent anti-vinculin antibodies, respectively. Cells were imaged with confocal microscopy. Actin stress fiber density, edge actin bundle density, focal adhesion density, cellular area and circularity (a morphological descriptor relating area and perimeter) were quantified by a program developed with Matlab software. GGTI-2166, Y-27632, and alendronate reduced actin stress fibers, FA density, and FA size, but had no effect on edge actin bundle density, cellular area, or circularity. GGOH completely antagonized the effects of alendronate, but did not significantly affect responses to GGTI-2166 or Y-27632. Constitutively active RhoA antagonized the effects of alendronate and GGTI-2166, but not those of Y-27632. The effects of alendronate were also antagonized by Nleu(8,18) Tyr(34) PTH 3-34 amide, but not by PTH 1-34. The results indicate that RhoA is involved in the maintenance of stress fibers and focal adhesions in osteoblastic cells, that PTH can affect this pathway independently of cAMP, and that a N-containing bisphosphonate can affect the actin cytoskeleton and focal adhesions through actions on geranylgeranyl groups and potentially through RhoA. In view of the importance of the actin cytoskeleton, the findings constitute evidence that N-containing bisphosphonates, when they attain certain concentrations, have effects on osteoblasts that could influence bone remodeling.

Filamentous network mechanics and active contractility determine cell and tissue shape

For both cells and tissues, shape is closely correlated with function presumably via geometry-dependent distribution of tension. In this study, we identify common shape determinants spanning cell and tissue scales. For cells whose sites of adhesion are restricted to small adhesive islands on a micropatterned substrate, shape resembles a sequence of inward-curved circular arcs. The same shape is observed for fibroblast-populated collagen gels that are pinned to a flat substrate. Quantitative image analysis reveals that, in both cases, arc radii increase with the spanning distance between the pinning points. Although the Laplace law for interfaces under tension predicts circular arcs, it cannot explain the observed dependence on the spanning distance. Computer simulations and theoretical modeling demonstrate that filamentous network mechanics and contractility give rise to a modified Laplace law that quantitatively explains our experimental findings on both cell and tissue scales. Our model in conjunction with actomyosin inhibition experiments further suggests that cell shape is regulated by two different control modes related to motor contractility and structural changes in the actin cytoskeleton.

Allicin inhibits cell polarization, migration and division via its direct effect on microtubules

Allicin (diallyl thiosulfinate) is a major biologically active component of garlic that is known to inhibit cell proliferation and induce apoptosis. The effects of allicin are attributed to its ability to react with thiol groups. However, the mechanism underlying the cytostatic activity of allicin, as well as the identity of the relevant subcellular targets, are not known. In the present study, we found that the effects of allicin on cell polarization, migration, and mitosis are similar to the effects of microtubule-depolymerizing drugs such as nocodazole. Moreover, treatment of cultured fibroblasts with micromolar doses of allicin results in microtubule depolymerization in cells within minutes of its application, without disrupting the actin cytoskeleton or inducing direct cytotoxic effects. Furthermore, allicin blocks the polymerization of pure tubulin in vitro in a concentration-dependent manner, suggesting that it acts directly on tubulin dimers. Sulfhydryl (SH)-reducing reagents such as 2-mercaptoethanol and dithiothreitol abolish the effect of allicin on microtubule polymerization. Thus, allicin is a potent microtubule-disrupting reagent interfering with tubulin polymerization by reaction with tubulin SH groups.

Kinetic-structural analysis of neuronal growth cone veil motility

Neuronal growth cone advance was investigated by correlative light and electron microscopy carried out on chick dorsal root ganglion cells. Advance was analyzed in terms of the two principal organelles responsible for protrusive motility in the growth cone – namely, veils and filopodia. Veils alternated between rapid phases of protrusion and retraction. Electron microscopy revealed characteristic structural differences between the phases. Our results provide a significant advance in three respects: first, protruding veils are comprised of a densely branched network of actin filaments that is lamellipodial in appearance and includes the Arp2/3 complex. On the basis of this structural and biomarker evidence, we infer that the dendritic nucleation and/or array-treadmilling mechanism of protrusive motility is conserved in veil protrusion of growth cones as in the motility of fibroblasts; second, retracting veils lack dendritic organization but contain a sparse network of long filaments; and third, growth cone filopodia have the capacity to nucleate dendritic networks along their length, a property consistent with veil formation seen at the light microscopic level but not previously understood in supramolecular terms. These elements of veil and filopodial organization, when taken together, provide a conceptual framework for understanding the structural basis of growth cone advance.

Cell distribution of stress fibres in response to the geometry of the adhesive environment

Cells display a large variety of shapes when plated in classical culture conditions despite their belonging to a common cell type. These shapes are transitory, since cells permanently disassemble and reassemble their cytoskeleton while moving. Adhesive micropatterns are commonly used to confine cell shape within a given geometry. In addition the micropattern can be designed so as to impose cells to spread upon adhesive and nonadhesive areas. Modulation of the pattern geometry allows the analysis of the mechanisms governing the determination of cell shape in response to external adhesive conditions. In this study, we show that the acquisition of cell shape follows two stages where initially the cell forms contact with the micropattern. Here, the most distal contacts made by the cell with the micropattern define the apices of the cell shape. Then secondly, the cell borders that link two apices move so as to minimise the distance between the two apices. In these cell borders, the absence of an underlying adhesive substrate is overcome by stress fibres forming between the apices, which in turn are marked by an accumulation of focal adhesions. By inhibiting myosin function, cell borders on nonadhesive zones become more concave, suggesting that the stress fibres work against the membrane tension in the cell border. Moreover, this suggested that traction forces are unevenly distributed in stationary, nonmigrating, cells. By comparing the stress fibres in cells with one, two, or three nonadherent cell borders it was reasoned that stress fibre strength is inversely proportional to number. We conclude that cells of a given area can generate the same total sum of tractional forces but that these tractional forces are differently spaced depending on the spatial distribution of its adherence contacts.

How actin crosslinking and bundling proteins cooperate to generate an enhanced cell mechanical response

Actin-crosslinking proteins organize actin filaments into dynamic and complex subcellular scaffolds that orchestrate important mechanical functions, including cell motility and adhesion. Recent mutation studies have shown that individual crosslinking proteins often play seemingly non-essential roles, leading to the hypothesis that they have considerable redundancy in function. We report live-cell, in vitro, and theoretical studies testing the mechanical role of the two ubiquitous actin-crosslinking proteins, alpha-actinin and fascin, which co-localize to stress fibers and the basis of filopodia. Using live-cell particle tracking microrheology, we show that the addition of alpha-actinin and fascin elicits a cell mechanical response that is significantly greater than that originated by alpha-actinin or fascin alone. These live-cell measurements are supported by quantitative rheological measurements with reconstituted actin filament networks containing pure proteins that show that alpha-actinin and fascin can work in concert to generate enhanced cell stiffness. Computational simulations using finite element modeling qualitatively reproduce and explain the functional synergy of alpha-actinin and fascin. These findings highlight the cooperative activity of fascin and alpha-actinin and provide a strong rationale that an evolutionary advantage might be conferred by the cooperative action of multiple actin-crosslinking proteins with overlapping but non-identical biochemical properties. Thus the combination of structural proteins with similar function can provide the cell with unique properties that are required for biologically optimal responses.

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.

Glial growth factor 2 induces proliferation and structural changes in ensheathing cells

Ensheathing cells were isolated from neonatal rat olfactory bulbs and cultured in the presence of glial growth factor 2 (GGF2). Proliferation assay showed that at concentrations of up to 60 ng/ml GGF2, ensheathing cells underwent a modest increase in proliferation rate. This stimulation was not maintained at high doses of GGF2 at 100 ng/ml or more. Chemotaxis chambers and scanning electron microscopy were used to determine whether GGF2 was a chemoattractant for ensheathing cells. Although the results showed no chemotactic response to GGF2, ensheathing cells demonstrated structural changes when cultured in the presence of 20 ng/ml GGF2. Ultrastructural observations revealed that GGF2 promoted increased deposition of extracellular matrix on the cell membrane, more cytoskeletal elements in the processes and as a possible consequence, contributed to a more rigid support. Ensheathing cells cultured in the absence of GGF2 often extended thinner and curved processes. Reverse transcription-polymerase chain reaction confirmed the presence of GGF2 transcripts in ensheathing cells, suggesting that ensheathing cells themselves are a source of GGF2.

Pearling in cells: a clue to understanding cell shape

Gradual disruption of the actin cytoskeleton induces a series of structural shape changes in cells leading to a transformation of cylindrical cell extensions into a periodic chain of "pearls." Quantitative measurements of the pearling instability give a square-root behavior for the wavelength as a function of drug concentration. We present a theory that explains these observations in terms of the interplay between rigidity of the submembranous actin shell and tension that is induced by boundary conditions set by adhesion points. The theory allows estimation of the rigidity and thickness of this supporting shell. The same theoretical considerations explain the shape of nonadherent edges in the general case of untreated cells.

Locomotory behaviour of epitheliocytes and fibroblasts on metallic grids

Behaviour of epitheliocytes and fibroblasts on special discontinuous substrata (metallic grids with square openings of 45x45 microm2) was examined in order to compare the ability of these cells to spread in two mutually perpendicular directions and to stretch over the void spaces. Two cell types with typical fibroblastic morphology, the AGO 1523 line of human foreskin fibroblasts and secondary cultures of mouse embryo fibroblasts, and three cell types with typical epithelial morphology, primary mouse hepatocytes, the IAR-2 line of rat liver cells and the MDCK line of canine kidney epithelial cells (clone 20) were used. We also examined the epitheliocytes (MDCK cells, clone 20) transformed to fibroblast-like morphology by treatment with hepatocyte growth factor/scatter factor (HGF/SF). Time-lapse video microscopy, scanning electron microscopy and immunofluorescence microscopy were used to examine cell reorganizations at various stages of spreading. It was found that early stages of spreading of fibroblasts and epitheliocytes were similar: the cell spread along two bars, perpendicular to each other (bar and crossbar), with the formation of a small triangular lamellar cytoplasm stretched over the opening. Later central parts of the bodies of the fibroblasts retracted from the bars so that the cells remained attached only by their polar lamellae. Successive expansions and partial retractions of these lamellae led to elongation of the cell body crossing several openings of the grid. Epitheliocytes, in contrast to fibroblasts, at the late stages of spreading did not retract their bodies and did not contract polar lamellae. As a result, their central lamellae stretched progressively over the openings. As a result of the treatment of MDCK epitheliocytes with HGF/SF the behaviour of the cells on the grids became similar to that of fibroblasts. It is suggested that these distinct spreading patterns of epitheliocytes and fibroblasts are due to the type-specific differences in the actin-myosin cortex. Experiments with microtubule-specific drugs, colcemid and taxol, indicate that the organization of this cortex is under microtubular control.

Direct observations of the mechanical behaviors of the cytoskeleton in living fibroblasts

Cytoskeletal proteins tagged with green fluorescent protein were used to directly visualize the mechanical role of the cytoskeleton in determining cell shape. Rat embryo (REF 52) fibroblasts were deformed using glass needles either uncoated for purely physical manipulations, or coated with laminin to induce attachment to the cell surface. Cells responded to uncoated probes in accordance with a three-layer model in which a highly elastic nucleus is surrounded by cytoplasmic microtubules that behave as a jelly-like viscoelastic fluid. The third, outermost cortical layer is an elastic shell under sustained tension. Adhesive, laminin-coated needles caused focal recruitment of actin filaments to the contacted surface region and increased the cortical layer stiffness. This direct visualization of actin recruitment confirms a widely postulated model for mechanical connections between extracellular matrix proteins and the actin cytoskeleton. Cells tethered to laminin-treated needles strongly resisted elongation by actively contracting. Whether using uncoated probes to apply simple deformations or laminin-coated probes to induce surface-to-cytoskeleton interaction we observed that experimentally applied forces produced exclusively local responses by both the actin and microtubule cytoskeleton. This local accomodation and dissipation of force is inconsistent with the proposal that cellular tensegrity determines cell shape.

Involvement of the protein kinase C substrate, SSeCKS, in the actin-based stellate morphology of mesangial cells

Activation of protein kinase C is a key signal transduction event in mesangial cell dedifferentiation and proliferation, yet little is known about downstream substrates or their roles in normal or diseased glomeruli. SSeCKS, a novel protein kinase C substrate originally isolated as a src-suppressed negative mitogenic regulator in fibroblasts, controls actin-based cytoskeletal architecture and scaffolds key signaling kinases such as protein kinase C and protein kinase A. Based on the morphologic similarity between SSeCKS-overexpressing fibroblasts and stellate mesangial cells, we hypothesized that SSeCKS might play a role in mesangial cell morphology in a protein kinase C-dependent manner. Immunoblotting, in situ staining and northern blotting detected abundant expression of SSeCKS in human and rodent mesangial cells and glomerular parietal cells but not in renal tubular epithelia. Immunofluorescence analysis showed enrichment of SSeCKS in mesangial cell podosomes and along a cytoskeletal network distinct from F-actin. Activation of protein kinase C by phorbol ester resulted in a rapid serine phosphorylation of SSeCKS and its subsequent translocation to perinuclear sites, coincident with the retraction of stellate processes. These effects were blocked by concentrations of bis-indolylmaleimide that selectively inhibit protein kinase C. Finally, ablation of SSeCKS expression using retroviral anti-sense vectors induced (1) an elongated, fibroblastic cell morphology, (2) production of thick, longitudinal stress fibers and (3) repositioning of vinculin-associated focal complexes away from the cell edges. These data suggest a role for SSeCKS as a downstream mediator of protein kinase C-controlled, actin-based mesangial cell cytoskeletal architecture.

The influence of micro-topography on cellular response and the implications for silicone implants

Tissue attachment to substratum surfaces is of central importance to the in vivo performance of prosthetic implant materials. It is not yet understood why connective tissue does not attach to the surface of silicone or any other polymeric material. Recently the authors have conclusively demonstrated that micro-range surface roughness modifies cellular responses in cell culture and modifies biocompatibility and tissue attachment in vivo significantly. In order to better understand the basic interactions between living cells or tissues on one hand and man-made substratum surfaces on the other hand, the germane literature is reviewed here. Cells adhere to substratum surfaces mainly through focal adhesions which are a complex of intracellular transmembrane and extracellular proteins. Adhesion is facilitated and modified by proteins adsorbed to the substratum surface. Protein adsorption in turn is modified by the underlying substratum surface properties including surface chemistry, charge, and free energy. When silicone and other polymeric implants having well-defined surface topographic features including pores, pillars, or grooves were implanted, the tissue response to these implants was strongly influenced by the dimensions of these features as well as by other geometric details. Highest biocompatibility along with tissue attachment was seen when topographic features had dimensions of 1-3 microns and a uniform distribution. Cell culture studies revealed that topographic features affect cellular alignment, direction of proliferation, cellular attachment, growth rate, metabolism, and cytoskeletal arrangement. Since discontinuities or curvatures associated with topographic features may represent local changes in surface free energy, it is hypothesized that these discontinuities trigger changes in protein adsorption, protein configuration, and cellular response.

Mechanical perturbation of webbed edges in 3T3 cells

We have previously described actin edge-bundles (AEBs) as cables of microfilaments lining the webbed edges of 3T3 cells (Zand and Albrecht-Buehler: Cell Motil. Cytoskeleton 13:195-211, 1989). We have suggested that AEBs, along with their cell-substratum adhesions, resist cortical tension and prevent the collapse of cytoplasm towards the nucleus. In this paper, we report several stages of AEB disassembly and re-formation induced by the following micro-manipulations: (1) Scoring of the webbed edge of a 3T3 cells with a micro-needle. As a result the sides of the score retracted and the severed AEB appeared to disassemble down to its terminal adhesion points. The retraction stopped after 20-40 seconds and the cells formed a webbed edge with large curvature. Over a period of 20-80 minutes, the new web decreased in length and depth, until it regained its approximate original shape. (2) Bending of cell processes at acute angles. As a result the processes moved until they projected at right angles to the side of the cell and formed new webs gradually expanded their area. In both cases, the nascent webs were lined by actin edge-bundles.

Ca(2+)-regulated actin and phospholipid binding protein (68 kD-protein) from bovine liver: identification as a homologue for annexin VI and intracellular localization

An F-actin binding protein was purified from bovine liver by means of DNase I affinity, hydroxylapatite and DEAE-cellulose column chromatographies. It consisted of a single polypeptide chain having an apparent molecular weight of 68,000 with a Stokes radius of 35 A. Electron microscopy of rotary shadowed specimens showed that the 68 kD protein is a globular protein. This protein showed a higher affinity for F-actin in the presence of Ca2+ than in its absence, which is opposite to the actin-binding property shown by nonmuscle alpha-actinin or fimbrin. The 68 kD protein had no F-actin severing and capping activity. Interestingly, the 68 kD protein was found to aggregate liposomes at micromolar Ca2+ concentrations. Immunoblot analysis and partial protein sequence data identified the 68 kD protein as an annexin VI (p68) homologue. Immunocytochemical studies showed that the 68 kD protein was localized along stress fibers as well as membrane ruffles, microspikes and focal contacts, raising the possibility that annexin VI may contribute to control membrane-microfilament interaction in the cell.

Spreading of mouse fibroblasts on the substrate with multiple spikes

Mouse embryo fibroblasts were cultivated on special substrates with discontinuous surfaces. The substrates were silicon plates with multiple vertical (65-90 microns height) spike-like silicon microcrystals evenly distributed on the plate surfaces. It was shown that the cells were successfully spread and flattened on these substrates. The spread cells formed several discrete attachment zones at the tops and side surfaces of the spikes; these zones were separated from one another by distances considerably greater than the diameter of the unspread cell. At early stages of spreading the unspread cells attached to the tops of single spikes and extended long filopodia attached to the distant spikes. At later stages the lamellae were formed between the filopodia: probably these filopodia served as guidelines for extension of lamellae and progressive cell spreading. These experiments demonstrated that continuity of substrate surface is not a necessary condition for advanced cell spreading.

Mechanism of cell protrusion formation in electrical field: the role of actin

An intense alternating current electrical field that imposes membrane-applied force on the cell surface can induce formation of cell protrusions (Popov, S.V. and Margolis, L.B. (1988) J. Cell Sci. 90, 379-389). This technique has been used to investigate the role of actin in the cell protrusion formation. Platinum replicas of the cytoskeleton were prepared to characterize the organization of the cytoskeleton in external force-induced protrusions. Bundles of microfilaments were found in the processes. A specific inhibitor of actin polymerization, cytochalasin B, as well as inhibitors of ATP synthesis (sodium azide and carbonyl m-chlorophenylhydrazone) did not change the morphology of electrical field-generated protrusions, revealed by scanning electron microscopy. However, organization of the cytoskeleton inside the processes changed drastically using these inhibitors. The results of these experiments demonstrate that (i) Membrane-applied force is sufficient to produce native-like cell protrusions, even in conditions where activity of the cytoskeleton is inhibited; (ii) Actin microfilaments can be organized into bundles directly under the action of membrane-applied force. The significance of these observations to cell protrusion formation under normal physiological conditions is discussed.

Actin filament alterations in rat hepatocytes induced in vivo and in vitro by microcystin-LR, a hepatotoxin from the blue-green alga, Microcystis aeruginosa

The morphologic effects of microcystin-LR (MCLR) were examined in vitro and in vivo to identify the specific cell type(s) affected and to characterize the actin filament changes occurring in hepatocytes. Male Sprague Dawley rats were used for all studies. For in vitro studies, hepatic cells were isolated by collagenase perfusion of liver, while parenchymal cells (hepatocytes) and nonparenchymal cells were prepared by pronase digestion and metrimazide gradient centrifugation. Cell suspensions and and primary hepatocyte monolayer cultures were treated with MCLR at doses up to 10 micrograms/ml; cultured hepatocytes were also treated with phalloidin or cytochalasin B at a dose of 10 micrograms/ml; and rats were treated intraperitoneally with MCLR at 180 mg/kg. Cultured hepatocyte preparations and frozen liver sections were stained with rhodamine-labeled phalloidin for filamentous actin. In cell suspensions, MCLR did not affect nonparenchymal cells but caused rapid, progressive, blebbing of the plasma membrane in hepatocytes. In cultured hepatocytes, MCLR caused plasma membrane blebbing as well as marked reorganization of actin microfilaments. These alterations were dose and time dependent. Cultured hepatocytes treated with phalloidin or cytochalasin B also showed extensive plasma membrane blebbing and actin filament alterations; however, actin filament changes were morphologically distinct from those induced by MCLR. In vivo, MCLR-induced hepatocyte actin alterations occurred at the same time as, or slightly preceded, histologic changes that began 30 minutes after dosing. These studies suggest that early MCLR-induced morphologic changes occurring both in vivo and in vitro are due to alterations in hepatocyte actin filaments.

Acoustic microscopy of cultured cells. Distribution of forces and cytoskeletal elements

The scanning acoustic microscope (SAM) allows one to measure mechanical parameters of living cells with high lateral resolution. By analyzing single acoustic images' sound attenuation and sound velocity, the latter corresponding to stiffness (elasticity) of the cortical cytoplasm can be determined. In this study, measurements of stiffness distribution in XTH-2 cells were compared with the organization of F-actin and microtubules. Single XTH-2 cells exhibit relatively high stiffness at the free margins; toward the cell center this value decreases and reaches a sudden minimum where the slope of the surface topography enlargens at the margin of the dome-shaped cell center. The steepness of the increase in slope is linearly related to the decrease in sound velocity at this site. Thus, a significant determinant of cell shape is paralleled by an alteration of stiffness. In the most central parts, no interferences could be distinguished, therefore, this region had to be excluded from the calculations. Stiffness distribution roughly coincided with the distribution of F-actin, but no correlation to microtubule arrangement was found. Following the treatment of XTH-2 cells with ionomycin in the presence of calcium (in the culture medium), the cell cortex first contracted as indicated by shape changes and by a marked increase in stiffness (deduced from sound velocity). This contraction phase was followed by a phase of microtubule and F-actin disassembly. Concomittantly, sound velocity decreased considerably, indicating the loss of elasticity in the cell cortex. No structural equivalent to sound attenuation has been identified.

Reorganization of cytoskeletal and junctional proteins during cochlear hair cell degeneration

Experiments were carried out to elucidate changes in cytoskeletal elements and intercellular junctions in the organ of Corti, when hair cells degenerate and phalangeal scars form. Hair cell damage was induced by exposing guinea pigs to high intensity noise. The spatial and temporal changes in the organization of microfilaments, intermediate filaments, and tight junction-specific proteins were investigated using scanning and transmission electron microscopy and histochemistry. The results show that microfilaments, cytokeratins, adherens junctions, and tight junctions rearrange their distribution in damaged areas. From the temporal sequence of these changes it appears that phalangeal scars develop simultaneous with hair cell degeneration, and that the integrity of the luminal membranes in the organ of Corti is not interrupted. Each scar is formed by two supporting cells which expand and invade the sub-apical region of the dying hair cell. This region becomes cytokeratin-positive. The two supporting cells meet at the mid-line of the scar, where a new junctional complex is formed. The junctional complex consists of tight junction and adherens-type junction, but desmosomes are absent.

Dynamic changes of microtubule and actin structures in CV-1 cells during electrofusion

To study the involvement of the cytoskeletal system in the fusion of animal cells, we examined the dynamic changes of cytoskeletal proteins during the various stages of cell fusion. CV-1 cells were fused by applying a radio-frequency electrical pulse. Structural changes of microtubules (MTs) and F-actin were monitored simultaneously by double-label fluorescence microscopy. It was observed that in a few minutes after the initiation of cell fusion, MT bundles began to extend into the cytoplasmic bridges which were formed by fusing the membranes of neighboring cells. Later, a network of parallel MT bundles appeared between the adjacent nuclei of the fusing cells; such MT bundles may provide the mechanical links that are responsible for nuclear aggregation. The structural changes of F-actin during cell fusion were more complicated. We observed many different patterns of actin distribution in the fusing cells, including some giant, ring-shaped structures. Reorganization of actin is unlikely to be involved in the nuclear aggregation process. Instead, actin bundles condensed at the cell edges may help to widen the cytoplasmic bridges to allow merging of cellular contents between the fusing cells.