Loops versus lines and the compression stiffening of cells

Both animal and plant tissue exhibit a nonlinear rheological phenomenon known as compression stiffening, or an increase in moduli with increasing uniaxial compressive strain. Does such a phenomenon exist in single cells, which are the building blocks of tissues? One expects an individual cell to compression soften since the semiflexible biopolymer-based cytoskeletal network maintains the mechanical integrity of the cell and in vitro semiflexible biopolymer networks typically compression soften. To the contrary, we find that mouse embryonic fibroblasts (mEFs) compression stiffen under uniaxial compression via atomic force microscopy studies. To understand this finding, we uncover several potential mechanisms for compression stiffening. First, we study a single semiflexible polymer loop modeling the actomyosin cortex enclosing a viscous medium modeled as an incompressible fluid. Second, we study a two-dimensional semiflexible polymer/fiber network interspersed with area-conserving loops, which are a proxy for vesicles and fluid-based organelles. Third, we study two-dimensional fiber networks with angular-constraining crosslinks, i.e. semiflexible loops on the mesh scale. In the latter two cases, the loops act as geometric constraints on the fiber network to help stiffen it via increased angular interactions. We find that the single semiflexible polymer loop model agrees well with the experimental cell compression stiffening finding until approximately 35% compressive strain after which bulk fiber network effects may contribute. We also find for the fiber network with area-conserving loops model that the stress-strain curves are sensitive to the packing fraction and size distribution of the area-conserving loops, thereby creating a mechanical fingerprint across different cell types. Finally, we make comparisons between this model and experiments on fibrin networks interlaced with beads as well as discuss implications for single cell compression stiffening at the tissue scale.

Compression stiffening in biological tissues: On the possibility of classic elasticity origins

Compression stiffening, or an increase in shear modulus with increasing compressive strain, has been observed in recent rheometry experiments on brain, liver, and fat tissues. Here we extend the known types of biomaterials exhibiting this phenomenon to include agarose gel and fruit flesh. The data reveal a linear relationship between shear storage modulus and uniaxial prestress, even up to 40% strain in some cases. We focus on this less-familiar linear relationship to show that two different results from classic elasticity theory can account for the phenomenon of linear compression stiffening. One result is due to Barron and Klein, extended here to the relevant geometry and prestresses; the other is due to Birch. For incompressible materials, there are no adjustable parameters in either theory. Which one applies to a given situation is a matter of reference state, suggesting that the reference state is determined by the tendency of the material to develop, or not develop, axial stress (in excess of the applied prestress) when subjected to torsion at constant axial strain. Our experiments and analysis also strengthen the notion that seemingly distinct animal and plant tissues can have mechanically similar behavior at the quantitative level under certain conditions.

Vimentin enhances cell elastic behavior and protects against compressive stress

Vimentin intermediate filament expression is a hallmark of epithelial-to-mesenchymal transitions, and vimentin is involved in the maintenance of cell mechanical properties, cell motility, adhesion, and other signaling pathways. A common feature of vimentin-expressing cells is their routine exposure to mechanical stress. Intermediate filaments are unique among cytoskeletal polymers in resisting large deformations in vitro, yet vimentin's mechanical role in the cell is not clearly understood. We use atomic force microscopy to compare the viscoelastic properties of normal and vimentin-null (vim(-/-)) mouse embryo fibroblasts (mEFs) on substrates of different stiffnesses, spread to different areas, and subjected to different compression patterns. In minimally perturbed mEF, vimentin contributes little to the elastic modulus at any indentation depth in cells spread to average areas. On a hard substrate however, the elastic moduli of maximally spread mEFs are greater than those of vim(-/-)mEF. Comparison of the plastic deformation resulting from controlled compression of the cell cortex shows that vimentin's enhancement of elastic behavior increases with substrate stiffness. The elastic moduli of normal mEFs are more stable over time than those of vim(-/-)mEFs when cells are subject to ongoing oscillatory compression, particularly on a soft substrate. In contrast, increasing compressive strain over time shows a greater role for vimentin on a hard substrate. Under both conditions, vim(-/-)mEFs exhibit more variable responses, indicating a loss of regulation. Finally, normal mEFs are more contractile in three-dimensional collagen gels when seeded at low density, when cell-matrix contacts dominate, whereas contractility of vim(-/-)mEF is greater at higher densities when cell-cell contacts are abundant. Addition of fibronectin to gel constructs equalizes the contractility of the two cell types. These results show that the Young's moduli of normal and vim(-/-)mEFs are substrate stiffness dependent even when the spread area is similar, and that vimentin protects against compressive stress and preserves mechanical integrity by enhancing cell elastic behavior.

Application of multiple levels of fluid shear stress to endothelial cells plated on polyacrylamide gels

Measurements of endothelial cell response to fluid shear stress have previously been performed on unphysiologically rigid substrates. We describe the design and implementation of a microfluidic device that applies discrete levels of shear stress to cells plated on hydrogel-based substrates of physiologically-relevant stiffness. The setup allows for measurements of cell morphology and inflammatory response to the combined stimuli, and identifies mechanisms by which vascular stiffening leads to pathological responses to blood flow. We found that the magnitude of shear stress required to affect endothelial cell morphology and inflammatory response depended on substrate stiffness. Endothelial cells on 100 Pa substrates demonstrate a greater increase in cell area and cortical stiffness and decrease in NF-κB nuclear translocation in response to TNF-α treatment compared to controls than cells plated on 10 kPa substrates. The response of endothelial cells on soft substrates to shear stress depends on the presence of hyaluronan (HA). These results emphasize the importance of substrate stiffness on endothelial function, and elucidate a means by which vascular stiffening in aging and disease can impact the endothelium.

Collagen remodeling by phagocytosis is determined by collagen substrate topology and calcium-dependent interactions of gelsolin with nonmuscle myosin IIA in cell adhesions

Cell adhesion to collagen presented on beads activates Ca²⁺ entry and promotes the formation of phagosomes enriched with NMMIIA and gelsolin. The Ca²⁺-dependent interaction of gelsolin and NMMIIA in turn enables actin remodeling and enhances collagen degradation by phagocytosis.

We examine how collagen substrate topography, free intracellular calcium ion concentration ([Ca²⁺]_i, and the association of gelsolin with nonmuscle myosin IIA (NMMIIA) at collagen adhesions are regulated to enable collagen phagocytosis. Fibroblasts plated on planar, collagen-coated substrates show minimal increase of [Ca²⁺]_i, minimal colocalization of gelsolin and NMMIIA in focal adhesions, and minimal intracellular collagen degradation. In fibroblasts plated on collagen-coated latex beads there are large increases of [Ca²⁺]_i, time- and Ca²⁺-dependent enrichment of NMMIIA and gelsolin at collagen adhesions, and abundant intracellular collagen degradation. NMMIIA knockdown retards gelsolin recruitment to adhesions and blocks collagen phagocytosis. Gelsolin exhibits tight, Ca²⁺-dependent binding to full-length NMMIIA. Gelsolin domains G4–G6 selectively require Ca²⁺ to interact with NMMIIA, which is restricted to residues 1339–1899 of NMMIIA. We conclude that cell adhesion to collagen presented on beads activates Ca²⁺ entry and promotes the formation of phagosomes enriched with NMMIIA and gelsolin. The Ca²⁺ -dependent interaction of gelsolin and NMMIIA in turn enables actin remodeling and enhances collagen degradation by phagocytosis.

Potential of ceragenin CSA-13 and its mixture with pluronic F-127 as treatment of topical bacterial infections

AIMS

Ceragenin CSA-13 is a synthetic mimic of cationic antibacterial peptides, with facial amphiphilic morphology reproduced using a cholic acid scaffold. Previous data have shown that this molecule displays broad-spectrum antibacterial activity, which decreases in the presence of blood plasma. However, at higher concentrations, CSA-13 can cause lysis of erythrocytes. This study was designed to assess in vitro antibacterial and haemolytic activity of CSA-13 in the presence of pluronic F-127.

METHODS AND RESULTS

CSA-13 bactericidal activity against clinical strains of bacteria associated with topical infections and in an experimental setting relevant to their pathophysiological environment, such as various epithelial tissue fluids and the airway sputum of patients suffering from cystic fibrosis (CF), was evaluated using minimum inhibitory and minimum bactericidal concentration (MIC/MBC) measurements and bacterial killing assays. We found that in the presence of pluronic F-127, CSA-13 antibacterial activity was only slightly decreased, but CSA-13 haemolytic activity was significantly inhibited. CSA-13 exhibits bacterial killing activity against clinical isolates of Staphylococcus aureus, including methicillin-resistant strains, Pseudomonas aeruginosa present in CF sputa, and biofilms formed by different Gram (+) and Gram (-) bacteria. CSA-13 bactericidal action is partially compromised in the presence of plasma, but is maintained in ascites, cerebrospinal fluid, saliva, and bronchoalveolar lavage fluid. The synergistic action of CSA-13, determined by the use of a standard checkerboard assay, reveals an increase in CSA-13 antibacterial activity in the presence of host defence molecules such as the cathelicidin LL-37 peptide, lysozyme, lactoferrin and secretory phospholipase A (sPLA).

CONCLUSION

These results suggest that CSA-13 may be useful to prevent and treat topical infection.

SIGNIFICANCE AND IMPACT OF THE STUDY

Combined application of CSA-13 with pluronic F-127 may be beneficial by reducing CSA-13 toxicity.

A simple indentation device for measuring micrometer-scale tissue stiffness

Mechanical properties of cells and extracellular matrices are critical determinants of function in contexts including oncogenic transformation, neuronal synapse formation, hepatic fibrosis and stem cell differentiation. The size and heterogeneity of biological specimens and the importance of measuring their mechanical properties under conditions that resemble their environments in vivo present a challenge for quantitative measurement. Centimeter-scale tissue samples can be measured by commercial instruments, whereas properties at the subcellular (nm) scale are accessible by atomic force microscopy, optical trapping, or magnetic bead microrheometry; however many tissues are heterogeneous on a length scale between micrometers and millimeters which is not accessible to most current instrumentation. The device described here combines two commercially available technologies, a micronewton resolution force probe and a micromanipulator for probing soft biological samples at sub-millimeter spatial resolution. Several applications of the device are described. These include the first measurement of the stiffness of an intact, isolated mouse glomerulus, quantification of the inner wall stiffness of healthy and diseased mouse aortas, and evaluation of the lateral heterogeneity in the stiffness of mouse mammary glands and rat livers with correlation of this heterogeneity with malignant or fibrotic pathology as evaluated by histology.

Microtubule-independent regulation of neurofilament interactions in vitro by neurofilament-bound ATPase activities

Neurofilaments (NFs), the major neuronal intermediate filaments, form networks in vitro that mimic the axonal NF bundles. This report presents evidence for previously unknown regulation of the interactions between NFs by NF-associated ATPases. Two opposite effects on NF gelation in vitro occur at low and high ATP concentration. These findings support the hypothesis that NF bundles in situ are dynamic structures, and raise the possibility that ATP-hydrolyzing mechanoenzymes regulate their organization.

A series of cationic sterol lipids with gene transfer and bactericidal activity

A family of cationic lipids was synthesized via direct amide coupling of spermine to the C-24 position of cholic acid analogs. Four monosubstituted spermines and a bis-substituted spermine were evaluated as plasmid transfection reagents, as bacteriostatic agents, and as bactericidal agents. The incorporation of a double bond in the sterol moiety enhanced transfection efficiency significantly and produced two compounds with little cytotoxicity and transfection potency comparable to Lipofectamine2000. Inclusion of the double bond had no effect on the general trend of increasing bactericidal activity with increasing sterol hydrophobicity. Co-formulation of the most hydrophilic of the compounds with its bis-substituted analogue led to enhancement in transfection activity. The bis-substituted compound, when tested alone, emerged as the most bacteriostatic compound in the family with minimum inhibitory concentrations (MIC) of 4 microM against Bacillus subtilis and 16 microM against Escherichia coli and therapeutic indexes (minimum hemolytic concentration/minimum inhibitory concentration) of 61 and 15, respectively. Cationic lipids can be optimized for both gene delivery and antibacterial applications by similar modifications.

MAP2-mediated in vitro interactions of brain microtubules and their modulation by cAMP

Microtubule-associated proteins (MAPs) are involved in microtubule (MT) bundling and in crossbridges between MTs and other organelles. Previous studies have assigned the MT bundling function of MAPs to their MT-binding domain and its modulation by the projection domain. In the present work, we analyse the viscoelastic properties of MT suspensions in the presence or the absence of cAMP. The experimental data reveal the occurrence of interactions between MT polymers involving MAP2 and modulated by cAMP. Two distinct mechanisms of action of cAMP are identified, which involve on one hand the phosphorylation of MT proteins by the cAMP-dependent protein kinase A (PKA) bound to the end of the N-terminal projection of MAP2, and on the other hand the binding of cAMP to the RII subunit of the PKA affecting interactions between MTs in a phosphorylation-independent manner. These findings imply a role for the complex of PKA with the projection domain of MAP2 in MT-MT interactions and suggest that cAMP may influence directly the density and bundling of MT arrays in dendrites of neurons.

Electrostatic contribution to the surface pressure of charged monolayers containing polyphosphoinositides

Structural and functional studies of lateral heterogeneity in biological membranes have underlined the importance of membrane organization in biological function. Most inquiries have focused on steric determinants of membrane organization, such as headgroup size and acyl-chain saturation. This manuscript reports a combination of theory and experiment that shows significant electrostatic contributions to surface pressures in monolayers of phospholipids where the charge spacing is smaller than the Bjerrum length. For molecules with steric cross sections typical of phospholipids in the cell membrane (approximately 50 A(2)), only polyphosphoinositides achieve this threshold. The most abundant such lipid is phosphatidylinositol bisphosphate, which has between three and four charged groups at physiological conditions. Theory and experiment show that surface pressure increases linearly with phosphatidylinositol bisphosphate net charge and reveal crossing of high and low ionic strength pressure-area isotherms, due to opposing effects of ionic strength in compressed and expanded monolayers. Theory and experiment show that electrostatic effects are negligible for monolayers of univalent lipids, emphasizing the unique importance of electrostatic effects for lateral organization of polyphosphoinositides. Quantitative differences between theory and experiment suggest that attractive interactions between polyphosphoinositides, possibly mediated by hydrogen bonding, can lessen the effect of electrostatic repulsions.

Phosphatidylinositol-4,5 bisphosphate produced by PIP5KIgamma regulates gelsolin, actin assembly, and adhesion strength of N-cadherin junctions

Phosphoinositides regulate several actin-binding proteins but their role at intercellular adhesions has not been defined. We found that phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) was generated at sites of N-cadherin-mediated intercellular adhesion and was a critical regulator of intercellular adhesion strength. Immunostaining for PI(4,5)P2 or transfection with GFP-PH-PLCdelta showed that PI(4,5)P2 was enriched at sites of N-cadherin adhesions and this enrichment required activated Rac1. Isoform-specific immunostaining for type I phosphatidylinositol 4-phosphate 5 kinase (PIP5KI) showed that PIP5KIgamma was spatially associated with N-cadherin-Fc beads. Association of PIP5KIgamma with N-cadherin adhesions was in part dependent on the activation of RhoA. Transfection with catalytically inactive PIP5KIgamma blocked the enrichment of PI(4,5)P2 around beads. Catalytically inactive PIP5KIgamma or a cell-permeant peptide that mimics and competes for the PI(4,5)P2-binding region of the actin-binding protein gelsolin inhibited incorporation of actin monomers in response to N-cadherin ligation and reduced intercellular adhesion strength by more than twofold. Gelsolin null fibroblasts transfected with a gelsolin severing mutant containing an intact PI(4,5)P2 binding region, demonstrated intercellular adhesion strength similar to wild-type transfected controls. We conclude that PIP5KIgamma-mediated generation of PI(4,5)P2 at sites of N-cadherin contacts regulates intercellular adhesion strength, an effect due in part to PI(4,5)P2-mediated regulation of gelsolin.

Mechanical and structural properties of in vitro neurofilament hydrogels

Neurofilaments belong to the class of cytoskeletal intermediate filaments and are the predominant structural elements in axons. They are composed of a semiflexible backbone and highly charged anionic sidearms protruding from the surface of the filaments. Here, the rheology of in-vitro networks of neurofilaments purified from pig spinal cord was determined. The mechanical properties of these networks are qualitatively similar to other hydrogels of semiflexible polymers. The low-deformation storage modulus G'(omega) showed a concentration (c) dependence of G' approximately c (1.3) that is consistent with a model for semiflexible networks, but was also observed for polyelectrolyte brushes. A terminal relaxation was not observed in the frequency range investigated (0.007-5 Hz), supporting the notion that sidearms act as cross-links hindering slip between filaments on a time scale of many minutes. The mesh size distribution of the network was measured by analysis of Brownian motion of embedded beads. The concentration dependence of the mesh size follows the same power law behaviour as found for F-actin networks, but shows a significantly wider distribution attributable to the smaller persistence length of neurofilaments. The attractive interaction between filaments is increased by addition of Al(3+) ions resulting in a reduction of the linear response regime from strains bigger than 80% to less than 30%.

HIV infection changes glomerular podocyte cytoskeletal composition and results in distinct cellular mechanical properties

In addition to forming the selective filtration barrier for the renal glomerulus, podocytes maintain glomerular capillary architecture by opposing distending hemodynamic forces. To understand the relationship of cytoskeletal properties and the mechanical characteristics of podocytes, we studied filamin expression and distribution and measured cell membrane deformability in conditionally immortalized wild-type (WT) mouse podocytes, and in podocytes derived from a mouse model of HIV-associated nephropathy (HIVAN). In the WT cells, filamin and F-actin were localized at the periphery and in prominent stress fibers. In the HIVAN cells, filamin expression was reduced, and stress fibers were sparse. In a microaspiration assay, HIVAN cells ruptured under minimal negative pressure. Atomic force microscopy demonstrated that the WT cells had a stiffness of 17 kPa, whereas the value for the HIVAN cells was 4 kPa. These results demonstrate that the mechanical properties of WT and HIVAN podocytes are markedly different in a manner that is consistent with differences in the composition and arrangement of their cytoskeletons. The mechanical properties of the WT podocytes suggest that these cells can better maintain capillary integrity than the HIVAN podocytes and implicate pathological assembly of the cytoskeleton as a mechanism of HIVAN.

Release of the antimicrobial peptide LL-37 from DNA/F-actin bundles in cystic fibrosis sputum

Cationic antibacterial peptides (ABPs) are secreted in the airways and function in the first line of defence against infectious agents. They attack multiple molecular targets to cooperatively penetrate and disrupt microbial surfaces and membrane barriers. Antibacterial properties of ABPs, including cathelicidin LL-37, are reduced in cystic fibrosis (CF) airways as a result of direct interaction with DNA and filamentous (F)-actin. Microscopic evaluation of a mixed solution of DNA and F-actin, after the addition of rhodamine-B-labelled LL-37 peptide, revealed the presence of a bundle structure similar to that present in CF sputum. Analysis of CF sputum after centrifugation showed that LL-37 was mostly bound to components of the pellet fraction containing DNA, F-actin and cell remnants. Factors that dissolve DNA/actin bundles and fluidise CF sputum, such as Dornase alfa (recombinant human DNase I), gelsolin, polyaspartate or their combinations, increased the amount of LL-37 peptide detected in the supernatant of CF sputum. The presence of the bacterial endotoxin lipopolysaccharide (LPS) in CF sputum and the ability of LPS to inhibit the antibacterial activity of LL-37 suggests that inactivation of LL-37 function in CF sputum partially results from its interaction with LPS. LL-37-LPS interaction was prevented by an LPS-binding protein (LBP)-derived peptide known for its ability to neutralise LPS, whereas LBPW91A, a mutant peptide that lacks ability to bind LPS, had no effect. A combination of factors that dissolve DNA/filamentous-actin aggregates together with lipopolysaccharide-binding agents may represent a potential treatment for the chronic infections that occur in cystic fibrosis airways.

Biophysical properties of lipids and dynamic membranes

The lipid bilayer is a 3D assembly with a rich variety of physical features that modulate cell signaling and protein function. Lateral and transverse forces within the membrane are significant and change rapidly as the membrane is bent or stretched and as new constituents are added, removed or chemically modified. Recent studies have revealed how differences in structure between the two leaflets of the bilayer and between different areas of the bilayer can interact together with membrane deformation to alter the activities of transmembrane channels and peripheral membrane binding proteins. Here, we highlight some recent reports that the physical properties of the membrane can help control the function of transmembrane proteins and the motor-dependent elongation of internal organelles, such as the endoplasmic reticulum.

Counterion-mediated attraction and kinks on loops of semiflexible polyelectrolyte bundles

The formation of kinks in a loop of bundled polyelectrolyte filaments is analyzed in terms of the thermal fluctuations of charge density due to polyvalent counterions adsorbed on the polyelectrolyte filaments. It is found that the counterion-mediated attraction energy of filaments depends on their bending. By consideration of curvature elasticity energy and counterion-mediated attraction between polyelectrolyte filaments, the characteristic width of the kink and the number of kinks per loop is found to be in reasonable agreement with existing experimental data for rings of bundled actin filaments.

Separate functions of gelsolin mediate sequential steps of collagen phagocytosis

Collagen phagocytosis is a critical mediator of extracellular matrix remodeling. Whereas the binding step of collagen phagocytosis is facilitated by Ca2+-dependent, gelsolin-mediated severing of actin filaments, the regulation of the collagen internalization step is not defined. We determined here whether phosphatidylinositol-4,5-bisphosphate [PI(4,5)P2] regulation of gelsolin is required for collagen internalization. In gelsolin null fibroblasts transfected with gelsolin severing mutants, actin severing and collagen binding were strongly impaired but internalization and actin monomer addition at collagen bead sites were much less affected. PI(4,5)P2 accumulated around collagen during internalization and was associated with gelsolin. Cell-permeable peptides mimicking the PI(4,5)P2 binding site of gelsolin blocked actin monomer addition, the association of gelsolin with actin at phagosomes, and collagen internalization but did not affect collagen binding. Collagen beads induced recruitment of type 1 gamma phosphatidylinositol phosphate kinase (PIPK1gamma661) to internalization sites. Dominant negative constructs and RNA interference demonstrated a requirement for catalytically active PIPK1gamma661 for collagen internalization. We conclude that separate functions of gelsolin mediate sequential stages of collagen phagocytosis: Ca2+-dependent actin severing facilitates collagen binding, whereas PI(4,5)P2-dependent regulation of gelsolin promotes the actin assembly required for internalization of collagen fibrils.

Viscoelasticity of single wall carbon nanotube suspensions

We investigate the viscoelastic properties of an associating rigid rod network: aqueous suspensions of surfactant stabilized single wall carbon nanotubes (SWNTs). The SWNT suspensions exhibit a rigidity percolation transition with an onset of solidlike elasticity at a volume fraction of 0.0026; the percolation exponent is 2.3+/-0.1. At large strain, the solidlike samples show volume fraction dependent yielding. We develop a simple model to understand these rheological responses and show that the shear dependent stresses can be scaled onto a single master curve to obtain an internanotube interaction energy per bond approximately 40k(B)T. Our experimental observations suggest SWNTs in suspension form interconnected networks with bonds that freely rotate and resist stretching. Suspension elasticity originates from bonds between SWNTs rather than from the stiffness or stretching of individual SWNTs.

Elongation and fluctuations of semiflexible polymers in a nematic solvent

We directly visualize single polymers with persistence lengths l(p), ranging from 0.05 to 16 microm, dissolved in the nematic phase of rodlike fd virus. Polymers with a sufficiently large persistence length undergo a coil-rod transition at the isotropic-nematic transition of the background solvent. We quantitatively analyze the transverse fluctuations of the semiflexible polymers and show that at long wavelengths they are driven by the fluctuating nematic background. We extract the Odijk deflection length and the elastic constant of the background nematic phase from the data.

Mechanisms of mitochondria-neurofilament interactions

Mitochondria are localized to regions of the cell where ATP consumption is high and are dispersed according to changes in local energy needs. In addition to motion directed by molecular motors, mitochondrial distribution in neuronal cells appears to depend on the docking of mitochondria to microtubules and neurofilaments. We examined interactions between mitochondria and neurofilaments using fluorescence microscopy, dynamic light scattering, atomic force microscopy, and sedimentation assays. Mitochondria-neurofilament interactions depend on mitochondrial membrane potential, as revealed by staining with a membrane potential sensitive dye (JC-1) in the presence of substrates/ADP or uncouplers (valinomycin/carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone) and are affected by the phosphorylation status of neurofilaments and neurofilament sidearms. Antibodies against the neurofilament heavy subunit disrupt binding between mitochondria and neurofilaments, and isolated neurofilament sidearms alone interact with mitochondria, suggesting that they mediate the interactions between the two structures. These data suggest that specific and regulated mitochondrial-neurofilament interactions occur in situ and may contribute to the dynamic distribution of these organelles within the cytoplasm of neurons.

Flavonoid inhibition of platelet procoagulant activity and phosphoinositide synthesis

Dietary flavonoids are known for their antiplatelet activity resulting in cardiovascular protection. Phosphatidylinositol 4,5-bisphosphate (PIP2) was previously reported to play a direct role in phosphatidylserine (PS) exposure, as a Ca2+ target. Thrombin formation and platelet procoagulant activity are dependent on PS exposure. As flavonoids can inhibit phosphoinositide (PPI) kinases, we examined whether changes in PPI metabolism in flavonoid-treated platelets could be involved in their antiplatelet effects. Treatment with the flavonoids quercetin or catechin reduced PS exposure, thrombin formation, PIP2 level and resynthesis after platelet activation with collagen, thrombin or calcium ionophore. Flavonoids also prevented [Ca2+]i increase induced by collagen, but not by the ionophore. The ability of flavonoids to decrease PS exposure induced by ionophore treatment could result from the diminution of PIP2 levels, whereas PS exposure induced by collagen could also be diminished by flavonoids' effects on calcium signaling dependent on PIP2 hydrolysis. These data favor a role for PIP2 in the antiplatelet effects of flavonoids.

Involvement of phosphatidylinositol 4,5-bisphosphate in phosphatidylserine exposure in platelets: use of a permeant phosphoinositide-binding peptide

During platelet activation, phosphatidylserine (PS) exposure on the extracellular face of the plasma membrane is associated with increased procoagulant activity. PS externalization is generally attributed to an increase in intracellular Ca(2+). Various phospholipid transporters, such as specific scramblases or proteins from the family of multidrug resistance proteins, and cofactors such as phosphatidylinositol 4,5-bisphosphate (PIP2) have been proposed to participate in this process. In this study, we used a membrane-permeant polycationic peptide (RhB-QRLFQVKGRR), derived from the PIP2-binding site of gelsolin (GS 160-169) and linked to rhodamine B, to investigate the role of PIP2 in PS externalization in whole platelets. The peptide penetrated rapidly into the platelets, specifically bound to PIP2, and induced PS exposure to a similar extent as thrombin or collagen, but independently of changes in intracellular Ca(2+) or phosphoinositide 3-kinase activity. A pretreatment of platelets with quercetin, an inhibitor of phosphoinositide metabolism, drastically decreased PS exposure induced by agonists or peptide. In large unilamellar vesicles (LUVs), the presence of PIP2 was strictly required for the induction of scrambling of NBD-labeled phospholipids (PC and PS) by the peptide. In inside-out vesicles from erythrocytes (IOVs), the peptide also induced redistribution of PC and PS. Our data suggest that, in intact platelets, PIP2 acts as a target of polycationic effectors, including Ca(2+), to promote PS exposure. The use of a membrane-permeant and fluorescent peptide which binds to PIP2 is a promising tool to investigate the role of PIP2 in various cellular processes.

Counterion-induced actin ring formation

Actin filaments form rings and loops when > 20 mM divalent cations are added to very dilute solutions of phalloidin-stabilized filamentous actin (F-actin). Some rings consist of very long single actin filaments partially overlapping at their ends, and others are formed by small numbers of filaments associated laterally. In some cases, undulations of the rings are observed with amplitudes and dynamics similar to those of the thermal motions of single actin filaments. Lariat-shaped aggregates also co-exist with rings and rodlike bundles. These polyvalent cation-induced actin rings are analogous to the toroids of DNA formed by addition of polyvalent cations, but the much larger diameter of actin rings reflects the greater bending stiffness of F-actin. Actin rings can also be formed by addition of streptavidin to crosslink sparsely biotinylated F-actin at very low concentrations. The energy of bending in a ring, calculated from the persistence length of F-actin and the ring diameter, provides an estimate for the adhesion energy mediated by the multivalent counterions, or due to the streptavidin-biotin bonds, required to keep the ring closed.

Cell permeant polyphosphoinositide-binding peptides that block cell motility and actin assembly

Polyphosphoinositides (PPIs) affect the localization and activities of many cellular constituents, including actin-modulating proteins. Several classes of polypeptide sequences, including pleckstrin homology domains, FYVE domains, and short linear sequences containing predominantly hydrophobic and cationic residues account for phosphoinositide binding by most such proteins. We report that a ten-residue peptide derived from the phosphatidylinositol 4,5-bisphosphate (PIP(2)) binding region in segment 2 of gelsolin, when coupled to rhodamine B has potent PIP(2) binding activity in vitro; crosses the cell membrane of fibroblasts, platelets, melanoma cells, and neutrophils by a process not involving endocytosis; and blocks cell motility. This peptide derivative transiently disassembles actin filament structures in GFP-actin-expressing NIH3T3 fibroblasts and prevents thrombin- or chemotactic peptide-stimulated actin assembly in platelets and neutrophils, respectively, but does not block the initial [Ca(2+)] increase caused by these agonists. The blockage of actin assembly and motility is transient, and cells recover motility within an hour after their immobilization by 5-20 microm peptide. This class of reagents confirms the critical relation between inositol lipids and cytoskeletal structure and may be useful to probe the location and function of polyphosphoinositides in vivo.

The distribution of polyphosphoinositides in lipid films

Fluorescence microscopy of Langmuir films is used to determine the effect of polyphosphoinositides (PPIs) on the structure of phosphatidylcholine-containing monolayers. Dramatic alterations in the texture of these films occur as the fraction of PPI in the film is altered. These changes depend on the ionic strength of the underlying subphase and can be accounted for by considering the electrostatic interactions among PPIs. Surface adsorption of a fluorescent peptide derivative based on the PPI binding site of the protein gelsolin co-localizes with PPI-rich domains. Localization of polyphosphoinositides in domains within the inner leaflet of the plasma membrane is proposed to be a key element in some aspects of intracellular signaling, and these data have implications for explaining the cause of restructuring of such domains.

Changes in the biomechanical properties of neutrophils and endothelial cells during adhesion

This study examined changes in the biomechanical properties of cultured pulmonary microvascular endothelial cells (ECs) and neutrophils induced by adhesion of neutrophils to these ECs. The biomechanical properties of cells were evaluated using magnetic twisting cytometry, which measures the angular rotation of ferromagnetic beads bound to cells through antibody ligation on application of a specified magnetic torque. Adhesion of neutrophils to 24-hour tumor necrosis factor-alpha (TNF-alpha)-treated ECs, but not to untreated ECs, induced an increase in EC stiffness within 2 minutes, which was accompanied by an increase and a reorganization of F-actin in ECs. A cell-permeant, phosphoinositide-binding peptide attenuated the EC stiffening response, suggesting that intracellular phosphoinositides are required. The stiffening response was not inhibited by ML-7, a myosin light-chain kinase inhibitor, or BAPTA, an intracellular Ca2+ chelator. Moreover, the phosphorylation pattern of the regulatory myosin light chains was unaltered within 15 minutes of neutrophil adherence. These data suggested that the EC stiffening response appeared not to be mediated by myosin light-chain-dependent mechanisms. Concomitantly, neutrophil adhesion to 24-hour TNF-alpha-treated ECs also induced changes in the biomechanical properties of neutrophils compared to neutrophils bound to untreated ECs. Taken together, these results demonstrated that neutrophil adhesion to TNF-alpha-treated ECs induces changes in the biomechanical properties of both cell types through actin cytoskeletal remodeling. These changes may modulate neutrophil transmigration across the endothelium during inflammation.

Purification of salmon clotting factors and their use as tissue sealants

Fibrin sealant prepared from the blood of farmed Atlantic salmon (Salmo salar) represents a potential source of well-controlled natural material with utility in a variety of clinical settings. A potential advantage of this material is a lower probability of viral or bacterial infection that has limited general approval of fibrin glues made from human or bovine proteins. This report describes the purification of fibrinogen from salmon blood, the use of fibrin glues derived from this material to promote wound healing in rats, and the antigenic response to this material. While the low ambient temperature of these cold water fish significantly lessens the probability of infectious transmission to humans, fibrinogen and factor XIII derived from S. salar are activated by human thrombin at 25 degrees C and 37 degrees C to form clots equivalent to those formed by human fibrin. We compare the reactivity of salmon and human fibrinogen with human and bovine thrombin and the structure and viscoelastic properties of the resulting fibrin gels over a range of pH and salt concentrations. The efficacy of salmon fibrin glues in a wound healing assay and the low antigenic response to salmon fibrinogen suggest that this material may substitute for proteins derived from mammalian sources with lower probability of infections.

Bidirectional translocation of neurofilaments along microtubules mediated in part by dynein/dynactin

Neuronal cytoskeletal elements such as neurofilaments, F-actin, and microtubules are actively translocated by an as yet unidentified mechanism. This report describes a novel interaction between neurofilaments and microtubule motor proteins that mediates the translocation of neurofilaments along microtubules in vitro. Native neurofilaments purified from spinal cord are transported along microtubules at rates of 100-1000 nm/s to both plus and minus ends. This motion requires ATP and is partially inhibited by vanadate, consistent with the activity of neurofilament-bound molecular motors. Motility is in part mediated by the dynein/dynactin motor complex and several kinesin-like proteins. This reconstituted motile system suggests how slow net movement of cytoskeletal polymers may be achieved by alternating activities of fast microtubule motors.

Signaling pathways and cell mechanics involved in wound closure by epithelial cell sheets

BACKGROUND

Sheets of cells move together as a unit during wound healing and embryonic tissue movements, such as those occurring during gastrulation and neurulation. We have used epithelial wound closure as a model system for such movements and examined the mechanisms of closure and the importance of the Rho family of Ras-related small GTPases in this process.

RESULTS

Wounds induced in Madin-Darby canine kidney (MDCK) epithelial cell monolayers close by Rac- and phosphoinositide-dependent cell crawling, with formation of lamellipodia at the wound margin, and not by contraction of a perimarginal actomyosin purse-string. Although Rho-dependent actin bundles usually form at the margin, neither Rho activity nor formation of these structures is required for wound closure to occur at a normal rate. Cdc42 activity is also not required for closure. Inhibition of Rho or Cdc42 results, however, in statistically significant decreases in the regularity of wound closure, as determined by the ratio of wound margin perimeter over the remaining denuded area at different times. The Rac-dependent force generation for closure is distributed over several rows of cells from the wound margin, as inhibition of motility in the first row of cells alone does not inhibit closure and can be compensated for by generation of motile force in cells behind the margin. Furthermore, we observed high levels of Rac-dependent actin assembly in the first few rows of cells from the wound margin.

CONCLUSIONS

Wounds in MDCK cell sheets do not close by purse-string contraction but by a crawling behavior involving Rac, phosphoinositides and active movement of multiple rows of cells. This finding suggests a new distributed mode of signaling and movement that, nevertheless, resembles individual cell motility. Although Rho and Cdc42 activities are not required for closure, they have a role in determining the regularity of closure.

Signaling pathways and cell mechanics involved in wound closure by epithelial cell sheets

BACKGROUND

Sheets of cells move together as a unit during wound healing and embryonic tissue movements, such as those occurring during gastrulation and neurulation. We have used epithelial wound closure as a model system for such movements and examined the mechanisms of closure and the importance of the Rho family of Ras-related small GTPases in this process.

RESULTS

Wounds induced in Madin-Darby canine kidney (MDCK) epithelial cell monolayers close by Rac- and phosphoinositide-dependent cell crawling, with formation of lamellipodia at the wound margin, and not by contraction of a perimarginal actomyosin purse-string. Although Rho-dependent actin bundles usually form at the margin, neither Rho activity nor formation of these structures is required for wound closure to occur at a normal rate. Cdc42 activity is also not required for closure. Inhibition of Rho or Cdc42 results, however, in statistically significant decreases in the regularity of wound closure, as determined by the ratio of wound margin perimeter over the remaining denuded area at different times. The Rac-dependent force generation for closure is distributed over several rows of cells from the wound margin, as inhibition of motility in the first row of cells alone does not inhibit closure and can be compensated for by generation of motile force in cells behind the margin. Furthermore, we observed high levels of Rac-dependent actin assembly in the first few rows of cells from the wound margin.

CONCLUSIONS

Wounds in MDCK cell sheets do not close by purse-string contraction but by a crawling behavior involving Rac, phosphoinositides and active movement of multiple rows of cells. This finding suggests a new distributed mode of signaling and movement that, nevertheless, resembles individual cell motility. Although Rho and Cdc42 activities are not required for closure, they have a role in determining the regularity of closure.

Hierarchical self-assembly of F-actin and cationic lipid complexes: stacked three-layer tubule networks

We describe a distinct type of spontaneous hierarchical self-assembly of cytoskeletal filamentous actin (F-actin), a highly charged polyelectrolyte, and cationic lipid membranes. On the mesoscopic length scale, confocal microscopy reveals ribbonlike tubule structures that connect to form a network of tubules on the macroscopic scale (more than 100 micrometers). Within the tubules, on the 0.5- to 50-nanometer length scale, x-ray diffraction reveals an unusual structure consisting of osmotically swollen stacks of composite membranes with no direct analog in simple amphiphilic systems. The composite membrane is composed of three layers, a lipid bilayer sandwiched between two layers of actin, and is reminiscent of multilayered bacterial cell walls that exist far from equilibrium. Electron microscopy reveals that the actin layer consists of laterally locked F-actin filaments forming an anisotropic two-dimensional tethered crystal that appears to be the origin of the tubule formation.

Domain analysis of cortexillin I: actin-bundling, PIP(2)-binding and the rescue of cytokinesis

Cortexillins are actin-bundling proteins that form a parallel two-stranded coiled-coil rod. Actin-binding domains of the alpha-actinin/spectrin type are located N-terminal to the rod and unique sequence elements are found in the C-terminal region. Domain analysis in vitro revealed that the N-terminal domains are not responsible for the strong actin-filament bundling activity of cortexillin I. The strongest activity resides in the C-terminal region. Phosphatidylinositol 4,5-bisphosphate (PIP(2)) suppresses this bundling activity by binding to a C-terminal nonapeptide sequence. These data define a new PIP(2)-regulated actin-bundling site. In vivo the PIP(2)-binding motif enhances localization of a C-terminal cortexillin I fragment to the cell cortex and improves the rescue of cytokinesis. This motif is not required, however, for translocation to the cleavage furrow. A model is presented proposing that cortexillin translocation is based on a mitotic cycle of polar actin polymerization and midzone depolymerization.

Controlling cytoskeleton structure by phosphoinositide-protein interactions: phosphoinositide binding protein domains and effects of lipid packing

Cell movement and resistance to mechanical forces are largely governed by the cytoskeleton, a three-dimensional network of protein filaments that form viscoelastic networks within the cytoplasm. The cytoskeleton underlying the plasma membrane of most cells is rich in actin filaments whose assembly and disassembly are regulated by actin binding proteins that are stimulated or inhibited by signals received and transmitted at the membrane/cytoplasm interface. Inositol phospholipids, or phosphoinositides, are potent regulators of many actin binding proteins, and changes in the phosphorylation of specific phosphoinositide species or in their spatial localization are associated with cytoskeletal remodeling in vitro. This review will focus on recent studies directed at defining the structural features of phosphoinositide binding sites in actin binding proteins and on the influence of the physical state of phosphoinositides on their ability to interact with their target proteins.

A role for gelsolin in stress fiber-dependent cell contraction

Gelsolin is an abundant actin binding protein that mediates the rapid remodeling of cortical actin filaments through severing, capping, and nucleating activities. Most of the attention on the intracellular function of gelsolin has focused on the remodeling of the cortical actin meshwork but the localization of gelsolin to other regions of the cell suggests that it may have other important functions elsewhere. In cultured fibroblasts, gelsolin is heavily enriched in stress fibers, where its function in these contractile organelles is unknown. To study gelsolin function during stress fiber formation and cell contraction, we first assessed gelsolin levels in stress fiber preparations from lysophosphatidic acid (LPA)-treated human fibroblasts. LPA induced a large, time-dependent, calcium-independent increase of actin, gelsolin, alpha-actinin, and tropomyosin in stress fiber preparations. A microinjected gelsolin antibody that inhibits severing by gelsolin reduced stress fibers. Anti-sense-transfected gelsolin-depleted 3T3 cell lines treated with LPA after serum starvation required approximately 6 h to form stress fibers and focal adhesions, in contrast to control lines transfected with vector only, which formed stress fibers 15 min after addition of LPA. In cells microinjected with the gelsolin antibody that inhibits severing, Mg-ATP-induced cell contraction was greatly reduced in approximately 90% of injected cells compared to cells injected with an irrelevant antibody. Gelsolin-depleted cells were incapable of collagen gel contraction and showed no apparent Mg-ATP-induced cell contraction compared to cell lines transfected with vector only. The involvement of gelsolin in cell contraction and remodeling of collagen gels suggests a novel role for gelsolin in stress fiber-dependent cell function.

Fluorescent phosphoinositide derivatives reveal specific binding of gelsolin and other actin regulatory proteins to mixed lipid bilayers

Fluorescent derivatives of phosphatidyl inositol (PtdIns)-(4,5)-P2 were synthesized and used to test the effects of the PtdIns-(4, 5)-P2-regulated proteins gelsolin, tau, cofilin, and profilin on labeled PtdIns-(4,5)-P2 that was either in micellar form or mixed with phosphatidylcholine (PtdCho) in bilayer vesicles. Gelsolin increased the fluorescence of 7-nitrobenz-2-oxa-1,3-diazole (NBD)- or pyrene-labeled PtdIns-(4,5)-P2 and NBD-PtdIns-(3,4,5)-P3. Cofilin and profilin produced no detectable change at equimolar ratios to PtdIns-(4,5)-P2, while tau decreased NBD-PtdIns-(4,5)-P2 fluorescence. Fluorescence enhancement by gelsolin of NBD-PtdIns-(4, 5)-P2 in mixed lipid vesicles depended on the mole fraction of PtdIns-(4,5)-P2 in the bilayer. Specific enhancement of 3% NBD-PtdIns-(4,5)-P2 : 97% PtdCho was much lower than that of 10% PtdIns-(4,5)-P2 : 90% PtdCho, but the enhancement of 3% NBD-PtdIns-(4,5)-P2 could be increased by addition of 7% unlabeled PtdIns-(4,5)-P2. The gelsolin-dependent increase in NBD-PtdIns-(4, 5)-P2 fluorescence was reversed by addition of Ca2+ or G-actin. Significant, but weaker, fluorescence enhancement was observed with the gelsolin N-terminal domain (residues 1-160) and a peptide comprised of gelsolin residues 150-169. Fluorescence energy transfer from gelsolin to pyrene-PtdIns-(4,5)-P2 was much stronger with intact gelsolin than the N-terminal region of gelsolin containing the PtdIns-(4,5)-P2 binding sites, suggesting that PtdIns-(4,5)-P2 may bind near a site formed by the juxtaposition of the N- and C-terminal domains of gelsolin.

The polyelectrolyte behavior of actin filaments: a 25Mg NMR study

Under physiological conditions, filamentous actin (F-actin) is a polyanionic protein filament. Key features of the behavior of F-actin are shared with other well-characterized polyelectrolytes, in particular, duplex DNA. For example, the bundle formation of F-actin by polyvalent cations, including divalent metal ions such as Mg2+, has been proposed to be a natural consequence of the polyelectrolyte nature of actin filaments [Tang and Janmey (1996) J. Biol. Chem. 271, 8556-8563]. This recently proposed model also suggests that weak interactions between F-actin and Mg2+ ions reflect a nonspecific trapping of counterions in the electric field surrounding F-actin due to its polyelectrolyte nature. To test this hypothesis, we have performed 25Mg NMR measurements in F-actin solutions. Based on the NMR data, we estimate that the rotational correlation times of Mg2+ are independent of the overall rotational dynamics of the actin filaments. Moreover, competitive binding experiments demonstrate a facile displacement of F-actin-bound Mg2+ by Co(NH3)63+. At higher Co(NH3)63+ concentrations, a fraction of the magnesium ions are trapped as actin filaments aggregate. ATP also competes effectively with actin filaments for binding to Mg2+. These results support the hypothesis that magnesium ions bind loosely and nonspecifically to actin filaments, and thus show a behavior typical of counterions in polyelectrolyte solutions. The observed features mimic to some extent the well-documented behavior of counterions in DNA solutions.

Cell crawling two decades after Abercrombie

In response to extracellular signals, cells remodel actin networks. Monomeric actin subunits at the cell's leading edge assemble into linear polymers that are cross-linked by accessory proteins into three-dimensional structures that are contracted by myosins to generate hydraulic force; elsewhere in the cell, actin networks dismantle. Actin subunit sequestering proteins prevent spontaneous actin nucleation, but not the growth of actin sub-units on to fast-growing filament ('barbed') ends, and at least half of the actin in most cells is filamentous. Therefore regulation of cellular actin assembly also requires proteins that block ('cap') actin filament barbed ends. Members of the capping protein gelsolin family also sever actin filaments mechanically. Calcium and protons activate gelsolin for severing and capping. Phosphoinositides reverse such capping, and a pathway has been defined in which receptor perturbation operates through GTP-Rac1 to stimulate the synthesis of endogenous phosphoinositides that uncap actin filaments. Other GTPases (and other signalling pathways) target phosphoinositide synthesis where other protrusions (e.g. filopodia) emerge. Cells maintain adequate, albeit compromised, locomotion in the absence of some, but not all, important machine parts. For example, gelsolin-null fibroblasts crawl using predominantly filopodia rather than lamellae. However, ABP-280 (actin-binding protein of 280 kDa), which promotes orthogonal branching of short actin filaments, seems to be necessary for membrane stability and translational locomotion. ABP-null cells hardly crawl at all, although they are viable and engage in surface movements.

Thiol oxidation of actin produces dimers that enhance the elasticity of the F-actin network

Slow oxidation of sulfhydryls, forming covalently linked actin dimers and higher oligomers, accounts for increases in the shear elasticity of purified actin observed after aging. Disulfide-bonded actin dimers are incorporated into F-actin during polymerization and generate cross-links between actin filaments. The large gel strength of oxidized actin (>100 Pa for 1 mg/ml) in the absence of cross-linking proteins falls to within the theoretically predicted order of magnitude for uncross-linked actin filament networks (1 Pa) with the addition of sufficient concentrations of reducing agents such as 5 mM dithiothreitol or 10 mM beta-mercaptoethanol. As little as 1 gelsolin/1000 actin subunits also lowers the high storage modulus of oxidized actin. The effects of gelsolin may be both to increase filament number as it severs F-actin and to cover the barbed end of an actin filament, which otherwise might cross-link to the side of another filament via an actin dimer. These new findings may explain why previous studies of actin rheology report a wide range of values when purified actin is polymerized without added regulatory proteins.

Enhancement of phosphoinositide 3-kinase (PI 3-kinase) activity by membrane curvature and inositol-phospholipid-binding peptides

The phosphorylation of phosphatidylinositol (PtdIns) on the 3' position of the inositol ring by phosphoinositide 3-kinase (PI 3-kinase) is shown to depend strongly on the curvature of liposomes containing a mixture of phosphatidylcholine (PtdCho) and PtdIns. Vesicles with an average diameter of 50 nm are phosphorylated 100 times faster than chemically identical vesicles with an average diameter greater than 300 nm. The low reactivity of large vesicles is not due to the difference in vesicle number for large and small vesicles at constant total lipid, nor to occlusion of lipid surfaces in multilammelar structures, and can be reversed by addition of low (< 1:100) molar ratios of either the PtdIns transfer protein sec14p or a ten-residue peptide derived from the inositol-phospholipid-binding site of gelsolin. Similar measurements using PI 4-kinase showed a weak dependence on vesicle size. The strong dependence of PI 3-kinase function on membrane curvature suggests possible localization of PI 3-kinase activity at sites where clustering of receptors, for example, may locally deform the membrane, and suggests that once PI 3-kinase is localized and activated at surface sites, the reaction may become self-accelerating.

Cofilin and gelsolin segment-1: molecular dynamics simulation and biochemical analysis predict a similar actin binding mode

An understanding of the actin-depolymerizing function attributed to members of the ADF/cofilin/destrin superfamily requires a structural model of these proteins in complex with actin. As a step toward defining actin-cofilin interactions, the complex of yeast cofilin with monomeric actin was predicted, starting with the actin-gelsolin segment-1 binding mode recently suggested for the actin-destrin complex. After refinement by molecular dynamics simulation, the structure of cofilin converged in a new binding mode that required only minimal changes induced in the actin-cofilin interface. The predicted complex exhibits strong interactions between the N termini of actin and cofilin, mediated by a salt bridge of cofilin Arg3 with actin Asp1. The forming of this salt bridge could be prevented by the phosphorylation of cofilin Ser4, which is believed to inhibit cofilin depolymerization activity. Recent mutagenesis studies, crosslinking experiments and peptide binding studies are consistent with the predicted model of the actin-cofilin complex. The structural homology between cofilin and gelsolin segment-1 binding to actin was confirmed experimentally by two types of competitive binding assays.