On the possible role of cytoskeletal filamentous networks in intracellular signaling: an approach based on percolation

A polarized cell: the root statocyte

In the gravity-perceiving cells (statocytes), located in the centre of the root cap, polarity is expressed in the arrangement of the organelles since, in most genera, the nucleus and the endoplasmic reticulum are maintained at the opposite ends of each cell by actin. Polarity is also evident in the distribution of plasmodesmata, which are more numerous in the transverse walls than in the longitudinal walls. The centre of each statocyte is depleted of microtubules (they are only located at the periphery) but is occupied by numerous amyloplasts (statoliths), denser than the cytoplasm. The amyloplasts do not contribute to the inherent structural polarity since their position is dependent upon the gravity vector. This article focuses on new microscopic analyses and on data obtained from experiments performed in microgravity, which have contributed to our better understanding of the architecture of the actin web implicated in the perception of gravity. Depending upon the plant, the actin network seems to be formed of single filaments arranged in various ways, or, of thin bundles of actin filaments. The amyloplasts are enmeshed in this web of actin and their envelopes are associated with it, but they can have autonomous movement via myosin in the absence of gravity. From calculations of the value of the force necessary to move one amyloplast in the lentil root, and from videomicroscopy performed with living statocytes of maize roots, it is hypothesized that actin microfilaments could be orientated in an overall diagonal direction in the statocyte. These observations could help in understanding how slight amyloplast movements may trigger and transmit the gravitropic signal.

Tensegrity I. Cell structure and hierarchical systems biology

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

Type III intermediate filament proteins interact with four-way junction DNA and facilitate its cleavage by the junction-resolving enzyme T7 endonuclease I

The isolation from proliferating mouse and human embryo fibroblasts of SDS-stable crosslinkage products of vimentin with DNA fragments containing inverted repeats capable of cruciform formation under superhelical stress and the competitive effect of a synthetic Holliday junction on the binding of cytoplasmic intermediate filament (cIF) proteins to supercoiled DNA prompted a detailed investigation of the proteins' capacity to associate with four-way junction DNA and to influence its processing by junction-resolving endonucleases. Electrophoretic mobility shift analysis of reaction products obtained from vimentin and Holliday junctions under varying ionic conditions revealed efficient complex formation of the filament protein not only with the unstacked, square-planar configuration of the junctions but also with their coaxially stacked X-conformation. Glial fibrillary acidic protein (GFAP) was less efficient and desmin virtually inactive in complex formation. Electron microscopy showed binding of vimentin tetramers or octamers almost exclusively to the branchpoint of the Holliday junctions under physiological ionic conditions. Even at several hundredfold molar excess, sequence-related single- and double-stranded DNAs were unable to chase Holliday junctions from their complexes with vimentin. Vimentin also stimulated bacteriophage T7 endonuclease I in introducing single-strand cuts diametrically across the branchpoint and thus in the resolution of the Holliday junctions. This effect is very likely due to vimentin-induced structural distortion of the branchpoint, as suggested by the results of hydroxyl radical footprinting of Holliday junctions in the absence and the presence of vimentin. Moreover, vimentin, and to a lesser extent GFAP and desmin, interacted with the cruciform structures of inverted repeats inserted into a supercoiled vector plasmid, thereby changing their configuration via branch migration and sensibilizing them to processing by T7 endonuclease I. This refers to both plasmid relaxation caused by unilateral scission and, particularly, linearization via bilateral scission at primary and cIF protein-induced secondary cruciform branchpoints that were identified by T7 endonuclease I footprinting. cIF proteins share these activities with a variety of other architectural proteins interacting with and structurally modulating four-way DNA junctions. In view of the known and hypothetical functions of four-way DNA junctions and associated protein factors in DNA metabolism, cIF proteins as complementary nuclear matrix proteins may play important roles in such nuclear matrix-associated processes as DNA replication, recombination, repair, and transcription, with special emphasis on both the preservation and evolution of the genome.

Toward a generalised tensegrity model describing the mechanical behaviour of the cytoskeleton structure

The control of many cell functions including growth, migration and mechanotransduction, depends crucially on stress-induced mechanical changes in cell shape and cytoskeleton (CSK) structure. Quantitative studies have been carried out on 6-bar tensegrity models to analyse several mechanical parameters involved in the mechanical responses of adherent cells (i.e. strain hardening, internal stress and scale effects). In the present study, we attempt to generalize some characteristic mechanical laws governing spherical tensegrity structures, with a view of evaluating the mechanical behaviour of the hierarchical multi-modular CSK-structure. The numerical results obtained by studying four different tensegrity models are presented in terms of power laws and point to the existence of unique and constant relationships between the overall structural stiffness and the local properties (length, number and internal stress) of the constitutive components.

Mechanotransduction through the cytoskeleton

We constructed a model cytoskeleton to investigate the proposal that this interconnected filamentous structure can act as a mechano- and signal transducer. The model cytoskeleton is composed of rigid rods representing actin filaments, which are connected with springs representing cross-linker molecules. The entire mesh is placed in viscous cytoplasm. The model eukaryotic cell is submitted to either shock wave-like or periodic mechanical perturbations at its membrane. We calculated the efficiency of this network to transmit energy to the nuclear wall as a function of cross-linker stiffness, cytoplasmic viscosity, and external stimulation frequency. We found that the cytoskeleton behaves as a tunable band filter: for given linker molecules, energy transmission peaks in a narrow range of stimulation frequencies. Most of the normal modes of the network are spread over the same frequency range. Outside this range, signals are practically unable to reach their destination. Changing the cellular ratios of linker molecules with different elastic characteristics can control the allowable frequency range and, with it, the efficiency of mechanotransduction.

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

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

Mechanical behavior in living cells consistent with the tensegrity model

Alternative models of cell mechanics depict the living cell as a simple mechanical continuum, porous filament gel, tensed cortical membrane, or tensegrity network that maintains a stabilizing prestress through incorporation of discrete structural elements that bear compression. Real-time microscopic analysis of cells containing GFP-labeled microtubules and associated mitochondria revealed that living cells behave like discrete structures composed of an interconnected network of actin microfilaments and microtubules when mechanical stresses are applied to cell surface integrin receptors. Quantitation of cell tractional forces and cellular prestress by using traction force microscopy confirmed that microtubules bear compression and are responsible for a significant portion of the cytoskeletal prestress that determines cell shape stability under conditions in which myosin light chain phosphorylation and intracellular calcium remained unchanged. Quantitative measurements of both static and dynamic mechanical behaviors in cells also were consistent with specific a priori predictions of the tensegrity model. These findings suggest that tensegrity represents a unified model of cell mechanics that may help to explain how mechanical behaviors emerge through collective interactions among different cytoskeletal filaments and extracellular adhesions in living cells.

Invited review: engineering approaches to cytoskeletal mechanics

An outstanding problem in cell biology is how cells sense mechanical forces and how those forces affect cellular functions. Various biophysical and biochemical mechanisms have been invoked to answer this question. A growing body of evidence indicates that the deformable cytoskeleton (CSK), an intracellular network of interconnected filamentous biopolymers, provides a physical basis for transducing mechanical signals into biochemical signals. Therefore, to understand how mechanical forces regulate cellular functions, it is important to know how cells respond to changes in the CSK force balance and to identify the underlying mechanisms that control transmission of mechanical forces throughout the CSK and bring it to equilibrium. Recent developments of new experimental techniques for measuring cell mechanical properties and novel theoretical models of cellular mechanics make it now possible to identify and quantitate the contributions of various CSK structures to the overall balance of mechanical forces in the cell. This review focuses on engineering approaches that have been used in the past two decades in studies of the mechanics of the CSK.

Mechanical properties of cultured human airway smooth muscle cells from 0.05 to 0.4 Hz

We investigated the rheological properties of living human airway smooth muscle cells in culture and monitored the changes in rheological properties induced by exogenous stimuli. We oscillated small magnetic microbeads bound specifically to integrin receptors and computed the storage modulus (G') and loss modulus (G") from the applied torque and the resulting rotational motion of the beads as determined from their remanent magnetic field. Under baseline conditions, G' increased weakly with frequency, whereas G" was independent of the frequency. The cell was predominantly elastic, with the ratio of G" to G' (defined as eta) being approximately 0. 35 at all frequencies. G' and G" increased together after contractile activation and decreased together after deactivation, whereas eta remained unaltered in each case. Thus elastic and dissipative stresses were coupled during changes in contractile activation. G' and G" decreased with disruption of the actin fibers by cytochalasin D, but eta increased. These results imply that the mechanisms for frictional energy loss and elastic energy storage in the living cell are coupled and reside within the cytoskeleton.

Trafficking and signaling through the cytoskeleton: a specific mechanism

A specific mechanism for the intracellular translocation of nonvesicle-associated proteins is proposed. This movement machinery is based on the assumption that the cytoskeleton represents an interconnected network of filamentous macromolecules, which extends over the entire cytoplasm. Diffusion along the filaments provides an efficient way for movement and with this, for signal transduction, between various intracellular compartments. We calculate the First Passage Time (FPT), the average time it takes a signaling molecule, diffusing along the cytoskeleton, to arrive from the cell surface to the nucleus for the first time. We compare our results with the FPT of free diffusion and of diffusion in the permeating cytoplasm. The latter is hindered by intracellular organelles and the cytoskeleton itself. We find that for filament concentrations even below physiological values, the FPT along cytoskeletal filaments converges to that for free diffusion. When filaments are considered as obstacles, the FPT grows steadily with filament concentration. At realistic filament concentrations the FPT is insensitive to local modifications in the cytoskeletal network, including bundle formation. We conclude that diffusion along cytoskeletal tracks is a reliable alternative to other established ways of intracellular trafficking and signaling, and therefore provides an additional level of cell function regulation.

Do cytoskeletal components control fatty acid translocation into liver mitochondria?

For two decades it has been assumed that inhibition of carnitine palmitoyltransferase I (CPT-I) by malonyl-CoA represents the main regulatory mechanism of liver ketogenesis. However, recent evidence indicates that CPT-I activity is also controlled by interactions between mitochondria and cytoskeletal components. This newly recognized mechanism emphasizes the emerging role of the cytoskeleton in the regulation of metabolic pathways.

The role of prestress and architecture of the cytoskeleton and deformability of cytoskeletal filaments in mechanics of adherent cells: a quantitative analysis

Mechanical properties of adherent cells were investigated using methods of engineering mechanics. The cytoskeleton (CSK) was modeled as a filamentous network and key mechanisms and corresponding molecular structures which determine cell elastic behavior were identified. Three models of the CSK were considered: open-cell foam networks, prestressed cable nets, and a tensegrity model as a special case of the latter. For each model, the modulus of elasticity (i.e. an index of resistance to small deformation) was given as a function of mechanical and geometrical properties of CSK filaments whose values were determined from the data in the literature. Quantitative predictions for the elastic modulus were compared with data obtained previously from mechanical tests on adherent cells. The open-cell foam model yielded the elastic modulus (10(3)-10(4)Pa) which was consistent with measurements which apply a large compressive stress to the cell. This suggests that bending of CSK filaments is the key mechanism for resisting large compression. The prestressed cable net and tensegrity model yielded much lower elastic moduli (10(1)-10(2)Pa) which were consistent with values determined from equilibrium measurements at low applied stress. This suggests that CSK prestress and architecture are the primary determinants of the cell elastic response. The tensegrity model revealed the possibility that buckling of microtubules of the CSK also contributed to cell elasticity.

Effects of stress on cellular infrastructure and metabolic organization in plant cells

Ample evidence shows the role of cytoskeleton mainly in cell division, cell form, and general orientation by the perception of physical forces such as gravity and mechanical ones in plant cells. However, the problem of how cytoskeleton organization and its dynamics at the cellular level in turn affects main metabolic pathways of gene expression and cellular energetics is yet unsolved. The response given by cells to environmental challenges such as stress responses is crucially dependent on the organization of their architecture. Drought, high salinity, and low temperature are sensed by plants as a water stress condition. The latter is known to entrain a series of physiological and metabolic changes at the cellular level. This review hypothesizes that the cytoskeletal network of plant cells and tissues may transduce environmental stress into changes in the organization and dynamics of metabolism and gene expression. Accordingly, experimental evidence concerning the current models of cytoplasmic architecture that have emerged in recent years and the effects of stress on the cytostructure are analyzed.

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.

A tensegrity model of the cytoskeleton in spread and round cells

Measurements on adherent cells have shown that spreading affects their mechanics. Highly spread cells are stiffer than less spread cells. The stiffness increases approximately linearly with increasing applied stress and more so in highly spread cells than in less spread cells. In this study, a six-strut tensegrity model of the cytoskeleton is used to analyze the effect of spreading on cellular mechanics. Two configurations are considered: a "round" configuration where a spherically shaped model is anchored to a flat rigid surface at three joints, and a "spread" configuration, where three additional joints of the model are attached to the surface. In both configurations a pulling force is applied at a free joint, distal from the anchoring surface, and the corresponding deformation is determined from equations of equilibrium. The model stiffness is obtained as the ratio of applied force to deformation. It is found that the stiffness changes with spreading consistently with the observations in cells. These findings suggest the possibility that the spreading-induced changes of the mechanical properties of the cell are the result of the concomitant changes in force distribution and microstructural geometry of the cytoskeleton.

Tyrosine phosphorylation of 40 kDa proteins in osteoblastic cells after mechanical stimulation of beta1-integrins

Using a method for the mechanical stimulation of cells which was adapted from one developed by Wang and Ingber employing magnetic microbeads [Wang, N. D., D. E. Ingber: Control of cytoskeletal mechanics by extracellular matrix, cell shape, and mechanical tension. Biophys. J. 66, 2181-2189 (1994)], mechanical stress could be applied to specific receptors on the cell surface. To achieve this, ferromagnetic microbeads coated with different ligands were magnetized after adhesion to the cells. The beads were then 'twisted' using a second magnetic field oriented perpendicular to the magnetizing one. Contrary to most current methods, it was possible to confer the strain without deforming the cell as a whole, thus being able to observe the individual reactions of transmembrane receptors to mechanical stress. An increase in tyrosine phosphorylation of proteins migrating at approximately 40 kDa could be observed as a reaction to stress on the beta1-subunits of the integrin family, while stress to other transmembrane molecules like the transferrin or low density lipoprotein receptors with no connection to the cytoskeleton did not give this reaction. Fibroblastic cells showed, contrary to osteoblastic cells, no reaction to stress applied on transmembrane proteins.

Malonyl-CoA-independent acute control of hepatic carnitine palmitoyltransferase I activity. Role of Ca2+/calmodulin-dependent protein kinase II and cytoskeletal components

The mechanism of malonyl-CoA-independent acute control of hepatic carnitine palmitoyltransferase I (CPT-I) activity was investigated. In a first series of experiments, the possible involvement of the cytoskeleton in the control of CPT-I activity was studied. The results of these investigations can be summarized as follows. (i) Very mild treatment of permeabilized hepatocytes with trypsin produced around 50% stimulation of CPT-I activity. This effect was absent in cells that had been pretreated with okadaic acid (OA) and seemed to be due to the action of trypsin on cell component(s) distinct from CPT-I. (ii) Incubation of intact hepatocytes with 3, 3'-iminodipropionitrile, a disruptor of intermediate filaments, increased CPT-I activity in a non-additive manner with respect to OA. Taxol, a stabilizer of the cytoskeleton, prevented the OA- and 3, 3'-iminodipropionitrile-induced stimulation of CPT-I. (iii) CPT-I activity in isolated mitochondria was depressed in a dose-dependent fashion by the addition of a total cytoskeleton fraction and a cytokeratin-enriched cytoskeletal fraction, the latter being 3 times more potent than the former. In a second series of experiments, the possible link between Ca2+/calmodulin-dependent protein kinase II (Ca2+/CM-PKII) and the cytoskeleton was studied in the context of CPT-I regulation. The data of these experiments indicate that (i) purified Ca2+/CM-PKII activated CPT-I in permeabilized hepatocytes but not in isolated mitochondria, (ii) purified Ca2+/CM-PKII abrogated the inhibition of CPT-I of isolated mitochondria induced by a cytokeratin-enriched fraction, and (iii) the Ca2+/CM-PKII inhibitor KN-62 prevented the OA-induced phosphorylation of cytokeratins in intact hepatocytes. Results thus support a novel mechanism of short-term control of hepatic CPT-I activity which may rely on the cascade Ca2+/CM-PKII activation --> cytokeratin phosphorylation --> CPT-I de-inhibition.

The cytoskeleton and cell signaling: component localization and mechanical coupling

The three-dimensional intracellular network formed by the filamentous polymers comprising the cytoskeletal affects the way cells sense their extracellular environment and respond to stimuli. Because the cytoskeleton is viscoelastic, it provides a continuous mechanical coupling throughout the cell that changes as the cytoskeleton remodels. Such mechanical effects, based on network formation, can influence ion channel activity at the plasma membrane of cells and may conduct mechanical stresses from the cell membrane to internal organelles. As a result, both rapid responses such as changes in intracellular Ca2+ and slower responses such as gene transcription or the onset of apoptosis can be elicited or modulated by mechanical perturbations. In addition to mechanical features, the cytoskeleton also provides a large negatively charged surface on which many signaling molecules including protein and lipid kinases, phospholipases, and GTPases localize in response to activation of specific transmembrane receptors. The resulting spatial localization and concomitant change in enzymatic activity can alter the magnitude and limit the range of intracellular signaling events.

Is cytoskeletal tension a major determinant of cell deformability in adherent endothelial cells?

We tested the hypothesis that mechanical tension in the cytoskeleton (CSK) is a major determinant of cell deformability. To confirm that tension was present in adherent endothelial cells, we either cut or detached them from their basal surface by a microneedle. After cutting or detachment, the cells rapidly retracted. This retraction was prevented, however, if the CSK actin lattice was disrupted by cytochalasin D (Cyto D). These results confirmed that there was preexisting CSK tension in these cells and that the actin lattice was a primary stress-bearing component of the CSK. Second, to determine the extent to which that preexisting CSK tension could alter cell deformability, we developed a stretchable cell culture membrane system to impose a rapid mechanical distension (and presumably a rapid increase in CSK tension) on adherent endothelial cells. Altered cell deformability was quantitated as the shear stiffness measured by magnetic twisting cytometry. When membrane strain increased 2.5 or 5%, the cell stiffness increased 15 and 30%, respectively. Disruption of actin lattice with Cyto D abolished this stretch-induced increase in stiffness, demonstrating that the increased stiffness depended on the integrity of the actin CSK. Permeabilizing the cells with saponin and washing away ATP and Ca2+ did not inhibit the stretch-induced stiffening of the cell. These results suggest that the stretch-induced stiffening was primarily due to the direct mechanical changes in the forces distending the CSK but not to ATP- or Ca(2+)-dependent processes. Taken together, these results suggest preexisting CSK tension is a major determinant of cell deformability in adherent endothelial cells.

Examination of temperature-induced 'gel-sol' transformation of alpha-actinin/cross-linked actin networks by static light scattering

We studied the gel-sol transformation of F-actin/alpha-actinin solutions. Cross-linking of actin filaments by alpha-actinin shows a temperature-dependent increase in light scatter signal, (I)T. Higher F-actin/alpha-actinin molar ratios, r(A alpha) as well as increases in F-actin concentration, [A], and reduction of actin filament lengths, rAG, augment the maximal light intensity, I and shift the gel-sol transition point, Tg to higher temperatures. This behavior is interpreted in terms of the model developed by Tempel, M., Isenberg, G. and Sackmann, E. (1996) (Physical Review E 54, 1802-1810) based on the percolation theory. Using the temperature-dependent binding model of this theory allows instant prediction of the equilibrium constant, K for F-actin/alpha-actinin solutions at temperatures T < Tg.

Alterations of hepatocellular intermediate filaments during extrahepatic cholestasis in rat liver

Intermediate filaments (IF) maintain the structural and functional integrity of cells. To investigate whether IF change as a consequence of increased mechanical pressure and what the significance of such alterations is for the integrity of hepatocytes, we investigated alterations of IF in rat liver following common bile duct ligation (CBDL). Immunofluorescence of cytokeratin 18 was performed on extracted cryostat sections which were also used for electron microscopy. Ultrathin sections of mildly extracted liver tissue were applied to reveal the relationship between IF and intercellular junctions and cytoplasmic organelles. Our results showed that hepatocellular IF underwent striking changes during CBDL. The so-called pericanalicular sheath disappeared and IF were rigidly rearranged at the cell periphery, appearing as honeycomb-like structures. Increased amounts of IF were found in close association with increased numbers of desmosomes at the lateral membranes of hepatocytes, and electron-dense desmosome-like bodies were even observed in the ectoplasm at bile canaliculi. Rearrangement of IF in the cytoplasm resulted in segregation of subcellular compartments. The increased density of the IF network and desmosomes are compensatory mechanisms of hepatocytes to resist increased mechanical load and disperse the tension. However, the intracellular rearrangement of IF leading to segregation of subcellular compartments may also have distinct effects on hepatocellular metabolic functions.

Intracellular detection of laminin alpha 2 chain in skin by electron microscopy immunocytochemistry: comparison between normal and laminin alpha 2 chain deficient subjects

The aim of this study is to localize the alpha 2 laminin chain in normal human skin. The methods used were immuno-gold cytochemistry on cryo-ultramicrotomy sections and thin-section-fracture-label, together with electron microscopy observation. Results were compared with light microscopy peroxidase immuno-staining. Both normal skin samples and skin biopsies from laminin alpha 2 chain deficient congenital muscular dystrophy affected patients were studied. The results show that, in normal skin, the laminin alpha 2 chain is spread throughout the cytoplasm of basal keratinocytes, while it appears associated with desmosomal tonofilaments in the spinous and granular epidermal layers; in skin samples from dystrophic patients the laminin alpha 2 chain was not detectable. These data suggest that the function of the laminin alpha 2 chain is different in the epidermis as compared to that in muscle and peripheral nerve, where it is localized in the basement membrane.

Demonstration of mechanical connections between integrins, cytoskeletal filaments, and nucleoplasm that stabilize nuclear structure

We report here that living cells and nuclei are hard-wired such that a mechanical tug on cell surface receptors can immediately change the organization of molecular assemblies in the cytoplasm and nucleus. When integrins were pulled by micromanipulating bound microbeads or micropipettes, cytoskeletal filaments reoriented, nuclei distorted, and nucleoli redistributed along the axis of the applied tension field. These effects were specific for integrins, independent of cortical membrane distortion, and were mediated by direct linkages between the cytoskeleton and nucleus. Actin microfilaments mediated force transfer to the nucleus at low strain; however, tearing of the actin gel resulted with greater distortion. In contrast, intermediate filaments effectively mediated force transfer to the nucleus under both conditions. These filament systems also acted as molecular guy wires to mechanically stiffen the nucleus and anchor it in place, whereas microtubules acted to hold open the intermediate filament lattice and to stabilize the nucleus against lateral compression. Molecular connections between integrins, cytoskeletal filaments, and nuclear scaffolds may therefore provide a discrete path for mechanical signal transfer through cells as well as a mechanism for producing integrated changes in cell and nuclear structure in response to changes in extracellular matrix adhesivity or mechanics.

Neutrophil activation by adhesion: mechanisms and pathophysiological implications

Neutrophil adhesion plays an essential role in the formation of an inflammatory exudate. Moreover, adhesion activates selective neutrophil functions and regulates the cell response to additional stimuli. In this review we summarize the information available on adhesion molecules involved in neutrophil adhesion to endothelial cells and extracellular matrix proteins and the experimental approaches which have been developed to block neutrophil adhesion and neutrophil mediated tissue damage. We also address the mechanisms of activation of selective neutrophil functions by adhesion molecules and, in particular the mechanisms of signal transduction by neutrophil integrins. On the basis of recent results obtained in our and other laboratories we propose a model hypothesizing mechanisms of signaling by neutrophil integrins involved in regulation of selective functions.

Pasteurella multocida toxin stimulates mitogenesis and cytoskeleton reorganization in Swiss 3T3 fibroblasts

Pasteurella multocida toxin (PMT) causes cytoplasmic retraction in epithelial cells, activates osteoclast neoformation, and is a potent mitogen for Swiss 3T3 fibroblasts. In the present study designed to further investigate the effects of PMT on cell shape and proliferation, we report that the mitogenic effect of affinitypurified PMT on quiescent 3T3 cells was even superior at 5 ng/ml to that of fetal bovine serum or bombesin. This positive effect was inhibited by heat denaturation and methylamine treatment (this agent blocks internalization). Preincubation of PMT with gangliosides GM1, GM2, or GM3 counteracted its effect on DNA synthesis, suggesting that the toxin binds to GM-type ceramides on target cells. The distribution of F-actin was analyzed in control/treated cells using FITC-conjugated phalloidin. In comparison with FBS and bombesin, PMT triggered a more rapid and profound reorganization of cortical actin into prominent stress fibers after only 5-10 min. This event lead to the retraction of cells after only 30 min and ultimately to the induction of mitotic figures. Interestingly, methylamine blocked the effects of PMT on stress fiber formation and cell retraction but not the ruffling response, suggesting that some early events may not require toxin internalization. In summary, these findings indicate that PMT concomitantly exerts a strong mitogenic activity and a rapid stimulation of cytoskeletal rearrangements, possibly after binding to membrane gangliosides and subsequent internalization. We propose that this toxin could be used in the future as a defined inducer of transduction signals involved in cellular proliferation and control of cell shape.

Vimentin in a cold-water fish, the rainbow trout: highly conserved primary structure but unique assembly properties

We have isolated from a rainbow trout (Oncorhynchus mykiss) spleen cDNA library a clone coding for vimentin. The deduced amino acid sequence reveals a high degree of identity with vimentin from carp (81%), frog (71%), chick and human (73% each). Large stretches in the central alpha-helical rod are identical within all four classes of vertebrates, but in 17 residues spread over the entire rod, the two fish differ distinctly from the tetrapod species. In addition, in the more diverged non-helical head domain, a nonapeptide motif previously shown to be important for regular filament formation is conserved. Recombinant trout vimentin assembles into bona fide filaments in vitro, with a temperature optimum between 18 and 24 degrees C. Above 27 degrees C, however, filament assembly is abruptly abolished and short filaments with thickened ends as well as structures without typical intermediate filament appearance are formed. This distinguishes its assembly properties significantly from amphibian, avian and mammalian vimentin. Also in vivo, after cDNA transfection into vimentin-free mammalian epithelial cells, trout vimentin does not form typical intermediate filament arrays at 37 degrees C. At 28 degrees C, and even more pronounced at 22 degrees C, the vimentin-positive material in the transfected cells is reorganized in the perinuclear region with a partial fibrillar appearance, but typical intermediate filament arrays are not formed. Together with immunoblotting and immunolocalization data from trout tissues, where vimentin is predominantly found in glial and white blood cells, we conclude that vimentin is indeed important in its filamentous form in fish and other vertebrates, possibly fulfilling cellular functions not directly evident in gene targeting experiments carried out in mice.

Biological specificity and measurable physical properties of cell surface receptors and their possible role in signal transduction through the cytoskeleton

It is proposed that the binding specificities of cell adhesion molecules are manifested in their measurable physical properties. A method specifically designed to measure the interfacial tension of cell aggregates is described. With the introduction of a statistical mechanical model, the measured values of tensions for aggregates consisting of genetically engineered cells with controlled adhesive properties are used to obtain information on the strength of individual receptor-ligand bonds. The strength of binding must depend on the receptor and its ligand and reflects the amino acid sequence of the binding proteins. Many of the cell surface receptors, being transmembrane proteins, are attached to the various macromolecular networks of the cytoskeleton; therefore, it is suggested that their ligation and ensuing conformational change may substantially affect the mechanical state of the cytoskeletal assemblies. Since these assemblies are believed to actively participate in intracellular signaling by transmitting signals from the cell membrane into the nucleus, the cell adhesion molecules may influence signaling in a predictable way through their measurable physical characteristics. In particular, varying bond strength at the cell surface may lead to differential gene regulation.