The actin content of fibroblasts

Transcriptome-proteome compendium of the Antarctic krill (Euphausia superba): Metabolic potential and repertoire of hydrolytic enzymes

The Antarctic krill (Euphausia superba Dana) is a keystone species in the Southern Ocean that uses an arsenal of hydrolases for biomacromolecule decomposition to effectively digest its omnivorous diet. The present study builds on a hybrid-assembled transcriptome (13,671 ORFs) combined with comprehensive proteome profiling. The analysis of individual krill compartments allowed detection of significantly more different proteins compared to that of the entire animal (1,464 vs. 294 proteins). The nearby krill sampling stations in the Bransfield Strait (Antarctic Peninsula) yielded rather uniform proteome datasets. Proteins related to energy production and lipid degradation were particularly abundant in the abdomen, agreeing with the high energy demand of muscle tissue. A total of 378 different biomacromolecule hydrolysing enzymes were detected, including 250 proteases, 99 CAZymes, 14 nucleases and 15 lipases. The large repertoire in proteases is in accord with the protein-rich diet affiliated with E. superba's omnivorous lifestyle and complex biology. The richness in chitin-degrading enzymes allows not only digestion of zooplankton diet, but also the utilization of the discharged exoskeleton after moulting. This article is protected by copyright. All rights reserved.

Qualitative analysis of contribution of intracellular skeletal changes to cellular elasticity

Cells are dynamic structures that continually generate and sustain mechanical forces within their environments. Cells respond to mechanical forces by changing their shape, moving, and differentiating. These reactions are caused by intracellular skeletal changes, which induce changes in cellular mechanical properties such as stiffness, elasticity, viscoelasticity, and adhesiveness. Interdisciplinary research combining molecular biology with physics and mechanical engineering has been conducted to characterize cellular mechanical properties and understand the fundamental mechanisms of mechanotransduction. In this review, we focus on the role of cytoskeletal proteins in cellular mechanics. The specific role of each cytoskeletal protein, including actin, intermediate filaments, and microtubules, on cellular elasticity is summarized along with the effects of interactions between the fibers.

Metabolic switch in energy metabolism mediates the sublethal effects induced by glyphosate-based herbicide on tadpoles of a farmland frog Microhyla fissipes

Glyphosate-based herbicides (GBHs) are widely-used agricultural chemicals, bringing potential detriments to aquatic organisms. Currently, our understanding of sublethal effects and underlying toxicologic mechanisms of GBHs are still limited, especially in amphibians. Here, the sublethal effects of a commercial GBH (KISSUN®) on tadpoles of a farmland dwelling frog, Microhyla fissipes, were investigated. The 10-d LC50 of "KISSUN®" GBH was 77.5 mg/L. Tadpoles exposed to 60-120 mg/L showed increased preference to higher temperature. After 10 days exposure, obvious growth suppression was observed in survived GBH-stressed tadpoles, characterized by dosage depended decrement in body mass, body width, total length, etc. GBH-stressed tadpoles also showed decreased tail length/snout-vent length ratio and smaller tail muscle fiber diameter. Comparative transcriptomics (control, 60 mg/L and 90 mg/L groups) was conducted to analyze the underlying molecular processes. GBH-stressed tadpoles showed downregulated transcription of ribosomal proteins and cytoskeleton proteins, which could explain their suppressed whole body and tail muscle growth. Moreover, GBH-stressed tadpoles showed transcriptional downregulation of carbohydrate and lipid catabolism, but upregulation of amino acid catabolism. It suggested a metabolic switch from carbohydrate and lipid to amino acid in these tadpoles. Accordingly, there was a trade-off between protein synthesis and energy production in respect to amino acid allocation, and it provided a metabolic explanation for why protein synthesis was downregulated and growth was suppressed in GBH-stressed tadpoles. In combination with existing literatures, we speculated that GBH might directly target the enzymes in carbohydrate and lipid catabolism, and this metabolic effect of GBH might be common to fish and amphibians. In conclusion, our study provided a systematic insight into the sublethal symptoms of GBH-stressed tadpoles, and a metabolic switch from carbohydrate and lipid to amino acid likely underlay some common toxic symptoms of GBHs on both fish and tadpoles.

IL1β and TNFα promote RANKL-dependent adseverin expression and osteoclastogenesis

Adseverin is an actin-binding protein involved in osteoclastogenesis, but its role in inflammation-induced bone loss is not well-defined. Here, we examined whether IL1β and TNFα regulate adseverin expression to control osteoclastogenesis in mouse primary monocytes and RAW264.7 cells. Adseverin was colocalized with subcortical actin filaments and was enriched in the fusopods of fusing cells. In precursor cells, adseverin overexpression boosted the formation of RANKL-induced multinucleated cells. Both IL1β and TNFα enhanced RANKL-dependent TRAcP activity by 1.6-fold and multinucleated cell formation (cells with ≥3 nuclei) by 2.6- and 3.3-fold, respectively. However, IL1β and TNFα did not enhance osteoclast formation in adseverin-knockdown cells. RANKL-dependent adseverin expression in bone marrow cells was increased by both IL1β (5.4-fold) and TNFα (3.3-fold). Luciferase assays demonstrated that this expression involved transcriptional regulation of the adseverin promoter. Activation of the promoter was restricted to a 1118 bp sequence containing an NF-κB binding site, upstream of the transcription start site. TNFα also promoted RANKL-induced osteoclast precursor cell migration. We conclude that IL1β and TNFα enhance RANKL-dependent expression of adseverin, which contributes to fusion processes in osteoclastogenesis.

Experimental and computational assessment of F-actin influence in regulating cellular stiffness and relaxation behaviour of fibroblasts

In biomechanics, a complete understanding of the structures and mechanisms that regulate cellular stiffness at a molecular level remain elusive. In this paper, we have elucidated the role of filamentous actin (F-actin) in regulating elastic and viscous properties of the cytoplasm and the nucleus. Specifically, we performed colloidal-probe atomic force microscopy (AFM) on BjhTERT fibroblast cells incubated with Latrunculin B (LatB), which results in depolymerisation of F-actin, or DMSO control. We found that the treatment with LatB not only reduced cellular stiffness, but also greatly increased the relaxation rate for the cytoplasm in the peripheral region and in the vicinity of the nucleus. We thus conclude that F-actin is a major determinant in not only providing elastic stiffness to the cell, but also in regulating its viscous behaviour. To further investigate the interdependence of different cytoskeletal networks and cell shape, we provided a computational model in a finite element framework. The computational model is based on a split strain energy function of separate cellular constituents, here assumed to be cytoskeletal components, for which a composite strain energy function was defined. We found a significant influence of cell geometry on the predicted mechanical response. Importantly, the relaxation behaviour of the cell can be characterised by a material model with two time constants that have previously been found to predict mechanical behaviour of actin and intermediate filament networks. By merely tuning two effective stiffness parameters, the model predicts experimental results in cells with a partly depolymerised actin cytoskeleton as well as in untreated control. This indicates that actin and intermediate filament networks are instrumental in providing elastic stiffness in response to applied forces, as well as governing the relaxation behaviour over shorter and longer time-scales, respectively.

Actin carbonylation: from cell dysfunction to organism disorder

Protein carbonylation is an important event in the context of proteostasis because of its frequency, non-enzymatic nature and irreversible effects. The carbonylation of proteins disturbs their function and leads to protein aggregates, which may precede cellular senescence and cell death. Actin, an evolutionarily conserved cytoskeletal protein that is involved in important cellular processes, is one of the proteins most susceptible to carbonylation. Conditions resulting in oxidative stress are likely to lead to its carbonylation, loss of function and aggregate formation. In this review, we summarise actin susceptibility to carbonylation, as verified in cell free extracts, cell lines and animal models, and review its fate through the activation of cell mechanisms aimed at removing damaged proteins. Their insufficient activity may underlie age-related diseases and the ageing process. This article is part of a Special Issue entitled: Posttranslational Protein modifications in biology and Medicine.

Relationships between actin regulatory mechanisms and measurable state variables

In this report we extend our recent mathematical formulation of the actin cycle model [Bindschadler et al. Biophys. J. 86 (2004) 2720] to predict the influence of key regulatory mechanisms on network-scale state variables estimable in live cell experiments. Specifically, we examine the influence of regulation by cofilin, profilin, capping protein and proteins that adjust filament number through nucleation and/or filament severing, on the higher order variables of average filament length, polymer fraction, and filament turnover rate. Importantly, we find that severing/nucleation, the acceleration of ADP-subunit disassembly by cofilin, and the catalytic and shuttle functions of profilin have 'signature' effects on the higher order state variables. In this way, measurement of the state variables in live cells can allow inference of regulatory mechanism(s) underlying changes in cell state. Our results compare favorably to published data for endothelial cells undergoing a transition from non-motile confluent cells to highly motile subconfluent cells. The extension of our model to higher order state variables allows us to investigate other important issues such as the distinction between basic and higher order measures of filament dynamics, the influence of thymosin beta4 on network state variables, the interplay between thymosin beta4 and profilin, and the synergystic effects of cofilin and profilin.

A master relation defines the nonlinear viscoelasticity of single fibroblasts

Cell mechanical functions such as locomotion, contraction, and division are controlled by the cytoskeleton, a dynamic biopolymer network whose mechanical properties remain poorly understood. We perform single-cell uniaxial stretching experiments on 3T3 fibroblasts. By superimposing small amplitude oscillations on a mechanically prestressed cell, we find a transition from linear viscoelastic behavior to power law stress stiffening. Data from different cells over several stress decades can be uniquely scaled to obtain a master relation between the viscoelastic moduli and the average force. Remarkably, this relation holds independently of deformation history, adhesion biochemistry, and intensity of active contraction. In particular, it is irrelevant whether force is actively generated by the cell or externally imposed by stretching. We propose that the master relation reflects the mechanical behavior of the force-bearing actin cytoskeleton, in agreement with stress stiffening known from semiflexible filament networks.

Use of fluorescently labelled deoxyribonuclease I to spatially measure G-actin levels in migrating and non-migrating cells

Lamellipodium protrusion is linked to actin filament disassembly in migrating fibroblasts [Cramer, 1999: Curr. Biol. 9:1095-1105]. To further study this relationship, we have identified a method to specifically and sensitively detect G-actin in distinct spatial locations in motile cells using deoxyribonuclease I (DNase I). Although DNase I can bind both G- and F-actin in vitro [Mannherz et al., 1980: Eur. J. Biochem. 95:377-385], when cells were fixed in formaldehyde and permeabilized in detergent, fluorescently-labelled DNase I specifically stained G-actin and not F-actin. 92-98% of actin molecules were stably retained in cells during fixation and permeabilization. Further, increasing or decreasing cellular G-actin concentration by treating live cells with latrunculin-A or jasplakinolide, respectively, caused a respective increase and decrease in DNase I cell-staining intensity as expected. These changes in DNase I fluorescence intensity accurately reflected increases and decreases in cellular G-actin concentration independently measured in lysates prepared from drug-treated live cells (regression coefficient = 0.98). This shows that DNase I cell-staining is very sensitive using this method. Applying this method, we found that the ratio of G-/F-actin is lower in both the lamellipodium and in a broad band immediately behind the lamellipodium in migrating compared to non-migrating fibroblasts. Thus, we predict that protrusion of the lamellipodium in migrating fibroblasts requires tight coupling to filament disassembly at least in part because G-actin is relatively limited within and behind the lamellipodium. This is the first report to directly demonstrate high sensitivity of cell-staining for any G-actin probe and this, together with the ready commercial accessibility of fluorescently-labelled DNase I, make it a simple, convenient, and sensitive tool for cell-staining of G-actin.

Reduced protein diffusion rate by cytoskeleton in vegetative and polarized dictyostelium cells

Fluorescence recovery after photobleaching measurements with high spatial resolution are performed to elucidate the impact of the actin cytoskeleton on translational mobility of green fluorescent protein (GFP) in aqueous domains of Dictyostelium discoideum amoebae. In vegetative Dictyostelium cells, GFP molecules experience a 3.6-fold reduction of their translational mobility relative to dilute aqueous solutions. In disrupting the actin filamentous network using latrunculin-A, the intact actin cytoskeletal network is shown to contribute an effective viscosity of 1.36 cP, which accounts for 53% of the restrained molecular diffusion of GFP. The remaining 47% of hindered protein motions is ascribed to other mechanical barriers and the viscosity of the cell liquid. A direct correlation between the density of the actin network and its limiting action on protein diffusion is furthermore established from measurements under different osmotic conditions. In highly locomotive polarized cells, the obstructing effect of the actin filamentous network is seen to decline to 0.46 cP in the non-cortical regions of the cell. Our results indicate that the meshwork of actin filaments constitutes the primary mechanical barrier for protein diffusion and that any noticeable reorganization of the network is accompanied by altered intracellular protein mobility.

Cytoarchitecture and physical properties of cytoplasm: volume, viscosity, diffusion, intracellular surface area

Classical biochemistry is founded on several assumptions valid in dilute aqueous solutions that are often extended without question to the interior milieu of intact cells. In the first section of this chapter, we present these assumptions and briefly examine the ways in which the cell interior may depart from the conditions of an ideal solution. In the second section, we summarize experimental evidence regarding the physical properties of the cell cytoplasm and their effect on the diffusion and binding of macromolecules and vesicles. While many details remain to be worked out, it is clear that the aqueous phase of the cytoplasm is crowded rather than dilute, and that the diffusion and partitioning of macromolecules and vesicles in cytoplasm is highly restricted by steric hindrance as well as by unexpected binding interactions. Furthermore, the enzymes of several metabolic pathways are now known to be organized into structural and functional units with specific localizations in the solid phase, and as much as half the cellular protein content may also be in the solid phase.

Crowding-induced organization of cytoskeletal elements: II. Dissolution of spontaneously formed filament bundles by capping proteins

Through calculations of molecular packing constraints in crowded solutions, we have previously shown that dispersions of filament forming proteins and soluble proteins can be unstable at physiological concentrations, such that tight bundles of filaments are formed spontaneously, in the absence of any accessory binding proteins. Here we consider the modulation of this phenomenon by capping proteins. The theory predicts that, by shortening the average filament length, capping alleviates the packing problem. As a result, the dispersed isotropic solution is stable over an expanded range of compositions.

Dependence of collagen remodelling on alpha-smooth muscle actin expression by fibroblasts

To study the relation between expression of the putative myofibroblast marker alpha-smooth muscle actin and the remodelling of extracellular matrix, immunocytochemical, gel electrophoresis, and collagen gel contraction studies were performed on two human fibroblast subtypes. Double immunolabelling for total actins and alpha-smooth muscle (sm) actin as well as affinity labelling of filamentous and monomeric actins in gingival fibroblasts demonstrated that alpha-sm was colocalized in stress fibres and in regions with high levels of monomeric actin throughout the cytoplasm. alpha-sm comprised up to 14% of total cellular actin as assessed by 2D gel electrophoresis. Thirteen different gingival and seven different periodontal ligament fibroblast lines constitutively expressed on alpha-sm actin. These cells exhibited up to 60% inter-line variations of fluorescence due to alpha-sm actin and up to 70% and 45% inter-line variation in the rate of collagen gel contraction. Quantitative, single cell fluorimetry of alpha-sm actin immunoreactivity demonstrated a linear relation between gel contraction and alpha-sm actin (correlation coefficients of 0.71 for gingival and 0.61 for periodontal ligament cells), but there was no detectable relationship between total actin content and gel contraction. In contrast, flow cytometry demonstrated that 99% of the total gated cells from cell lines exhibiting rapid gel contraction showed alpha-sm actin staining above background fluorescence as compared to only 35% of cells with slow rates of gel contraction. Contracting collagen gels stained with FITC-phalloidin showed cells with well-developed stress fibres that were progressively more compact and elongated during the time of maximal gel contraction. To examine the dependence of gel contraction on assembly of monomeric actin into actin filaments, cells were electroporated in the presence of phalloidin or cytochalasin D. Collagen gels exhibited up to 100% inhibition of gel contraction that was dose-dependent. Gel contraction was inhibited 93% by electroinjection of cells with alpha-sm actin antibody prior to incubation, but the antibody did not inhibit actin assembly after attachment and spreading on substrates. These data indicate that gel contraction is dependent on alpha-sm actin expression and that alpha-sm actin is a functional marker for a fibroblast subtype that rapidly remodels the extracellular matrix.

Effects of fibronectin on actin organization and respiratory burst activity in neutrophils, monocytes, and macrophages

Previous studies have shown that fibronectin (Fn) enhances phagocytosis and killing of antibody-coated bacteria by neutrophils and macrophages. In an attempt to understand the mechanism of this enhancement, we have investigated the effects of Fn on phagocytosis-related actin organization as well as respiratory burst activity in neutrophils, monocytes and culture-derived macrophages. Employing an NBD-phallacidin flow cytometric analysis of filamentous actin formation, we found that Fn promotes rapid actin polymerization within 30 seconds in neutrophils, monocytes, and macrophages, but not lymphocytes. Enhancement of actin polymerization by Fn was concentration-dependent and mediated by a pertussis toxin- but not cholera toxin-sensitive G protein. Inhibition of protein kinase C by sphingosine (20 microM), calcium influx by verapamil (0.1 mM), or intracellular calcium mobilization by 8-(N,N-diethyl-amino) octyl-3,4,5-trimethoxybenzoate HCl (TMB-8; 0.1 mM) did not block Fn-enhanced actin polymerization in phagocytes. Incubation of neutrophils and macrophages on microtiter plates precoated with Fn suppressed superoxide (O2-) production induced by IgG- and IgA- opsonized group B streptococci. In contrast, Fn significantly enhanced IgA- and IgG-mediated O2- production by freshly isolated monocytes. These data suggest that Fn enhances phagocytosis, presumably through G protein-coupled cytoskeleton reorganization and augments O2- production by circulating monocytes. In contrast, it appears to suppress O2- production by the active phagocytic cells, neutrophils and macrophages. This may result in enhanced phagocytosis and intracellular killing of microorganisms without damaging interstitial tissues.

The wily ways of a parasite: induction of actin assembly by Listeria

The intracellular pathogen Listeria has a spectacular mode of transport within and between host cells. By inducing host cell actin to assemble from its surface, the bacterium forms a tail composed of many short, crossbridged actin filaments. With this tail Listeria is propelled across the cytoplasm like a comet streaking across the sky. Here we discuss the antics of Listeria and some of the bacterial genes instrumental in maintaining it in the host.

Ultrastructural and immunohistochemical studies of Wharton's jelly umbilical cord cells

In order to determine the significance of Wharton's jelly, the characteristics of these cells were examined by means of electron microscopy and immunohistochemistry. These cells possessed ultrastructural characteristics of both fibroblasts and smooth muscle cells, indicating that they are modified, rather than typical fibroblasts. Immunohistochemically those 'myofibroblasts' stained positive for actin, non-muscle myosin, vimentin and desmin. Staining for muscle myosin was negative, supporting the ultrastructural findings. As our results indicate that these cells can function in both fibrogenesis and cell contraction, we speculate that they may contribute to the elasticity of Wharton's jelly, by synthesizing collagen fibers, and participate in the regulation of umbilical blood flow by virtue of their contractile properties.

How Listeria exploits host cell actin to form its own cytoskeleton. II. Nucleation, actin filament polarity, filament assembly, and evidence for a pointed end capper

After Listeria, a bacterium, is phagocytosed by a macrophage, it dissolves the phagosomal membrane and enters the cytoplasm. The Listeria than nucleates actin filaments from its surface. These newly assembled actin filaments show unidirectional polarity with their barbed ends associated with the surface of the Listeria. Using actin concentrations below the pointed end critical concentration we find that filament elongation must be occurring by monomers adding to the barbed ends, the ends associated with the Listerial surface. If Listeria with tails are incubated in G actin under polymerizing conditions, the Listeria is translocated away from its preformed tail by the elongation of filaments attached to the Listeria. This experiment and others tell us that in vivo filament assembly must be tightly coupled to filament capping and cross-bridging so that if one process outstrips another, chaos ensues. We also show that the actin filaments in the tail are capped on their pointed ends which inhibits further elongation and/or disassembly in vitro. From these results we suggest a simple picture of how Listeria competes effectively for host cell actin. When Listeria secretes a nucleator, the host's actin subunits polymerize into a filament. Host cell machinery terminate the assembly leaving a short filament. Listeria overcomes the host control by nucleating new filaments and thus many short filaments assemble. The newest filaments push existing ones into a growing tail. Thus the competition is between nucleation of filaments caused by Listeria and the filament terminators produced by the host.

Effect of picibanil (OK432) on neutrophil-mediated antitumor activity: implication of monocyte-derived neutrophil-activating factors

Picibanil (OK432), an extract from streptococci, has been widely utilized to treat malignant ascites and pleural effusions. The antitumor mechanism is believed to include complement-mediated neutrophil activation. Employing a flow-cytometric analysis of actin polymerization as an indicator of cell activation as well as a tumor proliferation assay, we have found that monocyte-derived neutrophil-activating factors were involved in OK432-induced neutrophil activation as well as antitumor activity. OK432-stimulated (0.1 KE/ml; 0.01 mg/ml) monocyte supernatants (OKMS) induced neutrophil actin polymerization and chemotaxis. OKMS were responsible for neutrophil-mediated inhibition of human leukemic (CEM) cell proliferation and stimulated neutrophils to produce superoxide in the presence of CEM leukemic cells at an effector/target ratio higher than 20/1. In contrast, OK432 alone, OK432-stimulated lymphocyte supernatants, or OK432-stimulated neutrophil supernatants had no effect on neutrophil activation or suppression of tumor cell proliferation. OK432 in combination with mononuclear cells also had no effect on the inhibition of CEM cell proliferation. Pretreatment of OKMS at 56 degrees C for 30 min did not affect its ability to activate neutrophils, implying that complement activation is not responsible for the neutrophil activation. Supernatants from OK432-stimulated mononuclear cells, as determined by enzyme-linked immunosorbent assays and radioimmunoassays, contained high levels of interleukin-8 (IL-8; 1567 +/- 145 pg/ml) and tumor necrosis factor (TNF alpha; 2105 +/- 152 pg/ml), low levels of leukotriene B4 (800 +/- 45 pg/ml) and IL-1 beta (180 +/- 22 pg/ml), but interferon gamma was not detectable. IL-1 beta, IL-8, and TNF alpha transcripts, undetectable in untreated monocytes, increased significantly after 30-60 min exposure to OK432. These results suggest that neutrophil-activating factors from monocytes or resident macrophages may play an important role in the OK432-induced neutrophil activation and antitumor activity.

Control of actin polymerization in live and permeabilized fibroblasts

We have investigated the spatial control of actin polymerization in fibroblasts using rhodamine-labeled muscle actin in; (a) microinjection experiments to follow actin dynamics in intact cells, and (b) incubation with permeabilized cells to study incorporation sites. Rhodamine-actin was microinjected into NIH-3T3 cells which were then fixed and stained with fluorescein-phalloidin to visualize total actin filaments. The incorporation of newly polymerized actin was assayed using rhodamine/fluorescein ratio-imaging. The results indicated initial incorporation of the injected actin near the tip and subsequent transport towards the base of lamellipodia at rates greater than 4.5 microns/min. Furthermore, both fluorescein- and rhodamine-intensity profiles across lamellipodia revealed a decreasing density of actin filaments from tip to base. From this observation and the presence of centripetal flux of polymerized actin we infer that the actin cytoskeleton partially disassembles before it reaches the base of the lamellipodium. In permeabilized cells we found that, in agreement with the injection studies, rhodamine-actin incorporated predominantly in a narrow strip of less than 1-microns wide, located at the tip of lamellipodia. The critical concentration for the rhodamine-actin incorporation (0.15 microM) and its inhibition by CapZ, a barbed-end capping protein, indicated that the nucleation sites for actin polymerization most likely consist of free barbed ends of actin filaments. Because any potential monomer-sequestering system is bypassed by addition of exogenous rhodamine-actin to the permeabilized cells, these observations indicate that the localization of actin incorporation in intact cells is determined, at least in part, by the presence of specific elongation and/or nucleation sites at the tips of lamellipodia and not solely by localized desequestration of subunits. We propose that the availability of the incorporation sites at the tips of lamellipodia is because of capping activities which preferentially inhibit barbed-end incorporation elsewhere in the cell, but leave barbed ends at the tips of lamellipodia free to add subunits.

Actin depolymerization in the cyclic AMP-stimulated toad bladder epithelial cell, determined by the DNAse method

Previous studies with the rhodamine phalloidin binding assay have shown that antidiuretic hormone and 8-Br-cAMP rapidly depolymerize F-actin in toad bladder epithelial cells. We have extended these studies with the DNAse inhibition assay and have found that in isolated epithelial cell suspensions, G-actin increases from 37 to 56% of total actin following 8-br-cAMP stimulation. The G-actin concentration in the epithelial cell greatly exceeds its critical concentration, indicating the requirement for a G-actin sequestering protein or proteins in this system.

Tracer diffusion in F-actin and Ficoll mixtures. Toward a model for cytoplasm

We have previously reported that self-diffusion of inert tracer particles in the cytoplasm of living Swiss 3T3 cells is hindered in a size-dependent manner (Luby-Phelps, K., D.L. Taylor, and F. Lanni. 1986. J. Cell Biol. 102:2015-2022; Luby-Phelps, K., P.E. Castle, D.L. Taylor, and F. Lanni. 1987. Proc Natl. Acad. Sci. USA. 84:4910-4913). Lacking a theory that completely explains our data, we are attempting to understand the molecular architecture responsible for this phenomenon by studying tracer diffusion in simple, reconstituted model systems. This report contains our findings on tracer diffusion in concentrated solutions of Ficoll 70 or Ficoll 400, in solutions of entangled F-actin filaments, and in solutions of entangled F-actin containing a background of concentrated Ficoll particles or concentrated bovine serum albumin (BSA). A series of size-fractionated fluorescein-Ficolls were used as tracer particles. By fluorescence recovery after photobleaching (FRAP), we obtained the mean diffusion coefficients in a dilute, aqueous reference phase (Do), the mean diffusion coefficients in the model matrices (D), and the mean hydrodynamic radii (RH) for selected tracer fractions. For each model matrix, the results were compared with similar data obtained from living cells. As in concentrated solutions of globular proteins (Luby-Phelps et al., 1987), D/Do was not significantly size-dependent in concentrated solutions of Ficoll 400 or Ficoll 70. In contrast, D/Do decreased monotonically with increasing RH in solutions of F-actin ranging in concentration from 1 to 12 mg/ml. This size dependence was most pronounced at higher F-actin concentrations. However, the shape of the curve and the extrapolated value of D/Do in the limit, RH----O did not closely resemble the cellular data for tracers in the same size range (3 less than RH less than 30 nm). In mixtures of F-actin and Ficoll or F-actin and BSA, D/Do was well approximated by D/Do for the same concentration of F-actin alone multiplied by D/Do for the same concentrations of Ficoll or BSA alone. Based on these results, it is possible to model the submicroscopic architecture of cytoplasm in living cells as a densely entangled filament network (perhaps made up of F-actin and other filamentous structures) interpenetrated by a fluid phase crowded with globular macromolecules, which in cytoplasm would be primarily proteins.

Instability of the myofibroblast phenotype in culture

Myofibroblasts are cells that have features of both smooth muscle cells and fibroblasts. Myofibroblasts from rat granulation tissue were studied using a coordinated biochemical and morphological approach. These myofibroblasts were maintained in culture for more than 17 passages. After establishing that these cells were indeed myofibroblasts by immunohistochemical and ultrastructural criteria, their biochemical parameters and phenotypic stability were studied. Rat dermal fibroblasts grown under similar conditions served as controls. Biochemically, it was found that both cells produced similar types of procollagens, namely collagens I and III, in similar proportions at or near a 1:1 ratio. However, myofibroblasts at early passage produced threefold more procollagens than did fibroblasts at the same stage of passage, while similar quantities were produced in each at late passage. These observations for procollagens were confirmed by separate studies for collagens, as measured by hydroxyproline determinations. Such differences were also reflected ultrastructurally. Secretory vesicles were identified in myofibroblasts at early stage, but not at late stage of passaging. No such vesicles were seen in fibroblasts at any stage of passage. In conclusion, myofibroblasts are active secretory variants of fibroblasts which have an unstable phenotype in culture and become indistinguishable from fibroblasts at late passage.

Chicken growth-associated protein GAP-43 is tightly bound to the actin-rich neuronal membrane skeleton

We have identified the chicken equivalent of growth-associated protein GAP-43 in a detergent-resistant membrane skeleton from cultures of chick neurones and embryonic chick brain. Antisera to the membrane skeleton protein, the 3D5 antigen, precipitate the translation product of chick GAP-43 cDNA, and the 3D5 antigen is also detected by antisera against synthetic peptides from the known amino acid sequence of rat GAP-43. The chick protein and the rat GAP-43 are biochemically similar proteins that both serve as major targets of phosphorylation by endogenous protein kinase C. The detergent-resistant complex in which GAP-43 is found also contains actin (approximately 5% of the total protein) and a neurone-specific cell surface glycoprotein. We suggest that the membrane skeleton of neurones may be a primary site of action of GAP-43.

Incorporation and turnover of biotin-labeled actin microinjected into fibroblastic cells: an immunoelectron microscopic study

We investigated the mechanism of turnover of an actin microfilament system in fibroblastic cells on an electron microscopic level. A new derivative of actin was prepared by labeling muscle actin with biotin. Cultured fibroblastic cells were microinjected with biotinylated actin, and incorporated biotin-actin molecules were detected by immunoelectron microscopy using an anti-biotin antibody and a colloidal gold-labeled secondary antibody. We also analyzed the localization of injected biotin-actin molecules on a molecular level by freeze-drying techniques. Incorporation of biotin-actin was rapid in motile peripheral regions, such as lamellipodia and microspikes. At approximately 1 min after injection, biotin-actin molecules were mainly incorporated into the distal part of actin bundles in the microspikes. Heavily labeled actin filaments were also observed at the distal fringe of the densely packed actin networks in the lamellipodium. By 5 min after injection, most actin polymers in microspikes and lamellipodia were labeled uniformly. These findings suggest that actin subunits are added preferentially at the membrane-associated ends of preexisting actin filaments. At earlier times after injection, we often observed that the labeled segments were continuous with unlabeled segments, suggesting the incorporation of new subunits at the ends of preexisting filaments. Actin incorporation into stress fibers was a slower process. At 2-3 min after injection, microfilaments at the surface of stress fibers incorporated biotin-actin, but filaments in the core region of stress fibers did not. At 5-10 min after injection, increasing density of labeling along stress fibers toward their distal ends was observed. Stress fiber termini are generally associated with focal contacts. There was no rapid nucleation of actin filaments off the membrane of focal contacts and the pattern of actin incorporation at focal contacts was essentially identical to that into distal parts of stress fibers. By 60 min after injection, stress fibers were labeled uniformly. We also analyzed the actin incorporation into polygonal nets of actin bundles. Circular dense foci, where actin bundles radiate, were stable structures, and actin filaments around the foci incorporated biotin-actin the slowest among the actin-containing structures within the injected cells. These results indicate that the rate and pattern of actin subunit incorporation differ in different regions of the cytoplasm and suggest the possible role of rapid actin polymerization at the leading margin on the protrusive movement of fibroblastic cells.

The fibroblast-like nature of myofibroblasts

The myofibroblast is found in normal tissue as well as in a wide variety of pathological processes. We have cultured myofibroblasts and dermal fibroblasts and have found that they secrete similar type-specific procollagens into the culture media. These were primarily type I and III procollagens with a predominance of type I procollagen. These patterns are distinctly different from those of smooth muscle cells, which synthesize predominantly type III procollagen. Cultured fixed cells were also examined by immunohistochemistry. Both myofibroblasts and fibroblasts stained positively with antibodies to type I and III procollagens. Reaction to type V procollagen antibodies was prominent only in the myofibroblast, as was immunostaining with anti-muscle actin antibodies. Immunostaining with desmin antibodies was negative in both cell types. By electron microscopy, the myofibroblast had well-developed dense microfilament fibers of 40-80 degrees that were prominent in the long axes of the cells near the cellular margins. Although the myofibroblast has properties of both smooth muscle cells and fibroblasts, it appears to be most likely a modified fibroblast that has undergone differentiation, probably in response to specific signals from the extracellular matrix.

Evaluation of an enzyme-linked immunosorbent assay to actin in cultured fibroblasts

The amount of actin in cultured human skin fibroblasts was determined by an enzyme-linked immunosorbent assay. Absorbance values (OD), resulting from specific binding of an antiactin monoclonal antibody to intracellular actin, were converted to cell concentrations (pg/cell) from an actin standard curve. Actin concentration for cultured human skin fibroblasts by this technique was estimated to be 55.5 picograms/cell and constituted approximately 3.9% of the total cellular protein. This assay procedure offers the following advantages: (1) It is time efficient, can be completed in 2-3 hours; (2) cells are unaltered except for membrane permeabilization; (3) the sensitivity is equal to or greater than previous procedures involving gel electrophoresis; and (4) the assay is easy and inexpensive to perform.

Oncogenic potential in fibroblasts from individuals genetically predisposed to cancer

There is now considerable evidence to suggest that genetic factors can influence the incidence of cancer. Although expression of this susceptibility to cancer appears to be tissue-specific, the normal skin fibroblasts from individuals predisposed to cancer (predisposed fibroblasts) have also been shown to express the risk of the target cell in the development of cancer. In the context of the 2-stage theory of chemical carcinogenesis predisposed fibroblasts may, therefore, exist in a pre-neoplastic or initiated state. The purpose of the present study was to determine whether predisposed fibroblasts would be oncogenically transformed in vitro by the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) alone. TPA treatment induced similar changes in cellular morphology, cytoskeleton, and epidermal growth-factor binding, in predisposed and normal cells. None of these cell lines acquired anchorage-independent growth or an unlimited growth potential in culture after chronic application of TPA. Fluorescent microscopy with an F-actin probe, in the absence of TPA, showed a disorganization of the microfilament and intermediate filament network in skin fibroblasts from individuals with familial polyposis coli, hereditary and sporadic retinoblastoma, basal cell nevus syndrome, and Gardner's syndrome, as compared to normal skin fibroblasts. Single and 2-dimensional electrophoresis also indicated that the incorporation of 35S-methionine into actin in predisposed fibroblasts was 2-fold greater than in normal fibroblasts, and the turnover rate of actin in predisposed fibroblasts was less than 5 h, compared to 48 h in normal fibroblasts. These observations clearly suggest that predisposed fibroblasts may not exist in a pre-neoplastic or initiated state, and that the mechanism of genetic susceptibility to cancer may be different from that of chemical carcinogenesis. In contrast, the results of this study indicate that genetic susceptibility to a variety of cancers may be associated with a rapid turnover of actin and a disorganization of the microfilament and intermediate filament networks.

Characterization of DNA-protein complexes induced in intact cells by the carcinogen chromate

Potassium chromate induced the formation of DNA-protein complexes in cultured Chinese hamster ovary cells. The DNA-protein complexes were isolated by ultracentrifugal sedimentation in the presence of 2% sodium dodecyl sulfate (SDS) and 5 M urea. Two-dimensional SDS-polyacrylamide gel electrophoresis analysis of the chromate-induced DNA-protein complexes revealed that two acidic proteins of 53 and 45 kDa and a basic protein of 54 kDa were selectively complexed to the DNA. Numerous other proteins also became associated with the DNA to a lesser degree as the chromate concentration was increased. Nuclease digestion was not a prerequisite for the resolution of the protein component of the DNA-protein complexes using two-dimensional gel electrophoresis. Ultracentrifugal analysis of the DNA-protein complexes in the presence of proteinase K, nucleases, or a chelating agent demonstrated that protein aggregation was not responsible for the increased protein recovery in chromate-treated samples and that the complexes were disrupted by EDTA. These data suggest that the selectively complexed proteins were associated with the DNA through strong interactions that may be mediated by the trivalent form of chromium.

Scanning microfluorometric measurement of TRITC-phalloidin labelled F-actin. Dependence of F-actin content on density of normal and transformed cells

A method is described for the determination of cellular F-actin content using fluorometry of TRITC-phalloidin at very high dilution. In this case no saturation of all binding sites available is reached, however the staining is highly specific and the specificity is not affected by the preparative procedure as may be the case at high concentrations of TRITC-phalloidin. The method is based on calculation of fluorescence intensity at equilibrium conditions from measurements at two different numbers of exchange of the staining solution by using Lineweaver-Burk plotting. The relative content of F-actin has been determined for three established cell lines, an amphibian cell line XTH-2 of endothelial origin, 3T3 cells and SV 40 transformed 3T3 cells. In the two non transformed lines F-actin decreases with increasing cell density starting with the onset of confluency of the culture. SV 40 transformed 3T3 cells generally contain less F-actin and do not show any significant point of change. The decrease in F-actin with increasing cell density is accompanied by a disappearance of stress fibres. SV 40 3T3 cells generally are devoid of stress fibres. The observations are discussed considering a possible involvement of F-actin in growth control.

Interactions of pig plasma gelsolin with G-actin

Pig plasma gelsolin forms a ternary complex with monomeric actin in 0.1 mM CaCl2 and a binary complex in EGTA (less than 0.01 microM calcium), as shown by gel filtration and fluorescence changes when actin which had been treated with N-ethylmaleimide and 7-chloro-4-nitrobenzeno-2-oxa-1,3-diazole (NBD-actin) or with N-(1-pyrenyl)iodoacetamide (PI-actin) binds to gelsolin. The fluorescence enhancement per actin molecule bound is similar in the binary and ternary complexes, but the affinity of gelsolin for labelled actin is very much greater in the presence of calcium. Furthermore, the formation of ternary complex exhibits strong positive cooperativity.

The architecture of actin filaments and the ultrastructural location of actin-binding protein in the periphery of lung macrophages

A highly branched filament network is the principal structure in the periphery of detergent-extracted cytoskeletons of macrophages that have been spread on a surface and either freeze or critical point dried, and then rotary shadowed with platinum-carbon. This array of filaments completely fills lamellae extended from the cell and bifurcates to form 0.2-0.5 micron thick layers on the top and bottom of the cell body. Reaction of the macrophage cytoskeletons with anti-actin IgG and with anti-IgG bound to colloidal gold produces dense staining of these filaments, and incubation with myosin subfragment 1 uniformly decorates these filaments, identifying them as actin. 45% of the total cellular actin and approximately 70% of actin-binding protein remains in the detergent-insoluble cell residue. The soluble actin is not filamentous as determined by sedimentation analysis, the DNAase I inhibition assay, and electron microscopy, indicating that the cytoskeleton is not fragmented by detergent extraction. The spacing between the ramifications of the actin network is 94 +/- 47 nm and 118 +/- 72 nm in cytoskeletons prepared for electron microscopy by freeze drying and critical point drying, respectively. Free filament ends are rare, except for a few which project upward from the body of the network or which extend down to the substrate. Filaments of the network intersect predominantly at right angles to form either T-shaped and X-shaped overlaps having striking perpendicularity or else Y-shaped intersections composed of filaments intersecting at 120-130 degrees angles. The actin filament concentration in the lamellae is high, with an average value of 12.5 mg/ml. The concentration was much more uniform in freeze-dried preparations than in critical point-dried specimens, indicating that there is less collapse associated with the freezing technique. The orthogonal actin network of the macrophage cortical cytoplasm resembles actin gels made with actin-binding protein. Reaction of cell cytoskeletons and of an actin gel made with actin-binding protein with anti-actin-binding protein IgG and anti-IgG-coated gold beads resulted in the deposition of clusters of gold at points where filaments intersect and at the ends of filaments that may have been in contact with the membrane before its removal with detergent. In the actin gel made with actin-binding protein, 75% of actin-fiber intersections labeled, and the filament spacing between intersections is consistent with that predicted on theoretical grounds if each added actin-binding protein molecule cross-links two filaments to form an intersection in the gel.(ABSTRACT TRUNCATED AT 400 WORDS)

Cell motion, contractile networks, and the physics of interpenetrating reactive flow

In this paper we propose a physical model of contractile biological polymer networks based on the notion of reactive interpenetrating flow. We show how our model leads to a mathematical formulation of the dynamical laws governing the behavior of contractile networks. We also develop estimates of the various parameters that appear in our equations, and we discuss some elementary predictions of the model concerning the general scaling principles that pertain to the motions of contractile networks.

Probing the structure of cytoplasm

We have used size-fractionated, fluorescent dextrans to probe the structure of the cytoplasmic ground substance of living Swiss 3T3 cells by fluorescence recovery after photobleaching and video image processing. The data indicate that the cytoplasm of living cells has a fluid phase viscosity four times greater than water and contains structural barriers that restrict free diffusion of dissolved macromolecules in a size-dependent manner. Assuming these structural barriers comprise a filamentous meshwork, the combined fluorescence recovery after photobleaching and imaging data suggest that the average pore size of the meshwork is in the range of 300 to 400 A, but may be as small as 200 A in some cytoplasmic domains.

A method for quantifying F-actin in chemotactic peptide activated neutrophils: study of the effect of tBOC peptide

The studies presented here characterize a simple, quantitative NBDphallacidin extraction assay for determining the F-actin content of fMLP-activated neutrophils. The NBDphallacidin extraction assay is based upon the specificity of NBDphallacidin binding to F-actin and the solubility of NBDphallacidin in methanol. Cells are fixed, permeabilized, and stained with NBDphallacidin; the cells are then pelleted, the bound NBDphallacidin is extracted into methanol, and the RFI (excite 465; emit 535) of the solution is determined. Binding of NBDphallacidin to neutrophils is saturable and 90% of bound NBDphallacidin is displaced by nonfluorescent phalloidin. The extraction of bound NBDphallacidin into methanol is complete and the excitation/emission characteristics of NBDphallacidin are not altered by extraction. The assay is relatively inexpensive, applicable to the study of cells in suspension or on substratum, allows kinetic studies with 5-10s time resolution, and is not affected by the shape of the cell or the distribution of the probe. We used the NBDphallacidin extraction assay to study the kinetics of fMLP-induced change in the F-actin content of neutrophils and the effect of tBOC peptide, an inhibitor of fMLP binding, on these changes. The extraction assay reveals a rapid, sequential fMLP-induced increase followed by a decrease in F-actin content. The tBOC peptide inhibits fMLP-induced actin polymerization. Addition of tBOC during fMLP-induced polymerization or at times when F-actin content is maximal enhances F-actin depolymerization. The rate of F-actin depolymerization is greater than or equal to fourfold faster in the presence than in the absence of tBOC. The results show that The NBDphallacidin extraction assay is useful for studying the kinetics of change in F-actin content of nonmuscle cells; fMLP receptor occupancy is required for fMLP-dependent polymerization but not depolymerization; and both the actin polymerizing and depolymerizing processes are active in the cell within 5 s after fMLP stimulation. Implications of these observations for understanding the observed increase and, then, decrease in F-actin content of fMLP-activated cells are discussed.

Improved fixation for immunofluorescence microscopy using light-activated 1,3,5-triazido-2,4,6-trinitrobenzene (TTB)

A new fixation method has been developed for immunofluorescent microscopy using the photosensitive compound 1,3,5-triazido-2,4,6-trinitrobenzene (TTB). Our results show that TTB-fixed cells are well preserved morphologically and that the cellular antigens are better preserved than conventionally fixed cells. By altering one condition at a time in the TTB fixation procedure and analyzing resulting fluorescent antitubulin staining patterns in mammalian tissue culture cells, an optimal procedure was developed. Cells fixed with TTB and stained with antitubulin, antiprekeratin, anti-intermediate filament, anti-alpha-actinin, anti-myosin, antiactin, or anticlathrin were compared with cells fixed by conventional methods and stained with the same antibody. The quality of immunofluorescence images of TTB fixed cells was the same as or better than that of conventionally fixed cells. The most dramatic improvement in image quality was seen when using antiprekeratin or antitubulin. In dividing cells, particularly in metaphase, fluorescent staining with antiactin and anti-alpha-actinin was relatively excluded from the spindle. Antimyosin, on the other hand, stained the spindle and surrounding area more heavily than the subcortical region. We suggest that after TTB fixation, the immunofluorescent patterns of these contractile proteins more closely reflect their relative concentrations in living cells. The exact mechanism for fixation by TTB is not yet known. However, our studies indicated that TTB fixation was not caused by the typical fast photoinduced nitrene diradical mechanism, but rather by some slower, temperature-dependent reaction of a photoactivation product of TTB with the cell.

Chemotactic peptide modulation of actin assembly and locomotion in neutrophils

To determine the relationship between the state of actin polymerization in neutrophils and the formyl-methionyl-leucyl-phenylalanine (fMLP)-induced changes in the locomotive behavior of neutrophils, the mean rate of locomotion (mROL), the percent G-actin, and the relative F-actin content of neutrophils were determined. The mROL was quantified by analysis of the locomotion of individual cells; the percentage of total actin as G-actin was measured by DNase I inhibition; and the F-actin was determined by fluorescence-activated cell sorter (FACS) analysis of nitrobenzoxadiazol (NBD)-phallacidin-stained neutrophils. Neutrophils stimulated with fMLP exhibit a change in their mROL that is biphasic and dose dependent. The mROL of neutrophils exposed to 10(-8) M fMLP, the KD, is 11.9 +/- 2.0 micron/min (baseline control 6.2 +/- 1.0 micron/min). At 10(-6) M fMLP, the mROL returns to baseline levels. Stimulation of neutrophils with fMLP also induces action polymerization. Evidence for actin polymerization includes a 26.5% reduction in G-actin and a twofold increase in the amount of NBD-phallacidin staining of cells as determined by FACS analysis. The NBD-phallacidin staining is not due to phagocytosis, is inhibited by phalloidin, requires cell permeabilization, and is saturable at NBD-phallacidin concentrations greater than 10(-7)M. The fMLP-induced increase in NBD-phallacidin staining occurs rapidly (less than 2 min), is temperature dependent, and is not due to cell aggregation. Since NBD-phallacidin binds specifically to F-actin, the increase in fluorescent staining of cells likely reflects an increase in the F-actin content of fMLP-stimulated cells. FACS analysis of NBD-phallacidin-stained cells shows that the relative F-actin content of neutrophils stimulated with 10(-11)-10(-8)M fMLP increases twofold and remains increased at concentrations greater than 10(-8)M fMLP. Therefore, the fMLP-induced increase in F-actin content of neutrophils as determined by FACS analysis of NBD-phallacidin-stained cells coincides with a decrease in G-actin and correlates with increased mROL of neutrophils under some (10(-11)-10(-8)M fMLP) but not all (greater than 10(-8)M fMLP) conditions of stimulation. Quantification of the F-actin content of nonmuscle cells by FACS analysis of NBD-phallacidin-stained cells may allow rapid assessment of the state of actin polymerization and correlation of that state with the motile behavior of nonmuscle cells.

The quantitation of G- and F-actin in cultured cells

An improved method to quantitate the amounts of filamentous (F-actin) and monomeric (globular) actin (G-actin) in cultured cells was developed. Cells are lysed into a myosin-containing buffer and F-actin is removed by centrifugation. The pelleted F-actin is then depolymerized to G-actin in a 1 mM ATP-containing buffer for 1 h before measuring the levels of G-actin using the DNase I inhibition assay. Partitioning of G-actin in the supernatant (greater than 95%) and recovery of actin in both fractions (greater than 85%) were measured by adding [3H]actin to cultured cells. Actin in the separated fractions is stable for at least 72 h at 0 degree C. Asynchronous monolayer cultures of Chinese hamster ovary (CHO) cells contain 2.5 +/- 0.2% of the total protein as actin with 72.4 +/- 5.7% as F-actin. About 10% of this F-actin is not associated with the readily sedimented Triton-cytoskeleton. CHO cells grown in suspension contain 55.8% of the actin as F-actin; following plating about 90 min is required for these cells to flatten and for the F-actin level to reach the monolayer value of about 70%.

Subcellular distribution of rhodamine-actin microinjected into living fibroblastic cells

The time course and pattern of incorporation of rhodamine-labeled actin microinjected into cultured fibroblastic cells were examined by fluorescence microscopy. Following microinjection, the fluorescent probe was incorporated rapidly into ruffling membranes, and within 5 min faintly fluorescent stress fibers were observed. Levels of fluorescence in ruffling membranes then tended to remain constant while fluorescence of the stress fibers continued to increase until approximately 20-min postinjection. Small, discrete regions of some microinjected cells displayed high levels of fluorescence that appeared initially approximately 5-10 min postinjection. I observed these small areas of intense fluorescence frequently near the cell periphery, which corresponded to focal contacts when examined with interference reflection optics. The results of this study show that a relationship exists between patterns of fluorescent actin incorporation in these cells and cellular areas or structures presumed to play a role in cell movement. These findings suggest that actin within stress fibers and the microfilament network of ruffling membranes undergoes a rapid turnover that may relate directly to the motility of the cell.

Cytoskeleton-associated glycoproteins from chicken sympathetic neurons and chicken embryo brain

A non-ionic detergent-insoluble fraction was obtained from pure cultures of chicken sympathetic neurons and further purified at the 10%-30% interface of a discontinuous density gradient. This fraction contains actin as its major component and approximately 20 further polypeptides some of which are glycosylated. Two conspicuous glycoproteins in this fraction, of molecular masses 130 kDa and 90 kDa, have been shown to bind to concanavalin A; in cultured neurons the 130-kDa glycoprotein may also be labelled with [3H]glucosamine and [3H]fucose. Both are restricted to one interface of the stepped sucrose gradient when cells are lysed in low ionic strength buffer and eluted with actin in the void volume of a Sepharose 6B column. Glycoproteins of the same molecular weight have been obtained by the same isolation procedure from 10-day-old chicken embryo brains. One-dimensional peptide maps show that the carbohydrate-containing peptides from brain and sympathetic neurons are closely similar if not identical. The glycoproteins are also present in sciatic nerve but cannot be detected in a detergent-insoluble form in rounded neurons - lacking axons - or fibroblasts. They might, therefore, be involved in the linkage of the axonal cytoskeleton to the plasma membrane.

Levels of filamentous and globular actin in Chinese hamster ovary cells throughout the cell cycle

Synchronous Chinese hamster ovary (CHO) cells were obtained by mitotic selection and the levels of globular (G) actin, filamentous (F) actin, and cytoskeletal-associated F-actin were determined as cells progressed through the cell cycle. Total actin levels remained quite constant when expressed as a percent of the total protein. An increase in F-actin occurred upon plating the mitotic cells, but this increase was shown to be a result of attachment to the substratum, since cells which remained attached during the second mitosis failed to show these changes. No large variation in the levels of either F-actin or cytoskeletal-associated F-actin occurred throughout the cell cycle. Therefore, changes in the morphology of the CHO cells which are accompanied by a reorganization of actin-containing microfilaments during the cell cycle are not accompanied by significant changes in the size of the monomeric actin pool.

Radiolabeled DNase, a potential indicator for noninvasive detection of tissue damage

Pancreatic DNase I was labeled with 131I or fluorescamine and injected IV into NMRI mice bearing a sarcoma 180. Of the injected tracer, 1.5%-2% was found to accumulate per g tumor. In sections of tumor tissue DNase was localized in damaged cells in solid and necrotic tumor regions. This binding is most probably due to specific interaction of DNase with actin, an ubiquitous cytoskeletal protein. Two-component blood clearance with a rapid first component (two-thirds of applied radioactivity) was observed. The labeled tumor could easily be visualized by gamma camera imaging. The findings suggest DNase to be a potent radiopharmaceutical for imaging damaged tissue, occurring in malignant tumors as well as in infarcts, and inflammatory lesions.

Heat(40 degrees C)-induced polypeptides in human embryonic fibroblasts

The synthesis of 3 polypeptides with molecular weights of 103-, 80- and 78-kdalton (K) was dramatically accelerated when the cultures of human embryonic fibroblasts were transferred from 37 to 40 degrees C. The induction of these polypeptides was not observed if actinomycin D was added to the cultures at the beginning of the rising of the temperature, indicating that this response may be mediated by increased transcription of their messenger RNA.

Actin and tubulin of normal and leukaemic lymphocytes

Double labelling and the isolation of actin- and tubulin-derived peptides were used to determine the amounts of these proteins in peripheral lymphocytes from normal donors and from patients with chronic lymphocytic leukaemia. As a precaution against proteolysis, samples were boiled before assay. The actin content of chronic-lymphocytic-leukaemia (CLL) lymphocytes was 8.1 +/- 2.1% of total protein, which was lower (P less than 0.05) than the amount (12.8 +/- 3.0%) of actin found in normal lymphocytes. The tubulin content of CLL lymphocytes was 4.4 +/- 1.5% of total protein, which was also significantly less (P less than 0.05) than that of normal lymphocytes, which was found to be 6.1 +/- 1.1%.

Creatine kinase from the bovine myometrium: purification and characterization

Creatine kinase from the smooth muscle of the cow uterus was extracted and purified by procedures involving precipitation of the enzyme in the presence of ethanol, cation exchange chromatography on phosphocellulose, gel filtration in Sephadex G-150 and anion exchange chromatography on DEAE-cellulose. The purified enzyme eluted as a single active peak after rechromatography on Sephadex G-150 with a molecular weight of about 82 000. Electrophoresis in polyacrylamide gels in tris-glycine buffer (pH 8.6) under non-denaturing conditions revealed a single enzymatically active protein band. In the presence of sodium dodecyl sulphate, the enzyme migrated as a single band in polyacrylamide gels at a mobility corresponding to a molecular weight of about 40 000 per subunit. Reaction with iodoacetamide indicated the presence of sulphydryl groups of differing susceptibility to alkylation. The purified enzyme was optimally active over a wide pH range (6.5-8.0). The Michaelis constants (Km) of the enzyme for MgADP and phosphoryl creatine (PCr) are 0.12 mM and 0.7 mM respectively, which are significantly lower than those for skeletal muscle CK. MgADP lowered the dissociation constant of the enzyme for PCr (from about 3.6 mM to 0.7 mM). Evidence is presented that the high affinity of the smooth muscle CK to MgADP and the MgADP-mediated facilitation of PCr binding might be key factors in the role of this enzyme in harnessing the energy reserves of the cell.

Vimentin and 70K neurofilament protein co-exist in embryonic neurones from spinal ganglia

The mesenchymal intermediate filament protein vimentin and the 70K component of neurofilament were detected by two-dimensional gel electrophoresis in cultures of pure sensory and sympathetic neurones derived from chick embryos. The identities of these neuronal intermediate filament proteins were confirmed by comparison of their molecular weights, isoelectric points, and peptide patterns from limited papain digestions with those of the corresponding proteins from fibroblasts and brain, respectively. A specific antibody to vimentin stained filamentous structures and colcemid-induced coils in both neurones and associated satellite cells. In contrast, a specific antibody to the 70K neurofilament protein stained these structures solely in neurones. This neurone-specific staining, as well as its molecular weight and isoelectric point, distinguishes the 70K neurofilament protein from the 68K neurofilament associated protein described by others, which has been claimed to resemble the tubulin assembly protein.