Interaction of the cytoskeleton with the plasma membrane

A role for the MAPK/ERK pathway in oligodendroglial differentiation in vitro: stage specific effects on cell branching

The mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) pathway is important for both long-term survival and timing of the progression of oligodendrocyte differentiation. Oligodendroglial cells treated with MEK inhibitor were distinguished by using stage specific markers: NG2 proteoglycan, A2B5, 2'3'nucleotide-cyclic 3'phosphodiesterase (CNPase) and myelin basic protein (MBP), and classified according to their morphology into different developmental stages. Treatment significantly increased the number of cells with more immature morphologies and decreased the number of mature cells. Furthermore, it increased the number of rounded cells that could not be classified into any of the oligodendroglial developmental stages. The strongest effects were usually observed shortly after treatment. Rounded cells were CNPase/MBP positive and they were not stained by anti-NG2 or A2B5, indicating that they were mature cells unable either to extend and/or to maintain their processes. These data showed an effect of the MAPK/ERK pathway on oligodendroglial branching, with possible consequences for the formation of the myelin sheath.

Advanced glycation end products induce actin rearrangement and subsequent hyperpermeability of endothelial cells�THIS ARTICLE HAS BEEN RETRACTED

This study aimed to determine the effects of advanced glycation end products (AGEs) on endothelial cytoskeleton morphology and permeability, and to detect the underlying signaling mechanisms involved in these responses. Cultured endothelial cells (ECs) were exposed to AGE-modified human serum albumin (AGE-HSA), and EC cytoskeletal changes were evaluated by observing fluorescence of F-actin following ligation with labeled antibodies. Endothelial permeability was detected by measuring the flux of TRITC-albumin across the EC monolayers. To explore the signaling pathways behind AGE-induced EC alteration, ECs were treated with either soluble anti-AGE receptor (RAGE) IgG, or the MAPK inhibitors PD98059 and SB203580 before AGE-HSA administration. To further elucidate possible involvement of the ERK and p38 pathways in AGE-induced EC changes, adenovirus-carried recombinant constitutive dominant-negative forms of upstream ERK and p38 kinases, namely MEK1(A) and MKK6b(A), were pre-infected into ECs 24 h prior to AGE-HSA exposure. AGE-HSA induced actin cytoskeleton rearrangement, as well as EC hyperpermeability, in a dose and time-dependent manner. The effects were attenuated in cells pretreated with anti-RAGE IgG, PD98059 or SB203580, respectively. EC pre-infection with MEK1(A) and MKK6b(A) also alleviated the effect of AGEs. Furthermore, adenovirus-mediated administration of activated forms of either MEK1 or MKK6b alone induced rearrangement of F-actin and hyperpermeability. The results indicate that ERK and p38 MAPK play important roles in the mediation of AGE-induced EC barrier dysfunction associated with morphological changes of the F-actin.

Effect of the polar headgroup of phospholipids on their interaction with actin

It is generally admitted that actin filaments are anchored to a membrane by membranar actin-binding-proteins. However, we found that actin may also interact directly with membrane phospholipids. The actin-phospholipid complex has been investigated at the air-water interface using a film balance technique. In order to probe the effect of the phospholipid headgroup on the actin-phospholipid interaction, we focus mainly on phospholipids that have the same acyl chain length but different headgroups. For all the phospholipids, the apparent area per molecule (the total surface divided by the number of lipid molecules) increases after the injection of the protein into the subphase, which suggests an intercalation of actin between the phospholipid molecules. This effect seems to be more important for DMPE and DMPS than for DMPG, suggesting that the headgroup plays an important role in this intercalation. The critical surface pressure associated to the liquid expanded-liquid condensed (LE-LC) phospholipid transition increases with the concentration of G-actin and thus suggests that G-actin acts as an impurity, simply competing as a surfactant at the air-water interface. On the other hand, F-actin affects the LE to LC transition of phospholipids differently. In this case, the LE to LC transition is broader and F-actin slightly decreases the critical surface pressure, which suggests that electrostatic interactions are involved.

Adsorption of actin at the air-water interface: A monolayer study

The intrinsic surface activity of the contractile protein actin has been determined from surface tension measurements using the Wilhelmy hanging-plate method. Actin, a very soluble protein, moves from the subphase to the air-water interface to make a film. In the absence of magnesium, actin is monomeric and is known as G-actin. During the compression the monomers change their conformation or orientation at the interface and they are then pushed reversibly into the subphase upon further compression. No collapse occurs. Actin monomers in the presence of magnesium become activated; at concentrations greater than some critical value, actin polymerizes to form filaments of F-actin. The actin filaments have a higher surface activity than the actin monomers either because they are more hydrophobic or because F-actin, a rigid polymer, is much more efficient at creating excluded volume. The actin filaments then form a rigid film at the interface that collapses when the surface area is decreased. At less than the critical concentration, the actin monomers are present in the subphase in their activated form. However, their concentration increases at the interface during film compression until the critical concentration is reached. The surface pressure isotherm in this case has the characteristics of a G-actin film at the beginning of the compression and of an F-actin film at the end of the compression process.

Surface film pressure of actin: interactions with lipids in mixed monolayers

The interactions of actin with neutral lipid films made from DLPC, and with positively charged films built from DLPC and stearylamine (SA), have been characterized by the monolayer technique. Injection of actin underneath an expanded lipid film produces an increase in the surface pressure that is consistent with a penetration of the lipid molecules by actin. This adsorption of actin to the lipid is more pronounced either with positively charged films or with Mg(2+) present in the sub-phase, suggesting that the mechanism involves an electrostatic attraction. During compression, the actin molecules are squeezed out into the sub-phase, carrying along some lipid molecules; this suggests a strong affinity of the lipids for actin. An analysis of the dilational modulus shows that when actin is found as monomers at the interface, the mixed actin-lipid film undergoes three phase changes upon compression. On the other hand, when actin is polymerized at the interface, the actin and the lipid form a rigid film for which the compressibility is mostly dominated by actin.

Cytoskeletal reorganization during the formation of oligodendrocyte processes and branches

During oligodendrocyte development, signals relevant to process formation must be transduced into appropriate changes in cytoskeletal organization. We have explored how microtubules and microfilaments interact during the outgrowth and branching of oligodendrocyte processes in culture. We observed that microfilaments are enriched in the peripheral region beneath the plasma membrane and constitute the major cytoskeletal element at the leading edge of the process, which is also enriched in the B-isoform of the non-muscle myosin II heavy chain. Microtubules form a dense bundle within the process and splay before extending into the leading edge and branches, following tracks laid by microfilaments. Pharmacologic disruption of microfilaments and microtubules compromised normal process outgrowth and branching. However, microtubules rapidly reinvaded most processes after removal of both antimicrotubule and antimicrofilament drugs, but the reinvasion was severely compromised if the antimicrofilament drug was retained. These results are consistent with the hypothesis that microfilaments guide the local reorganization of microtubules for the elongation of oligodendrocyte processes and the formation of new branches.

Temperature dependence of membrane sealing following transection in mammalian spinal cord axons

Using an in vitro sucrose-gap recording chamber, sealing of cut axons in isolated strips of white matter from guinea pig spinal cord was measured by recording the "compound membrane potential". This functional sealing was found to correlate well with anatomical resealing, measured by a horseradish peroxidase uptake assay. Near-complete functional and anatomical recovery of the axonal membrane occurred routinely within 60 min following transection at 37 degrees C in regular Krebs' solution. The rate of membrane potential recovery is exponential, with a time-constant of 20+/-5 min. The sealing process at 31 degrees C was similar to that at 37 degrees C, and was effectively blocked at 25 degrees C, under which condition most axons continued to take up horseradish peroxidase for more than 1h, and failed to substantially recover their membrane potential. Seventy-five percent of the cords transected at 40 degrees C had similar sealing behavior to those at 37 degrees C and 31 degrees C. The balance failed to seal the cut end. Two-dimensional morphometric analysis has shown that raising the temperature from 25 degrees C to above 31 degrees C significantly decreases axonal permeabilization to horseradish peroxidase (increases the sealing of transected ends) across all areas of a transverse section of spinal cord. Moreover, this enhancement of sealing exists across all axon calibers. Since severe cooling compromises membrane resealing, caution needs to be taken when hypothermic treatment (below 25 degrees C) is applied within the first 60 min following mechanical injury. In summary, we have found that at normal temperature (37 degrees C), nerve fibers repair their damaged membrane following physical injury with an hour. This is similar at mildly lower (31 degrees C) and relatively higher (40 degrees C) temperature, although some fibers tend to collapse under this febrile temperature. Moreover, severely low temperature (25 degrees C) hindered the repair of damaged membranes. Based on our study, caution is needed in treating spinal cord injury with low temperatures.

A putative spectrin-containing membrane skeleton in hyphal tips of Neurospora crassa

The apical plasma membrane (PM) is important in hyphal tip growth, where it may regulate tip extensibility via its association with an appropriate membrane skeleton (MS). By cell fractionation and immunocytochemistry we show that proteins with characteristics of actin, spectrin, and integrin are associated in a MS-like manner with the PM of Neurospora crassa hyphae. The spectrin-like protein in particular is highly concentrated at the PM in the region of maximum apical expansion. This protein shares with other spectrins immunoreactivity, molecular weight, PM association, and actin binding capacity. Its distribution in hyphae suggests that it is a dominant component of the MS in true fungi and is critical to hyphal tip growth.

The influence of microtubule integrity on plasma membrane fluidity in L929 cells

The aim of this work was to examine the possible influence of the integrity of the microtubule network on the plasma membrane fluidity of L929 mouse fibroblasts. The L929 cell line was selected for the ease of culture and the stability of its characteristics. The cells were treated with colchicine, nocodazole and vinblastine, three microtubule-depolymerizing drugs, at various concentrations and for various times. Membrane fluidity was assessed from fluorescence depolarization measurements with the plasma membrane probe TMA-DPH. Each of the drugs induced a significant, dose-dependent decrease in fluorescence anisotropy. The effect levelled off (5-7% decrease) after approximately 90 min of treatment, and could be unambiguously interpreted as resulting from an increase in membrane fluidity. The cumulative action of the drugs did not significantly increase the effect. The effects of colchicine and nocodazole could be reversed by incubation in drug-free medium, but not that of vinblastine. The results are discussed in correlation with the kinetics of the three drugs interaction with tubulin or microtubules. It is concluded that the microtubule integrity contributed to the high plasma membrane lipidic order, but less than other factors, like the lipid composition and the cholesterol content.

Disruption of filamentous actin diminishes hormonally evoked Ca2+ responses in rat liver

Previous studies have suggested a role for the actin cytoskeleton in hormonally evoked Ca2+ signaling in the liver. Here, we present evidence supporting a connection between filamentous actin (F-actin) organization and the ability of vasopressin and glucagon to increase cytosolic free-Ca2+ ([Ca2+]i) levels. F-actin was disrupted in hepatic cells by perfusion of rat liver with cytochalasin D. Epifluorescence microscopy of subsequently isolated cells showed reduced cortical fluorescent phalloidin staining in cytochalasin D-treated liver cells. Cytochalasin D pretreatment of liver cells reduced the vasopressin-stimulated elevation of [Ca2+]i by 60% and of glucagon by 50%. Experiments performed on cytochalasin D-treated cells using Mn2+ as an indicator of Ca2+ influx quenched fura-2 fluorescence signals following vasopressin administration. This indicates that a structurally intact cortical F-actin web is not a prerequisite for the influx of calcium. Therefore, the attenuation of the increase in cytosolic calcium observed in cytochalasin D-treated liver cells was likely caused either by the depletion of the calcium store by treatment with cytochalasin D or by the need for an intact cytoskeletal structure for its release. Because the resting level of calcium did not change in cells exposed to cytochalasin D, the latter is likely. The reduced [Ca2+]i response may be the mechanism by which cytochalasin D pretreatment inhibits vasopressin-induced metabolic effects. Cytochalasin D pretreatment also decreased the ability of glucagon to stimulate gluconeogenesis and reduced the stimulation of O2 uptake usually observed following glucagon administration. In conclusion, these results suggest that the hormonal elevation of [Ca2+]i and resultant activation of specific metabolic pathways require normal F-actin organization.

Resealing of transected myelinated mammalian axons in vivo: evidence for involvement of calpain

The mechanisms underlying resealing of transected myelinated rat dorsal root axons were investigated in vivo using an assay based on exclusion of a hydrophilic dye (Lucifer Yellow-biocytin conjugate). Smaller caliber axons (<5 microm outer diameter) resealed faster than larger axons. Resealing was Ca2+ dependent, requiring micromolar levels of extracellular [Ca2+] to proceed, and further accelerated in 1 mM Ca2+. Two hours after transection, 84% of axons had resealed in saline containing 2 mM Ca2+, 28% had resealed in saline containing no added Ca2+ and only 3% had resealed in the Ca2+ buffer BAPTA (3 mM). The enhancing effect of Ca2+ could be overcome by both non-specific cysteine protease inhibitors (e.g., leupeptin) and inhibitors specific for the calpain family of Ca2+ -activated proteases. Resealing in 2 mM Ca2+ was not inhibited by an inhibitor of phospholipase A2. Resealing in low [Ca2+] was not enhanced by agents which disrupt microtubules, but was enhanced by dimethylsulfoxide (0.5-5%). These results suggest that activation of endogenous calpain-like proteases by elevated intra-axonal [Ca2+] contributes importantly to membrane resealing in transected myelinated mammalian axons in vivo.

Protein crystals in Phycomyces sporangiophores are involved in graviperception

The sporangiophores of the zygomycete fungus Phycomyces blakesleeanus contain octahedral crystals with diameters of up to 5 micrometers in their vacuole. The crystals are associated with the intracellular membrane system. In tilted or horizontally placed sporangiophores, the crystals sediment to the respective lower face of the vacuole with a velocity of up to 100 micrometers per minute. The sedimentation is completed within about 2 minutes, well within the latency period for the negative gravitropic response of Phycomyces. Crystal-lacking mutant strains display a smaller maximal bending angle and a reduced gravitropic bending rate in comparison to the wild type. We therefore conclude that the crystals serve as statoliths for gravitropism in Phycomyces.

FTIR study of the thermal denaturation of alpha-actinin in its lipid-free and dioleoylphosphatidylglycerol-bound states and the central and N-terminal domains of alpha-actinin in D2O

Fourier transform infrared (FTIR) spectroscopy has been carried out to investigate the thermal denaturation of alpha-actinin and its complexes with dioleoylphosphatidylglycerol (DOPG) vesicles. The amide I regions in the deconvolved spectra of alpha-actinin in the lipid-free and DOPG-bound states are both consistent with predominantly alpha-helical secondary structure below the denaturation temperatures. Studies of the temperature dependence of the spectra revealed that for alpha-actinin alone the secondary structure was unaltered up to 40 degrees C. But, in the presence of DOPG vesicles, the thermal stability of the secondary structure of alpha-actinin increased to 55 degrees C. The thermal denaturation mechanisms of the lipid-free and DOPG-bound states of alpha-actinin also vary. The secondary structure of the lipid-free alpha-actinin changed to be predominantly unordered upon heating to 65 degrees C and above. Whereas, the original alpha-helical structure in the DOPG-bound alpha-actinin retained even at 70 degrees C, the highest temperature we examined. Analysis of the reduction in amide II intensities, which is due to peptide H-D exchange upon heating alpha-actinin in D2O, showed that partially unfolded states with increased solvent accessibility but substantial secondary structures could be observed from 35 to 40 degrees C only if DOPG vesicles were present. A so-called "protamine precipitation" method has been developed to purify the N-terminal domain of alpha-actinin by use of the fact that the central domain of alpha-actinin is negatively charged but the N-terminal domain is positively charged. Thermal denaturation of the central and N-terminal domains of alpha-actinin were then investigated with FTIR. The secondary structure of the N-terminal domain of alpha-actinin was found to be thermally sensitive below 35 degrees C, which is characterized as the increase of the alpha-helical structure at the expense of the random coil upon heating the N-terminal domain from 4 to 35 degrees C. The membrane-binding ability of the N-terminal domain of alpha-actinin was proposed in terms of the analysis of the local electrostatic properties of alpha-actinin and the assignment of the amide II bands in the FTIR spctra of alpha-actinin.

Interaction of cytoskeletal proteins with membrane lipids

Rapid and significant progress has been made in understanding lipid/protein interactions involving cytoskeletal components and the plasma membrane. Covalent and noncovalent lipid modifications of cytoskeletal proteins mediate their interaction with lipid bilayers. The application of biophysical techniques such as differential scanning colorimetry, neutron reflection, electron spin resonance, CD spectroscopy, nuclear magnetic resonance, and hydrophobic photolabeling, allow various folding stages of proteins during electrostatic adsorption and hydrophobic insertion into lipid bilayers to be analyzed. Reconstitution of proteins into planar lipid films and liposomes help to understand the architecture of biological interfaces. During signaling events at plasma membrane interfaces, lipids are important for the regulation of catalytic protein functions. Protein/lipid interactions occur selectively and with a high degree of specificity and thus have to be considered as physiologically relevant processes with gaining impact on cell functions.

Multiple forms of tubulin: different gene products and covalent modifications

Tubulin, the subunit protein of microtubules, is an alpha/beta heterodimer. In many organisms, both alpha and beta exist in numerous isotypic forms encoded by different genes. In addition, both alpha and beta undergo a variety of posttranslational covalent modifications, including acetylation, phosphorylation, detyrosylation, polyglutamylation, and polyglycylation. In this review the distribution and possible functional significance of the various forms of tubulin are discussed. In analyzing the differences among tubulin isotypes encoded by different genes, some appear to have no functional significance, some increase the overall adaptability of the organism to environmental challenges, and some appear to perform specific functions including formation of particular organelles and interactions with specific proteins. Purified isotypes also display different properties in vitro. Although the significance of all the covalent modification of tubulin is not fully understood, some of them may influence the stability of modified microtubules in vivo as well as interactions with certain proteins and may help to determine the functional role of microtubules in the cell. The review also discusses isotypes of gamma-tubulin and puts various forms of tubulin in an evolutionary context.

Immunocytochemical colocalization of desmin and vimentin in human fetal skeletal muscle cells

BACKGROUND

Desmin and vimentin are the major intermediate filaments in muscle. They have been extensively studied in animal experiments. This study is the first to identify the distribution and to analyse the correlation of desmin and vimentin in human fetal skeletal muscle. Vimentin might be replaced by or transformed into desmin during myogenesis in chick embryo, although the precise process remains to be elucidated. The aim of this report is to evaluate the ratio of desmin to vimentin in human fetal muscle.

METHODS

By double-labeling immunoelectron microscopy, desmin and vimentin intermediate filaments were localized in developing skeletal muscles of 20-29-week-old human fetuses. The ratio of desmin and vimentin was analyzed statistically.

RESULTS

Two sizes of colloidal gold particles, 5 nm (vimentin) and 10 nm (desmin), were distributed along the intermediate filaments. The commonest distance between gold particles was approximately 40-50 nm. Desmin and vimentin labeled with gold particles were arranged very close together in the same intermediate filament. The ratio of vimentin to desmin varied but the amount of vimentin decreased progressively from the undifferentiated myoblast to the differentiated myocytes. As the fetuses developed, desmin increased and vimentin decreased. Desmin and vimentin intermediate filaments were identified in the intermyofibrils of differentiated myocytes, in subsarcolemmal space, and in myoblast.

CONCLUSIONS

Desmin and vimentin were colocalized in the same intermediate filaments. More vimentin existed in the less differentiated myocytes, although a small number of desmin filaments were already found in undifferentiated myoblasts. These intermediate filaments may not only connect myofibril bundles, cell organelles, and cell membrane but also provide a basis for myofibrillogenesis that is similar to relation between connective fibers and parenchymal cells.

Targeting of nitric oxide synthase to endothelial cell caveolae via palmitoylation: implications for nitric oxide signaling

The membrane association of endothelial nitric oxide synthase (eNOS) plays an important role in the biosynthesis of nitric oxide (NO) in vascular endothelium. Previously, we have shown that in cultured endothelial cells and in intact blood vessels, eNOS is found primarily in the perinuclear region of the cells and in discrete regions of the plasma membrane, suggesting trafficking of the protein from the Golgi to specialized plasma membrane structures. Here, we show that eNOS is found in Triton X-100-insoluble membranes prepared from cultured bovine aortic endothelial cells and colocalizes with caveolin, a coat protein of caveolae, in cultured bovine lung microvascular endothelial cells as determined by confocal microscopy. To examine if eNOS is indeed in caveolae, we purified luminal endothelial cell plasma membranes and their caveolae directly from intact, perfused rat lungs. eNOS is found in the luminal plasma membranes and is markedly enriched in the purified caveolae. Because palmitoylation of eNOS does not significantly influence its membrane association, we next examined whether this modification can affect eNOS targeting to caveolae. Wild-type eNOS, but not the palmitoylation mutant form of the enzyme, colocalizes with caveolin on the cell surface in transfected NIH 3T3 cells, demonstrating that palmitoylation of eNOS is necessary for its targeting into caveolae. These data suggest that the subcellular targeting of eNOS to caveolae can restrict NO signaling to specific targets within a limited microenvironment at the cell surface and may influence signal transduction through caveolae.

Phosphorylation of calmodulin by plasma-membrane-associated protein kinase(s)

Plasma-membrane-associated protein kinase(s) from normal rat liver phosphorylates exogenous bovine brain calmodulin in the absence of Ca2+ and in the presence of histone or poly(L-lysine). Maximum levels of calmodulin phosphorylation are obtained at a poly(L-lysine)/calmodulin molar ratio of 0.4. Phosphoamino acid analysis revealed that calmodulin is phosphorylated on serine, threonine and tyrosine residues. Endogenous plasma-membrane-associated calmodulin was also phosphorylated by plasma-membrane-associated protein kinase(s) in the absence of added cationic protein or polypeptide. The identity of endogenous phosphocalmodulin was confirmed by immunoprecipitation with a specific anti-calmodulin monoclonal antibody. Ehrlich ascites tumor cell plasma membranes do not contain endogenous calmodulin. However, membrane-associated protein kinase(s) from these tumor cells phosphorylates bovine brain calmodulin in the presence of poly(L-lysine). These data demonstrate that phosphocalmodulin is present in liver plasma membranes and suggest that this post-translational modification could have a physiological role in this location.

Immunocytochemical localization of vinculin in muscle and nerve

A complete survey of the immunofluorescence distribution of the cytoskeletal protein vinculin in the normal skeletal muscle and peripheral nerve of humans and of different rodent species was performed. Our results enable us to localize vinculin in different types of adhesion plaques such as sarcolemmal costameres and neuromuscular and myotendinous junctions, but also in a fine intermyofibrillar lattice, possibly associated with intermediate filaments and/or with the triads. Moreover, we describe the presence of vinculin in junctional domains of several, previously unrecognized, specialized cells such as: the outer sheath of the muscle spindle capsule, the multilayered flat cells of the perineurium, the smooth muscle cells of epineurial blood vessels, and the endothelial cells in the endoneurium. These data call for a major role of vinculin in mechanisms of adhesion between cells, between cell and substrate and between intermyofibrillar components in the neuromuscular system. Such knowledge provides an anatomical background for studies on the possible pathological effects induced by an impairment in vinculin function.

Protein 1a: a major wheat germ agglutinin binding protein on the surface of human granulocytes associated with the cytoskeleton

Lectin-receptors on leukocyte and endothelial surfaces are becoming more important in the light of increasing evidence which implicates lectin-carbohydrate interactions in diverse physiological phenomena. This study reports the identification of a major 118 kDa granulocyte surface protein, (Protein 1 a) which binds the lectin wheat germ agglutinin (WGA), and is distinctly different from reported WGA binding granulocyte membrane proteins. Protein 1 a has been isolated from the Triton-soluble and Triton-insoluble lysates of normal individuals and patients with Chronic Myeloid Leukemia (CML) using a combination of differential solubilization, lectin affinity, ion exchange chromatography and HPLC. The protein from the detergent lysates of both normal and CML granulocytes has similar pI values, lectin affinities, and hydrophobicity. However, its solubility in Triton is different in the two cell types. In 71% of CML cases examined, Protein 1 a exhibits decreased Triton solubility suggesting its increased association with the cytoskeleton (CSK). Stimulation of normal granulocytes with WGA leads to the translocation of the soluble form of Protein 1 a to the Triton-insoluble fraction. This cytoskeletal recruitment of Protein 1 a is sustained only under conditions of excess WGA and occupied receptor. The CSK disruptive agent dihydrocytochalasin B (H2CB) releases the insoluble form of the receptor into the Triton-soluble fraction. Investigation of a CSK-involving process such as ligand internalization revealed that CML granulocytes exhibit slower kinetics of internalization of fluorescent WGA molecules.(ABSTRACT TRUNCATED AT 250 WORDS)

Internal actin filament dynamics in the presence of vinculin: a dynamic light scattering study

Analyses of dynamic light scattering data by stretched exponential fit show that vinculin has a negligible influence on internal actin filament dynamics and actin bending stiffness which contrasts with our previous observations with talin, another actin and vinculin-binding protein from focal adhesions. The results here agree with kinetic and rheologic measurements.

Mechanism of interaction between actin and membrane lipids: a pressure-tuning infrared spectroscopy study

Using pressure-tuning Fourier transform infrared spectroscopy to study an in vitro system consisting of actin and distearoyl-phosphatidylcholine (DSPC) liposomes, we have determined the mechanism of interaction between actin and membrane lipids. This interaction results in a significant conformational change in actin molecules. Analysis of the amide I band of actin shows an increase in the beta-sheets to alpha-helix ratio, in random turns, and in interactions between actin monomers. In the absence of lipids, the actin molecules are denatured by pressures of 8 x 10(8) Pa and more, which give rise to a random organization of the peptide chain. However, in the presence of DSPC liposomes, pressure greater than 2 x 10(8) Pa induces a change in actin conformation, which is dominated by strongly interacting beta-sheets. As the spectra of the lipid molecules are not changed by the presence of actin, the organization of the lipid molecules in the bilayer is not affected by the protein. It is concluded from these results that this interaction of actin with membrane lipids involves very few lipid molecules. These lipid molecules may interact with actin at a few specific sites on the protein.

Cytoskeletal modulation of the response to mechanical stimulation in human vascular endothelial cells

Possible interactions of cytoskeletal elements with mechanically induced membrane currents and Ca2+ signals were studied in human endothelial cells by using a combined patch-clamp and Fura II technique. For mechanical stimulation, cells were exposed to hypotonic solution (HTS). The concomitant cell swelling activates a Cl- current, releases Ca2+ from intracellular stores and activates Ca2+ influx. To interfere with the cytoskeleton, cells were loaded either with the F-actin-stabilizing agent phalloidin (10 mumol/l), or the F-actin-depolymerizing substance cytochalasin B (50 mumol/l). These were administered either in the bath or the pipette solutions. The tubulin structure of the endothelial cells was modulated by taxol (50 mumol/l), which supports polymerization of tubulin, or by the depolymerizing agent colcemid (10 mumol/l) both applied to the bath. Immunofluorescence experiments show that under the chosen experimental conditions the cytoskeletal modifiers employed disintegrate the F-actin and microtubuli cytoskeleton. Neither of these cytoskeletal modifiers influenced the HTS-induced Cl- current. Ca2+ release was not affected by cytochalasin B, taxol or colcemid, but was suppressed if the cells were loaded with phalloidin. Depletion of intracellular Ca2+ stores by thapsigargin renders the intracellular [Ca2+] sensitive to the extracellular [Ca2+], which is indicative of a Ca2+ entry pathway activated by store depletion. Neither cytochalasin B nor phalloidin affected this Ca2+ entry. We conclude that F-actin turnover or depolymerization is necessary for Ca2+ release by mechanical activation. The tubulin network is not involved. The Ca2+ release- activated Ca2+ entry is not modulated by the F-actin cytoskeleton.

Identification of functional domains in the cytoskeletal protein talin

The cytoskeletal protein talin potentially plays a key role in actin-membrane linkage. It is able to nucleate actin filament growth in vitro while binding simultaneously to lipid bilayers. Thrombin digestion of human platelet talin yields tow polypeptide domains of 200 kDa and 47 kDa. We have purified these fragments and analyzed their functional properties: the 200-kDa fragment was active in nucleating actin filament formation and reduced the viscosity of filamentous actin, comparable to the effects of the intact protein. The 47-kDa fragment was inactive in this respect. However, the 47-kDa polypeptide, but not the 200-kDa fragment, interacted specifically with large liposomes containing acidic phospholipids. This is demonstrated by selective, hydrophobic photolabeling of the 47-kDa fragment using phosphatidylserine liposomes containing trace amounts of a photoactivable phospholipid analogue and by selective co-sedimentation of this domain with the liposomes. The 200-kDa fragment, whether alone or in conjunction with the small fragment, neither incorporated significant amounts of label nor co-sedimented with the liposomes. We thus are able to attribute specialized functions to distinct domains on the talin molecule. These enable the protein to interact simultaneously with actin filaments and lipid membranes.

Lipid dynamics and peripheral interactions of proteins with membrane surfaces

A large body of evidence strongly indicates biomembranes to be organized into compositionally and functionally specialized domains, supramolecular assemblies, existing on different time and length scales. For these domains and intimate coupling between their chemical composition, physical state, organization, and functions has been postulated. One important constituent of biomembranes are peripheral proteins whose activity can be controlled by non-covalent binding to lipids. Importantly, the physical chemistry of the lipid interface allows for a rapid and reversible control of peripheral interactions. In this review examples are provided on how membrane lipid (i) composition (i.e., specific lipid structures), (ii) organization, and (iii) physical state can each regulate peripheral binding of proteins to the lipid surface. In addition, a novel and efficient mechanism for the control of the lipid surface association of peripheral proteins by [Ca2+], lipid composition, and phase state is proposed. The phase state is, in turn, also dependent on factors such as temperature, lateral packing, presence of ions, metabolites and drugs. Confining reactions to interfaces allows for facile and cooperative large scale integration and control of metabolic pathways due to mechanisms which are not possible in bulk systems.

Dystrophin-dependent efficiency of metabolic pathways in mouse skeletal muscles

Muscles from the mdx mouse (X-linked genetic disorder similar to Duchenne muscular dystrophy) lack dystrophin-associated transsarcolemmal proteins and show reduced maintenance metabolic rates. Here, microcalorimetric comparisons of metabolic stimulation by exogenous substrates in isolated muscles revealed substrate-selective limitation of chemical reaction rates through both glycolytic and TCA-cycle pathways, identical in slow- and fast-twitch mdx muscles. This systemic approach, as opposed to comparisons of single-enzyme activities, sheds new light on the function of dystrophin and associated proteins. The in vivo efficiency of metabolic pathways may depend on stabilization of enzyme complexes by dystrophin-associated elements of the cytoskeleton.

Neutrophil chemoattractant receptors and the membrane skeleton

Signal transduction via receptors for N-formylmethionyl peptide chemoattractants (FPR) on human neutrophils is a highly regulated process which involves participation of cytoskeletal-elements. Evidence exists suggesting that the cytoskeleton and/or the membrane skeleton controls the distribution of FPR in the plane of the plasma membrane, thus controlling the accessibility of FPR to different proteins in functionally distinct domains. In desensitized cells, FPR are restricted to domains which are depleted of G proteins but enriched in cytoskeletal proteins such as actin and fodrin. Thus, the G protein signal transduction partners of FPR become inaccessible to the agonist-occupied receptor, preventing cell activation. The mechanism of interaction of FPR with the membrane skeleton is poorly understood but evidence is accumulating that suggests a direct binding of FPR (and other receptors) to cytoskeletal proteins such as actin.

Cytoskeletal regulation of ion channel distribution in the tip-growing organism Saprolegnia ferax

Growing hyphal tips of the oomycete Saprolegnia ferax possess a tip-high gradient of stretch-activated ion channels permeable to calcium. These mechanosensitive channels appear to play a direct role in the polarized tip growth process. Treatment of S. ferax hyphae with cytochalasin E leads to the disruption of plasmalemma-associated, peripheral cytoplasmic actin populations and altered morphology of apical protoplasts, and eliminates the tip-high gradient of stretch-activated channels. Cytochalasin E did not alter the normal aggregation of stretch-activated channels. The density of spontaneous K+ channels was decreased in all regions of the hyphae after treatment with cytochalasin E. These results suggest that the peripheral F-actin network in the growing tip of S. ferax hyphae establishes or maintains the tip-high gradient of SA channels, either by the delivery of channel-bearing vesicles to the apex or by the interactions between the channels and the peripheral actin network.

Actin in stratified squamous keratinized epithelium

Actin filament distribution patterns were revealed in a stratified squamous keratinized epithelium using phalloidin-fluorescent and immunogold labeling techniques applied on bovine ruminal pilar as a model tissue. In non-keratinized cell types, actin concentrates on the microfilament-rich cellular cortex as well as on cytoplasmic processes and protrusions. In cornified cells labeling is distributed diffusely over the amorphous cytoplasm. A constant feature in all cell types is plasmalemmal labeling. Desmosomes exhibit deposition on their plasmalemmal leaflets, the dense central stratum and plaques. Desmosomal as well as cytoplasmic keratin filament bundles also label for actin, the latter often in a cross-banded manner. These cellular distribution patterns of actin filaments are discussed with respect to the significance of the microfilaments in the process of cell shape determination, stratification, and cell adhesion.

Cytoskeletal regulation of chemotactic receptors: molecular complexation of N-formyl peptide receptors with G proteins and actin

Signal transduction via receptors for N-formylmethionyl peptide chemoattractants (FPR) on human neutrophils is a highly regulated process. It involves direct interaction of receptors with heterotrimeric G-proteins and may be under the control of cytoskeletal elements. Evidence exists suggesting that the cytoskeleton and/or the membrane skeleton determines the distribution of FPR in the plane of the plasma membrane, thus controlling FPR accessibility to different proteins in functionally distinct membrane domains. In desensitized cells, FPR are restricted to domains which are depleted of G proteins but enriched in cytoskeletal proteins such as actin and fodrin. Thus, the G protein signal transduction partners of FPR become inaccessible to the agonist-occupied receptor, preventing cell activation. We are investigating the molecular basis for the interaction of FPR with the membrane skeleton, and our results suggest that FPR, and possibly other receptors, may directly bind to cytoskeletal proteins such as actin.

Deformations in the cytoplasmic membrane of Escherichia coli direct the synthesis of peptidoglycan. The hernia model

To explain the growth of the Gram-negative envelope and in particular how it could be strengthened where it is weakest, we propose in the hernia model that local weakening of the peptidoglycan sacculus allows turgor pressure to cause the envelope to bulge outwards in a hernia; the consequent local alteration in the radius of curvature of the cytoplasmic membrane causes local alterations in phospholipid structure and composition that determine both the synthesis and hydrolysis of peptidoglycan. This proposal is supported by evidence that phospholipid composition determines the activity of phospho-N-acetylmuramic acid pentapeptide translocase, UDP-N-acetylglucosamine:N-acetylmuramic acid-(pentapeptide)-P-P-bactoprenyl-N-acetylglucosamine transferase, bactoprenyl phosphate phosphokinase, and N-acetylmuramyl-L-alanine amidase. We also propose that the shape of Escherichia coli is maintained by contractile proteins acting at the hernia. Given the universal importance of membranes, these proposals have implications for the determination of shape in eukaryotic cells.

Evidence for a ternary interaction between alpha-actinin, (meta)vinculin and acidic-phospholipid bilayers

The cytoskeletal component vinculin has been demonstrated by hydrophobic photoradiolabelling, to insert into bilayers containing acidic phospholipids and trace amounts of a photoactivatable analogue of lecithin. It is shown in this study that the higher-molecular-mass variant metavinculin and alpha-actinin, also share this property. alpha-Actinin and vinculin were also shown to associate with phosphatidylserine liposomes by chromatography of protein/lipid mixtures on a Bio-Gel A-5m column. Furthermore, interesting differences in the behaviour of binary mixtures of these proteins, in the presence of phosphatidylserine liposomes, are shown. Thus, incubation of alpha-actinin with vinculin or metavinculin, prior to the addition of liposomes, strongly inhibited the photoradiolabelling of alpha-actinin under conditions in which the liposome surface was non-limiting, but enhanced the labelling of vinculin. In contrast, vinculin and metavinculin did not mutually influence their labelling. Using gel-filtration chromatography, it was shown that alpha-actinin still bound to the vinculin-liposome complex, under conditions similar to those used for hydrophobic photolabelling with a non-limiting lipid surface. In the presence of limiting amounts of liposomes, the alpha-actinin/vinculin ratio was markedly decreased in the liposome fractions. Our results suggest the formation of a ternary complex consisting of vinculin, alpha-actinin and phospholipids. In this complex, both proteins interact at the bilayer, resulting in an altered conformation of the two proteins and, as a consequence, in modified bilayer interactions.

Mevalonate controls cytoskeleton organization and cell morphology in thyroid epithelial cells

Blockade of mevalonate synthesis by the 3-hydroxy-3-methylglutaryl Coenzyme A reductase inhibitor mevinolin (lovastatin) causes FRTL-5 thyroid cells to undergo significant morphological changes; these include a transition from a flat, polygonal to a round shape, the development of cytoplasmic arborizations, and the loss of contact between neighboring cells. Immunofluorescence studies of cytoskeletal structures show that, at early times after administering the drug, and before the round phenotype develops, stress fibers disassemble while the peripheral actin filaments, which are adjacent to the cytoplasmic face of the plasma membrane, appear largely unaffected. Subsequently, when this cortical actin network becomes fragmented, cells start to round up and become separated from neighbors. Microtubules become disconnected from the plasma membrane and retract toward the cell center, although they do not appear depolymerized; indeed, at this stage, cytoplasmic elongations contain mostly intact microtubules. After exposure to mevinolin FRTL-5 cells also lose vinculin-related substrate contacts. Treatment of cells with either cycloheximide or colchicine abolishes morphological changes induced by mevinolin, suggesting that ongoing protein synthesis and microtubule integrity are prerequisites for the drug to be effective. Both cytoskeletal and morphological perturbations can be reversed by mevalonate, but not by cholesterol or the non-sterol derivatives of mevalonate such as dolichol, ubiquinone, and isopentenyladenine, individually or in combination. It is suggested that mevalonate deficiency may impair formation of isoprenylated proteins important for cytoskeletal organization and stability.

Statoliths pull on microfilaments: experiments under microgravity

Previous videomicroscopy of Chara rhizoids during parabolic flights of rockets showed that the weightless statoliths moved basipetally. A hypothesis was offered that the removal of gravity force disturbed the initial balance between this force and the basipetally acting forces generated in a dynamic interaction of statoliths with microfilaments (MFs). The prediction of this hypothesis that the statoliths would not be displaced basipetally during the microgravity phase (MG-phase) after disorganizing the MFs was tested by videomicroscopy of a rhizoid treated with cytochalasin D (CD) immediately before the flight. The prediction was fully supported by the flight experiment. Additionally, by chemical fixation of many rhizoids at the end of the MG-phase it was shown that all rhizoids treated with CD before the flight had statoliths at the same location. i.e., sedimented an the apical cell wall, while all untreated rhizoids had statoliths considerably displaced basipetally from their normal position. Thus, a dynamical interaction involving shearing forces between MFs and statoliths appears highly probable.

Zinc deficiency in the rat alters the lipid composition of the erythrocyte membrane Triton shell

The effect of dietary zinc deficiency on the lipid composition of the erythrocyte membrane Triton shell was determined. Weanling male Wistar rats were fed an egg white-based diet containing < 1.0 mg Zn/kg diet ad libitum. Control rats were either pair-fed or ad libitum-fed the basal diet supplemented with 100 mg Zn/kg diet. A Zn refed group was fed the -Zn diet until day 18 and then pair-fed the +Zn diet until day 21. Dietary Zn deficiency caused an increased cholesterol/phospholipid ratio in Triton shells compared to those from pair-fed controls. Zn deficiency caused a decreased double bond index of fatty acids in phosphatidylinositol (PI) and phosphatidylcholine (PC); there was a decreased proportion of 18:2n-6 and 22:4n-6 in PC and 20:4n-6 in PI as compared to that found in pair-fed controls. All glycerophospholipids that were retained in the shell had a lower double bond index and increased content of 16:0 and/or 18:0 relative to the phospholipid in the intact membrane.

Cytoskeleton--plasma membrane interactions

Proteins at the boundary between the cytoskeleton and the plasma membrane control cell shape, delimit specialized membrane domains, and stabilize attachments to other cells and to the substrate. These proteins also regulate cell locomotion and cytoplasmic responses to growth factors and other external stimuli. This diversity of cellular functions is matched by the large number of biochemical mechanisms that mediate the connections between membrane proteins and the underlying cytoskeleton, the so-called membrane skeleton. General organizational themes are beginning to emerge from examination of this biochemical diversity.

Phosphorylation of vinculin in human platelets spreading on a solid surface

Vinculin is a cytoskeletal protein believed to be involved in linking microfilaments to the cell membrane. It is a substrate for the Ca(2+)- and phospholipid-dependent protein kinase C. We show here that when human platelets attach and spread on a solid surface, the alpha isoforms of vinculin become phosphorylated at serine and/or threonine residues. Phosphorylation is dependent on adhesion to a surface, since suspended, unattached platelets can produce filopodia but no phosphorylation of vinculin. Phosphorylation is also dependent on actin polymerization, as it does not occur when platelets had been pretreated with cytochalasin B. Most likely, protein kinase C is responsible for the phosphorylation of vinculin, since phosphorylation also occurs when platelets are treated with a phorbol ester, which activates protein kinase C, and is blocked by treatment with a staurosporine derivative which inhibits this enzyme. These results suggest that phosphorylation plays a role in anchoring vinculin at sites of microfilament-membrane interaction.

Effect of hypothyroidism on different forms of actin in rat cerebral neuronal cultures studied by an improved DNase I inhibition assay

An improved DNase I inhibition assay for the filamentous actin (F-actin) and monomeric actin (G-actin) in brain cells has been developed. Unlike other methods, the cell lysis conditions and postlysis treatments, established by us, inhibited the temporal inactivation of actin in the cell lysate and maintained a stable F-actin/G-actin ratio for at least 4-5 h after lysis. The new procedure allowed separate quantitation of the noncytoskeletal F-actin in the Triton-soluble fraction (12,000 g, 10 min supernatant) that did not readily sediment with the Triton-insoluble cytoskeletal F-actin (12,000 g, 10 min pellet). We have applied this modified assay system to study the effect of hypothyroidism on different forms of actin using primary cultures of neurons derived from cerebra of neonatal normal and hypothyroid rats. Our results showed a 20% increase in the Triton-insoluble cytoskeletal F-actin in cultures from hypothyroid brain relative to normal controls. In the Triton-soluble fraction, containing the G-actin and the noncytoskeletal F-actin, cultures from hypothyroid brain showed a 15% increase in G-actin, whereas the F-actin remained unaltered. The 10% increase in total actin observed in this fraction from hypothyroid brain could be totally accounted for by the enhancement of G-actin. The mean F-actin/G-actin ratio in this fraction was about 30% higher in the cultures from normal brain compared to that of the hypothyroid system, which indicates that hypothyroidism tends to decrease the proportion of noncytoskeletal F-actin relative to G-actin.

Mycoplasma pneumoniae cytadherence phase-variable protein HMW3 is a component of the attachment organelle

The subcellular location of the phase-variable cytadherence-accessory protein HMW3 in Mycoplasma pneumoniae has been examined by biochemical and immunoelectron microscopic techniques. Analysis by Western blot (immunoblot) with HMW3-specific antiserum established the presence of this protein within the M. pneumoniae Triton X-100-insoluble fraction or triton shell. Immunogold labeling of Triton-extracted mycoplasmas with affinity-purified antibodies localized HMW3 to the terminal knob on the rodlike extensions of the triton shell, a location that would correspond to the adherence organelle in whole mycoplasmas. Treatment of triton shells with KI resulted in the selective removal of the adherence-accessory proteins HMW1 to HMW4. Analysis of these triton shells by transmission electron microscopy revealed dramatic ultrastructural changes in the filamentous network and core structure. Immunogold labeling of KI-extracted shells reflected the removal of HMW3 from the disrupted tip structure. An examination of ultrathin sections of wild-type cells by transmission electron microscopy following labeling with HMW3-specific antibodies provided further evidence for the nonrandom distribution of HMW3 and its localization to the terminal portion of filamentous cell extensions. Most colloidal gold molecules were associated with the cell interior, but limited peripheral labeling of the terminal region was also observed. Postfixation antibody labeling of whole cells suggested limited exposure of HMW3 on the mycoplasma surface at the tip structure. However, prefixation antibody labeling failed to indicate surface exposure, raising some uncertainty regarding the relationship of HMW3 with the mycoplasma membrane.

Potential role of the viral protease in human immunodeficiency virus type 1 associated pathogenesis

Infection with the human immunodeficiency virus type 1 (HIV-1) results in a variety of pathological changes culminating in the acquired immune deficiency syndrome (AIDS). While most of these changes can readily be accounted for either by direct effects of HIV-1 on the immune system or by indirect effects of secondary infectious agents as a result of faulty immune surveillance, the direct cause for a number of disease states, including some neuropathies, myopathies, nephropathy, thrombocytopenia, wasting syndromes and increased incidence of cancers (primarily lymphoma) has remained an enigma. We have recently shown that the HIV-1 protease, a viral encoded enzyme necessary for virus maturation and infectivity, can cleave a variety of host cell cytoskeletal proteins in vitro. Potential substrates for the HIV-1 protease are found in all of the cell types affected in these unexplained diseases. Recent proposals suggest that elements of the cytoskeleton may play an important role in the regulation of large scale genetic regulation. We propose that some of the degenerative changes associated with infection by HIV-1 are a direct consequence of cleavage of host cell cytoskeletal proteins, which in turn may be responsible for the increased incidence of cancer in HIV-1 infected individuals as a result of the perturbation of the regulation of gene expression by cytoskeletal components.

Association of blebbing with assembly of cytoskeletal proteins in ATP-depleted EL-4 ascites tumour cells

ATP depletion in EL-4 ascites tumour cells rapidly induced the changes in cell morphology (blebbing), cytoskeletal protein assembly and finally resulted in cell death. After 1 hr of incubation with 2 microM rotenone (inhibitor of respiration) in glucose-free medium, when ATP level was 4% of the initial level, there were increases in triton-insoluble actin and vinculin levels (2.5-fold and 2.8-fold, respectively) and 44% of cells showed blebs; such treatment damaged cells irreversibly. Ca2+ removal did not diminish the effect of ATP depletion on cytoskeleton, blebbing and cell death, although the elevation of free intracellular Ca2+ in rotenone-treated cells was prevented. The role of ATP in maintaining cytoskeleton and cell shape is discussed.

Insulin induces changes in the subcellular distribution of actin and 5'-nucleotidase

An increase in the amount of actin associated with the plasma membrane was visualized by immunocytochemistry 5 min after the addition of insulin to Krebs II ascites tumour cells maintained in serum-free medium. At 1 h of incubation the rim of fluorescence at the plasma membrane as measured by image analysis, was about 30% more intense than in control cells indicating that the initial accumulation of actin at the plasma membrane was not of a transient nature. Since an increase in the total cellular actin content in ascites cells did not occur until after a lag period of about 15 min then the increased amount of actin at the plasma membrane seen at 5 min was attributed to a stimulation of the polymerization of actin. An increase in the association of actin at the plasma membrane was also observed in 3T3 fibroblasts in areas of membrane ruffling, while in some cells there was also increased actin accumulation in the perinuclear area. The putative plasma membrane-microfilament linking protein 5'-nucleotidase was shown to be present in association with actin in the cytoskeletal fraction. Incubation of cells with insulin resulted in a shift of the enzyme toward the bottom of gradients indicating association with actin filaments of a greater length. The results demonstrate that insulin causes a stimulation of actin polymerization and that the hormone can be therefore assigned a role in the regulation of the cytoskeleton.

Spreading depression increases immunohistochemical staining of glial fibrillary acidic protein

Reactive astrocytosis is a process by which astrocytes respond to brain injury by showing an increase in glial fibrillary acidic protein (GFAP) staining that is associated with hypertrophy and/or hyperplasia of these cells. Because spreading depression (SD) is a perturbation uncomplicated by neuronal necrosis and is seen in both in vivo and in vitro neural structures, we sought to determine whether SD was a sufficient stimulus to induce enhanced GFAP staining. SD was elicited in anesthetized rats by application of KCI to parietal cortex for 3 hr; equimolar NaCI was applied to contralateral cortex. SD was confirmed by monitoring DC potentials in frontal neocortices. Animals were allowed to recover for 48 hr, and their brains were processed for semiquantitative and computer-based analyses of GFAP staining intensity. Experimental GFAP staining was referenced to contralateral control levels. Neocortical SD (13-37 SDs) was associated with a significant (p less than 10(-4)), 43% increase in GFAP staining intensity, which remained statistically greater than normal for more than 2 weeks. If SD was inhibited by combined hyperoxia and hypercarbia, only a nonsignificant (p greater than 0.20), 7% increase in GFAP staining was seen. Thus, SD may be a useful physiologic process with which to begin to explore the cellular mechanisms that induce the transformation of normal astrocytes into reactive species.

Adenosine diphosphate-ribosylation of G-actin by botulinum C2 toxin increases endothelial permeability in vitro

The endothelial cytoskeleton is believed to play an important role in the regulation of endothelial permeability. We used botulinum C2 toxin to perturb cellular actin and determined its effect on the permeability of endothelial cell monolayers derived from porcine pulmonary arteries. The substrate for botulinum C2 toxin is nonmuscle monomeric actin which becomes ADP-ribosylated. This modified actin cannot participate in actin polymerization and, in addition, acts as a capping protein. Exposure of endothelial cell monolayers to botulinum C2 toxin resulted in a dose- (3-100 ng/ml) and time-dependent (30-120 min) increase in the hydraulic conductivity and decrease in the selectivity of the cell monolayers. The effects of C2 toxin were accompanied by a time- and dose-dependent increase in ADP-ribosylatin of G-actin. G-Actin content increased and F-actin content decreased time- and dose-dependently in C2 toxin-treated endothelial cells. Phalloidin which stabilizes filamentous actin prevented the effects of botulinum C2 toxin on endothelial permeability. Botulinum C2 toxin induced interendothelial gaps. The effects occurred in the absence of overt cell damage and were not reversible within 2 h. The data suggest that the endothelial microfilament system is important for the regulation of endothelial permeability.