Interaction of the plasma membrane with the cytoskeleton: an overview

Effects of humidified and dry air on corneal endothelial cells during vitreal fluid-air exchange

PURPOSE

To report the immediate anatomic and functional alterations in corneal endothelial cells following use of humidified air and dry air during vitreal fluid-air exchange in rabbits.

DESIGN

Experimental study.

METHODS

Rabbits undergoing pars plana vitrectomy and lensectomy were perfused with either dry or humidified air during fluid-air exchange for designated durations. Three different experiments were performed. First, control and experimental corneas were examined by scanning electron microscopy (SEM). Second, corneas were stained with Phalloidin-FITC and examined by fluorescein microscopy. Finally, third, transendothelial permeability for carboxyfluorescein was determined using a diffusion chamber.

RESULTS

While different from the corneal endothelial cells, those cells exposed to humidified air were less stressed than cells exposed to dry air by SEM. Actin cytoskeleton was found highly disorganized with dry air exposure. Humidified air maintained the normal actin cytoskeleton throughout the 20 minutes of fluid-air exchange. Paracellular carboxyfluorescein leakage was significantly higher in dry air insufflated eyes compared with that of the humidified air after 5, 10, and 20 minutes of fluid-air exchange (P =.002, P =.004, and P =.002, respectively).

CONCLUSIONS

Dry air stress during fluid-air exchange causes significant immediate alterations in monolayer appearance, actin cytoskeleton, and barrier function of corneal endothelium in aphakic rabbit eyes. Use of humidified air largely prevents the alterations in monolayer appearance, actin cytoskeleton, and barrier function of corneal endothelial cells.

Association of globular beta-actin with intracellular lipid droplets in rat adrenocortical cells and adipocytes

Proteins located on the surface of lipid droplets may mediate intracellular lipid metabolism. In the present study, immunofluorescent staining and polyacrylamide gel electrophoresis demonstrated that actin (43 kD) is associated with isolated intracellular lipid droplets of rat adrenocortical cells and adipocytes. Two-dimensional gel electrophoresis and immunoblot analysis further confirmed that the lipid droplet-associated actin is the beta isoform. In cultured adrenocortical cells, stress fibers and the surface of intracellular lipid droplets were labeled with anti-beta-actin monoclonal antibody, whereas FITC-phalloidin staining did not mark the rim of lipid droplets. The present results provide the first morphological evidence that globular beta-actin is associated with intracellular lipid droplets. This significant association of actin with the surface of lipid droplets suggests that beta-actin might be involved in the regulation of intracellular lipid metabolism, particularly providing insight into the important transport of lipid constituents.

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.

Cytotoxic effects of 2-butoxyethanol in vitro are related to butoxyacetaldehyde, an intermediate oxidation product

Ethylene glycol ethers belong to a group of solvents with a wide spectrum of applications, particularly because of their compatibility to both hydrophilic and lipophilic systems. Especially ethylene glycol monobutyl ether (2-butoxyethanol, BE) is widely used as a key ingredient in many industrial and consumer cleaning products. Therefore, the risk of human exposure and toxicity by BE as well as its potential for environmental contamination have to be carefully evaluated. By using an established kidney epithelial cell line from the proximal tubule (opossum kidney cells), we investigated the effects of BE on viability, proliferative activity, volume and the organization of the intracellular cytoskeleton of the cells. The experiments were performed with freshly used BE and BE that had been stored at room temperature in the original packing for 3 months after use. After this period of storage the latter BE contained-besides butyraldehyde and n-butanol-0.5 vol% butoxyacetaldehyde (BAL) as measured by capillary gas chromatography and mass spectrometry. Freshly used BE did not cause a toxic effect in the in vitro assays at all concentrations tested (up to 1 mg/ml). In contrast, stored BE which contained BAL reduced cell viability and mitotic activity in a dose-dependent manner. The effective concentration of stored BE causing a 50% loss in cell viability (EC(50/24h)) was calculated to be 1 mg/ml. The toxic effect of stored BE also resulted in alterations of cell morphology and a depolymerization of actin-containing stress fibers. Moreover, administration of stored BE also caused a dose-dependent cell volume increase by the uptake of water, pointing to a necrotic process. In addition, synthesized BAL with a purity of 73.5% (gas chromatography) was also tested and caused an EC(50/24h) of 15 μg/ml, which is a 70-fold lower concentration when compared with stored BE. The present study provides evidence that BE possesses only a low cytotoxic potential in vitro, whereas the corresponding BAL, an intermediate in the oxidation process of BE to butoxyacetic acid, has marked toxic effects. The occurrence of the aldehyde might explain the predominant hematological effects of BE observed in vivo.

Functional significance of cell volume regulatory mechanisms

To survive, cells have to avoid excessive alterations of cell volume that jeopardize structural integrity and constancy of intracellular milieu. The function of cellular proteins seems specifically sensitive to dilution and concentration, determining the extent of macromolecular crowding. Even at constant extracellular osmolarity, volume constancy of any mammalian cell is permanently challenged by transport of osmotically active substances across the cell membrane and formation or disappearance of cellular osmolarity by metabolism. Thus cell volume constancy requires the continued operation of cell volume regulatory mechanisms, including ion transport across the cell membrane as well as accumulation or disposal of organic osmolytes and metabolites. The various cell volume regulatory mechanisms are triggered by a multitude of intracellular signaling events including alterations of cell membrane potential and of intracellular ion composition, various second messenger cascades, phosphorylation of diverse target proteins, and altered gene expression. Hormones and mediators have been shown to exploit the volume regulatory machinery to exert their effects. Thus cell volume may be considered a second message in the transmission of hormonal signals. Accordingly, alterations of cell volume and volume regulatory mechanisms participate in a wide variety of cellular functions including epithelial transport, metabolism, excitation, hormone release, migration, cell proliferation, and cell death.

Activation of protein kinase C in rat basophilic leukemia cells stimulates increased production of phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate: correlation with actin polymerization

Cross-linking of the immunoglobulin E receptor on rat basophilic leukemia (RBL)1 cells by multivalent antigen activates phosphatidylinositol (PI) kinase and phosphatidylinositol 4-phosphate (PIP) kinase leading to the increased production of PIP and phosphatidylinositol 4,5-bisphosphate (PIP2). Activators of protein kinase C (PKC), such as phorbol myristate acetate (PMA) and the synthetic diacylglycerol, 1,2-dioctanoyl-sn-glycerol (diC8), were found to have the same effect even though PMA and diC8 do not cause the activation of phospholipase C. Although the kinetics are different depending on the stimulant, activation of PKC using multivalent antigen, PMA or diC8 also causes the polymerization of actin and an increase in the F-actin content of the cells. In all cases, a good correlation was observed between F-actin levels, activation of PI and PIP kinases, and the increased production of PIP and PIP2. However, in the case of antigen, there is no correlation between actin polymerization and the total amount of PIP and PIP2. Staurosporine, an inhibitor of protein kinases, blocks the F-actin response and the increased synthesis of PIP and PIP2 with similar dose dependencies. Furthermore, depletion of PKC activity through long-term exposure to PMA, inhibited both the F-actin response and the increased synthesis of PIP and PIP2 induced by either DNP-BSA or diC8. These results suggest that activation of PKC precedes the activation of PI and PIP kinases and that under certain circumstances activation of the kinases and the increased synthesis of PIP and PIP2 may be involved in the polymerization of actin in RBL cells, possibly through the interaction of the polyphosphoinositides with actin-binding proteins such as gelsolin and profilin.

Photodynamic effects of the cationic porphyrin, mesotetra(4N-methylpyridyl)porphine, on microtubules of HeLa cells

The treatment of HeLa human carcinoma cells with mesotetra(4N-methylpyridyl)porphine (T4MPyP) and blue light led to damage of the microtubules (MTs). The morphologies of interphase MTs and the mitotic spindle apparatus were analysed by immunofluorescence staining of alpha-tubulin. The extent of MT damage depended on the light dose and the time after photodynamic treatment. After a period of 1 h after irradiation with doses of 0.3 or 1.5 J cm-2 (sublethal conditions, corresponding to survival rates of 90% and 60% respectively), the normal MT network arrangement of interphase cells and the mitotic spindle apparatus of many cells were clearly affected. However, these effects were found to be transient, and several hours after irradiation most MTs resumed control morphology. Higher irradiation doses (4.5 J cm-2, lethal conditions, less than 10% cell survival) resulted in the irreversible alteration of interphase and mitotic MTs. The change in MT organization appeared to be the reason for the observed increase in the mitotic index (MI) after sublethal doses. The largest increase in MI was detected 6 h after treatment (twofold increase over untreated cells) for both sublethal light doses. Most of the cells in mitosis corresponded to metaphase, the number of ana-telophase cells being greatly reduced for the first hours after irradiation with a dose of 1.5 J cm-2. The results, which resemble those observed with inhibitors of MT assembly, suggest that MTs might represent an important target for T4MPyP action.

Actin, alpha-actinin, and spectrin with specific associations with the postacrosomal and acrosomal domains of bovine spermatozoa

BACKGROUND

Characteristic membrane changes in spermatozoa culminating in acrosome reaction and sperm-egg fusion, and suspected involvement of actin-containing cytoskeleton in membrane changes in general, prompted us to investigate subcellular distribution of actin and actin-binding proteins in bovine spermatozoa subjected to various extractions which sequentially denude the sperm investments.

METHODS

Spermatozoa were treated with either 1% SDS, 0.1% Triton X-100, 0.1% Hyamine, or 1 M MgCl2 or were sonicated. Immunostaining of actin, alpha-actinin, spectrin, and acrosin as well as electron microscopic analysis of extracted spermatozoa were carried out.

RESULTS

Extractions caused evagination of the acrosomal lamina which retained focal contacts with the inner acrosomal membrane. Extractions further revealed lateral prongs at the anterior border of the postacrosomal sheath. Labeling for alpha-actinin and spectrin was localized in the acrosin-positive acrosomal lamina, neck, and principal piece, the latter containing also relatively extraction-resistant oligomeric or polymerized actin. In the postacrosomal area, actin was accumulated in the extraction-resistant posterior ring structure and anteriorly at the sites apparently related to the lateral prongs. Notably, spectrin reactivity was enhanced by MgCl2 in head, neck, and principal piece, and sonication abolished cytoskeletal immuno-reactivity in the head.

CONCLUSIONS

Destabilization of membranes with selected extractions induces changes in the acrosomal lamina mimicking acrosomal vesicle formation. The lateral prongs and posterior ring structure, respectively, may serve as anterior and posterior anchors for the extraction-resistant post-acrosomal sheath. The lateral prongs may also be a merger zone for actin, alpha-actinin, and spectrin with important implication on sperm function. The latter two proteins may be involved in acrosomal vesicle formation. It is apparent that extractions have a significant effect on the detectability of sperm cytoskeletal elements.

Morphological alterations and cytoskeletal reorganization in opossum kidney (OK) cells during osmotic swelling and volume regulation

Cells from a variety of tissues regulate their volume when exposed to anisotonic conditions. After exposure of cells to hypotonic conditions, the rapid phase of cell swelling is followed by a slower phase of cell shrinkage towards the initial volume. The present study investigates morphological alterations of adherent and fully spread cells after exposure to hypotonic conditions and the reorganization of cytoskeletal components such as F-actin, actin-binding proteins, microtubules and intermediate-sized filaments. We used cells of a continuous epithelial cell line from the opossum kidney (OK cells), which were exposed to hypotonic conditions for a period of 60 min at 25 degrees C. The osmolarity was reduced by 40% from 320 mosmol/l (isotonic conditions) to 192 mosmol/l (hypotonic conditions). The initial swelling after exposure of OK cells to hypotonic conditions caused enhanced ruffling membrane activity, formation of lamellipodia and an extended space between adjacent cells which was caused by a more rounded cell shape. Moreover, the height of cells located in the centre of cell clusters increased by 32 +/- 8% (mean value +/- SEM) as checked by morphometric analysis of the vertical distance between the apical and basolateral F-actin domain. Although the fluorescence intensity and organization of F-actin in a horizontal direction remained unaltered during cell swelling, we observed a loss of periodicity and irregular distribution of myosin aggregates and a partial rearrangement of vimentin filaments in the form of short fragments. In all experiments the organization of microtubles was observed to be unaltered.(ABSTRACT TRUNCATED AT 250 WORDS)

Polymerization of actin in RBL-2H3 cells can be triggered through either the IgE receptor or the adenosine receptor but different signaling pathways are used

Crosslinking of the IgE receptor on rat basophilic leukemia (RBL) cells using the multivalent antigen DNP-BSA leads to a rapid and sustained increase in the filamentous actin content of the cells. Stimulation of RBL cells through the adenosine receptor also induces a very rapid polymerization of actin, which peaks in 45-60 s and is equivalent in magnitude to the F-actin response elicited through stimulation of the IgE receptor. However, in contrast to the IgE mediated response, which remains elevated for over 30 min, the F-actin increase induced by the adenosine analogue 5'-(N-ethylcarboxamido)-adenosine (NECA) is relatively transient and returns to baseline values within 5-10 min. While previous work has shown that the polymerization of actin in RBL cells stimulated through the IgE receptor is mediated by protein kinase C (PKC), protein kinase inhibitors have no effect on the F-actin response activated through the adenosine receptor. In contrast, pretreatment of the cells with pertussis toxin completely inhibits the F-actin response to NECA but has relatively little effect on the response induced through the IgE receptor. Stimulation of RBL cells through either receptor causes increased production of phosphatidylinositol mono-phosphate (PIP) and phosphatidylinositol bis-phosphate (PIP2), which correlates with the F-actin response. Production of PIP and PIP2 may be important downstream signals since these polyphosphoinositides are able to regulate the interaction of gelsolin and profilin with actin. Thus the polymerization of actin can be triggered through either the adenosine receptor or the IgE receptor, but different upstream signaling pathways are being used. The IgE mediated response requires the activation of PKC while stimulation through the adenosine receptor is PKC independent but involves a G protein.

Effect of soman and sarin on phosphatidylcholine asymmetry in the electroplax from the electric eel

Some effects of organophosphorus anticholinesterase compounds that are unrelated to cholinesterase inhibition and that are sometimes long lasting may be due to alterations at the cellular membrane level. Phosphatidylcholine exchange protein was used to assess the effects of sarin and soman on phosphatidylcholine asymmetry in the inner and outer leaflets of the plasma membrane bilayer of the electroplax. Exposure of electroplax (30 min in vitro) to soman (10(-4), 10(-6) M) or sarin (10(-4), 10(-6), 5 x 10(-9) M) increased the percentage of phosphatidylcholine in the outer monolayer of the innervated plasma membrane bilayer and decreased the percentage in the inner monolayer. These changes by sarin were observed at concentrations that produced 100% cholinesterase inhibition (10(-4), 10(-6) M) and at a concentration (5 x 10(-9) M) where no inhibition occurred, suggesting that these effects are not directly due to cholinesterase inhibition. A 1-week exposure of live eels to soman (10(-8) M) in vivo caused an increase in phosphatidylcholine labeling in the outer monolayer of the innervated and noninnervated surfaces of the electroplax. Two weeks after stopping exposure to soman, increased labeling was still observed, suggesting that this may be a long-term effect. Because the organophosphates did not increase the permeability of the electroplax, all of these changes in labeling appear to be due to a redistribution of phosphatidylcholine from the inner to the outer monolayer of the plasma membrane bilayer.

Binding of actin to liver cell membranes: the state of membrane-bound actin

Previous work has shown that actin binds specifically and saturably to liver membranes stripped of endogenous actin (Tranter, M. P., S. P. Sugrue, and M. A. Schwartz. 1989. J. Cell Biol. 109:2833-2840). Scatchard plots of equilibrium binding data were linear, indicating that binding is not cooperative, as would be expected for F- or G-actin. To determine the state of membrane-bound actin, we have analyzed the binding of F- and G-actin to liver cell membranes. G-actin in low salt depolymerization buffer and EF-actin, a derivative that polymerizes very poorly in solution, bind to liver cell membranes as well as untreated actin in polymerization buffer. Phalloidin-stabilized F-actin binds, but to a lesser extent. The binding of F- and G-actins are mutually competitive and are inhibited by ATP, suggesting that both forms of actin bind to the same sites. For untreated actin in polymerization buffer, the time course of binding is biphasic, with an initial rapid component which is followed by a plateau phase, then a second, slower component. The binding kinetics of pure F-actin and pure G-actin are both monophasic and match the fast and slower components, respectively, of untreated actin. In the reconstituted system, membrane-bound actin does not stain with rhodamine-phalloidin, nor are actin filaments detected by EM. Distinct regions of amorphous material, however, are visible, which stain with an anti-actin antibody. The exact nature of this material has yet to be determined. A model of actin binding is presented.

On the principles of functional ordering in biological membranes

Integrating the available data on lipid-protein interactions and ordering in lipid mixtures allows to emanate a refined model for the dynamic organization of biomembranes. An important difference to the fluid mosaic model is that a high degree of spatiotemporal order should prevail also in liquid crystalline, "fluid" membranes and membrane domains. The interactions responsible for ordering the membrane lipids and proteins are hydrophobicity, coulombic forces, van der Waals dispersion, hydrogen bonding, hydration forces and steric elastic strain. Specific lipid-lipid and lipid-protein interactions result in a precisely controlled yet highly dynamic architecture of the membrane components, as well as in its selective modulation by the cell and its environment. Different modes of organization of the compositionally and functionally differentiated domains would correspond to different functional states of the membrane. Major regulators of membrane architecture are proposed to be membrane potential controlled by ion channels, intracellular Ca2+, pH, changes in lipid composition due to the action of phospholipase, cell-cell coupling, as well as coupling of the membrane with the cytoskeleton and the extracellular matrix. Membrane architecture is additionally modulated due to the membrane association of ions, lipo- and amphiphilic hormones, metabolites, drugs, lipid-binding peptide hormones and amphitropic proteins. Intermolecular associations in the membrane and in the membrane-cytoskeleton interface are further selectively controlled by specific phosphorylation and dephosphorylation cascades involving both proteins and lipids, and regulated by the extracellular matrix and the binding of growth factors and hormones to their specific receptor tyrosine kinases. A class of proteins coined architectins is proposed, as a notable example the pp60src kinase. The functional role of architectins would be in causing specific changes in the cytoskeleton-membrane interface, leading to specific configurational changes both in the membrane and cytoskeleton architecture and corresponding to (a) distinct metabolic/differentiation states of the cell, and (b) the formation and maintenance of proper three dimensional membrane structures such as neurites and pseudopods.

Co-distribution of annexin VI and actin in secretory ameloblasts and odontoblasts of rat incisor

Annexin VI and actin were detected by immunoblot analysis in the enamel- and dentin-related portions of dental tissues. Annexin VI was found mainly in the particulate fraction whereas actin was detected in both the soluble and particulate fractions. By immunoelectron microscopy, annexin VI antibodies conjugated with colloidal gold were seen to label the mitochondria, the cytosol and the nucleus of secretory ameloblasts and odontoblasts of rat incisor. In the processes of these cells, the plasmalemmal undercoat was labeled. Anti-actin antibodies labeled the desmosome-like junctions, the cytosol, and the mitochondria of the cell bodies. Extensive labeling was seen at the periphery of the Tomes' processes and odontoblast processes. These results suggest that annexin VI may play a role in Ca2(+)-regulation in the cell bodies, especially as a calcium receptor protein in the mitochondria. Moreover, annexin VI and actin seem to be co-distributed in secretory processes. Thus, these proteins might be both involved in exocytotic and endocytotic events.

Interaction of epidermal growth factor receptors with the cytoskeleton is related to receptor clustering

Recently it has been established that cytoskeleton-associated epidermal growth factor (EGF) receptors are predominantly of the high-affinity class and that EGF induces a recruitment of low-affinity receptors to the cytoskeleton. The nature of this EGF-induced receptor-cytoskeleton interaction, however, is still unknown. Therefore, we have studied the association of mutated EGF receptors with the cytoskeleton. Receptor deletion mutants lacking almost all intracellular amino acid residues displayed no interaction with the cytoskeleton, demonstrating that the cytoplasmic receptor domain is involved in this interaction. Further analysis revealed that receptor-cytoskeleton interaction is independent of receptor kinase activity and the C-terminal 126 amino acid residues, which include the auto-phosphorylation sites. Furthermore, it is shown that the high-affinity receptor subclass is not essential for association of low-affinity receptors to the cytoskeleton. EGF receptor-cytoskeleton interaction was increased, however, by treatment with sphingomyelinase, an enzyme known to induce membrane protein clustering, indicating that EGF receptor clustering may cause the association to the cytoskeleton.

Redistribution of membranes and cytoskeletal proteins in chicken oxyntic cells during the HCl secretory cycle: ultrastructural and immunofluorescence study

Changes in ultrastructure and cytoskeletal organization by avian oxyntic cells, at the onset of HCl secretion, were analysed. Cells in resting state, induced by fasting and cimetidine, were compared with histamine stimulated secreting cells. Ultrastructural studies were done by transmission electron microscopy; the distribution of prekeratin, myosin, and filamin-like protein, by immunofluorescence; and that of F-actin using FITC-phalloidin. Resting cells show short pericellular clefts. These are increasingly deepened in secreting cells by a reorganization of the lateral cell borders involving displacement of the junctional complexes toward the cell base and incorporation of the tubular system to the luminal plasma membrane. In secreting cells, the processes of the secretory surface are concentrated in a pericellular groove. Histamine stimulation induces a drastic redistribution of cytoskeletal proteins. In chicken oxyntic cells, in addition to the F-actin cytoskeleton associated with the membranes of the secretory surface, there is a cytoskeletal ring containing F-actin, myosin, and a filamin-like protein, located at the level of the junctional complexes. In resting cells, filaments and masses of cytoskeletal matrix are associated with the zonula adherens. In secreting cells, the junctional complexes maintain their association with the filamentous ring, while the amorphous matrix is replaced by microfilaments that support the processes of the luminal surface. Intermediate filaments form a peripheral ring probably associated with the zonula adherens, and project from the ring toward the cell cytoplasm. Thus, with the onset of HCl secretion, the apical cytoskeletal ring of resting cells displaces toward the cell base. A role for this cytoskeletal ring in the changes in shape parallel to HCl secretion is discussed.

Cellular inhibition of microtubule assembly by photoactivated sulphonated meso-tetraphenylporphines

This work relates to studies on modes of phototoxicity by sulphonated mesotetraphenylporphines on cultured cells. Toxicity appears to be related to inhibition of microtubule function. Treatment of human cervix carcinoma cells of the line NHIK 3025 incubated for 18 h with meso-tetraphenylporphine sulphonates (TPPSn where n = 2a, 2o or 4) and exposed to light, inhibits multiplication for the first hours after light exposure, a significant fraction of the cells accumulating in mitosis. The maximal number of cells in mitosis after treatment (approximately 20%) is dependent on the fluence but is similar for all three photosensitizers. For the first hours after treatment the mitotic cells were always mainly in metaphase; mainly seen as c-metaphases and three-group metaphases. During this time anaphase and telophase cells were absent or greatly reduced in number. Indirect immunofluorescence staining of beta-tubulin showed that the spindle apparatus of mitotic cells was perturbed in all cases. Results are presented which indicate that photoactivation of TPPSn located on the plasma membrane destroys microtubules in interphase cells and leads to arrest of the cells in mitosis. The localization of the dye which sensitizes the photoinduced perturbation of microtubules is further discussed.

Evidence for a direct, nucleotide-sensitive interaction between actin and liver cell membranes

We have investigated the association of actin with membranes isolated from rat liver. A plasma membrane-enriched fraction prepared by homogenization in a low salt/CaCl2 buffer was found to contain a substantial amount of residual actin which could be removed by treatment with 1 M Na2CO3/NaHCO3, pH 10.5. Using a sedimentation binding assay that uses gelsolin to shorten actin filaments and render membrane binding saturable (Schwartz, M. A., and E. J. Luna. 1986. J. Cell Biol. 102:2067-2075), we found that membranes stripped of endogenous actin bound 125I-actin in a specific and saturable manner. Scatchard plots of binding data were linear, indicating a single class of binding sites with a Kd of 1.6 microns; 66 micrograms actin bound/mg membrane protein at saturation. Binding of actin to liver cell membranes was negligible with unstripped membranes, was competed by excess unlabeled actin, and was greatly reduced by preheating or proteolytic digestion of the membranes. Kinetic measurements showed that binding had an initial lag phase and was strongly temperature dependent. The binding of actin to liver cell membranes was also found to be competitively inhibited by ATP and other nucleotides, including the nonhydrolyzable analogue AMP-PNP. We conclude that we have reconstituted an interaction between actin and integral membrane proteins from the rat liver. This interaction exhibits a number of distinctive features which have not been observed in other actin-membrane systems.

K+-induced swelling of the dogfish shark (Squalus acanthias) rectal gland cells is associated with changes of the cytoskeleton

Dogfish shark (Squalus acanthias) rectal gland cells swell massively when incubated in elasmobranch media in which Na+ was equivalently replaced by K+; this swelling was abolished when the impermeant gluconate replaced Cl-, while the cell depolarization was comparable in both media. The K+-effect was associated with (a) an increase of the steady-state 42K (and 86Rb) efflux (particularly of the rate constant of the fast cellular efflux component) and a rearrangement of the respective cellular pools of K+; (b) an alteration of cell morphology and the pattern of the F-actin staining along the basolateral cell membrane as revealed with fluorescent analogs of phallacidin. These changes were independent of cell volume, being identical in KCl and K-gluconate media. The observations were specific for K+ (and Rb+): replacement of media Na+ by Li+ (which is not actively extruded by the cells), or the presence of ouabain, produced only minor swelling without affecting cell morphology and F-acting distribution. The results are consistent with the following view: as opposed to Na+ or Li+ media, the K+-induced changes of the cortical F-actin component of the cytoskeleton permit the observed massive cell swelling due to the osmotic contribution of intracellular impermeant anion(s).

Binding of plasma membrane glycoproteins to the cytoskeleton during patching and capping is consistent with an entropy-enhancement model

Concentrations of concanavalin A that induced patching and capping of cell surface receptors on Dictyostelium discoideum also induce binding of the receptors to the cortical cytoskeleton, which was isolated by density-gradient centrifugation. The receptors were solubilized by deoxycholate, purified by affinity chromatography, and used to determine whether the receptors bound directly to the cytoskeletal protein, actin. As the concentration of actin was increased, many of the receptors became bound to purified filamentous rabbit muscle actin, even in the absence of concanavalin A. As in the ligation-induced binding of receptors to the cortical cytoskeleton in cells, concanavalin A induced much stronger binding of the purified receptors to filamentous actin. The results were consistent with a previously stated hypothesis that induction of receptor binding to the cytoskeleton during their patching and capping is driven by clustering the receptors, which reduces their translational entropy and by doing so enhances their avidity for the cytoskeleton.

Interaction of intermediate filaments with nuclear lamina and cell periphery

Ultrastructural observations of the cytoskeleton suggest that the connection of the intermediate filaments (IFs) to actin microfilaments (MFs) at the plasma membrane and the nuclear lamina inside the nuclear membrane link signals received at the cell periphery to the nucleus. When these connections are viewed in three dimensions using detergent extracted cytoskeletal preparations from tissue cultures or slices made from tissue, the IFs are seen to run without interruption from the cell periphery to the nucleus and back. The IFs form side to side connections with the nuclear lamina and pore complexes. The nucleus and the centrioles are supported and held suspended in these extracted cells where all organelles and cytosol have been removed. The IFs are particularly dense in the ectoplasm where they form a sheet and provide the scaffolding which maintains the shape of the extracted cells. The IFs in the ectoplasm are attached to desmoplakin at cell-cell desmosome adhesions and to MFs where the cells are attached to the fibronectin substratum possibly through integrin linkages at adhesion plaques. This was graphically shown by immunogold labelling of IF cells treated with nickel. In this way, it was possible to visualize the loss of the cell-cell connections at desmosomes and the disruption of the IF-MF connections in the ectoplasm. The MFs after losing their connections with the IFs, redistribute to cover the entire cell periphery. The nickel treatment of primary liver cell cultures lead to the loss of several functions including formation of the bile canaliculus, the ability to secrete fluorescein diacetate and the ability to take up horseradish peroxidase (HRP) by endocytosis. These observations support the conclusion that the IF-MF connections at the cell periphery provide both structural and functional polarity of the liver cells including uptake and secretion and the formation of bile canaliculi.

Localization of actin in human, bull, rabbit, and hamster sperm by immunoelectron microscopy

Mammalian spermatozoa have previously been shown to contain actin, but its subcellular localization and function have not been elucidated. In this study, actin has been localized at the ultrastructural level in human, bull, rabbit, and golden hamster spermatozoa by a monoclonal antiactin antibody and a preembedding immunogold labeling technique. Specific labeling was localized 1) around the connecting piece in the neck region of sperm from all four species, although a species-specific pattern was evident; 2) on the external surface of the fibrous sheath of human sperm; 3) in the perinuclear space underlying the postacrosomal sheath of bull and rabbit sperm; and 4) between the plasma membrane and outer acrosomal membrane along the concave margin of the hamster sperm head. SDS-PAGE and Western blots immunostained with the monoclonal antibody confirmed the presence of actin in SDS extracts of Percoll-purified sperm from each species.

A 36 kDa monomeric protein and its complex with a 10 kDa protein both isolated from bovine aorta are calpactin-like proteins that differ in their Ca2+-dependent calmodulin-binding and actin-severing properties

Interaction of plasma membrane with the cytoskeleton involves a large number of proteins, among them a 36 kDa protein that was found to be involved in the interaction with actin filaments. We have isolated a 36 kDa protein from bovine aorta as a monomer and in a complex with a 10 kDa protein. Partial amino acid sequence determinations show that the 36 kDa and 10 kDa proteins isolated from bovine aorta are analogous to or identical with corresponding proteins purified from bovine intestine already described by Kristensen, Saris, Hunter, Hicks, Noonan, Glenney & Tack [(1986) Biochemistry 25, 4497-4503]. We report here that the association of the 10 kDa protein with the 36 kDa protein confers specific calmodulin-binding and actin-severing properties on the complex that are not possessed by the 36 kDa monomer alone. These findings suggest that the protein complex could be involved in thin-filament-related structures or could modulate some Ca2+-regulated events mediated by calmodulin.

Quantitative subcellular study of apical pole membranes from chicken oxyntic cells in resting and HCl secretory state

Vertebrate oxyntic cells, responsible for gastric HCl production, undergo a remarkable morphological reorganization in relation to their secretory cycle. In resting state, the luminal surface of the cells is smooth; a peculiar system of endocellular membranes, the tubular system, occupies the luminal cytoplasm. Actin filaments frame a cortical network between the tubular system and the luminal plasma membrane. With the onset of HCl secretion, the tubular system becomes incorporated into the luminal plasma membrane. Villous processes containing microfilaments fill the secretory surface. This morphological reorganization of membranes and cytoskeletal matrix could regulate HCl secretion by translocation of membranes containing the proton pump from the endocellular compartment to the secretory surface. In this paper, we describe the isolation of membranes that selectively belong to the tubular system or to the cytoplasmic processes of the secretory surface of chicken oxyntic cells. Chicken oxyntic cells are the main cellular component of the proventricular glands. A resting state was obtained after cimetidine treatment, whereas the HCl-secretory state was induced by histamine. We present a comparative analysis of resting and stimulated chicken gastric glands by quantitative subcellular fractionation. The HCl secretory state was related to specific modifications in membrane fractions derived from the secretory pole of oxyntic cells. Morphological and functional reorganization of oxyntic cells was closely correlated with changes in: the sedimentation pattern of the marker enzyme of the apical pole membrane (K-NPPase), the total activity of K-NPPase and nonmitochondrial Mg-ATPase, the valinomycin dependence of K-ATPase, and polypeptides that cosediment in purified membrane fractions. Changes in the distribution pattern of K-NPPase after fractionation of histamine-stimulated glands were consistent with the replacement of the small vesicles typical of resting glands by dense membrane profiles, analogous to the luminal processes of stimulated oxyntic cells. SDS-PAGE showed that, in purified membrane fractions of stimulated glands, the concentration of 28-, 43-, and 200-kD polypeptides increased while that of 95- and 250-kD polypeptides decreased. The present results define the tubular system of oxyntic cells as an organelle with properties different from those of endoplasmic reticulum, mitochondria, and plasma membrane. The biochemical and physico-chemical properties of this membraneous system changed when the organization of the membranes and the cytoskeletal matrix of the apical pole was modified by the onset of HCl secretion.

F-actin binds to the cytoplasmic surface of ponticulin, a 17-kD integral glycoprotein from Dictyostelium discoideum plasma membranes

F-actin affinity chromatography and immunological techniques are used to identify actin-binding proteins in purified Dictyostelium discoideum plasma membranes. A 17-kD integral glycoprotein (gp17) consistently elutes from F-actin columns as the major actin-binding protein under a variety of experimental conditions. The actin-binding activity of gp17 is identical to that of intact plasma membranes: it resists extraction with 0.1 N NaOH, 1 mM dithiothreitol (DTT); it is sensitive to ionic conditions; it is stable over a wide range of pH; and it is eliminated by proteolysis, denaturation with heat, or treatment with DTT and N-ethylmaleimide. gp17 may be responsible for much of the actin-binding activity of plasma membranes since monovalent antibody fragments (Fab) directed primarily against gp17 inhibit actin-membrane binding by 96% in sedimentation assays. In contrast, Fab directed against cell surface determinants inhibit binding by only 0-10%. The actin-binding site of gp17 appears to be located on the cytoplasmic surface of the membrane since Fab against this protein continue to inhibit 96% of actin-membrane binding even after extensive adsorption against cell surfaces. gp17 is abundant in the plasma membrane, constituting 0.4-1.0% of the total membrane protein. A transmembrane orientation of gp17 is suggested since, in addition to the cytoplasmic localization of the actin-binding site, extracellular determinants of gp17 are identified. gp17 is surface-labeled by sulfo-N-hydroxy-succinimido-biotin, a reagent that cannot penetrate the cell membrane. Also, gp17 is glycosylated since it is specifically bound by the lectin, concanavalin A. We propose that gp17 is a major actin-binding protein that is important for connecting the plasma membrane to the underlying microfilament network. Therefore, we have named this protein "ponticulin" from the Latin word, ponticulus, which means small bridge.

Elevation in cytosolic free calcium concentration early in myocardial ischemia in perfused rat heart

Changes in cytosolic free calcium concentration during myocardial ischemia were measured by 19F NMR in 5FBAPTA-loaded perfused rat hearts. The hearts were perfused with Krebs-Henseleit buffer containing 5 microM of the acetoxymethyl ester of 5FBAPTA, which was hydrolyzed by cytosolic esterases to achieve cytosolic concentrations of 5FBAPTA of 0.12 to 0.65 mM. Cytosolic free calcium concentrations were calculated as the product of the ratio of peak areas for bound and free 5FBAPTA in the NMR spectra and the dissociation constant (708 nM). The basal cytosolic calcium concentration, measured in potassium or magnesium arrested hearts, was 252 nM, and the time-average calcium concentration in beating hearts was 630 nM. Following the onset of total ischemia, there was no immediate substantial change in cytosolic calcium despite a rapid decline in creatine phosphate and ATP and a marked increase in inorganic phosphate as monitored by 31P NMR, but by 10 minutes, there was a substantial increase in free calcium concentration. The ratio of peak areas of bound and free 5FBAPTA returned to the preischemic value during reperfusion, and there was no detectable loss of 5FBAPTA from the heart. Creatine phosphate was also restored to its preischemic level during reperfusion. These results indicate that cytosolic free calcium increases during ischemia and is not immediately associated with lethal injury. This increase in cytosolic calcium may activate degradative enzymes that eventually could compromise myocyte viability.

Cytoskeletal damage during myocardial ischemia: changes in vinculin immunofluorescence staining during total in vitro ischemia in canine heart

The role of cytoskeletal damage in the disruption of the plasma membrane observed during myocardial ischemia has been studied using antibodies to vinculin to identify changes in the distribution of this membrane associated cytoskeletal protein. Vinculin is a component of the cytoskeletal attachment complex between the plasma membrane and the Z-line of the underlying myofibrils. The effects of varying periods of total ischemia on the localization of vinculin were assessed by immunofluorescence and evidence of membrane disruption was evaluated by electron microscopy. Thin tissue slices prepared from the ischemic tissue were incubated in oxygenated Krebs-Ringer phosphate buffer at 37 degrees C to assess inulin permeability, ultrastructure, and any changes in the distribution of vinculin associated with incubation. The previously reported costameric pattern of vinculin staining was observed in longitudinal sections of control myocardium, myocardium subjected to 60 minutes of total ischemia, and myocardium subjected to 60 minutes of ischemia followed by 60 minutes of incubation in oxygenated media. Electron microscopy and inulin permeability measurements confirmed that plasma membrane integrity was preserved under these conditions. However, when the duration of total ischemia was extended to 120 minutes or longer, there was a progressive loss of vinculin staining along the lateral margin of myocytes. This change correlates with the appearance of subsarcolemmal blebs and breaks in the plasma membranes observed by electron microscopy and confirmed by the increase in inulin permeability observed in tissue slices.(ABSTRACT TRUNCATED AT 250 WORDS)

The cytoskeleton of chick retinal pigment epithelial cells in situ

Gelatin-coated slides were used to obtain en face preparations of retinal pigment epithelium (RPE) from 6- to 21-day-old chick embryos in order to study the distribution of F-actin in microfilaments (MF) and the MF-associated proteins, myosin, tropomyosin, alpha-actinin and vinculin in situ at different stages of development by fluorescence microscopy. The epithelial sheets were fixed in formaldehyde and then extracted in a solution containing 0.1% Triton X-100. NBD-Phallacidin was used to visualize the F-actin in MF, and antisera against myosin, tropomyosin, alpha-actinin and vinculin were used to determine the distribution of these four MF-associated proteins. F-actin, myosin, tropomyosin, alpha-actinin and vinculin were present in cortical rings around the apical ends of the RPE cells throughout this period of development. Of these proteins, only F-actin was identified in the apical processes of RPE cells. The increase in the amount of F-actin could be followed as the length and the number of apical processes increased with age and maturation of RPE cells. F-actin was first detected in numerous short apical processes on the surface of each RPE cell on day 12. From day 12 to day 17, they were at an intermediate stage of elongation and from day 17 onward all of the RPE cells had long F-actin-containing apical processes. These results indicate that the F-actin-containing MF assemble much later in the apical processes than in the cortical rings.(ABSTRACT TRUNCATED AT 250 WORDS)

Using antibodies against Dictyostelium membranes to identify an actin-binding membrane protein

Polyclonal antibodies made against Dictyostelium discoideum membranes were used to block the interaction of those membranes with actin. As expected, actin interacted mostly with the internal surface of the membrane, demonstrated by the fact that whole cells could only absorb out a minor fraction of the blocking antibody. The antibody was used to show that the membrane component(s) which interacted with actin were probably integral; they could be extracted with detergent but not with solutions designed to extract peripheral membrane proteins. To identify the responsible protein(s), Western transfers of membranes were cut into fractions which were tested for their ability to absorb out the blocking activity of the antibody. We observed a single peak at a molecular weight of approximately 20,000, and thus conclude that a 20,000-mol-wt protein is a major integral membrane actin-binding protein in Dictyostelium. This approach to the identification of proteins involved in actin-membrane interaction has allowed us to make the first identification of an actin-binding membrane protein which is based on its activity in native membranes.

Calpactins: two distinct Ca++-regulated phospholipid- and actin-binding proteins isolated from lung and placenta

Three forms of calpactin, the 36,000 Mr Ca++-binding cytoskeletal protein, were isolated in large amounts from bovine lung and human placenta using cycles of calcium-dependent precipitation followed by solubilization with EGTA-containing buffers. Calpactin-I as a tetramer of heavy (36 kD) and light (11 kD) chains was the predominant form of calpactin isolated, however milligram amounts of the calpactin-I heavy chain monomer and calpactin-II, a related but distinct molecule, were also isolated by this method. Calpactin-II was characterized in some detail and found to bind two Ca++ ions with Kd's of 10 microM in the presence of phosphatidylserine. Both calpactin-I and -II were found to aggregate liposomes at micromolar Ca++ concentrations, suggesting that at least two phospholipid-binding sites are present on these molecules. Both calpactin monomers bind to and bundle actin filament at high (1 mM) but not low (less than 1 microM) Ca++ concentrations. Amino-terminal sequence analysis of a lower molecular mass variant of calpactin-II revealed that this protein was the previously identified human "lipocortin" molecule. Antibodies were elicited to calpactin-I and -II and the cell and subcellular distribution of each was compared. Calpactin-II was only present at high levels in tissues (lung, placenta) which contained high levels of calpactin-I. Other tissues (intestine) contained high calpactin-I and undetectable levels of calpactin-II. Double-label immunofluorescence microscopy on human fibroblasts revealed that, like calpactin-I, calpactin-II is present in a submembraneous reticular network, although the distribution of the two calpactins is not identical.

The mobility of concanavalin A receptors and surface immunoglobulins on rat hepatocyte plasma membranes

Lateral mobilities of lectin receptors and surface immunoglobulins were measured in plasma membranes of hepatocytes prepared by smearing small pieces of rat liver tissue and then using the fluorescence recovery after photobleaching (FRAP) technique. Smears were treated with various doses of fluorescein isothiocyanate (FITC) conjugated concanavalin A (ConA), succinylated ConA (SConA), wheat germ agglutinin (WGA), and soybean agglutinin (SBA), as well as with rabbit anti-rat IgG (RARa/IgG) and goat anti-rat IgM(Fc) (GARa/IgM(Fc] antisera. 10 micrograms/ml ConA and SConA concentrations and a 55 X dilution of the GARa/IgM(Fc) antiserum were found to be suitable for measuring the lateral mobilities dependent on age. Diffusion constant and mobile fractions of receptor complexes were measured in different age groups of female Fisher rats (from 1 to 26 month-old). The FRAP measurements revealed that at least two major receptor sites can be distinguished in cell membranes of compact tissue (similar to the cultured and isolated cells), forming a mobile and an immobile fraction. The mobile fractions of both the lectin receptors and the surface immunoglobulins tended to decrease with age, while the age differences of the diffusion constants were not statistically significant. The observed alterations could be due to the covalent crosslinking of the mobile receptors to immobile patches and/or to the retardation of free diffusion by the cytoskeleton, dependent on age.

Effect of colchicine on the ultrastructure of secretory-state smooth muscle cells from the rabbit artery wall

Little is known about the possible role of microtubules and related structures with respect to the structural and functional heterogeneity of arterial smooth muscle cells (SMC). This study demonstrates that a short-term treatment with an antitubulin, colchicine, resulted in two major structural changes in arterial SMC in the aortic arch, the thoracic aorta and the pulmonary trunk of 20-d-old rabbits. The SMC of control rabbits showed well-developed rough endoplasmic reticulum and Golgi complexes, and numerous microtubules. Colchicine treatment affected (a) microtubules in a monotypic way (all SMC contained no microtubules after colchicine), and (b) Golgi complexes and rough endoplasmic reticulum in a pleiotypic way. In effect two major structural subpopulations of SMC were obtained; the first subpopulation displayed markedly altered Golgi complexes, whereas the rough endoplasmic reticulum was unaltered; the second subpopulation showed a vacuolar dilation of rough endoplasmic reticulum (or extensively-developed smooth sarcoplasmic reticulum), but a disappearance of Golgi complex. These data suggest that an organelle-specific sensitivity to colchicine (most likely to its antitubulin action) may contribute to the heterogeneity of SMC. They also suggest that within control SMC two subpopulations might exist which can only be detected under special conditions like colchicine treatment. The disassembly of microtubules detected in parallel may be one trigger, but not the main mechanism in the heterogeneous changes of SMC in reaction to colchicine.

Localization of actin in mammalian spermatozoa: a comparison of eight species

The distribution of monomeric and polymeric actin in spermatozoa from the bull, boar, rabbit, human, rat, mouse, golden hamster, and guinea pig has been examined by using a monoclonal antiactin antibody and NBD-phallacidin. Actin was present in sperm from each species. When the monoclonal antibody was used, there was a species-specific distribution and intensity of fluorescence, but no generalized pattern. Specific fluorescence was noted in the neck and principal piece of human sperm; in the postacrosomal region, neck, and midpiece of bull and boar sperm; in the postacrosomal region, neck, and principal/equatorial segment border of rabbit sperm; in the neck region of hamster sperm; and in the neck, midpiece, and principal piece of rat, mouse, and guinea pig sperm. Sperm from all eight species displayed no specific fluorescence with NBD-phallacidin, indicating that actin was present in a nonfilamentous form. SDS extracts of sperm were analyzed by SDS-PAGE and Western blotting; in sperm from each species, a 42-kD protein with specific affinity for the monoclonal antibody was present.

Distribution of actin in Sertoli cell ectoplasmic specializations and associated spermatids in the ground squirrel testis

We have investigated the possibility that the complex patterns of fluorescence associated with spermatids of the ground squirrel labeled with 7-nitrobenz-2-oxa-1,3-diazole-phallacidin (NBD-phallacidin) are due to the presence of filamentous actin within the spermatids themselves rather than to actin in attached Sertoli cell ectoplasmic specializations, as previously reported (J. Cell Biol., 100:814-825). Enzymatic treatments (trypsin, DNAase 1) freed Sertoli cell ectoplasmic specializations from spermatids and resulted in a loss, from the spermatids, of the complex fluorescence patterns, suggesting that the latter were generated by labeled actin in ectoplasmic specializations. Moreover, ectoplasmic specializations that were detached enzymatically from spermatids demonstrated the same fluorescence patterns as those emitted from spermatids in the intact or mechanically fragmented seminiferous epithelium. Most spermatids, however, do display a weak and diffuse pattern of fluorescence that changes during spermatogenesis and that is localized between the acrosomal cap and nucleus. S-1 decoration confirmed this subacrosomal localization and further demonstrated that the actin in adjacent Sertoli cell ectoplasmic specializations is arranged in a unipolar fashion. We conclude that the complex patterns of actin fluorescence associated with mechanically isolated spermatids are a superimposition of both Sertoli cell and germ cell actin; however, the latter is either poorly detected or not detected at all when Sertoli cell ectoplasmic specializations overlie the germ cells.

Clustering of cell surface laminin enhances its association with the cytoskeleton

In order to provide evidence for an association of cell surface laminin with the cytoskeleton, we have examined the detergent extractability of cell surface laminin on murine fibrosarcoma cells. We utilized indirect immunofluorescence with affinity-purified anti-laminin antibodies to determine the distribution, mobility and detergent extractability of laminin bound to the cell surface. We demonstrate that antibody induces clustering of cell surface laminin rendering it resistant to detergent extraction. At low receptor occupancy, approx. 80% of cell surface laminin is detergent-extractable. If cell surface laminin is induced to cluster with anti-laminin antibody, IB4 isolectin from Bandeiraea simplicifolia or by high receptor occupancy, then it is rendered resistant to detergent extraction. This process is temperature-sensitive and inhibited by cytochalasin D (CD). On the basis of these findings, we propose a model in which laminin anchored in the basement membrane in vivo affects the cellular cytoskeleton by facilitating the clustering of cell surface transmembrane laminin receptors which are able to interact with cellular actin.

Actin filament organization within dendrites and dendritic spines during development

The myosin S-1 subfragment was used to label actin filaments in the developing rat brain. The results show actin filaments present throughout the dendritic region with highest concentrations within growth cones and regions of spine development. Between 6 and 25 days postnatal, spines became more complex and actin filaments within them increased in number and formed a complex network. The observed organization of actin supports the hypothesis that actin has a role in the protrusion of spines from the dendrite during development.

Binding and assembly of actin filaments by plasma membranes from Dictyostelium discoideum

The binding of native, 125I-Bolton-Hunter-labeled actin to purified Dictyostelium discoideum plasma membranes was measured using a sedimentation assay. Binding was saturable only in the presence of the actin capping protein, gelsolin. In the presence of gelsolin, the amount of actin bound at saturation to three different membrane preparations was 80, 120, and 200 micrograms/mg of membrane protein. The respective concentrations of actin at half-saturation were 8, 12, and 18 micrograms/ml. The binding curves were sigmoidal, indicating positive cooperativity at low actin concentrations. This cooperativity appeared to be due to actin-actin associations during polymerization, since phalloidin converted the curve to a hyperbolic shape. In kinetic experiments, actin added as monomers bound to membranes at a rate of 0.6 microgram ml-1 min-1, while pre-polymerized actin bound at a rate of 3.0 micrograms ml-1 min-1. Even in the absence of phalloidin, actin bound to membranes at concentrations well below the normal critical concentration. This membrane-bound actin stained with rhodamine-phalloidin and was cross-linked by m-maleimidobenzoyl succinimide ester, a bifunctional cross-linker, into multimers with the same pattern observed for cross-linked F-actin. We conclude that D. discoideum plasma membranes bind actin specifically and saturably and that these membranes organize actin into filaments below the normal critical concentration for polymerization. This interaction probably occurs between multiple binding sites on the membrane and the side of the actin filament, and may be related to the clustering of membrane proteins.

Cell interactions during the in vitro neoformation of fetal rat pancreatic islets

As shown by scanning electron microscopy, transmission electron microscopy and membrane labeling analysis, the in vitro neoformation of rat pancreatic islets arose from two main processes: a budding from explants containing duct cells, and a competition between endocrine monolayers and fibroblasts on the culture substratum. The stronger cytoskeleton of fibroblasts and their higher adhesive properties, probably related to their more homogeneous distribution of membrane charges, may explain the spherization of the islets. The pure endocrine cell population of neoformed islets was composed mainly of insulin-secreting cells, and the other types of endocrine cells were distributed in the periphery. Preformed extracellular matrices of osmotically disrupted fibroblasts enhanced the yield of the cultures by increasing the anchorage of endocrine cells and slowing down the fibroblastic growth.

Linkage of a membrane skeleton to integral membrane glycoproteins in human platelets. Identification of one of the glycoproteins as glycoprotein Ib

Experiments were performed to determine whether platelets contain a membrane skeleton. Platelets were labeled by a sodium periodate/sodium [3H]borohydride method and lysed with Triton X-100. Much of the filamentous actin could be sedimented at low g forces (15,600 g, 4 min), but some of the actin filaments required high-speed centrifugation for their sedimentation (100,000 g, 3 h). The latter filaments differed from those in the low-speed pellet in that they could not be depolymerized by Ca2+ and could not be sedimented at low g forces even from Triton X-100 lysates of platelets that had been activated with thrombin. Actin-binding protein sedimented with both types of filaments, but 3H-labeled membrane glycoproteins were recovered mainly with the high-speed filaments. The primary 3H-labeled glycoprotein recovered with this "membrane skeleton" was glycoprotein (GP) Ib. Approximately 70% of the platelet GP Ib was present in this skeleton. Several other minor glycoproteins, including greater than 50% of the GP Ia and small amounts of three unidentified glycoproteins of Mr greater than 200,000, were also recovered with the membrane skeleton. The Triton X-100 insolubility of GP Ib, GP Ia, a minor membrane glycoprotein of 250,000 Mr, and actin-binding protein resulted from their association with actin filaments as they were rendered Triton X-100-soluble when actin filaments were depolymerized with deoxyribonuclease I and co-isolated with actin filaments on sucrose gradients. When isolated platelet plasma membranes were extracted with Triton X-100, actin, actin-binding protein, and GP Ib were recovered as the Triton X-100 residue. These studies show that unstimulated platelets contain a membrane skeleton composed of actin filaments and actin-binding protein that is distinct from the rest of the cytoskeleton and is attached to GP Ib, GP Ia, and a minor glycoprotein of 250,000 Mr on the plasma membrane.