The contractile basis of amoeboid movement. I. The chemical control of motility in isolated cytoplasm

The megakaryocyte in thrombocytopenia: a microscopic study which supports the theory that platelets are produced in the pulmonary circulation

Thrombocytopenia can increase platelet production and thereby facilitate investigations into the mechanism. Megakaryocytes in the bone marrow, lung, spleen and liver have been studied by light and transmission electron microscopy after 6 days of experimentally induced thrombocytopenia in the rabbit. The findings support the theory that platelets are produced in the pulmonary circulation by the physical fragmentation of megakaryocyte cytoplasm.

Abnormal intrapulmonary platelet production: a possible cause of vascular and lung disease

It is postulated that platelets are not produced by megakaryocyte budding within the bone marrow but by physical fragmentation in the pulmonary circulation. Alterations in the nature of the production site (the pulmonary vessels and their concomitant biochemical environment) or changes in the antecedent megakaryocyte cytoplasmic volumes with a concomitant alteration in protein structure can produce platelets with a mean volume larger than normal. If such platelets are more reactive then they may be involved in atherogenesis and arterial thrombosis. Furthermore several pulmonary pathological states may arise from dysfunction during pulmonary platelet production.

Microinjection into an identified axon to study the mechanism of fast axonal transport

Microinjection into an axon of an identified invertebrate neuron is shown to be a useful technique for analyzing the mechanisms of fast axonal transport. It permits direct assessment of the effect of agents that cannot permeate the plasma membrane on the translocation of material in the axon. The actin filament depolymerizer DNase I, when injected into the axon of the Aplysia neuron R2, caused a local block of fast transport of [3H]glycoprotein. Two agents that should interfere with the functioning of actin filaments without causing extensive depolymerization, tne N-ethylmaleimide-modified nuclease S1 fragment of myosin (injected) and dihydrocytochalasin B (applied externally). had no effect. Together these results suggest that actin plays a structural role in the axonal cytoskeleton rather than a role in transport force generation, the effect of DNase I being mediated by structural disordering of the axoplasm. Experiments were also done with inhibitors of dynein, the microtubule-associated ATPase. erythro-9-[3-(2-Hydroxynonyl)]adenine blocked transport but vanadate was ineffective.

Mobility of cytoplasmic and membrane-associated actin in living cells

We have combined fluorescent analogue cytochemistry with fluorescence photobleaching recovery to measure the mobility of fluorescently labeled actin and other labeled test proteins microinjected into living amoebae. Bovine serum albumin, ovalbumin, and ribonuclease A have a cytoplasmic mobility, expressed as a diffusion coefficient, that is 1/2 to 1/3 of that observed in aqueous solution; 90% of the actin has a mobility 1/2 to 1/8 of that of G-actin in aqueous solution, and approximately equal to 10% of the actin has a mobility comparable to that of F-actin in aqueous solution. Therefore, no more than 10% of the actin in the cytoplasm of amoebae can exist as static filaments. Microinjection of phalloidin decreases the diffusion coefficient of the mobile component of cytoplasmic actin, and it also increases the low-mobility fraction to 50% but has no effect on the mobility of labeled ovalbumin. By comparing the mobility of actin in different parts of amoebae and by separating cytoplasm from plasmalemma-ectoplasm, we found the low-mobility fraction of actin to be enriched in the tail, along the plasmalemma-ectoplasm, and in contracted cytoplasm.

pH changes in pinosomes and phagosomes in the ameba, Chaos carolinensis

Changes in pH are measured in pinosomes and phagosomes of single specimens of the giant, free-living ameba, Chaos carolinensis. Measurements of pH are made microfluorometrically, as previously described (Heiple and Taylor. 1980. J. Cell Biol. 86:885-890.) by quantitation of fluorescence intensity ratios (Ex489nm,/Ex452nm, Em520-560nm from ingested fluorescein thiocarbamyl (FTC)-ovalbumin. After 1 h of pinocytosis (induced in acid solution), FTC-ovalbumin is found in predominantly small ( less than or equal to 5 micrometers in diameter), acidic (pH less than or equal to 5.0-6.2) vesicles of various shape and density. As the length of ingestion time increases (up to 24 h), the probe is also found in vesicles of increasing size (up to 100 micrometers in diameter), increasing pH (up to pH approximately 8.0), and decreasing density. Co-localization of fluorescein and rhodamine fluorescence, after a pulse-chase with fluorescein- and rhodamine-labeled ovalbumin, suggests vesicle growth, in part, by fusion. The pH in a single phagosome is followed after ingestion of ciliates in neutral solutions of FTC-ovalbumin. A dramatic acidification (delta pH greater than or equal to - 2.0) begins within 5 min of phagosome formation and appears to be complete in approximately 20 min. Phagosomal pH then slowly recovers to more neutral values over the next 2 h. pH changes observed in more mature populations of pinosomes within a single cell may reflect those occurring within a single phagosome. Phagosomal and pinosomal pH changes may be required for lysosomal fusion and may be involved in regulation of lysosomal enzyme activity.

Spatial organization and fine structure of the cortical filament layer in normal locomoting Amoeba proteus

The fine structural organization of a cortical filament layer in normal locomoting Amoeba proteus was demonstrated using improved fixation and embedding techniques. Best results were obtained after application of PIPES-buffered glutaraldehyde in connection with substances known to prevent the depolymerization of F-actin, followed by careful dehydration and freeze-substitution. The filament layer is continuous along the entire surface; it exhibits a varying thickness depending on the cell polarity, measuring several nm in advancing regions and 0.5-1 micron in retracting ones. Two different types of filaments are responsible for the construction of the layer: randomly distributed thin (actin) filaments forming an unordered meshwork beneath the plasma membrane, and thick (myosin) filaments mostly restricted to the uroid region in close association with F-actin. The cortical filament layer generates the motive force for amoeboid movement by contraction at posterior cell regions and induces a pressure flow that continues between the uroid with a high hydrostatic pressure and advancing pseudopodia with low one. The local destabilization of the cell surface as a precondition for the formation of pseudopodia is enabled by the detachment of the cortical filament layer from the plasma membrane. This results in morphological changes by the active separation of peripheral hyaloplasmic and central granuloplasmic regions.

Demonstration of contractility of circumferential actin bundles and its morphogenetic significance in pigmented epithelium in vitro and in vivo

Each pigmented epithelial cell bears circumferential actin bundles at its apical level when the pigmented epithelium is established in eyes in situ or in culture in vitro. Well-differentiated pigmented epithelia in culture were treated with a 50% glycerol solution containing 0.1 M KCl, 5 mM EDTA, and 10 mM sodium phosphate buffer, pH 7.2, for 24 h or more at 4 degrees C. When the glycerinated epithelium was transferred to the ATP solution, each cell constituting the epithelium began to contract. The epithelium was cleaved into many cell groups as a result of contraction of each cell. The periphery of each cell group was lifted to form a cup or vesicle and eventually detached from the substratum. However, those cells that had not adhered tightly and not formed a monolayer epithelium with typical polygonal cellular pattern contracted independently as observed in the glycerinated fibroblasts. Contraction of the glycerinated pigmented epithelial cells was inhibited by N-ethylmaleimide but not by cytochalasin B. ITP and UTP also effected the contraction of the glycerinated cells, but GTP and ADP did not. Ca2+ was not required. This contractile model of pigmented epithelium provides a useful experimental system for analyzing the function of actin in cellular morphogenesis.

Mechanism of retraction of the trailing edge during fibroblast movement

Retraction of the taut, trailing portion of a moving chick heart fibroblast in vitro is an abrupt dynamic process. Upon retraction, the fibroblast tail always ruptures, leaving a small amount of itself attached to the substratum by focal contacts. Time-lapse cinemicrography shows that retraction produces a sudden, massive movement of both surface and cytoplasmic material toward a cluster of focal contacts near the main body of the cell. The appearance of folds on the upper cell surface at this time and the absence of endocytotic vesicles are consistent with this forward movement. Retraction of the trailing edge, either occurring naturally or produced artificially with a microneedle, consists of an initial fast component followed and overlapped by a slow component. Upon artificial detachment in the presence of iodoacetate, dinitrophenol, and sodium fluoride, and at 4 degrees C, the slow component is strongly inhibited and the fast one only slightly inhibited. Moreover of the bundles of microfilaments oriented parallel to the long axis of the tail seen in TEM. Most of the birefringence is lost during the fast phase and the rest during the slow phase of retraction. Concurrently, the bundles of microfilaments disappear during the fast phase of retraction and are replaced by a microfilament meshwork. All of these results are consistent with the hypothesis that the initial fast component of retraction is a passive elastic recoil, associated with the oriented bundles of microfilaments, and that the slow component of retraction is an active contraction, associated with a meshwork of microfilaments.

Cytoplasmic streaming in a heliozoan

A method is described utilizing deuterium oxide stabilization of microtubules that allows the separation of stable axopodia from the cell body. In some instances the isolated axopodia are broken open and display two classes of cytoplasmic linear elements, microtubules and microfilaments, lying side by side. This morphological arrangement is consistent with the hypothesis that microtubules support a microfilament-based contractile apparatus in heliozoans and probably other cell types as well. Also included is a description of pseudopodial formation from axopodial cytoplasm in response to mechanical stimulation. The pseudopodial motility is discussed in relation to the particle saltations also observed in these cells and to different levels of organization or activation of the contractile machinery.

Active movement in vitro of bundle of microfilaments isolated from Nitella cell

Subcortical fibrils composed of bundles of F-actin filaments and endoplasmic filaments are responsible for endoplasmic streaming. It is reported here that these fibrils and filaments move actively in an artificial medium containing Mg-ATP and sucrose at neutral pH, when the medium was added to the cytoplasm squeezed out of the cell. The movement was observed by phase-contrast microscopy or dark-field microscopy and recorded on 16-mm film. Chains of chloroplasts linked by subcortical fibrils showed translational movement in the medium. Even after all chloroplasts and the endoplasm were washed away by perfusion with fresh medium, free fibrils and/or filaments (henceforth, referred to as fibers) not attached to chloroplasts continued travelling in the direction of the fiber orientation. Sometimes the fibers formed rings and rotated. Chloroplast chains and free fibers or rings continued moving for 5-30 min at about half the rate of the endoplasmic streaming in vivo. Calcium ion concentrations < 10(-7) M permitted movement to take place. Electron microscopy revealed that both fibers and rings were bundles of F-actin filaments that showed the same polarity after decoration with heavy meromyosin.

Intracellular pH in single motile cells

Cytoplasmic pH in single living specimens of Chaos carolinensis is determined microfluorometrically by measuring the ratio of fluorescence intensity of microinjected fluorescein-thiocarbamyl (FTC)-ovalbumin at two different excitation wavelengths. The probe is evenly distributed throughout, and confined to, the cytoplasm, and the fluorescence intensity ratio depends only upon pH. It is independent of pathlength, concentration of probe, divalent cations, and ionic strength. Ratios are calibrated with a standard curve generated in situ by adjusting internal pH of FTC-ovalbumin-containing amebae with weak acid and weak base or by injection of strong buffers. With this technique, the average cytoplasmic pH of freely moving ameba is found to be 6.75 (SD +/- 0.3). The pH of a given spot relative to the morphology of a moving ameba remains fairly constant (+/- 0.05 U), whereas the pH of two different spots in the same cell may differ by as much as 0.4 U, and average pH in different amebae ranges from 6.3 to 7.4, with a suggestion of clustering about pH 6.5 and 6.8. During wound healing, there is a local, transient drop in pH (as great as 0.35 U) at the wound site upon puncture, proportional in extent to the degree of damage. Comparison of tails and advancing pseudopod tips reveals no significant difference in cytoplasmic pH at this level of spatial (50 microns diameter spot) and temporal (1.3 s) resolution. Fluctuations in intracellular pH and/or intracellular free Ca++ may be involved in regulation of cytoplasmic structure and contractility.

Contractile basis of ameboid movement. VII. Aequorin luminescence during ameboid movement, endocytosis, and capping

Aequorin luminescence has been utilized to determine the spatial and temporal fluctuations of the free calcium ion concentration [Ca++] in Chaos carolinensis during ameboid movement, pinocytosis, and capping. The [Ca++] increases above approximately 10(-7) M during normal ameboid movement. Three types of luminescent signals are detected in cells: continuous luminescence, spontaneous pulses, and stimulated pulses. Continuous luminescence is localized in the tails of actively motile cells, and spontaneous pulses occur primarily over the anterior regions of cells. We are sometimes able to correlate the spontaneous pulses with extending pseudopods, whereas stimulated pulses are induced by mechanical damage, electrical stimulation, concanavalin A-induced capping, and pinocytosis. The localization of both distinct actin structures and sites where [Ca++] increases suggests cellular sites of contractile activity. The independent evidence from localizing actin structures and the distribution of [Ca++] can also be viewed in relation to the solation-contraction coupling hypothesis defined in vitro.

Contractile basis of ameboid movement. VII. The distribution of fluorescently labeled actin in living amebas

The technique of molecular cytochemistry has been used to follow the distribution of fluorescently labeled actin in living Chaos carolinensis and Amoeba proteus during ameboid movement and various cellular processes. The distribution of 5-iodoacetamidofluorescein-labeled actin was compared with that of Lissamine rhodamine B sulfonyl chloride-labeled ovalbumin microinjected into the same cell and recorded with an image intensification microscope system. Actively motile cells demonstrated a rather uniform distribution of actin throughout most of the cytoplasm, except in the tail ectoplasm and in plasma gel sheets, where distinct actin structures were observed. In addition, actin-containing structures were induced in the cortex during wound healing, concanavalin A capping, pinocytosis, and contractions elicited by phalloidin injections. The formation of distinct fluorescent actin structures has been correlated with contractile activities.

Cytoplasmic free calcium and amoeboid movement

Measurements with the Ca2+ -sensitive photoprotein aequorin show that locomotion in the amoeba Chaos carolinense occurs without changes in the aequorin signal and that not more than 0.025% of the cytoplasm can exist at the micromolar threshold concentration for contraction. The results do not support the hypothesis that cytoplasmic streaming is under the control of changes in the cytoplasmic free Ca2+ concentration.

Viscometric analysis of the gelation of Acanthamoeba extracts and purification of two gelation factors

We have studied the kinetics of the gelation process that occurs upon warming cold extracts of Acanthamoeba using a low-shear falling ball assay. We find that the reaction has at least two steps, requires 0.5 mM ATP and 1.5 mM MgCl2, and is inhibited by micromolar Ca++. The optimum pH is 7.0 and temperature, 25 degrees-30 degrees C. The rate of the reaction is increased by cold preincubation with both MgCl2 and ATP. Nonhydrolyzable analogues of ATP will not substitute for ATP either in this "potentiation reaction" or in the gelation process. Either of two purified or any one of four partially purified Acanthamoeba proteins will cross-link purified actin to form a gel, but none can account for the dependence of the reaction in the crude extract on Mg-ATP or its regulation by Ca++. This suggests that the extract contains, in addition to actin-cross-linking proteins, factors dependent on Mg-ATP and Ca++ that regulate the gelation process.

Organization and spatial arrangement of fluorescein-labeled native actin microinjected into normal locomoting and experimentally influenced Amoeba proteus

Fully polymerization-competent fluorescein-labeled actin from skeletal muscle was microinjected into both normal moving and experimentally treated Amoeba proteus. Its intracellular distribution was followed by integral image intensification of the fluorescence on a television screen and compared with controls injected with rhodamine-labeled serum albumin. The labeled actin was incorporated into the endogenous actin pool and exhibited a characteristic redistribution depending on the cellular morphology. Increased amounts of labeled actin could be detected within a thin layer separating the hyalo- and granuloplasm or running immediately beneath the plasma membrane when hyaloplasmic regions were absent. The topography of the fluorescent layer demonstrated in living cells is in agreement with the cortical microfilament layer described ultrastructurally recently in corresponding cells. The combined results emphasize the important role of the cortical filament layer in both morphogenetic processes (e.g., hyalo-granuloplasm separation or changes in cell shape) and motive force generation for cytoplasmic streaming and amoeboid movement.

The contractile basis of ameboid movement. VI. The solation-contraction coupling hypothesis

The contracted pellets derived from a high-speed supernate of Dictyostelium discoideum (S3) were investigated to determine the functional activity associated with this specific subset of the cellular motile apparatus. A partially purified model system of gelation and contraction (S6) was prepared from the contracted pellets, and the presence of calcium- and pH-sensitive gelation and contraction in this model demonstrated that a functional cytoskeletal-contratile complex remained at least partially associated with the actin and myosin during contraction. Semi-quantitative assays of gelation and solation in the myosin-free preparation S6 included measurements of turbidity, relative viscosity, and strain birefringence. The extent of gelation was optimal at pH 6.8 and a free calcium ion concentration of approximately 3.0 x 10(-8) M. Solation was favored when the free calcium ion concentration was greater than 7.6 x 10(-7) M or when the pH was increased or decreased from pH 6.8. Gelation was reversibly inhibited by increasing the free calcium ion concentration to approxomately 4.6 x 10(-6) M at pH 6.8. The solation-gelation process of this model has been interpreted to involve the reversible cross-linking of actin filaments. The addition of purified D. discoideum myosin to S6 served to reconstitute calcium- and pH-regulated contraction. The results from this study indicate that contraction is coupled functionally to the local breakdown (solation) of the gel. Therefore, solation has been identified as a structural requirement for extensive shortening during contraction. We have called this concept the solation-contraction coupling hypothesis. Fractionation of a preparation derived from the contracted pellets yielded a fraction consisting of actin and a 95,000-dalton polypeptide that exhibited calcium-sensitive gelation at 28 degrees C and a fraction composed of actin and 30,000- and 18,000-dalton polypeptides that demonstrated calcium-sensitive genlation at 0 degrees C.

pH regulates the polymerization of actin in the sea urchin egg cortex

The state of actin in the isolated cortex of the unfertilized sea urchin egg can be controlled by experimentally manipulating the pH of the isolation medium. Cortices isolated at the pH of the unfertilized egg (6.5--6.7) do not contain filamentous actin, while those isolated at the pH of the fertilized egg (7.3--7.5) develop large numbers of microvilli which contain bundles of actin filaments. Cortices that are isolated at pH 6.5 and then transferred to isolation medium buffered at pH 7.5 also develop actin filaments. However, the filaments are not arranged in bundles and microvilli do not form. Although the cortical granules in cortices isolated at pH 6.5 discharge at a free Ca++ concentration of approximately 10 micrometer, actin polymerization is not induced by increasing the Ca++ concentration of the isolation medium. These results suggest that the increase in cytoplasmic pH which occurs following fertilization induces the polymerization of actin in the egg cortex.

N-ethylmaleimide-modified heavy meromyosin. A probe for actomyosin interactions

Treatment of rabbit skeletal muscle heavy meromyosin (HMM) with the sulfhydryl reagent N-ethylmaleimide (NEM) produces a species of HMM which remains tightly bound to actin in the presence of MgATP. NEM-HMM forms characteristic "arrowhead" complexes with actin which persist despite rinses with MgATP. NEM-HMM inhibits the actin activation of native HMM-ATPase activity, the superprecipitation of actomyosin, the contraction of glycerinated muscle myofibrils, and the contraction of cytoplasmic strands of the soil amoeba Chaos carolinensis. However, NEM-HMM does not interfere with in vitro microtubule polymerization or beating of demembranated cilia.

Actin in Xenopus oocytes

It has been found that a high-speed supernatant fraction from Xenopus oocytes extracted in the cold will form a clear, solid gel upon warming. Gel formation occurs within 60 min at 18 degrees-40 degrees C, and is, at least initially, temperature reversible. Gelation is strictly dependent upon the addition of sucrose to the extraction medium. When isolated in the presence of ATP, the gel consists principally of a 43,000-dalton protein which co-migrates with Xenopus skeletal muscle actin on SDS-polyacrylamide gels, and a prominent high molecular weight component of approx. 250,000 daltons. At least two minor components of intermediate molecular weight are also found associated with the gel in variable quantities. Actin has been identified as the major consituent of the gel by ultrastructural and immunological techniques, and comprises roughly 47% of protein in the complex. With time, the gel spontaneously contracts to form a small dense aggregate. Contraction requires ATP. In the absence of exogenous ATP, a polypeptide which co-migrates with the heavy chain of Xenopus skeletal muscle myosin becomes a prominent component of the gel. This polypeptide is virtually absent from gels which have contracted in ATP-containing extracts. It has also been found that Ca++ is required for gelation in oocyte extracts. At both low and high concentrations of Ca++ (defined as a ratio of Ca++/EGTA in the extraction medium), gelation is inhibited.

Molecular cytochemistry: incorporation of fluorescently labeled actin into living cells

Actin labeled with 5-iodoacetamidofluorescein has been incorporated into the functional pool of actin in Chaos carolinensis and Physarum polycephalum by direct microinjection. The functional activity of the labeled actin has been analyzed at three levels of organization as: (a) with the purified actin, (b) in motile extracts of cells, and (c) in living motile cells. The labeled actin exhibited normal polymerization and activated myosin ATPase to a similar extent as unlabeled controls. Labeled actin and endogenous actin were incorporated into contracted pellets to approximately the same extent in motile cell extracts. After labeled actin had been microinjected into single C. carolinensis cells, the fluorescent actin spread into both the endoplasm and etoplasm without forming distinct fibrils. In contrast, fluorescent bundles developed in the ectoplasm of P. polycephalum following microinjection of labeled actin. This experimental method in conjunction with fluorescence spectroscopic techniques could become a powerful tool for studying the intracellular distribution and structural changes of components in living cells.

Phagocytosis of bacteria by polymorphonuclear leukocytes. A freeze-fracture, scanning electron microscope, and thin-section investigation of membrane structure

The changes in membrane structure of rabbit polymorphonuclear (PMN) leukocytes during bacterial phagocytosis was investigated with scanning electron microscope (SEM), thin-section, and freeze-fracture techniques. SEM observations of bacterial attachment sites showed the involvement of limited areas of PMN membrane surface (0.01-0.25mum(2)). Frequently, these areas of attachment were located on membrane extensions. The membrane extensions were present before, during, and after the engulfment of bacteria, but were diminished in size after bacterial engulfment. In general, the results obtained with SEM and thin-section techniques aided in the interpretation of the three-dimensional freeze-fracture replicas. Freeze-fracture results revealed the PMN leukocytes had two fracture faces as determined by the relative density of intramembranous particles (IMP). Membranous extensions of the plasma membrane, lysosomes, and phagocytic vacuoles contained IMP's with a distribution and density similar to those of the plasma membrane. During phagocytosis, IMPs within the plasma membrane did not undergo a massive aggregation. In fact, structural changes within the membranes were infrequent and localized to regions such as the attachment sites of bacteria, the fusion sites on the plasma membrane, and small scale changes in the phagocytic vacuole membrane during membrane fusion. During the formation of the phagocytic vacuole, the IMPs of the plasma membrane appeared to move in with the lipid bilayer while maintaining a distribution and density of IMPs similar to those of the plasma membranes. Occasionally, IMPs were aligned to linear arrays within phagocytic vacuole membranes. This alignment might be due to an interaction with linearly arranged motile structures on the side of the phagocytic vacuole membranes. IMP-free regions were observed after fusion of lysosomes with the phagocytic vacuoles or plasma membrane. These IMP-free areas probably represent sites where membrane fusion occurred between lysosomal membrane and phagocytic vacuole membrane or plasma membrane. Highly symmetrical patterns of IMPs were not observed during lysosomal membrane fusion.

Effects of myosin and heavy meromyosin on actin-related gelation of HeLa cell extracts

The gelation induced by warming (to 25 degrees C) the 100,000 g supernatant fraction (extract) of HeLa cells lysed in a buffer containing sucrose, ATP, DTE, EGTA, imidazole, and Triton X-100 was studied in the presence of myosin and heavy meromyosin (HMM). Myosin mixed with extract induces shrinkage of the gel, but jelled extract or myosin alone does not shrink. In the concentration range, 0.14-1.04 mg/ml of myosin, the degree of shrinkage is roughly proportional to the concentration of myosin. Supplementa MgCl2 also promotes shrinkage. HMM (0.4-0.8 mg/ml) can inhibit gel formation by extract in tubes or floated on a sucrose cushion. Gel electrophoresis of gels shrunken by added myosin or electrophoresis of the proteins which can be sedimented from extract after incubation in the presence of HMM indicate that both myosin and HMM interfere with the changes in sedimentability of the high molecular weight protein (HMWP) thought to participate (together with actin) in gel formation in HeLa cell extracts (R. R. Weihing, 1976. J. Cell Biol. 71:303-307). These results, together with previous results showing that actin is present and that HMWP is enriched in the plasma membrane fraction of HeLa cells (R. R. Weihing, 1976. Cold Spring Harbor Conf. Cell Proliferation. 3:671-684), point to the possibility of dynamic changes in the interactions of HMWP or myosin with actin in processes of movement occurring at the cell surface.

Granule release by polymorphonuclear leukocytes treated with the ionophore A23187

Polymorphonuclear leukocytes (PMN's) incubate three to eight minutes at 37 degrees C in medium containing 1 X 10(-6) M of the ionophore antibiotic A23187 released their cytoplasmic granules into the extracellular medium. Transmission electron microscopy of treated cells showed microfilament bundles extending between adjacent granules within the cytoplasm and between granules and the plasma membrane. Tiny dense projections (beads) 8-12 nm in diameter were observed along segments of the cytoplasmic surface of the plasma membrane with a periodicity of 20-30 nm. These beads were observed on the plasma membrane only in the vicinity of intra- or extracytoplasmic granules. The structural relationships of the beads with the plasma membrane microfilaments suggest they play a role in the process of ionophore-induced granule release from polymorphonuclear leukocytes.

Macrophage deformability and phagocytosis

The influence of several metabolic inhibitors and pharmacologic agents on macrophage deformation (induced by fluid shear stress) was examined in relationship to changes in ATP content and phagocytosis of latex beads. Two relatively specific inhibitors of glycolysis (iodoacetate [IA], and sodium fluoride [NaF]) and a sulfhydryl-binding agent (N-ethylmaleimide [NEM] markedly inhibited phagocytosis and reduced cell deformability. A microtubule-disrupting agent (vinblastine) and a highly specific inhibitor of glycolysis (2-deoxyglucose) markedly inhibited phagocytosis without influencing cell deformability. An organomercurial sulfhydryl binding agent p-chloromercuribenzene (PCMBS) and a microfilament-disrupting agent (cytochalasin B) inhibited phagocytosis and increased cell deformability. The effects of these agents on phagocytosis and cell deformability bore no consistent relationship to alterations in cellular content of ATP. The observation that 2-deoxyglucose, the most specific inhibitor of glycolysis examined, reduced ATP content to levels far lower (15 percent of control values) than those achieved by any other agent examined and inhibited phagocytosis without altering cell deformability, suggests that alterations in cell deformability induced by NaF, IA, NEM, PCMBS, and cytochalasin B are not due to inhibition of glycolysis per se, but instead result from direct or indirect effects of these agents on cell constituents, possibly contractile proteins, which are determinants of cell deformability. The finding that cytochalasin B, NEM, PCMBS, and IA interfere with phagocytosis and alter cell deformability, together with evidence that these agents interact with isolated actin and myosin, suggests that contractile proteins are important both in phagocytosis and as determinants of cell deformability. The observation that vinblastine, colchicines, and heavy water (D(2)O) did not alter cell deformability, even though vinblastine caused formation of intracellular crystals of microtubular protein, indicates that microtubules are not major determinants of cell deformability. The observations that beads adhered normally to surfaces of cytochalasin B- and of PCMBS-treated cells and that shear-stress induced deformation was increased whereas phagocytosis was markedly inhibited, suggest that deformation of cells around beads associated with ingestion depends on some form of cellular (contractile?) activity, whereas deformation of cells by fluid shear stress is a passive phenomenon.

The contractile basis of amoeboid movement. V. The control of gelation, solation, and contraction in extracts from Dictyostelium discoideum

Motile extracts have been prepared from Dictyostelium discoideum by homogenization and differential centrifugation at 4 degrees C in a stabilization solution (60). These extracts gelled on warming to 25 degrees Celsius and contracted in response to micromolar Ca++ or a pH in excess of 7.0. Optimal gelation occurred in a solution containing 2.5 mM ethylene glycol-bis (beta-aminoethyl ether)N,N,N',N'-tetraacetate (EGTA), 2.5 mM piperazine-N-N'-bis [2-ethane sulfonic acid] (PIPES), 1 mM MgC1(2), 1 mM ATP, and 20 mM KCI at ph 7.0 (relaxation solution), while micromolar levels of Ca++ inhibited gelation. Conditions that solated the gel elicited contraction of extracts containing myosin. This was true regardless of whether chemical (micromolar Ca++, pH >7.0, cytochalasin B, elevated concentrations of KCI, MgC1(2), and sucrose) or physical (pressure, mechanical stress, and cold) means were used to induce solation. Myosin was definitely required for contraction. During Ca++-or pH-elicited contraction: (a) actin, myosin, and a 95,000-dalton polypeptide were concentrated in the contracted extract; (b) the gelation activity was recovered in the material sqeezed out the contracting extract;(c) electron microscopy demonstrated that the number of free, recognizable F-actin filaments increased; (d) the actomyosin MgATPase activity was stimulated by 4- to 10-fold. In the absense of myosin the Dictyostelium extract did not contract, while gelation proceeded normally. During solation of the gel in the absense of myosin: (a) electron microscopy demonstrated that the number of free, recognizable F- actin filaments increased; (b) solation-dependent contraction of the extract and the Ca++-stimulated MgATPase activity were reconstituted by adding puried Dictyostelium myosin. Actin purified from the Dictyostelium extract did not gel (at 2 mg/ml), while low concentrations of actin (0.7-2 mg/ml) that contained several contaminating components underwent rapid Ca++ regulated gelation. These results indicated : (a) gelation in Dictyostelium extracts involves a specific Ca++-sensitive interaction between actin and several other components; (b) myosin is an absolute requirement for contraction of the extract; (c) actin-myosin interactions capable of producing force for movement are prevented in the gel, while solation of the gel by either physical or chemical means results in the release of F-actin capable of interaction with myosin and subsequent contraction. The effectiveness of physical agents in producting contraction suggests that the regulation of contraction by the gel is structural in nature.

Relations between ameboid movement and membrane-controlled electrical currents

We have studied the pattern of electrical currents through amebas (mainly Chaos chaos) with an ultrasensitive extracellular vibrating probe. Amebas drive both steady currents and current pulses through themselves. Relatively steady current with an average surface density of 0.1-0.2 muA/cm2 enters the rear quarter of an ameba and leaves its pseudopods. Streaming reversals are preceded by changes in this current pattern and the region with the largest new inward current becomes the new tail. Ion substitution studies suggest that some of the steady inward current is carried by calcium ions. Characteristic stimulated pulses of current sometimes follow the close approach of the vibrating probe to the side of an advancing pseudopod. Such a pulse enters the cytoplasm through a small patch of membrane near the probe (and seems to leave through the adjacent membrane), is usually followed by hyaline cap and then by pseudopod initiation, is calcium dependent, lasts about 5-10 s, and has a peak density of about 0.4 muA/cm2. Spontaneous pulses of similar shape and duration may enter or leave any part of an animal. They are much less localized, tend to have higher peak densities, and occur in physiological salt solutions at about 0.2-4 times per minute. Retraction of a pseudopod is always accompanied or preceded by a spontaneous pulse which leaves its sides.

Dynamic aspects of filopodial formation by reorganization of microfilaments

The coelomocytes of the sea urchin, Strongylocentrotus droebachiensis, may be prevented from clotting with 50 mM ethylene glycol-bis(beta-aminoethyl)-N,N,N',N'-tetraacetate, 50 mM Tris-HCl, pH 7.8 and subsequently separated into various cell types on sucrose gradients. One cell type, the petaloid coelomocyte, spontaneously undergoes a striking morphological transformation to a form exhibiting numerous, t-in cytoplasmic projections (filopodia). Moreover, the transformation is reversible. Ultrastructurally, the formation of the filopodia results from a progressive reorganization of actin-containing filaments into bundles that are radially oriented. The formation of the filament bundles is initiated at the cell's periphery and proceeds inward. Simultaneously, the cytoplasm in between the bundles is withdrawn, exposing finger-like filopodia. Ultimately, the filopodia can be extended by up to four times their original length. Biochemically, actin is the most abundant protein in while cell homogenates and is extractable in milligram quantities via acetone powders. An actomyosin complex may also be isolated from these cells and is presumed to be active in producing the various forms of motility observed.

Ionophore A23187 and dibutyryl cyclic AMP effects on cell shape and morphology of B-16 melanoma

Melanoma cells treated with dibutyryl cyclic AMP (db cAMP) for 24 h resulted in dendritic cells possessing parallel assembled microtubules. A23187 treatments resulted in a biphasic response: Long term effects of the ionophore were characterized by small epitheloid cells while the immediate response produced elongated cells with parallel arranged 10 nm microgilaments, characteristic of dispersive melanocytes.

Fluorescent antibody localization of myosin in the cytoplasm, cleavage furrow, and mitotic spindle of human cells

We have studied the distribution of myosin molecules in human cells using myosin-specific antibody coupled with fluorescent dyes. Rabbits were immunized with platelet myosin or myosin rod. They produced antisera which precipitated only myosin among all the components in crude platelet extracts. From these antisera we isolated immunoglobulin-G (IgG) and conjugated it with tetramethylrhodamine or fluorescein. We separated IgG with 2-5 fluorochromes per molecule from both under- and over-conjugated IgG by ion exchange chromatography and used it to stain acetone-treated cells. The following controls established the specificity of the staining patterns: (a) staining with labeled preimmune IgG; (b) staining with labeled immune IgG adsorbed with purified myosin; (c) staining with labeled immune IgG mixed with either unlabeled preimmune or immune serum; and (d) staining with labeled antibody purified by affinity chromatography. In blood smears, only the cytoplasm of platelets and leukocytes stained. In spread Enson and HeLa cells, stress fibers stained strongly in closely spaced 0.5 mum spots. The cytoplasm stained uniformly in those cells presumed to be motile before acetone treatment. In dividing HeLa cells there was a high concentration of myosin-specific staining in the vicinity of the contractole ring and in the mitotic spindle, especially the region between the chromosomes and the poles. We detected no staining of erythrocytes, or nuclei of leukocytes and cultured cells, or the surface of platelets and cultured cells.

Actin polymerization and interaction with other proteins in temperature-induced gelation of sea urchin egg extracts

The gel which forms on warming the extracts of the cytoplasmic proteins of sea urchin eggs has been separated into two fractions, one containing F-actin and the other containing two proteins of 58,000 and 22,000 mol wt. When combined in 0.1 M KCl, even at 0 degrees C, these components will form gel material identical to that formed by warming extracts. This gel is a network of laterally aggregated F-actin filaments which are in register and which display a complex cross-banding pattern generated by the presence of the other two proteins. Low concentrations of calcium block the assembly of these proteins to form this complex structure, which may play some cytoskeletal role in the cytoplasm. This association of F-actin with the other proteins to form a gel is very likely the last step fo the process occurring in warmed extracts. At low temperatures, gelation of extracts is limited by the relative absence of F-actin, as demonstrated by the inability to sediment it at 100,000 g and also by the fact that gelation occurs immediately if exogenous F-actin is added to cold extracts. The transformation of the G-actin present in the extract to the F-form is apparently repressed at low temperatures. This is shown directly by the failure of added G-actin to polymerize at low temperatures in the presence of extract. These observations resemble those which have been reported on preparations from amoeboid cells and may be significant in the involvement of actin and these other proteins in cell division and later developmental processes.

Association of microfilament bundles with lysosomes in polymorphonuclear leukocytes

The juxtaposition of microfilament bundles and lysosomes seen both in thin-sectioned cells in the transmission electron microscope and in cryofractured cells in the scanning electron microscope, and the presence of short filamentous structures between lysosomes and microfilament bundles, suggest that microfilaments may be attached to lysosomal membranes and that these filaments may be involved in lysosomal movements. Further work is in progress to test these hypotheses.

Brush border motility. Microvillar contraction in triton-treated brush borders isolated from intestinal epithelium

The brush border of intestinal epithelial cells consists of an array of tightly packed microvilli. Within each microvillus is a bundle of 20-30 actin filaments. The basal ends of the filament bundles are embedded in and interconected by a filamentous meshwork, the terminal web, which lies directly beneath the microvilli. When calcium and ATP are added to isolated brush borders that have been treated with the detergent, Triton X-100, the microvillar filament bundles rapidly retract into and through the terminal web region. Biochemical studies of brush border contractile proteins suggest that the observed microvillar contraction is actomyosin mediated. We have shown previously that the major protein of the brush border's actin (Tilney, L. G., and M. S. Mooseker. 1971. Proc. Natl. Acad. Sci. U. S. A. 68:2611-2615). The brush border also contains a protein with the same molecular weight as the heavy chain subunit of myosin (200, 000 daltons). In addition, preparations of demembranated brush borders exhibit potassium-EDTA ATPase activity of 0.02 mumol phosphate/mg-min (22 degrees C); this assay is diagnostic for myosin-like ATPase isolated from vertebrate sources. Other proteins of the brush border include a 30,000 dalton protein with properties similar to those of tropomyosin, and a protein with the same molecular weight as the Z band protein, alpha-actinin (95,000 daltons). How these observations bear on the basis for microvillar movements in vivo is discussed within the framework of our recent model for the organization of actin and myosin in the brush border (Mooseker, M. S., and L. G. Tilney. 1975. J. Cell Biol. 67:725-743).

Microfilaments in Naegleria fowleri amoebae

Examination by electron microscopy has revealed 2 types of microfilament in the cytoplasm of 3 strains of axenically grown Naegleria fowleri amoebae. Thin, actin-like microfilaments 5-7 nm in diameter are randomly oriented in the nonmotile amoebae, and are concentrated near the plasma membrane. In the actively motile amoebae these microfilaments aggregate to form colateral bundles in close proximity to the plasma membrane. Thick, myosin-like microfilaments 17-19 nm in diameter also occur in the amoebae cytoplasm. The significance of these 2 kinds of microfilament in amoeboid motion is discussed.