Novel form of actin-based motility transports bacteria on the surfaces of infected cells

Enteropathogenic Escherichia coli (EPEC) attach to cells (attachment) lining the intestine and induce a decrease in the number of the cells' microvilli (effacement). This attachment and effacement is followed by diarrhea, which may be explained, at least in part, to the loss of microvilli and the decreased ability of the infected cells to absorb fluids. EPEC also attach to the surfaces of a number of cultured cells including CaCo-2, LLC-PK, and PtK2 cells. The extracellular, attached EPEC induce filaments of actin to form in the cytoplasm just underneath the EPEC surface attachment sites. Beneath some of the attached EPEC, the actin filaments become organized into membrane encased columns that extend up to 6 micrometers above the cell surface creating "pedestals" on which the EPEC rest. The raised pedestals can be readily observed in stereo pairs taken using the Intermediate Voltage Electron Microscope. The concentration of non-muscle isoforms of myosin II and tropomyosin near the base of the pedestals suggests a similarity of these structures to brush border microvilli. Video microscopy indicates that these EPEC pedestals can bend and undulate, alternately growing longer and shorter while remaining tethered in place on the cell surface. Some of the attached EPEC also translocate along the cell surface, reaching speeds up to 0.07 micrometers/sec. Both types of movement are inhibited by cytochalasin D, indicating that actin polymerization in the pedestals is required for the motility of EPEC on the host cell surface. In this respect, EPEC motility on host cells resembles the intracellular motility of Listeria, but there are differences in the actin filament bundles induced by the two different bacteria. The most obvious one is the interposition of the cell membrane between EPEC and the actin filaments in the pedestal in contrast to the close apposition of actin filaments to Listeria. The intensity of fluorescence of rhodamine phalloidin is nearly uniform along most of the length of the pedestals indicating a constant number of actin filaments, whereas the fluorescence intensity decreases along the length of Listeria tails reflecting the disassembly that occurs all along the tails. Epec's movements may be a hybrid of Listeria filopodia and Aplysia inductopodia movements. This paper is the first report of a microbe attached to the extracellular surface of an infected cell propelled by an intracellular actin polymerization-dependent mechanism.

Increasing intracellular concentrations of thymosin beta 4 in PtK2 cells: effects on stress fibers, cytokinesis, and cell spreading

Thymosin beta 4 (T beta 4) binds to G-actin in vitro and inhibits actin polymerization. We studied the effects of increasing T beta 4 concentration within living PtK2 cells, comparing its effects on the disassembly of stress fibers and membrane-associated actin with its ability to inhibit cytokinesis and cell spreading after mitosis. We chose PtK2 cells for the study because these cells have many striking actin bundles in both stress fibers and cleavage furrows. They also have prominent concentrations of membrane-associated actin and remain flattened during mitosis. We have found that PtK2 cells contain an endogenous homologue of T beta 4 at a concentration (approximately 28 microM) sufficient to complex a third or more of the cell's unpolymerized actin. Intracellular T beta 4 concentrations were increased by three different methods: 1) microinjection of an RSV vector containing a cDNA for T beta 4; 2) transfection with the same vector; and 3) microinjection of purified T beta 4 protein. The plasmid coding for T beta 4 was microinjected into PtK2 cells together with fluorescently labeled alpha-actinin as a reporter molecule. Immediately after microinjection fluorescently labeled alpha-actinin was detected in a periodic pattern along the stress fibers just as in control cells injected solely with the reporter. However, after 13 h, cells microinjected with reporter and plasmid showed marked disassembly of the fiber bundles. PtK2 cells transfected with this RSV vector for 2-3 days showed disassembly of stress fibers as detected by rhodamine-phalloidin staining; in these cells the membrane actin was also greatly diminished or absent and the border of the cells was markedly retracted. Microinjection of pure T beta 4 protein into interphase PtK2 cells induced disassembly of the stress fibers within 10 min, while membrane actin appeared only somewhat reduced. If the PtK2 cells were mitotic, similar microinjection of pure thymosin beta 4 protein at times from early prophase to metaphase resulted in an unusual pattern of delayed cytokinesis. Furrowing occurred but at a much slower rate than in controls and the amount of actin in the cleavage furrow was greatly reduced. The cells constricted to apparent completion, but after about 30 min the furrow regressed, forming a binucleate cell, much as after treatment with cytochalasin B or D. Postcytokinesis spreading of these T beta 4-injected cells was often inhibited. These experiments suggest that an insufficient number of actin filaments prolongs the contractile phase of cytokinesis and abolishes the final sealing process.

Listeria monocytogenes intracellular migration: inhibition by profilin, vitamin D-binding protein and DNase I

Infection of host cells by Listeria monocytogenes results in the recruitment of cytoplasmic actin into a tail-like appendage that projects from one end of the bacterium. Each filamentous actin tail progressively lengthens, providing the force which drives the bacterium in a forward direction through the cytoplasm and later results in Listeria cell-to-cell spread. Host cell actin monomers are incorporated into the filamentous actin tail at a discrete site, the bacterial-actin tail interface. We have studied the consequences of microinjecting three different actin monomer-binding proteins on the actin tail assembly and Listeria intracellular movement. Introduction of high concentrations of profilin (estimated injected intracellular concentration 11-22 microM) into infected PtK2 cells causes a marked slowing of actin tail elongation and bacterial migration. Lower intracellular concentrations of two other injected higher affinity monomer-sequestering proteins, Vitamin D-binding protein (DBP; 1-2 microM) and DNase I (6-7 microM) completely block bacterial-induced actin assembly and bacterial migration. The onset of inhibition by each protein is gradual (10-20 min) indicating that the mechanisms by which these proteins interfere with Listeria-induced actin assembly are likely to be complex. To exclude the possibility that Listeria recruits preformed actin filaments to generate the tails and that these monomer-binding proteins act by depolymerizing such performed actin filaments, living infected cells have been injected with fluorescently labeled phalloidin (3 microM). Although the stress fibers are labeled, no fluorescent phalloidin is found in the tails of the moving bacteria. These results demonstrate that Listeria-induced actin assembly in PtK2 cells is the result of assembly of actin monomers into new filaments and that Listeria's ability to recruit polymerization competent monomeric actin is very sensitive to the introduction of exogenous actin monomer-binding proteins.

Organization and structure of actin filament bundles in Listeria-infected cells

During its motion inside host cells, Listeria monocytogenes promotes the formation of a column of actin filaments that extends outward from the distal end of the moving bacterium. The column is constructed of short actin filaments that polymerize at the bacteria-column interface. To get a measure of filament organization in the column, Listeria grown in cultured PtK2 cells were studied with steady state fluorescence polarization, confocal microscopy, and whole cell intermediate voltage electron microscopy. Although actin filament ordering was higher in nearby stress fibers than in the Listeria-associated actin, four distinct areas of ordering could be observed in fluorescence polarization ratio images of bacteria: 1) the surface of the bacteria, 2) the cytoplasm next to the bacteria, 3) the outer shell of the actin column, and 4) the core of the column. Filaments were preferentially oriented parallel to the long axis of the column with highest ordering along the long axis of the bacterial surface and in the shell of the tail. The lowest ordering was in the core (where filaments are possibly also shorter with respect to the cup and the shell), whereas in the adjacent cytoplasm, filaments were oriented perpendicular to the column. A mutant of Listeria that can polymerize actin around itself but cannot move intracellularly does not have its actin organized along the bacterial surface. Thus the alignment of the actin filaments along the bacterial surfaces may be important for the intracellular movement. These conclusions are also supported by confocal microscopy and whole mount electron microscopic data that also reveal that actin filaments can be deposited asymmetrically around the long axis of the bacteria, a distribution that may affect the direction of motility of Listeria monocytogenes inside infected cells.

The premyofibril: evidence for its role in myofibrillogenesis

When cardiac muscle cells are isolated from embryonic chicks and grown in culture they attach to the substrate as spherical cells with disrupted myofibrils, and over several days in culture, they spread and extend lamellae. Based on antibody localizations of various cytoskeletal proteins within the spreading cardiomyocyte, three types of myofibrils have been identified: 1) fully formed mature myofibrils that are centrally positioned in the cell, 2) premyofibrils that are closest to the cell periphery, and 3) nascent myofibrils located between the premyofibrils and the mature myofibrils. Muscle-specific myosin is localized in the A-bands in the mature, contractile myofibrils, and along the nascent myofibrils in a continuous pattern, but it is absent from the premyofibrils. Antibodies to non-muscle isoforms of myosin IIB react with the premyofibrils at the cell periphery and with the nascent myofibrils, revealing short bands of myosin between closely spaced bands of alpha-actinin. In the areas where the nascent myofibrils border on the mature myofibrils, the bands of non-muscle myosin II reach lengths matching the lengths of the mature A-bands. With the exception of a small transition zone consisting of one myofibril, or sometimes several sarcomeres, bordering the nascent myofibrils, there is no reaction of these non-muscle myosin IIB antibodies with the mature myofibrils in spreading myocytes. C-protein is found only in the mature myofibrils, and its presence there may prevent co-polymerization of non-muscle and muscle myosins. Antibodies directed against the non-muscle myosin isoforms, IIA, do not stain the cardiomyocytes. In contrast to the cardiomyocytes, the fibroblasts in these cultures stain with antibodies to both non-muscle myosin IIA and IIB. The premyofibrils near the leading edge of the lamellae show no reaction with antibodies to either titin or zeugmatin, whereas the nascent myofibrils and mature myofibrils do. The spacings of the banded alpha-actinin staining range from 0.3 to 1.4 microns in the pre- and nascent myofibrils and reach full spacings (1.8-2.5 microns) in the mature myofibrils. Based on these observations, we propose a premyofibril model in which non-muscle myosin IIB, titin, and zeugmatin play key roles in myofibrillogenesis. This model proposes that pre- and nascent myofibrils are composed of minisarcomeres that increase in length, presumably by the concurrent elongation of actin filaments, the loss of the non-muscle myosin II filaments, the fusion of dense bodies or Z-bodies to form wide Z-bands, and the capture and alignment of muscle myosin II filaments to form the full spacings of mature myofibrils.

Dynamics of actin and alpha-actinin in the tails of Listeria monocytogenes in infected PtK2 cells

Listeria monocytogenes can penetrate and multiply within a variety of cell types, including the PtK2 kidney epithelial line. Once released within the cytoplasm, L. monocytogenes acquires the capacity for rapid movement through the host cell [Dabiri et al., 1990: Proc. Natl. Acad. Sci. 87:6068-6072]. In the process, actin monomers are inserted in proximity to one end of the bacterium, forming a column or tail of actin filaments [Sanger et al., 1992: Infect. Immun. 60:3609-3619]. The rate of new actin filament growth correlates closely with the speed of bacterial migration. In this study we have used fluorescently labeled actin and alpha-actinin to monitor the movement and turnover rate of actin and alpha-actinin molecules in the tails. The half-lives of the actin and alpha-actinin present in the tails are approximately the same: actin, 58.7 sec; alpha-actinin, 55.3 sec. The half-life of alpha-actinin surrounding a dividing bacterium was 30 sec, whereas its half-life in the tails that formed behind the two daughter cells was about 20-30% longer. We discovered that the speeds of the bacteria are not constant, but show aperiodic episodes of decreased and increased speeds. There is a fluctuation also in the intensities of the fluorescent probes at the bacterium/tail interface, implying that there is a fluctuation in the number of actin filaments forming there. There was no strong correlation, however, between these fluctuating intensities and changes in speed of the bacteria. These measurements suggest that while actin polymerization at the bacterial surface is coupled to the movement of the bacterium, the periodic changes in intracellular motility are not a simple function of the number of actin filaments nucleating at the bacterial surfaces.

Occurrence of fibers and their association with talin in the cleavage furrows of PtK2 cells

PtK2 cells of exceptionally large size were microinjected with fluorescently labeled probes for actin, myosin, filamin, and talin in order to follow the assembly of the contractile proteins into the cleavage furrows. Whereas in cells of normal size, there is usually a diffuse pattern of localization of proteins in the cleavage furrow, in these large, flat cells the labeled proteins localized in fibers in the cleavage furrow. Often, the fibers were striated in a pattern comparable to that measured in the stress fibers of the same cell type. The presence of talin in discrete plaques along fibers in the cleavage furrows of the large cells suggests a further similarity between cleavage furrow and stress fiber structure. The presence of filamin in the cleavage furrows also suggests the possibility of an overlapping mechanism in addition to that of a talin mediated mechanism for the attachment of actin filaments to the cell surfaces in the cleavage furrow. A model is presented that emphasizes the interrelationships between stress fibers, myofibrils, and cleavage furrows.

Intact alpha-actinin molecules are needed for both the assembly of actin into the tails and the locomotion of Listeria monocytogenes inside infected cells

After the infectious bacterium, Listeria monocytogenes, is phagocytosed by a host cell, it leaves the lysosome and recruits the host cell's cytoskeletal proteins to assemble a stationary tail composed primarily of actin filaments cross-linked with alpha-actinin. The continual recruitment of contractile proteins to the interface between the bacterium and the tail accompanies the propulsion of the bacterium ahead of the elongating tail. When a bacterium contacts the host cell membrane, it pushes out the membrane into an undulating tubular structure or filopodium that envelops the bacterium at the tip with the tail of cytoskeletal proteins behind it. Previous work has demonstrated that alpha-actinin can be cleaved into two proteolytic fragments whose microinjection into cells interferes with stress fiber integrity. Microinjection of the 53 kD alpha-actinin fragment into cells infected with Listeria monocytogenes, induces the loss of tails from bacteria and causes the bacteria to become stationary. Infected cells that possess filopodia when injected with the 53 kD fragment lose their filopodia. These results indicate that intact alpha-actinin molecules play an important role in the intracellular motility of Listeria, presumably by stabilizing the actin fibers in the stationary tails that are required for the bacteria to move forward. Fluorescently labeled vinculin associated with the tails when it was injected into infected cells. Talin antibody staining indicated that this protein, also, is present in the tails. These observations suggest that the tails share properties of attachment plaques normally present in the host cells. This model would explain the ability of the bacterium (1) to move within the cytoplasm and (2) to push out the surface of the cell to form a filopodium. The attachment plaque proteins, alpha-actinin, talin, and vinculin, may bind and stabilize the actin filaments as they polymerize behind the bacteria and additionally could also enable the tails to bind to the cell membrane in the filopodia.

Dynamics of organelles in the mitotic spindles of living cells: membrane and microtubule interactions

The distribution and dynamics of the membranous organelles in two cell types were investigated during cell division. Live cells (either PtK2 or LLC-PK1) labeled with the vital dye 3,3'-dihexyloxacarbocyanine iodide [DiOC6(3)] were observed via serial optical sectioning with the laser-scanning confocal microscope. Z-series of labeled, dividing cells were collected every 1-2 minutes throughout mitosis, beginning at prophase and extending to the spreading of the daughter cells. Membrane distribution began to change from the onset of prophase in both cell types. When the mitotic spindle formed in prometaphase, fine tubular membranes, similar to those extending out to the edges of interphase cells aligned along the kinetochore spindle fibers. The lacy polygonal network typical of interphase cells persisted beneath the spindle, and a membrane network was also associated with the dorsal layer of the cell. As PtK2 cells reached metaphase, their spindles were nearly devoid of membrane staining, whereas the spindles of LLC-PK1 cells contained many tubular and small vesicular membranous structures. X-Z series of the LLC-PK1 metaphase spindle revealed a small cone of membranes that was separated from the rest of the cytoplasm by kinetochore MTs. In both cell types, as chromosome separation proceeded, the interzone remained nearly devoid of membranes until the onset of anaphase B. At this time the elongating interzonal microtubules were closely associated with the polygonal network of endoplasmic reticulum. Cytokinesis caused a compression, and then an exclusion of organelles from the midbody. Immunofluorescence staining with anti-tubulin antibodies suggested that spindle membranes were associated with microtubules throughout mitosis. In addition, taxol induced a dense and extensive collection of small vesicles to collect at the spindle poles of both cell types. Nocodazole treatment induced a distinct loss of organization of the membranous components of the spindles. Together these results suggest that microtubules organize the membrane distribution in mitotic cells, and that this organization may vary in different cell types depending on the quantity of microtubules within the spindle.

Contractile protein dynamics of myofibrils in paired adult rat cardiomyocytes

The purpose of this study was to determine how quickly contractile proteins are incorporated into the myofibrils of freshly isolated cardiomyocytes and to determine whether there are regions of the cells that are more dynamic than others in their ability to incorporate the proteins. Paired cardiomyocytes joined at intercalated discs and single cells were isolated from adult rats, and microinjected 3 hours later with fluorescently labeled actin, alpha-actinin, myosin light chains and vinculin. The cells were fixed and permeabilized at various period, 5 seconds and longer, after microinjection. Actin became incorporated throughout the I-Bands in as short a time as 5 seconds. The free edges of the cells, which were formerly intercalated discs, exhibited concentrations of actin greater than that incorporated in the I-Bands. This extra concentration of actin was not detected, however, at intact intercalated discs connecting paired cells. Alpha-actinin was incorporated immediately into Z-Bands and intercalated discs. Vinculin, also, was localized at the Z-Bands and at intercalated discs, but in contrast to alpha-actinin, there was a higher concentration of vinculin in the region of the intact intercalated discs. Both alpha-actinin and vinculin were concentrated at the free ends of the cells that were formerly parts of intercalated discs. Myosin light chains were observed to incorporate into the A-Bands in periods as short as 5 seconds. These results suggest that the myofibrils of adult cardiomyocytes may be capable of rapid isoform transitions along the length of the myofibrils. The rapid accumulation of fluorescent actin, alpha-actinin, and vinculin in membrane sites that were previously parts of intercalated discs, may reflect the response to locomotory activity that is initiated in these areas as cells spread in culture. A similar response after an injury in the intact heart could allow repair to occur.

Costameres are sites of force transmission to the substratum in adult rat cardiomyocytes

Costameres, the vinculin-rich, sub-membranous transverse ribs found in many skeletal and cardiac muscle cells (Pardo, J. V., J. D. Siciliano, and S. W. Craig. 1983. Proc. Natl. Acad. Sci. USA. 80:363-367.) are thought to anchor the Z-lines of the myofibrils to the sarcolemma. In addition, it has been postulated that costameres provide mechanical linkage between the cells' internal contractile machinery and the extracellular matrix, but direct evidence for this supposition has been lacking. By combining the flexible silicone rubber substratum technique (Harris, A. K., P. Wild, and D. Stopak. 1980. Science (Wash. DC). 208:177-179.) with the microinjection of fluorescently labeled vinculin and alpha-actinin, we have been able to correlate the distribution of costameres in adult rat cardiac myocytes with the pattern of forces these cells exert on the flexible substratum. In addition, we used interference reflection microscopy to identify areas of the cells which are in close contact to the underlying substratum. Our results indicate that, in older cell cultures, costameres can transmit forces to the extracellular environment. We base this conclusion on the following observations: (a) adult rat heart cells, cultured on the silicone rubber substratum for 8 or more days, produce pleat-like wrinkles during contraction, which diminish or disappear during relaxation; (b) the pleat-like wrinkles form between adjacent alpha-actinin-positive Z-lines; (c) the presence of pleat-like wrinkles is always associated with a periodic, "costameric" distribution of vinculin in the areas where the pleats form; and (d) a banded or periodic pattern of dark gray or close contacts (as determined by interference reflection microscopy) has been observed in many cells which have been in culture for eight or more days, and these close contacts contain vinculin. A surprising finding is that vinculin can be found in a costameric pattern in cells which are contracting, but not producing pleat-like wrinkles in the substratum. This suggests that additional proteins or posttranslational modifications of known costamere proteins are necessary to form a continuous linkage between the myofibrils and the extracellular matrix. These results confirm the hypothesis that costameres mechanically link the myofibrils to the extracellular matrix. We put forth the hypothesis that costameres are composite structures, made up of many protein components; some of these components function primarily to anchor myofibrils to the sarcolemma, while others form transmembrane linkages to the extracellular matrix.

Host cell actin assembly is necessary and likely to provide the propulsive force for intracellular movement of Listeria monocytogenes

Listeria monocytogenes is able to escape from the phagolysosome and grow within the host cell cytoplasm. By 3 h after initiation of infection, actin filaments begin to concentrate at one end of the bacterium. Polarization of F-actin is associated with intracellular bacterial movement, long projections of actin filaments forming directly behind the moving bacteria. New actin monomers are added to the region of the projection in proximity to the bacterium. The rate of new actin filament growth correlates closely with the speed of bacterial migration. This actin structure is anchored within the cytoplasm, serving as a fixed platform for directional expansion of the actin filament network. The actin projection progressively lengthens as the bacterium migrates. Cytochalasin blocks both elongation of the projection and bacterial movement but does not result in complete depolymerization of the bacterially induced actin structure, residual actin and alpha-actinin persisting in proximity to one end of the bacterium. Bacteria initially migrate within the cortical cytoplasm but later move to the peripheral membrane, where they form filopodiumlike structures which pivot and undulate in the extracellular medium. In the filopodia, bacteria are occasionally seen to abruptly change direction, turn 180 degrees, and move back into the medullary region of the host cell. All filopodium movement ceases once the bacterium containing the F-actin projection returns to the cortical cytoplasm. These results indicate that host cell actin polymerization is necessary for intracellular migration of listeriae and suggest that directional actin assembly may in fact generate the propulsive force for bacterial and filopodial movement.

Expression of desmin cDNA in PtK2 cells results in assembly of desmin filaments from multiple sites throughout the cytoplasm

The assembly of intermediate filaments into a cytoplasmic network was studied by microinjecting into the nuclei and cytoplasms of PtK2 cells, plasmids that contained a full length desmin cDNA and an RSV promoter. Immunofluorescence was used to monitor the expression of desmin and its integration into the cells' vimentin intermediate filament network. We found that the expressed desmin co-localized with filaments of vimentin just as it does with fluorescently labelled desmin is microinjected into the cytoplasm of PtK2 cells. As early as two hours after microinjection of the plasmids, small discrete dots and short fragments of desmin could be detected throughout the cytoplasm of the cells. This initial distribution of desmin was superimposed on the filamentous pattern of vimentin in the cells. At 8 hours after microinjection of the plasmids, some of the desmin was present in long filaments that were coincident with vimentin filaments. By 18 hours, most of the desmin was in a filamentous network co-localizing with vimentin. There was no indication that desmin assembly began in the perinuclear region and proceeded toward the cell periphery. In some cells, excessively high levels of desmin were expressed. In these cases, overexpression led to clumping of desmin filaments as well as to an accumulation of diffusely distributed desmin protein in the center of the cells. This effect was apparent at approximately 18 hours after introduction of the plasmid. The native vimentin filaments in such cells were also aggregated around the nucleus, co-localizing with desmin. The microtubule networks in all injected cells appeared normal; microtubules were extended in typical arrays out to the periphery of the cells.

Incorporation of fluorescently labeled contractile proteins into freshly isolated living adult cardiac myocytes

When fluorescently labeled contractile proteins are injected into embryonic muscle cells, they become incorporated into the cells' myofibrils. In order to determine if this exchange of proteins is unique to the embryonic stage of development, we isolated adult cardiac myocytes and microinjected them with fluorescently labeled actin, myosin light chains, alpha-actinin, and vinculin. Each of these proteins was incorporated into the adult cardiomyocytes and was colocalized with the cells' native proteins, despite the fact that the labeled proteins were prepared from noncardiac tissues. Within 10 min of injection, alpha-actinin was incorporated into Z-bands surrounding the site of injection. Similarly, 30 sec after injection, actin was incorporated into the entire I-bands at the site of injection. Following a 3-h incubation, increased actin fluorescence was noted at the intercalated disc. Vinculin exchange was seen in the intercalated discs, as well as in the Z-bands throughout the cells. Myosin light chains required 4-6 h after injection to become incorporated into the A-bands of the adult muscle. Nonspecific proteins, such as fluorescent BSA, showed no association with the myofibrils or the former intercalated discs. When adult cells were maintained in culture for 10 days, they retain the ability to incorporate these contractile proteins into their myofibrils. T-tubules and the sarcoplasmic reticulum could be detected in periodic arrays in the freshly isolated cells using the membrane dye WW781 and DiOC6[3], respectively. In conclusion, the myofibrils in adult, as in embryonic, muscle cells are dynamic structures, permitting isoform transitions without dismantling of the myofibrils.

Differential effects of myosin-antibody complexes on contractile rings and circumferential belts in epitheloid cells

The role of myosin filaments during assembly and activity of microfilament rings was analyzed by microinjecting epitheloid cells (PtK2 and LLC-PK1 kidney cell lines) with specific anti-myosins. Six monoclonal antibodies directed against the light meromyosin (LMM) region of the myosin molecule were characterized with respect to epitope location, and their effects on actin-activated MgATPase as well as on assembly, structural integrity and stability of myosin filaments. All of these antibodies recognized LLC-PK1 myosin, but only three reacted with PtK2 myosin. The remaining three served as matching controls in experiments with this cell line. When injected in amounts sufficient to yield an excess of antibody over myosin, the reactive antibodies significantly delayed formation and constriction of the contractile ring in mitotic cells. These rings contained less myosin, but not less actin, than the controls. This indicates that the recruitment and alignment of actin in the cleavage furrow can occur independently of other components of the contractile ring. After completion of cytokinesis, the majority of the injected cells was unable to assemble a normal circumferential belt. This resulted in defective epitheloid sheets. Approximately one third of these cells showed grossly distorted cell shapes and an increase in locomotory activity. All these changes were fully reversible with time, suggesting that the effects of the antibodies were overcome by protein synthesis. The differential sensitivity seen between contractile rings and peripheral belts is discussed with respect to differences in their architecture, stability and proposed function.

Disruption of microfilament organization in living nonmuscle cells by microinjection of plasma vitamin D-binding protein or DNase I

Plasma vitamin D-binding protein (DBP), which binds to monomeric actin, causes the breakdown of stress fibers when it is microinjected into nonmuscle cells. Disruption of the stress fiber network is also accompanied by shape changes in the cell that resemble those seen after cytochalasin treatment. When DBP was coinjected with fluorescently labeled alpha-actinin, no fluorescent stress fibers or attachment plaques were visible 30 min after injection. Twelve hours later the cells regained their flattened shape and their stress fibers. Fluorescently labeled DBP causes the same reversible changes in cell shape as the unlabeled protein. Upon injection, the labeled DBP diffuses throughout the cytoplasm, becoming localized by 12 hr in a punctate pattern, presumably due to lysozomal sequestration. Similar injections of DBP into skeletal myotubes and cardiac myocytes did not lead to shape changes or breakdown of nascent and/or fully formed myofibrils, even though DBP has a 2-fold higher binding affinity for muscle actin over that of the nonmuscle isoactins. Similar differential effects in nonmuscle cells were also observed after the microinjection of DNase I, another protein capable of binding monomer actin. The effects of these microinjected monomer actin-binding proteins imply that an accessible pool of monomer actin is needed to maintain stress fiber integrity in nonmuscle cells but not the integrity of the nascent or fully formed myofibrils in muscle cells.

Angiopathic consequences of saturating the plasma scavenger system for actin

Two plasma proteins, vitamin D-binding protein (actin monomer sequestrant) and gelsolin (actin polymer severing), have been found in association with actin in plasma from ill humans and during experimental injury. In vitro, these are the only plasma proteins that display a high affinity for actin. We infused increasing amounts of globular actin intravenously to rats to evaluate its disposition in plasma and tissues. Intravascular filament formation, microthrombi, and endothelial injury were observed, especially in the pulmonary circulation. These pathological changes were not observed when the globular actin in the infusate had been preincubated with the vitamin D-binding protein in vitro. Complexes of actin with both proteins were found in the plasma, suggesting a saturable, plasma actin-binding system in vivo. Our findings suggest that in vivo saturation of these proteins' actin-binding capacities may serve as a paradigm for pulmonary vascular disorders seen during widespread tissue trauma and cell lysis.

Aberrant postendocytotic fate of a 34-kDa molecular mass growth factor from human trophoblasts

A 34-kDa growth factor expressed by trophoblasts and certain carcinomas binds to target fibroblastic cells through specific high-affinity receptors. Here we report studies on the cellular routing behavior of the receptor-bound 34-kDa protein. Internalization was visualized by using lissamine rhodamine-conjugated 34-kDa protein and was quantified by analyzing the acid dissociability of cell-bound radioiodinated protein after incubation at 37 degrees C. The protein was found to be rapidly internalized in a temperature-sensitive manner. However, in contrast with other protein ligands, the 34-kDa protein was not rapidly degraded. The extent of ligand degradation was small as quantified by gel filtration analysis. Studies on the receptor showed that there was an atypical up-regulation, i.e., increase in surface receptors in response to ligand binding at 37 degrees C. The up-regulation was partially blocked by cycloheximide, an inhibitor of protein biosynthesis, but not by known inhibitors of receptor recycling such as monensin, chloroquine, and methylamine, suggesting that enhanced receptor biosynthesis may be responsible for the process. These studies indicate that the cellular routing and receptor regulatory characteristics of the internalized 34-kDa growth factor are different from those of most growth factor ligands and imply the involvement of receptor up-regulation in signal transduction.

Analysis of cell division using fluorescently labeled actin and myosin in living PtK2 cells

Actin and the light chains of myosin were labeled with fluorescent dyes and injected into interphase PtK2 cells in order to study the changes in distribution of actin and myosin that occurred when the injected cells subsequently entered mitosis and divided. The first changes occurred when stress fibers in prophase cells began to disassemble. During this process, which began in the center of the cell, individual fibers shortened, and in a few fibers, adjacent bands of fluorescent myosin could be seen to move closer together. In most cells, stress fiber disassembly was complete by metaphase, resulting in a diffuse distribution of the fluorescent proteins throughout the cytoplasm with the greatest concentration present in the mitotic spindle. The first evidence of actin and myosin concentration in a cleavage ring occurred at late anaphase, just before furrowing could be detected. Initially, the intensity of fluorescence and the width of the fluorescent ring increased as the ring constricted. In cells with asymmetrically positioned mitotic spindles, both protein concentration and furrowing were first evident in the cortical regions closest to the equator of the mitotic spindle. As cytokinesis progressed in such asymmetrically dividing cells, fluorescent actin and myosin appeared at the opposite side of the cell just before furrowing activity could be seen there. At the end of cytokinesis, myosin and actin were concentrated beneath the membrane of the midbody and subsequently became organized in two rings at either end of the midbody.

Talin dynamics in living microinjected nonmuscle cells

To investigate the role of talin in the anchoring of actin-containing stress fibers to the cell membrane of nonmuscle cells, a fluorescent analog of the adhesion plaque protein talin was developed, characterized, and microinjected into living cells. Purified chicken gizzard talin was covalently labeled with the fluorescent dye lissamine rhodamine B sulfonyl chloride. The fluorescently labeled protein was then chromatographed on Sephadex G-25 and DEAE-cellulose in order to remove free dye and denatured protein. The fluorescent talin was able to bind purified vinculin and was localized in adhesion plaques, membrane ruffles, microspikes, and polygonal networks in acetone-permeabilized nonmuscle cells. In cells that were double-stained with fluorescent talin and an affinity-purified anti-talin antibody, a one-to-one correspondence of adhesion plaque staining was seen. Living epithelial cells (PtK2) were microinjected during interphase with fluorescent talin. Computer-enhanced video microscopy was used to document adhesion plaque dynamics such as 1) changes in plaque shape, 2) alterations in plaque positions, and 3) the appearance, growth, and dissolution of plaques. In cells that were followed during mitosis, the adhesion plaques disappeared during cell rounding and then subsequently reappeared upon spreading of the two daughter cells. Treatment of microinjected cells with DMSO in order to disassemble stress fibers resulted in an altered localization of the fluorescent talin. Upon recovery of the cell from the drug, the talin was visualized in its characteristic submembraneous position. These results are the first to document the role and distribution of talin in dynamic processes occurring in living microinjected nonmuscle cells.

Dynamics of the endoplasmic reticulum in living non-muscle and muscle cells

The dynamic changes of the endoplasmic reticulum (ER) in interphase and mitotic cells was detected by the vital fluorescent dye 3,3'-dihexyloxacarbocyanine iodide. Two types of arrays characterize the continuous ER system in the non-muscle PtK2 cell: 1) a lacy network of irregular polygons and 2) long strands of ER that are found aligned along stress fibers. In cross-striated myotubes there was a periodic localization of fluorescence over each I-band corresponding to the positions of the terminal cisternae of the sarcoplasmic reticulum (SR). In contrast to the arrangement in muscle cells, the alignment of the long strands of ER alon stress fibers showed no strict periodicity that could be correlated with the sarcomeric units of the stress fibers. The ER and SR arrays seen in living cells were also detected in fixed cells stained with antibodies directed against proteins of the endoplasmic reticulum and sarcoplasmic reticulum, respectively. Observations of vitally stained PtK2 cells at 1 to 2 minute intervals using low light level video cameras and image processing techniques enabled us to see the polygonal ER units form and undergo changes in their shapes. During cell division, the ER, rhodamine 123-stained mitochondria, and phagocytosed fluorescent beads were excluded from the mitotic spindle while soluble proteins were not. No obvious concentration or alignment of membranes could be found associated with the contractile proteins in the cleavage furrow. After completion of cell division there was a redeployment of the ER network in each daughter cell.

Visualization of intermediate filaments in living cells using fluorescently labeled desmin

Fluorescently labeled desmin was incorporated into intermediate filaments when microinjected into living tissue culture cells. The desmin, purified from chicken gizzard smooth muscle and labeled with the fluorescent dye iodoacetamido rhodamine, was capable of forming a network of 10-nm filaments in solution. The labeled protein associated specifically with the native vimentin filaments in permeabilized, unfixed interphase and mitotic PtK2 cells. The labeled desmin was microinjected into living, cultured embryonic skeletal myotubes, where it became incorporated in straight fibers aligned along the long axis of the myotubes. Upon exposure to nocodazole, microinjected myotubes exhibited wavy, fluorescent filament bundles around the muscle nuclei. In PtK2 cells, an epithelial cell line, injected desmin formed a filamentous network, which colocalized with the native vimentin intermediate filaments but not with the cytokeratin networks and microtubular arrays. Exposure of the injected cells to nocadazole or acrylamide caused the desmin network to collapse and form a perinuclear cap that was indistinguishable from vimentin caps in the same cells. During mitosis, labeled desmin filaments were excluded from the spindle area, forming a cage around it. The filaments were partitioned into two groups either during anaphase or at the completion of cytokinesis. In the former case, the perispindle desmin filaments appeared to be stretched into two parts by the elongating spindle. In the latter case, a continuous bundle of filaments extended along the length of the spindle and appeared to be pinched in two by the contracting cleavage furrow. In these cells, desmin filaments were present in the midbody where they gradually were removed as the desmin filament network became redistributed throughout the cytoplasm of the spreading daughter cells.

Somitogenesis in the mouse embryo

This report describes the initiation of somitogenesis in the mouse embryo. Correlations are made with fibronectin distribution around the unsegmented mesoderm and the distribution of cytoskeletal elements within the cells as they undergo morphogenetic movements. The same temporal and topological changes in fibronectin, laminin, and cytoskeletal elements are seen in mouse somitogenesis as in the chick embryo. A notable exception is that the epithelial stage of somitogenesis in the mouse does not form a closed vesicle as it does in the chick. In the mouse the mesial portion of the forming somite does not become epithelial before the migration of sclerotomal cells.