Myosin subfragment binding for the localization of actin-like microfilaments in cultured cells. A light and electron microscope study

Spindle-F-actin interactions in mitotic spindles in an intact vertebrate epithelium

Mitotic spindles are well known to be assembled from and dependent on microtubules. In contrast, whether actin filaments (F-actin) are required for or are even present in mitotic spindles has long been controversial. Here we have developed improved methods for simultaneously preserving F-actin and microtubules in fixed samples and exploited them to demonstrate that F-actin is indeed associated with mitotic spindles in intact Xenopus laevis embryonic epithelia. We also find that there is an “F-actin cycle,” in which the distribution and organization of spindle F-actin changes over the course of the cell cycle. Live imaging using a probe for F-actin reveals that at least two pools of F-actin are associated with mitotic spindles: a relatively stable internal network of cables that moves in concert with and appears to be linked to spindles, and F-actin “fingers” that rapidly extend from the cell cortex toward the spindle and make transient contact with the spindle poles. We conclude that there is a robust endoplasmic F-actin network in normal vertebrate epithelial cells and that this network is also a component of mitotic spindles. More broadly, we conclude that there is far more internal F-actin in epithelial cells than is commonly believed.

And the dead shall rise: actin and myosin return to the spindle

The spindle directs chromosome partitioning in eukaryotes and, for the last three decades, has been considered primarily a structure based on microtubules, microtubule motors, and other microtubule binding proteins. However, a surprisingly large body of both old and new studies suggests roles for actin filaments (F-actin) and myosins (F-actin-based motor proteins) in spindle assembly and function. Here we review these data and conclude that in several cases the evidence for the participation of F-actin and myosins in spindle function is very strong, and in the situations where it is less strong, there is nevertheless enough evidence to warrant further investigation.

Mitosis: new roles for myosin-X and actin at the spindle

Roles for actin and myosin in positioning mitotic spindles in the cell are well established. A recent study of myosin-X function in early Xenopus embryo mitosis now reports that this unconventional myosin is required for pole integrity and normal spindle length by localizing to poles and exerting pulling forces on actin filaments within the spindle.

Myosin-10 and actin filaments are essential for mitotic spindle function

Mitotic spindles are microtubule-based structures responsible for chromosome partitioning during cell division. Although the roles of microtubules and microtubule-based motors in mitotic spindles are well established, whether or not actin filaments (F-actin) and F-actin-based motors (myosins) are required components of mitotic spindles has long been controversial. Based on the demonstration that myosin-10 (Myo10) is important for assembly of meiotic spindles, we assessed the role of this unconventional myosin, as well as F-actin, in mitotic spindles. We find that Myo10 localizes to mitotic spindle poles and is essential for proper spindle anchoring, normal spindle length, spindle pole integrity, and progression through metaphase. Furthermore, we show that F-actin localizes to mitotic spindles in dynamic cables that surround the spindle and extend between the spindle and the cortex. Remarkably, although proper anchoring depends on both F-actin and Myo10, the requirement for Myo10 in spindle pole integrity is F-actin independent, whereas F-actin and Myo10 actually play antagonistic roles in maintenance of spindle length.

Microtissue elasticity: measurements by atomic force microscopy and its influence on cell differentiation

It is increasingly appreciated that the mechanical properties of the microenvironment around cells exerts a significant influence on cell behavior, but careful consideration of what is the physiologically relevant elasticity for specific cell types is required to produce results that meaningfully recapitulate in vivo development. Here we outline methodologies for excising and characterizing the effective microelasticity of tissues; but first we describe and validate an atomic force microscopy (AFM) method as applied to two comparatively simple hydrogel systems. With tissues and gels sufficiently understood, the latter can be appropriately tuned to mimic the desired tissue microenvironment for a given cell type. The approach is briefly illustrated with lineage commitment of stem cells due to matrix elasticity.

Structure of kinetochore fibres in crane-fly spermatocytes after irradiation with an ultraviolet microbeam: Neither microtubules nor actin filaments remain in the irradiated region

We studied chromosome movement after kinetochore microtubules were severed. Severing a kinetochore fibre in living crane-fly spermatocytes with an ultraviolet microbeam creates a kinetochore stub, a birefringent remnant of the spindle fibre connected to the kinetochore and extending only to the edge of the irradiated region. After the irradiation, anaphase chromosomes either move poleward led by their stubs or temporarily stop moving. We examined actin and/or microtubules in irradiated cells by means of confocal fluorescence microscopy or serial-section reconstructions from electron microscopy. For each cell thus examined, chromosome movement had been recorded continuously until the moment of fixation. Kinetochore microtubules were completely severed by the ultraviolet microbeam in cells in which chromosomes continued to move poleward after the irradiation: none were seen in the irradiated regions. Similarly, actin filaments normally present in kinetochore fibres were severed by the ultraviolet microbeam irradiations: the irradiated regions contained no actin filaments and only local spots of non-filamentous actin. There was no difference in irradiated regions when the associated chromosomes continued to move versus when they stopped moving. Thus, one cannot explain motion with severed kinetochore microtubules in terms of either microtubules or actin-filaments bridging the irradiated region. The data seem to negate current models for anaphase chromosome movement and support a model in which poleward chromosome movement results from forces generated within the spindle matrix that propel kinetochore fibres or kinetochore stubs poleward.

Effects of anti-myosin drugs on anaphase chromosome movement and cytokinesis in crane-fly primary spermatocytes

To investigate whether myosin is involved in crane-fly primary spermatocyte division, we studied the effects of myosin inhibitors on chromosome movement and on cytokinesis. With respect to chromosome movement, the myosin ATPase inhibitor 2,3-butanedione 2-monoxime (BDM) added during autosomal anaphase reversibly perturbed the movements of all autosomes: autosomes stopped, slowed, or moved backwards during treatment. BDM added before anaphase onset altered chromosome movement less than when BDM was added during anaphase: chromosome movements only rarely were stopped. They often were normal initially and then, if altered at all, were slowed. To confirm that the effects of BDM were due to myosin inhibition, we treated cells with ML-7, a drug that inhibits myosin light chain kinase (MLCK), an enzyme necessary to activate myosin. ML-7 affected anaphase movement only when added in early prometaphase: this treatment prevented chromosome attachment to the spindle. We treated cells with H-7 as a control for possible non-myosin effects of ML-7. H-7, which has a lower affinity than ML-7 for MLCK but a higher affinity than ML-7 for other potential targets, had no effect. These data confirm that the BDM effect is on myosin and indicate that the myosin used for chromosome movement is activated near the start of prometaphase. With respect to cytokinesis, BDM did not block furrow initiation but did block subsequent contraction of the contractile ring. When BDM was added after initiation of the furrow, the contractile ring either stalled or relaxed. ML-7 blocked contractile ring contraction when added at all stages after autosomal anaphase onset, including when added during cytokinesis. H-7 had no effect. These results confirm that the effects of BDM are on myosin and indicate that the myosin used for cytokinesis is activated starting from autosomal anaphase and continuing throughout cytokinesis.

Phallacidin stains the kinetochore region in the mitotic spindle of the green algae Oedogonium spp

We found previously that in living cells of Oedogonium cardiacum and O. donnellii, mitosis is blocked by the drug cytochalasin D (CD). We now report on the staining observed in these spindles with fluorescently actin-labeling reagents, particularly Bodipy FL phallacidin. Normal mitotic cells exhibited spots of staining associated with chromosomes; frequently the spots appeared in pairs during prometaphase-metaphase. During later anaphase and telophase, the staining was confined to the region between chromosomes and poles. The texture of the staining appeared to be somewhat dispersed by CD treatment but it was still present, particularly after shorter (< 2 h) exposure. Electron microscopy of CD-treated cells revealed numerous spindle microtubules (MTs); many kinetochores had MTs associated with them, often laterally and some even terminating in the kinetochore as normal, but the usual bundle of kinetochore MTs was never present. As treatment with CD became prolonged, the kinetochores became shrunken and sunk into the chromosomes. These results support the possibility that actin is present in the kinetochore of Oedogonium spp. The previous observations on living cells suggest that it is a functional component of the kinetochore-MT complex involved in the correct attachment of chromosomes to the spindle.

Actin, alpha-actinin, and vinculin are associated with septate junctions in Insecta

Cytoskeletal elements associated with the smooth septate junctions linking the midgut columnar cells of Manduca sexta larvae (Insecta, Lepidoptera) were characterized. Myosin subfragment 1 decoration and immunostaining for actin demonstrated that the filaments associated with the septate junctions were constituted of actin. Moreover, using a combination of immunochemical and immunolocalization techniques, evidence is presented that alpha-actinin, myosin II, and vinculin are localized close to the specialized plasma membranes. The insertion of microfilament bundles into submembranous F-actin/alpha-actinin/vinculin complexes, previously described in vertebrate junctions of adherens type, appears to be a more general organization, including the insect septate junction here examined.

Actin-plasma membrane associations in mouse eggs and oocytes

Using rhodamine-phalloidin stained preparations and extracted specimens labeled with heavy meromyosin or run on polyacrylamide gels, actin-plasma membrane associations in mouse mature eggs at the second metaphase of meiosis and oocytes at meiotic prophase have been examined. Cortices of extracted oocytes possessed numerous actin filaments that emanated from the plasma membrane delimiting regions between microvilli and from microvillar apices. The membrane anchorage sites of actin filaments were marked by an electron dense material on the inner leaflet of the plasma membrane. The free ends of filaments emanating from the plasma membrane of oocytes intermeshed to form a dense, cortical layer. With meiotic maturation, changes in the organization of cortical actin were first noted approximately 3 hr after the chromosomes had become localized at the oocyte's periphery. Fewer and shorter actin filaments, which did not form a well-defined layer as in oocytes, were connected with electron-dense material to the inner leaflet of the plasma membrane of extracted egg cortices in regions other than that associated with the meiotic spindle. Cortical actin adjacent to the meiotic spindle, however, was organized into a dense, cresentic aggregation in which clusters of filaments emanated from electron-dense regions associated with both the inner and outer leaflets of the plasma membrane. These observations indicate that mouse oocyte maturation not only involves changes in the distribution of cortical actin but also local alterations in the association of actin with the plasma membrane.

The relationship between intermediate filaments and microfilaments before and during the formation of desmosomes and adherens-type junctions in mouse epidermal keratinocytes

Actin, keratin, vinculin and desmoplakin organization were studied in primary mouse keratinocytes before and during Ca2+-induced cell contact formation. Double-label fluorescence shows that in cells cultured in low Ca2+ medium, keratin-containing intermediate filament bundles (IFB) and desmoplakin-containing spots are both concentrated towards the cell center in a region bounded by a series of concentric microfilament bundles (MFB). Within 5-30 min after raising Ca2+ levels, a discontinuous actin/vinculin-rich, submembranous zone of fluorescence appears at cell-cell interfaces. This zone is usually associated with short, perpendicular MFB, which become wider and longer with time. Later, IFB and the desmoplakin spots are seen aligned along the perpendicular MFB as they become redistributed to cell-cell interfaces where desmosomes form. Ultrastructural analysis confirms that before the Ca2+ switch, IFB and desmosomal components are found predominantly within the perimeter defined by the outermost of the concentric MFB. Individual IF often splay out, becoming interwoven into these MFB in the region of cell-substrate contact. In the first 30 min after the Ca2+ switch, areas of submembranous dense material (identified as adherens junctions), which are associated with the perpendicular MFB, can be seen at newly formed cell-cell contact sites. By 1-2 h, IFB-desmosomal component complexes are aligned with the perpendicular MFB as the complexes become redistributed to cell-cell interfaces. Cytochalasin D treatment causes the redistribution of actin into numerous patches; keratin-containing IFB undergo a concomitant redistribution, forming foci that coincide with the actin-containing aggregates. These results are consistent with an IF-MF association before and during desmosome formation in the primary mouse epidermal keratinocyte culture system, and with the temporal and spatial coordination of desmosome and adherens junction formation.

Characterization of filaments within Leydig cells of the rat testis

Rat Leydig cells were permeabilized and the cytoplasm partially extracted to visualize, describe, and characterize filamentous elements of the cytoskeleton. It was demonstrated by immunofluorescence microscopy that vimentin is abundant within Leydig cells. Ultrastructurally, intermediate filaments in Leydig cells were concentrated at perinuclear sites and comprised bundles that coursed through the cytoplasm. Actin was identified in Leydig cells with the F actin probe, NBD-phallacidin. Fluorescence was strongest at the cortex of the cell. With myosin S-1 subfragments, sparse actin was found positioned almost exclusively in cortical regions of the cell associated with coated pits and in Leydig cell processes.

Relationship between intermediate filaments and microfilaments in cultured fibroblasts: evidence for common foci during cell spreading

Spreading and fully spread chick embryo fibroblasts (CEF) were examined by double-label fluorescence microscopy using the actin-specific probe rhodamine-phalloidin and an antibody directed against CEF intermediate filaments (IF). During midspreading, a striking relationship became discernible: statistical analysis showed that approximately half of the cell population exhibited one or more phase-dense, phalloidin-binding nodules that appeared to act as foci from which IF diverged. Coincidence between actin-containing structures and IF was not limited to these centers; IF could also frequently be seen running in close parallel arrays with stress fibers. Ultrastructural analysis confirmed the presence of non-membrane-bound out-pocketings along the length of stress fibers from which 10-nm IF diverged. These structures varied in size and shape, and displayed a dense, fine fibrillar appearance. IF and microfilaments (MF) were distinguished by size and by decoration of MF with myosin subfragment-1. Other IF-MF interactions were seen in cells of all stages: IF were observed to loop through stress fibers, most frequently at the cell margins. In colchicine-treated cells, IF became redistributed into cables that often ran parallel and appeared to merge with stress fibers. Cytochalasin D-treated CEF exhibited loose aggregates of actin-containing material that appeared to be associated with IF. These results suggest the possibility of an interaction between actin-containing structures and IF, particularly during cell spreading in cultured fibroblasts.

Arrangement of actin filaments and myosin-like filaments in the contractile ring and of actin-like filaments in the mitotic spindle of dividing HeLa cells

We used a glutaraldehyde-tannic acid-saponin fixative to improve the preservation of actin filaments in dividing HeLa cells during preparation for thin sectioning. The contractile ring in the cleavage furrow is composed of a parallel array of actin filaments that circle the equator. We show that many of these actin filaments are arranged in small bundles. These bundles consist of about 25 filaments throughout cytokinesis. For comparison, filopodia on these cells have about 23 actin filaments packed at a higher density than the filaments in the contractile ring bundles. Some of the contractile ring actin filaments appear to radiate out from electron-dense sites on the plasma membrane. The contractile ring also has a large number of short filaments 13 nm in diameter that closely resemble filaments formed from purified human cytoplasmic myosin. These thick filaments are aligned circumferentially and interdigitate with the actin filaments, as expected for a sliding filament mechanism of tension generation. There are no long actin filaments in the mitotic spindle, but there are a large number (400 to 1000 per micron 3) of very short filaments identical in appearance to actin filaments in other parts of these cells. These short filaments may account for the reported staining of the mitotic spindle with fluorescent antibodies to actin and with fluorescent myosin fragments.

Morphological study of the mammalian stress response: characterization of changes in cytoplasmic organelles, cytoskeleton, and nucleoli, and appearance of intranuclear actin filaments in rat fibroblasts after heat-shock treatment

Using both electron microscopy and immunological methods, we have characterized a number of changes occurring in rat fibroblasts after heat-shock treatment. Incubation of the cells for 3 h at 42 degrees-43 degrees C resulted in a number of changes within the cytoplasm including: a disruption and fragmentation of the Golgi complex; a modest swelling of the mitochondria and subtle alterations in the packing of the cristae; and alterations in cytoskeletal elements, specifically a collapse and aggregation of the vimentin-containing intermediate filaments around the nucleus. A number of striking changes were also found within the nuclei of the heat-treated cells: (a) We observed the appearance of rod-shaped bodies consisting of densely packed filaments. Using biochemical and immunological methods, these nuclear inclusion bodies were shown to be comprised of actin filaments. (b) Considerable alterations in the integrity of the nucleoli were observed after the heat-shock treatment. Specifically, there appeared to be a general relaxation in the condensation state of the nucleoli, changes in both the number and size of the granular ribonucleoprotein components, and finally a reorganization of the nucleolar fibrillar reticulum. These morphological changes in the integrity of the nucleoli are of significant interest since previous work as well as studies presented here show that two of the mammalian stress proteins, the major stress-induced 72-kD protein and the 110-kD protein, localize within the nucleoli of the cells after heat-shock treatment. We discuss these morphological changes with regards to the known biological and biochemical events that occur in cells after induction of the stress response.

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

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

In situ localization of S1-labeled actin filaments in chick lens epithelial cells

The actin filaments in the lens epithelial cells of three-day and eight-day post-hatched chicks have been labeled in situ with myosin subfragment 1 (S1). Labeling was accomplished by injuring the lens transcorneally with an ultramicroneedle 5 min or 24 hr before detergent treatment and incubation in S1. A band of filaments found at the epithelio-fiber junction in normal, uninjured chick lens is labeled in the 5 min and 24 hr injury. A subcapsular labeled band is found only in the 24 hr injury, and may be the result of a healing process.

Effect of concanavalin A on lymph node macrophages: stimulation of endocytic cisternae

Incubation of isolated lymph node macrophages with concanavalin A (Con A) resulted in a dense and continuous labeling of the plasmalemma and filopodia which were closely adherent to each other and the cell surface. Within a short time period (3-5 min), membranes of the closely apposed filopodia became invaginated into the cytoplasm to form numerous interconnecting cisternae. After 10 min the system of internalized membranes had migrated into the deeper cytoplasm and was closely associated with numerous actin filaments and other components of the cytoskeleton. The internalized plasmalemma remained in the cytoplasm up to 24 hr without fusing with lysosomes. Concomitant with plasmalemmal invagination and the formation of cisternae there were also changes in the Golgi apparatus. These appeared in the form of hypertrophied Golgi saccules and the accumulation of numerous vesicles around the Golgi. Treatment of isolated lymph node macrophages with either succinylated Con A, alpha-methyl-D-mannoside, or ferritin particles alone failed to produce the cisternal structures. The results suggested that the tetravalency of Con A may be responsible for the binding of adjacent Con A-labeled membranes to each other and for maintaining a crosslinking of membranes during invagination and internalization. It is suggested that this process of extensive membrane internalization represents a specialized form of endocytosis. At 24 hr after incubation with Con A, cisternal structures in close proximity to the Golgi vesicles showed signs of degradation. By 48 hr there was a breakdown of cisternal membranes with a release of Con A marker particles into large phagocytic vesicles, which also showed reaction product for acid phosphatase, suggesting a fusion with lysosomes.

A simple technique for the visualization of whole mount cytoskeletons with transmission electron microscopy

Examination of whole mount cells in the transmission electron microscope has been useful in studies of cellular architecture. The common technique is to grow cells directly on formvar-coated, gold grids for direct observation through a cell. We report a technique for obtaining whole mount preparations which requires neither fragile formvar films nor expensive, gold grids. Cells are grown on palladium-coated coverslips and processed for electron microscopy. The cells and the palladium substrate are separated from the coverslip. The cell-palladium complex is then picked up on copper grids as in thin section processing. We compare images of the cytoskeleton using our technique with images using previously described techniques and present preliminary observations of contracting cell models. Such contractions would tear formvar films if attempted on cells grown in the conventional manner for whole mount examination. Our technique allows cells to contract without tearing the underlying substrate.

Elongation of the fertilization tubule in Chlamydomonas: new observations on the core microfilaments and the effect of transient intracellular signals on their structural integrity

Experimental manipulations of gametes of Chlamydomonas reinhardi and ultrastructural observation were used to examine the composition of the microfilaments in the fertilization tubule, their probable mode of formation, and their interaction with intracellular signals. Decoration with myosin subfragment-1 was used to demonstrate that the microfilaments in the fertilization tubule were actin filaments having uniform polarity: Myosin subfragment-1 arrowheads pointed away from the membrane at the tip of the process. Filaments were attached to the cone-shaped "doublet zone" at the base of the process by their pointed ends. Discrete attachment sites for filaments on the surface of the doublet zone were seen in stereo view. To test whether actin polymerization might accompany elongation of the fertilization tubule, mating gametes were exposed to cytochalasin D in an attempt to block actin polymerization. Treatment of mating type "plus" gametes with cytochalasin D prior to and during mating inhibited the appearance of actin filaments in fertilization tubules, suppressed fertilization tubule outgrowth, and lowered mating efficiency from 90 to 15%. The role of signals generated by flagellar adhesion in maintaining the structural integrity of the microfilament-doublet zone complex was examined by correlating flagellar disadhesion with the kinetics of breakdown of the complex. In zygotes, where flagellar disadhesion occurred after cell fusion, the complex disassembled within 3 h after mating. In gametes that had been agglutinated by isolated mating type "minus" flagella, microfilaments and fertilization tubules progressively disassembled over a 3-h time course following flagellar disadhesion. Disassembly of microfilaments was inhibited by maintaining flagellar agglutination, suggesting that signals generated by flagellar adhesion were necessary to maintain microfilaments intact.

The localization of actin in dividing corneal endothelial cells demonstrated with nitrobenzoxadiazole phallacidin

The distribution of actin in dividing endothelial cells of the rat cornea was studied by fluorescence microscopy by means of the nitrobenzoxadiazole conjugated derivative of the actin-binding toxin phallacidin (NBD-Ph). In normal noninjured tissue, fluorescence is limited to an area at or near the plasma membrane. Twenty-four hours after a corneal freeze injury, stress fibers are detected but only in those cells that are migrating into the wound area. By 48 h post-injury, cells in various stages of mitosis can be identified. During metaphase, anaphase, and telophase, diffuse cytoplasmic staining is observed, although the spindle region remains free of fluorescence. At various sites along the plasma membrane, fluorescence appears stronger compared to other regions. During the latter two stages of proliferation, NBD-Ph positive material can be seen within cell processes. In addition, a band of this material is observed within the region that corresponds to the cleavage furrow. As the daughter cells separate, actin can be detected within the cytoplasmic bridge. The results indicate that NBD-Ph can be used to study the distribution of actin in cells that were proliferating in vivo, and these patterns appear similar to those obtained with immunological methods on cultured cells.

Caenorhabditis elegans spermatozoan locomotion: amoeboid movement with almost no actin

The pseudopods of Caenorhabditis elegans spermatozoa move actively causing some cells to translocate when the sperm are dissected into a low osmotic strength buffered salts solution. On time-lapse video tapes, pseudopodial projections can be seen moving at 20-45 micrometers/min from the tip to the base of the pseudopod. This movement occurs whether or not the cell is attached to a substrate. Translocation of the cell is dependent on the substrate. Some spermatozoa translocate on acid-washed glass, but a better substrate is prepared by drying an extract of Ascaris uteri (the normal site of nematode sperm motility) onto glass slides. On this substrate more than half the spermatozoa translocate at a velocity (21 micrometers/min) similar to that observed in vivo. Translocating cells attach to the substrate by their pseudopodial projections. They always move toward the pseudopod; changes in direction are caused by changes in pseudopod shape that determine points of detachment and reattachment of the cell to the substrate. Actin comprises less than 0.02% of the proteins in sperm, and myosin is undetectable. No microfilaments are found in the sperm. Immunohistochemistry shows that some actin is localized in patches in the pseudopod. The movement of spermatozoa is unaffected by cytochalasins, however, so there is no evidence that actin participates in locomotion. Fertilization-defective mutants in genes fer-2, fer-4, and fer-6 produce spermatozoa with defective pseudopodial projections, and these spermatozoa are largely immotile. Mutants in the spermatozoa do not translocate. Thus pseudopod movement is correlated with the presence of normal projections. Twelve mutants with defective muscles have spermatozoa with normal movement, so these genes do not specify products needed for both muscle and nonmuscle cell motility.

Filamentous structures in adherent Mycoplasma pneumoniae cells treated with nonionic detergents

Mycoplasma pneumoniae cells adhering to glass or Parlodion-coated grids were extracted with Triton X-100. The extracted cells showed a cytoskeleton consisting of a rodlike tip structure and a filamentous network in the cytoplasm. The tip structure was up to 300 nm long and approximately 40 nm wide ending at the distal end in a bleb-like structure, and seemed to consist of filaments arranged in parallel, 4.8 +/- 0.5 nm wide. In the cytoplasm the filaments formed an irregular lattice. Similar filaments were found in platinum replicated critical-point dried extracted cells. An actinlike nature of the filaments is suggested by some of their properties, but the degree of homology with respect to eucaryotic actin is still unknown. The filaments were sensitive to protease treatment but stable in high molar KCl solutions. They were apparently destroyed by incubation in high molar KI solution, leaving only some parts of the tip structure. Formaldehyde-fixed M. pneumoniae cells treated with Triton X-100 bound rhodamine-labeled phalloidin specifically. Furthermore, they could be stained with antiactin antibodies. Binding of myosin subfragment 1 to the filaments was not observed.

Localization and organization of actin in melanophores

Melanophores of the angelfish, Pterophyllum scalare, were studied in an attempt to demonstrate the existence of actin in these cells although microfilaments had previously not been found. By use of a variety of procedures, including immunofluorescence microscopy of intact and detergent-extracted cells, transmission electron microscopy, high voltage electron microscopy of whole-mount preparations, and labeling with heavy meromyosin-subfragment 1, the presence of a loose cortical mesh of actin filaments is demonstrated. In addition, a more parallel array of filaments is detected in microspike- and microvillus-like surface projections. There seem to be no changes in the arrangement of these filaments as a function of the state of pigment distribution. No actin filaments could be found in association with pigment granules or microtubules in more central cell portions. For reasons presently unknown, the preservation of the cortical filament network in lysed cell preparations depends strongly on the presence of an intact microtubular system. The involvement of this subplasmalemmal actin filament network in pigment granule transport remains unclear.

Differential staining of actin in metaphase spindles with 7-nitrobenz-2-oxa-1,3-diazole-phallacidin and fluorescent DNase: is actin involved in chromosomal movement?

The distribution and polymerization state of actin in metaphase rat kangaroo cells was studied by fluorescence microscopy. Formaldehyde-fixed, acetone-extracted cells were labeled with either of two types of actin probes. The first, 7-nitrobenz-2-oxa-1,3-diazole-phallacidin, has high affinity for F actin and does not bind monomeric G actin. The second was a conjugate of DNase I labeled with either tetramethylrhodamine or fluorescein. DNase binds with high affinity to G actin and with lesser affinity to F actin. The polymerization state of actin was deduced by comparing the fluorescence distribution of the phallacidin derivative with that of the fluorescent DNase. The results indicate that the pole-to-chromosome region of the metaphase spindle contains G actin but little if any conventional F actin. F actin is found concentrated in a diffuse distribution outside the spindle region in metaphase cells and returns to the interzone area between the chromosomes by early telophase. These results exclude spindle models for chromosomal movement that require more than about five F actin filaments per chromosome, support the hypothesis that F actin is involved in force generation for cell cleavage, and are not inconsistent with the possibility that actin outside the spindle may be involved in chromosomal movement.

A permeabilized cell model for studying cell division: a comparison of anaphase chromosome movement and cleavage furrow constriction in lysed PtK1 cells

After lysis in a Brij 58-polyethylene glycol medium, PtK1 cells are permeable to small molecules, such as erythrosin B, and to proteins, such as rhodamine-labeled FAB, myosin subfragment-1, and tubulin. Holes are present in the plasma membrane, and the mitochondria are swollen and distorted, but other membrane-bounded organelles of the lysed cell model are not noticeably altered. After lysis, the mitotic apparatus is functional; chromosomes move poleward and the spindle elongates. Cells lysed while in cytokinesis will continue to divide for several minutes. Addition of crude tubulin extracts, MAP-free tubulin, or taxol to the lysis medium retards anaphase chromosome movements but does not affect cleavage. On the other hand, N-ethylmaleimide-modified myosin subfragment-1, phalloidin, and cytochalasin B inhibit cleavage but have no effect on anaphase chromosome movements under identical lysis conditions. These results suggest that actomyosin plays no functional role in anaphase chromosome movement in mammalian tissue culture cells and that microtubule depolymerization is a rate-limiting step for chromosome-to-pole movements.

The identification and localization of actin and actin-like filaments in lactating guinea pig mammary gland alveolar cells

Cytochalasin B, a microfilament-altering drug, inhibits lactose synthesis in lactating guinea pig mammary gland [Biochim. Biophys. Acta 392:20, 1975] but not primary by inhibiting glucose transport [Eur. J. Cell Biol. 20:150, 1979]. In order to study the possible role of microfilaments in lactose synthesis and secretion, we isolated both the alveolar (milk-secreting) and myoepithelial (contractile) cells from lactating mammary gland. Light microscopy shows that the alveolar cell fraction (viability approximately 71%) is homogenous and that the cells retain strong polarity of secretory structures in the apical region. Two proteins were extracted from the alveolar cell fraction. One (mol wt 42,000) comigrates with skeletal muscle actin on SDS-PAGE gels. The other, a high-molecular-weight (180,000) protein (HMWP) may be analogous to actin-binding protein or clathrin. An extract from the myoepithelial cell fraction also contains a protein that comigrates with actin but no HMWP. Whole tissue extract contains the 42K protein, and a 185K HMWP. Examination of the alveolar cell extract by electron microscopic (EM) negative staining revealed meshworks of multistranded, interconnecting filaments, with attached globular structures (100-200 A) (possibly the HMWP) and single filaments (40-60 A diameter) branching off. To localize these filamentous structures in situ, whole tissue was glycerinated and incubated with rabbit skeletal muscle heavy meromyosin (HMM). Masses of filaments in myoepithelial cells served as convenient standards for HMM decoration. Decorated filaments have cross-arms or projections, unlike the narrow, smooth filaments of control tissue. Decorated filaments in alveolar cells are located beneath the plasma membrane, in close association with secretory vacuoles, and near the Golgi apparatus; filaments near the latter two are often oriented perpendicular to the plasma membrane. Microvesicles are embedded in meshworks under the plasmalemma and near the Golgi apparatus. Intermediate-sized (85-115 A diameter), non-decorated filaments diverge from the meshworks of decorated filaments. Micro-vesicles are associated with intermediate-sized filaments as well. The association of actin-like filaments with secretory vacuoles and microvesicles and their location in areas of the cell concerned with biosynthetic activities suggest a possible function in the intracellular transport of secretory products.

Microfilaments and tropomyosin of cultured mammalian cells: isolation and characterization

Microfilaments were isolated from cultured mammalian cells, utilizing procedures similar to those for isolation of "native" thin filaments from muscle. Isolated microfilaments from rat embryo, baby hamster kidney (BHK- 21), and Swiss mouse 3T3 cells appeared structurally similar to muscle thin filaments, exhibiting long, 6 nm Diam profiles with a beaded, helical substructure. An arrowhead pattern was observed after reaction of isolated microfilaments with rabbit skeletal muscle myosin subfragment 1. Under appropriate conditions, isolated microfilaments will aggregate into a form that resembles microfilament bundles seen in situ cultured cells. Isolated microfilaments represent a complex of proteins including actin. Some of these components have been tentatively identified, based on coelectrophoresis with purified proteins, as myosin, tropomyosin, and a high molecular weight actin-binding protein. The tropomyosin components of isolated microfilaments were unexpected; polypeptides comigrated on SDS-polyacrylamide gels with both muscle and nonmuscle types of tropomyosin. In order to identify more specifically these subunits, we isolated and partially characterized tropomyosin from three cell types. BHK-21 cell tropomyosin was similar to other nonmuscle tropomyosins, as judged by several criteria. However, tropomyosin isolated from rate embryo and 3T3 cells contained subunits that comigrated with both skeletal muscle and nonmuscle types of myosin, whereas the BHK cell protein consistently contained a minor muscle-like subunit. The array of tropomyosin subunits present in a cell culture was reflected in the polypeptide chain pattern seen on SDS-polyacrylamide gels of microfilaments isolated from that culture. These studies provide a starting point for correlating changes in the ultrastructural organization of microfilaments with alterations in their protein composition.

Evidence for the participation of actin microfilaments and bristle coats in the internalization of gap junction membrane

Thin sections of rabbit granulosa, human SW-13 adrenal cortical adenocarcinoma, and mouse B-16 melanoma cells revealed an apparent single-layered basket of 4- to 7-nm filaments surrounding cytoplasmic gap junction vesicles. This interpretation was based upon apparent longitudinal, cross, and en face sections of structures surrounding the vesicle profiles in tissue treated with tannic acid-glutaraldehyde. In granulosa cells incubated with the S-1 fragment of heavy meromyosin, arrowhead-decorated filaments were observed at the periphery of the gap junction vesicles, suggesting that these filaments contained actin. In addition, we found that small gap junctional blebs and vesicles at the cell surface were coated with short electron-dense bristles similar in appearance to the cloathrin-containing coat of coated vesicles of nonjunctional membrane. The role of these and other cytoskeletal elements in the possible endocytosis of gap junction membrane is discussed.

Isolation of a high molecular weight actin-binding protein from baby hamster kidney (BHK-21) cells

A high molecular weight protein (HMWP) with properties similar to those of both actin-binding protein (ABP) and filamin has been isolated from cultured baby hamster kidney (BHK-21) cells. The protein was present in an actomyosin-depleted sucrose extract of the cells and was eluted, upon gel chromatography on Sepharose 4B, near the void volume. The subunit migration on sodium dodecyl sulfate/polyacrylamide gels and the amino acid composition of HMWP were similar to those of ABP and filamin. HMWP bound to and crosslinked F-actin from rabbit muscle, as shown by the formation of a gel that was sedimented with low-speed centrifugation. This interaction was insensitive to temperature and low concentrations of calcium ions, although it may depend on the presence of myosin. Observation of thin sections of the actin-HMWP gel revealed crosslinked complexes of laterally aggregated actin filaments. The axial period of the dense crosslinks was 34 nm. The HMWP may be involved in regulation of microfilament organization.

Tubulin and calmodulin. Effects of microtubule and microfilament inhibitors on localization in the mitotic apparatus

Indirect immunofluorescence was used to determine the distribution of calmodulin in the mitotic apparatus of rat kangaroo PtK2 and Chinese hamster ovary (CHO) cells. The distribution of calmodulin in PtK2 cells was compared to the distribution of tubulin, also as revealed by indirect immunofluorescence. During mitosis, calmodulin was found to be a dynamic component of the mitotic apparatus. Calmodulin first appeared in association with the forming mitotic apparatus during midprophase. In metaphase and anaphase, calmodulin was found between the spindle poles and the chromosomes. While tubulin was found in the interzonal region throughout anaphase, calmodulin appeared in the interzone region only at late anaphase. The interzonal calmodulin of late anaphase condensed during telophase into two small regions, one on each side of the midbody. Calmodulin was not detected in the cleavage furrow. In view of the differences in the localization of calmodulin, tubulin, and actin in the mitotic apparatus, experiments were designed to determine the effects of various antimitotic drugs on calmodulin localization. Cytochalasin B, an inhibitor of actin microfilaments, had no apparent effect on calmodulin or tubulin localization in the mitotic apparatus of CHO cells. Microtubule inhibitors, such as colcemid and N2O, altered the appearance of tubulin- and calmodulin-specific fluorescence in mitotic CHO cells. Cold temperature (0 degrees C) altered tubulin-specific fluorescence of metaphase PtK2 cells but did not alter calmodulin-specific fluorescence. From these studies, it is concluded that calmodulin is more closely associated with the kinetichore-to-pole microtubules than other components of the mitotic apparatus.

Ultrastructure of microfilament bundles in baby hamster kidney (BHK-21) cells. The use of tannic acid

After standard glutaraldehyde-osmium tetroxide fixation procedures, the majority of microfilament bundles in BHK-21 cells exhibit relatively uniform electron density along their long axes. The inclusion of tannic acid in the glutaraldehyde fixation solution results in obvious electron density shifts along the majority of microfilament bundles. Striated patterens are frequently observed which consist of regularly spaced electron dense (D) and electron lucid (L) bands. A striated pattern is also observed along many BHK-21 stress fibers after processing for indirect immunofluorescence utilizing BHK-21 myosin antiserum. A direct correlation of these periodicities seen by light and electron microscope techniques is impossible at the present time. However, comparative measurements indicate that the overall patterns seen in the immunofluorescence and electron microscope preparations are similar. The ultrastructural results provide an initial clue for the ultimate determination of the supramolecular organization of contracile proteins other than actin within the microfilament bundles of non-muscle cells.

Functions of cytoplasmic fibers in intracellular movements in BHK-21 cells

After trypsinization and replating, BHK-21 cells spread and change shape from a rounded to a fibroblastic form. Time-lapse movies of spreading cells reveal that organelles are redistributed by saltatory movements from a juxtanuclear position into the expanding regions of cytoplasm. Bidirectional saltations are seen along the long axes of fully spread cells. As the spreading process progresses, the pattern of saltatory movements changes and the average speed of saltations increases from 1.7 MICROMETER/S during the early stages of spreading to 2.3 micrometer/s in fully spread cells. Correlative electron microscope studies indicate that the patterns of saltatory movements that lead to the redistribution of organelles during spreading are closely related to changes in the degree of assembly, organization, and distribution of microtubules and 10-nm filaments. Colchicine (10 microgram/ml of culture medium) reversibly disassembles the microtubule-10-nm filament complexes which form during cell spreading. This treatment results in the disappearance of microtubules and the appearance of a juxtanuclear accumulation of 10-nm filaments. These changes closely parallel an inhibition of saltatory movements. Within 30 min after the addition of the colchicine, pseudopod-like extensions form rapidly at the cell periphery, and adjacent organelles are seen to stream into them. The pseudopods contain extensive arrays of actinlike microfilament bundles which bind skeletal-muscle heavy meromyosin (HMM). Therefore, in the presence of colchicine, intracellular movements are altered from a normal saltatory pattern into a pattern reminiscent of the type of cytoplasmic streaming seen in amoeboid organisms. The streaming may reflect either the activity or the contractility of submembranous microfilament bundles. Streaming activity is not seen in cells containing well-organized microtubule-10-nm filament complexes.