Blood platelet behaviour in patients with a type I diabetes mellitus

Platelets appear to be involved in the development of vascular diseases in diabetic patients. In a number of studies, platelet adhesion and aggregation has been found to be enhanced in diabetic patients indicating a platelet hyperreactivity. However, contradictory results on hyperreactivity of diabetic platelets towards agonists published in the literature leading to the problem of reliability of agonist-induced aggregation as a parameter which is able to reflect an altered platelet reactivity. Therefore, we introduced the quantification of actin filaments of platelets as an early indicator of platelet hyperreactivity. In 20 patients with type I diabetes mellitus agonist-induced aggregation, spontaneous aggregation, malondialdehyde (MDA)-formation, plasma lipid status and the ability of plasma to degrade platelet activating factor (PAF) as well as the G- to F-actin equilibrium of platelets were assayed. Compared to an age- and sex-matched control group F-actin values, spontaneous aggregation and PAF-hydrolase were significantly increased in diabetic patients.

Immunohistologic abnormalities of the microfibrillar-fiber system in the Marfan syndrome

BACKGROUND

Indirect-immunofluorescence studies of skin and cultured dermal fibroblasts from patients with the Marfan syndrome demonstrate apparent deficiency of one element of connective tissue--the microfibrillar-fiber system--in assays using specific antibodies against fibrillin, a major microfibrillar protein. This study was designed to test whether these immunostaining abnormalities are consistent and diagnostic features of the disease.

METHODS

We studied patients with either the Marfan syndrome or various other inherited connective-tissue disorders and normal subjects according to a single-blind protocol in which coded samples of skin, fibroblast cultures, or both were analyzed without knowledge of the clinical diagnosis and classified as "Marfan" or "non-Marfan" before the sample codes were broken.

RESULTS

Of the 27 patients with the Marfan syndrome, 24 were correctly identified by the decreased content of microfibrillar fibers in their skin, cultured fibroblasts, or both; in contrast, 19 of 25 patients with other heritable disorders of connective tissue and all 13 normal subjects were correctly classified as "non-Marfan" by these assays (P less than 0.001).

CONCLUSIONS

These results document consistent, relatively specific abnormalities of microfibrillar fibers in the Marfan syndrome. The biomechanical incompetence of these structural elements, due to quantitative or qualitative abnormalities, may account for the pleiotropic clinical manifestations of the disease. Therefore, various defects in the expression, structure, assembly, or degradation of the constituent structural glycoprotein (or glycoproteins) of microfibrils may be implicated in the causation of the Marfan syndrome.

Effect of ADP on the orientation of spin-labeled myosin heads in muscle fibers: a high-resolution study with deuterated spin labels

We have used electron paramagnetic resonance (EPR) to determine the effects of ADP on the orientational distribution of nitroxide spin labels attached to myosin heads in skinned rabbit psoas muscle fibers. To maximize the specificity of labeling, we spin-labeled isolated myosin heads (subfragment 1) on a single reactive thiol (SH1) and diffused them into unlabeled muscle fibers. To maximize spectral and orientational resolution, we used perdeuterated spin labels, 2H-MSL and 2H-IASL, eliminating superhyperfine broadening and thus narrowing the line widths. Two different spin labels were used, with different orientation relative to the myosin head, to ensure that the results are not affected by unfavorable probe orientation. In rigor, a very narrow three-line spectrum was observed for both spin labels, indicating a narrow orientational distribution, as reported previously (Thomas & Cooke, 1980). ADP induced very slight changes in the spectrum, corresponding to very slight (but significant) changes in the orientational distribution. These changes were quantified by a digital analysis of the spectra, using a two-step simplex fitting procedure (Fajer et al., 1990). First, the magnetic tensor values and line widths were determined by fitting the spectrum of a randomly oriented sample. Then the spectrum of oriented fibers was fit to a model by assuming a Gaussian distribution of the tilt angle (theta) and twist angle (phi) of the nitroxide principal axes relative to the fiber axis. A single-Gaussian distribution resulted in inadequate fits, but a two-component model gave excellent results. ADP induces a small (less than 5 degrees) rotation of the major components for both spin labels, along with a similarly small increase of disorder about the average positions.

Staurosporine induces dissolution of microfilament bundles by a protein kinase C-independent pathway

The protein kinase C (PKC) inhibitor staurosporine was found to dramatically alter the actin microfilament cytoskeleton of a variety of cultured cells, including PTK2 epithelial cells, Swiss 3T3 fibroblasts, and human foreskin fibroblasts. For example, PTK2 cells exposed to 20 nM staurosporine exhibited a progressive thinning and loss of cytoplasmic actin microfilament bundles over a 60-min period. During this time microtubule and intermediate filament systems remained intact (as shown by immunofluorescence and at higher resolution by photoelectron microscopy), and the cells remained spread even though microfilament bundles were absent. Higher doses of staurosporine or longer exposure times at lower doses resulted in morphological alterations, but even severely arborized cells recovered normal morphology and actin patterns after a wash and an incubation for several hours in fresh medium. The actin filament disruption induced by staurosporine was distinguishable from the actin reorganization induced by exposure to the tumor promoter (and activator of PKC) phorbol myristate acetate (PMA). Swiss 3T3 cells made deficient in PKC by prolonged exposure to PMA (PKC down-regulation) exhibited actin alterations in response to staurosporine which were comparable to those in cells which had not been exposed to the phorbol ester. In a parallel control experiment, the actin cytoskeleton of PKC-deficient 3T3 cells was unaffected in response to PMA, consistent with down-regulation of this kinase. While the exact mechanism of staurosporine-induced actin reorganization remains to be determined, the observed effects of staurosporine on PKC-deficient cells make a role for PKC unlikely. These results indicate the need for care when staurosporine is employed as an inhibitor of protein kinase C in studies involving intact cells.

Cytochemical detection of disulfide and sulfhydryl groups in lamprey aortic microfibrils

A cytochemical study was performed on the lamprey ventral aorta with special reference to disulfide and sulfhydryl groups of microfibrils, using the high-iron diamine-thiocarbohydrazide-silver proteinate (HID-TCH-SP) method combined with several other types of treatment. The HID-TCH-SP staining observed was classified into three categories: 1) weak staining in the periphery of collagen fibrils, 2) moderate staining in the boundaries of collagen fibrils, microfibrils and smooth muscle cells, and 3) intense staining of microfibrils. The first and second categories of staining were considered to represent chondroitin and/or heparan sulfate because of sensitivity of the staining to chondroitinase ABC (ChABC) and its specific localization. By contrast, the third category of staining was considered to represent disulfide and sulfhydryl groups of microfibrillar glycoprotein, because it was disclosed only after Oxone oxidation or thiosulfation and was not removed by ChABC digestion. Although this staining reactivity was not apparently altered by SH blockade prior to oxidation or thiosulfation, it was markedly diminished or completely inhibited by S-S reduction followed by SH blockade. These results indicate that lamprey aortic microfibrils contain more S-S groups than SH groups.

Intracellular assembly of newly synthesized canine cardiac myosins

To investigate how newly synthesized cardiac myosins are assembled into myofilaments, we analysed the distribution of newly produced alpha-myosin heavy chain isozyme in sarcomeres by immunoelectron microscopy using a monoclonal antibody (CMA19), which is specific for alpha-myosin heavy chain. Isozymic changes in myosin heavy chains from beta to alpha type were induced in canine ventricular muscles and cultured ventricular myocytes by administration of 1-thyroxine. We incubated the glycerinated ventricular muscles or cultured ventricular myocytes with the enzyme (horseradish peroxidase) labelled Fab fragment of CMA19. After the reaction with 3, 3'-diaminobenzidine and osmification, we prepared ultrathin sections of the ventricular muscles or cultured ventricular myocytes and analysed their staining patterns by electron microscopy. There was apparent heterogeneity in the staining intensity of the myofilaments among different cells, among different myofibrils and even intramyofibrillarly. Higher magnification revealed that there were scattered foci of strong reaction which appeared to be foci of assembly of the newly synthesized alpha-myosin heavy chain. Immunocytochemical study also showed heterogeneous reactions within myofilaments and that there were scattered foci of myofilament assembly, which were closely associated with polyribosomes producing newly induced alpha-myosin heavy chain. These data suggest that newly synthesized cardiac myosins are assembled into myofilaments from the sites of synthesis, that is polyribosomes. This may explain the heterogeneity of the assembly pattern of newly synthesized cardiac myosins at the subcellular level.

Excimer fluorescence of pyrenyliodoacetamide-labeled tropomyosin: a probe of the state of tropomyosin in reconstituted muscle thin filaments

Rabbit skeletal tropomyosin (Tm) specifically labeled at cysteine groups with N-(1-pyrenyl)-iodoacetamide (PIA) exhibits excimer fluorescence. The excimer fluorescence was sensitive to the local conformation of Tm, to actin binding, and, in reconstituted thin filaments, to the Tm state change induced by binding of myosin subfragment 1 (S1). The properties of PIATm were similar to previously studied pyrenylmaleimide-labeled Tm (PMTm) [Ishii, Y., & Lehrer, S.S. (1985) Biochemistry 24, 6631] except that S1 binding to actin-Tm increased the excimer fluorescence in contrast to the time-dependent decrease seen for PMTm. The fluorescence properties of PIATm are sensitive to the Tm chain-chain interaction via equilibria among pyrene configurations and nonfluorescent dimer as well as the monomer and excimer-forming configurations. The effect of bound troponin (Tn) on the excimer fluorescence of PIATm in the reconstituted systems was dependent on ionic strength with a slight Ca2+ dependence. S1 titrations in the absence and presence of Tn and Ca2+ indicated that the excimer fluorescence probes the state change of Tm from the weak S1 binding state to the strong S1 binding state which is facilitated by Ca2+ [Hill et al. (1980) Proc. Natl. Acad. Sci. U.S.A. 77, 3186]. Binding of MgADP-S1 and MgAMPPNP-S1 produced the same total excimer fluorescence change as for nucleotide-free S1, showing that the strong S1 binding state of Tm-actin is independent of nucleotide.(ABSTRACT TRUNCATED AT 250 WORDS)

Different cytoskeletal organization in two maturation stages of Discoglossus pictus (Anura) oocytes: thickness and stability of actin microfilaments and tropomyosin immunolocalization

In Discoglossus pictus oocytes, the germinative area (GA) contains long and irregular microvilli where actin microfilaments are located. In the egg, the funnel-shaped dimple that originates by invagination of the GA is present. In the dimple both microvilli and microfilament bundles have a very orderly appearance. This report extends previous observations (Campanella and Gabbiani, Gamete Res 3:99-114, 1980) and shows that GA microfilaments are thinner (36 A average) than dimple microfilaments (60 A average), as measured in ultrathin section. Moreover, the interfilament distance is smaller in GA bundles than in the dimple bundles. To get an insight into actin organization in oocytes and eggs, we used an actin-depolymerizing factor (ADF) in which cryostat sections were incubated prior to immunofluorescent staining with antiactin antibodies. The microfilaments of the GA microvilli and partially of the oocyte cortex are resistant to ADF when compared to those in the dimple and the rest of the egg cortex. We also investigated immunocytochemically the presence of tropomyosin and found that this protein is localized in the dimple and in the cortex of oocytes and eggs but is absent in the GA.

A new 400-kD protein from isolated adherens junctions: its localization at the undercoat of adherens junctions and at microfilament bundles such as stress fibers and circumferential bundles

In the previous study, we succeeded in isolating the cell-to-cell adherens junctions from rat liver (Tsukita, S., and S. Tsukita. 1989. J. Cell Biol. 108:31-41.). In this study, we have obtained mAbs specific to the 400-kD protein, which was identified as one of the major constituents of the undercoat of isolated adherens junctions. Immune blot analyses showed that this protein occurs in various types of tissues. Immunofluorescence microscopy and immune electron microscopy have revealed that this protein is distributed not only at the undercoat of adherens junctions but also along actin bundles associated with the junction in nonmuscle cells: stress fibers in cultured fibroblasts and circumferential bundles in epithelial cells. The partially purified protein molecule looks like a slender rod approximately 400 nm in length. By virtue of its molecular shape, we have named this protein 'tenuin' (from Latin 'tenuis', thin or slender).

Substructure and accessory proteins in scallop myosin filaments

Native myosin filaments from scallop striated muscle fray into subfilaments of approximately 100-A diameter when exposed to solutions of low ionic strength. The number of subfilaments appears to be five to seven (close to the sevenfold rotational symmetry of the native filament), and the subfilaments probably coil around one another. Synthetic filaments assembled from purified scallop myosin at roughly physiological ionic strength have diameters similar to those of native filaments, but are much longer. They too can be frayed into subfilaments at low ionic strength. Synthetic filaments share what may be an important regulatory property with native filaments: an order-disorder transition in the helical arrangement of myosin cross-bridges that is induced on activation by calcium, removal of nucleotide, or modification of a myosin head sulfhydryl. Some native filaments from scallop striated muscle carry short "end filaments" protruding from their tips, comparable to the structures associated with vertebrate striated muscle myosin filaments. Gell electrophoresis of scallop muscle homogenates reveals the presence of high molecular weight proteins that may include the invertebrate counterpart of titin, a component of the vertebrate end filament. Although the myosin molecule itself may contain much of the information required to direct its assembly, other factors acting in vivo, including interactions with accessory proteins, probably contribute to the assembly of a precisely defined thick filament during myofibrillogenesis.

Structural changes induced in Ca2+-regulated myosin filaments by Ca2+ and ATP

We have used electron microscopy and proteolytic susceptibility to study the structural basis of myosin-linked regulation in synthetic filaments of scallop striated muscle myosin. Using papain as a probe of the structure of the head-rod junction, we find that this region of myosin is approximately five times more susceptible to proteolytic attack under activating (ATP/high Ca2+) or rigor (no ATP) conditions than under relaxing conditions (ATP/low Ca2+). A similar result was obtained with native myosin filaments in a crude homogenate of scallop muscle. Proteolytic susceptibility under conditions in which ADP or adenosine 5'-(beta, gamma-imidotriphosphate) (AMPPNP) replaced ATP was similar to that in the absence of nucleotide. Synthetic myosin filaments negatively stained under relaxing conditions showed a compact structure, in which the myosin cross-bridges were close to the filament backbone and well ordered, with a clear 14.5-nm axial repeat. Under activating or rigor conditions, the cross-bridges became clumped and disordered and frequently projected further from the filament backbone, as has been found with native filaments; when ADP or AMPPNP replaced ATP, the cross-bridges were also disordered. We conclude (a) that Ca2+ and ATP affect the affinity of the myosin cross-bridges for the filament backbone or for each other; (b) that the changes observed in the myosin filaments reflect a property of the myosin molecules alone, and are unlikely to be an artifact of negative staining; and (c) that the ordered structure occurs only in the relaxed state, requiring both the presence of hydrolyzed ATP on the myosin heads and the absence of Ca2+.

Topography and microfilament core association of a cell surface glycoprotein of ascites tumor cell microvilli

Membrane-microfilament interactions are being investigated in microvilli isolated from 13762 rat mammary ascites tumor cells. These microvilli are covered by a sialomucin complex, composed of the sialomucin ascites sialoglycoprotein-1 (ASGP-1) and the associated concanavalin A (Con A)-binding glycoprotein ASGP-2. Limited proteolysis of the microvilli releases large, highly glycosylated fragments of ASGP-1 from the microvilli and increases the association of ASGP-2 with the Triton-insoluble microvillar microfilament core (Vanderpuye OA, Carraway CAC, Carraway, KL: Exp Cell Res 178:211, 1988). To analyze the topography of ASGP-2 in the membrane and its association with the microfilament core, microvilli were treated with proteinase K for timed intervals and centrifuged. The pelleted microvilli were extracted with Triton X-100 for the preparation of microfilament cores and Triton-soluble proteins or with 0.1 M carbonate, pH 11, for the preparation of microvillar membranes depleted of peripheral membrane proteins. These microvilli fractions were analyzed by dodecyl sulfate gel electrophoresis, lectin blotting with Con A and L-phytohemagglutinin, and immunoblotting with anti-ASGP-2. The earliest major proteolysis product from this procedure was a 70 kDa membrane-bound fragment. At longer times a 60 kDa released fragment, 30-40 kDa Triton-soluble fragments, and 25-30 kDa membrane- and microfilament-associated fragments were observed. Phalloidin shift analysis of microfilament-associated proteins on velocity sedimentation gradients indicated that the 25-30 kDa fragments were strongly associated with the microfilament core. From these studies we propose that ASGP-2 has a site for indirect association with the microfilament core near the membrane on a 15-20 kDa segment.

Biotinylphallotoxins: preparation and use as actin probes

We describe the synthesis of four phalloidin derivatives conjugated with biotin. An aminomethyldithiolane derivative of ketophalloidin was used as a reactive starter compound, and biotin residues were coupled to this molecule either directly, separated by spacer chains comprised of one or two glycyl residues, or of a 12-atom long chain constructed from succinic acid and hexamethylendiamine. Although all products still displayed a high affinity for F-actin, as seen in competition experiments with [3H]-demethylphalloidin, only the one with the longest spacer (BHPP) showed specific and high-affinity decoration of actin filaments in permeabilized cells, in conjunction with FITC-coupled avidin and fluorescence microscopy. Combined with gold-streptavidin, BHPP decorated the actin filament system at the light and electron microscopic level faithfully and with satisfactory density. Actin filaments polymerized in vitro from purified protein were not as densely labeled as had been expected. However, in all these experiments the new phalloidin probe, when combined with avidin or streptavidin, yielded clear and highly specific labeling of F-actin. Therefore, this system is useful to identify and localize actin unambiguously in microfilaments, independent of actin antibodies, and should facilitate double-label experiments on cytoskeletal components at the ultrastructural level.

Aggregation of membrane-associated actin filaments following localized adherence of enteropathogenic Escherichia coli to HeLa cells

We have previously observed that enteropathogenic Escherichia coli (EPEC) adhere to HeLa cells in a localized manner, which we designated localized adherence as opposed to the diffuse pattern of adhesion. In this paper we have examined the effects of localized adherence of EPEC on the actin microfilament system of host HeLa cells. Centrifugation of bacteria onto HeLa cells improved the localized adherence and rapid rearrangements of actin filaments were detected by immunofluorescence and electron microscopy. Aggregation of microfilaments is consistently observed at the sites of localized adherence, and is abolished by cytochalasin D and low temperatures. Scanning electron microscopy indicates that these aggregates are surface microvilli entangled with attached EPEC.

Enhanced development of striated myofibrils in Xenopus myoblasts cultured in an applied electric field

Cultured myoblasts from Xenopus laevis differentiating in a DC electric field (approximately 100 mV/mm) had more microfilaments, more striated myofibrils and an increased Z-disc diameter than myoblasts in control cultures.

35 kDa proteins are not components of vertebrate smooth muscle thin filaments

A 35 kDa protein present in vertebrate smooth muscle and capable of binding to purified actin does not appear to be a constituent of smooth-muscle thin filaments in vivo; instead, it is more likely to be a component easily solubilized from particulate material which then spuriously interacts with actin.

Novel redistribution of myosin-containing filaments in cultured keratinocytes identified by a human monoclonal autoantibody

A time-dependent redistribution of microfilaments was observed in cultured human keratinocytes using a human monoclonal autoantibody specific for myosin. Immunofluorescent staining revealed that 5 days after plating keratinocytes in either 0.1 mM or 2.0 mM Ca++, myosin was distributed uniformly throughout the cytoplasm. At day 6, parallel arrays of myosin-containing microfilaments were prominent in the cell peripheries. At day 7 the microfilaments formed circumferential rings. The distribution of the microfilaments was disrupted by cytochalasin but not by colchicine, indicating that this novel distribution of myosin was not dependent on colchicine-sensitive vimentin intermediate filaments. The time-dependent redistribution of myosin was not influenced by cell population density, cell shape or cell cycle phase, except for mitotic cells in which myosin was distributed diffusely through the cytoplasm. If, as suggested by Kolega (9), microfilaments align parallel to the direction of applied tension, the redistribution of myosin-containing microfilaments in cultured keratinocytes may reflect the increased tension between cells resulting from increasing strength of cell-cell junctions over time. In sectioned human skin, myosin was localized in the peripheral cytoplasm of stratified epidermal cells. Tensions arising from the numerous desmosomal junctions between cells in vivo could account for this distribution of myosin.

Postnatal development of the Sertoli cell barrier, tubular lumen, and cytoskeleton of Sertoli and myoid cells in the rat, and their relationship to tubular fluid secretion and flow

The postnatal development of the Sertoli cell barrier, tubular lumen, fluid flow, and cytoskeletal elements in Sertoli and myoid cells was investigated in the Sprague-Dawley rat. With the aid of hypertonic fixatives, a barrier to the rapid entry of fluid was noted in the majority of tubules on the 15th and 16th postnatal (p.n.) days and was completely formed in all tubules prior to p.n. day 18. The actin forming the ectoplasmic specialization (ES), a cytoskeletal complex related to the occluding junctions composing the barrier, began its development during the period of initial barrier formation (16 p.n. day) and progressively attained its adult prominence. The ES developed its characteristic adult pattern and adult fluorescent intensity at about p.n. day 22. Some seminiferous tubules showed very small lumina as early as p.n. day 10. All tubules were not open until p.n. day 30. The size (diameter) of the lumen increased slowly from p.n. day 10 until p.n. day 30 when it started to increase rapidly until about p.n. day 50. Fluid flow in seminiferous tubules was detected as early as p.n. day 20 and increased in amount thereafter. Myoid cell actin filament bundles, running in parallel, were present at p.n. day 10. Actin formed a meshwork pattern characteristic of the adult on, or slightly prior to, p.n. day 22. These data indicate that there is a temporal relationship between the development of the actin cytoskeleton within the Sertoli cell and initial formation of the Sertoli cell barrier. Similarly, there is a temporal relationship between the development of the actin cytoskeleton of myoid cells and tubular fluid flow. The rapid increase in tubular lumen diameter, however, does not correlate with the initial development of Sertoli and myoid cytoskeletal elements.

Alpha-smooth muscle actin, a differentiation marker of smooth muscle cells, is present in microfilamentous bundles of pericytes

alpha-Smooth muscle (alpha-sm) actin, an isoform typical of smooth muscle cells (SMC) and present in high amounts in vascular SMC, was demonstrated in the cytoplasm of pericytes of various rat and human organs by means of immunocytochemistry at the electron microscopic level. In SMC and pericytes, alpha-sm actin was localized in microfilament bundles, strengthening the assumption that it is the functional isoform in these cell types and supporting the assumption that pericytes exert contractile functions.

Surface spines of human blood flukes (Schistosoma mansoni) contain bundles of actin filaments having identical polarity

The spines of Schistosoma mansoni have crystalline structures that have been suggested to consist of actin filaments. In this ultrastructural study, binding of heavy meromyosin to the actin filament spines strongly supports this view. Moreover, we reveal that all the packed actin filaments in the spines have the same polarity pointing away from the apical plasma membrane toward the basal membrane of the surface syncytial epithelium of the parasites and that the spine filaments interact indirectly with both the apical and basal membranes.

What structures, besides adhesions, prevent spread cells from rounding up?

The outline of cells in sparse cultures consists predominantly of concave and convex segments; straight segments are rare and ephemeral. The convex segments are areas of active cell expansion. The concave segments are stationary and web-shaped, similar in profile to the cables of a suspension bridge. In 3T3 fibroblasts, we have found a single microfilament bundle following the outline of every webbed edge and have called it the actin edge-bundle (AEB). While the AEB is composed predominantly of actin, alpha-actinin and myosin are also present. In contrast to normal stress fibers, AEBs are more resistant to several treatments that depolymerize F-actin. Once an AEB disassembles, however, the webbed edge collapses and retracts, suggesting that the actin edge-bundle is a specialized cytoskeletal structure that supports the webbed edges of interphase 3T3 fibroblasts. The stability of AEBs is independent of microtubules. We suggest that the microfilament bundles that frequently line the lateral contacts between epithelial cells in vivo may be related to the actin edge-bundle.

Direct visualization of bipolar myosin filaments in stress fibers of cultured fibroblasts

The authors examined the molecular organization of myosin in stress fibers (microfilament bundles) of cultured mouse embryo fibroblasts. To visualize the organization of myosin filaments in these cells, fibroblast cytoskeletons were treated with gelsolin-like protein from bovine brain (hereafter called brain gelsolin), which selectively disrupts actin filaments. As shown earlier [Verkhovsky et al., 1987], this treatment did not remove myosin from the stress fibers. The actin-free cytoskeletons then were lightly sonicated to loosen the packing of the remaining stress fiber components and fixed with glutaraldehyde. Electron microscopy of platinum replicas of these preparations revealed dumbbell-shaped structures of approximately 0.28 micron in length, which were identified as bipolar myosin filaments by using antibodies to fragments of myosin molecule (subfragment 1 and light meromyosin) and colloidal gold label. Bipolar filaments of myosin in actin-free cytoskeletons were often organized in chains and lattices formed by end-to-end contacts of individual filaments at their head-containing regions. Therefore, after extraction of actin, it was possible for the first time to display bipolar myosin filaments in the stress fibers of cultured cells.

The organization of myosin and actin in rapid frozen nerve growth cones

Rapid freezing and freeze substitution were used in conjunction with immunofluorescence, whole mount EM, and immunoelectron microscopy to study the organization of myosin and actin in growth cones of cultured rat superior cervical ganglion neurons. The general cytoplasmic organization was determined by whole mount EM; tight microfilament bundles formed the core of filopodia while a dense meshwork formed the underlying structure of lamellipodia. Although the central microtubule and organelle-rich region of the growth cone had fewer microfilaments, dense foci and bundles of microfilaments were usually observed. Anti-actin immunofluorescence and rhodamine phalloidin staining of f-actin both showed intense staining of filopodia and lamellipodia. In addition, staining of bundles and foci were observed in central regions suggesting that the majority of the microfilaments seen by whole mount EM are actin filaments. Anti-myosin immunofluorescence was brightest in the central region and usually had a punctate pattern. Although less intense, anti-myosin staining was also seen in peripheral regions; it was most prominent at the border with the central region, in portions of lamellipodia undergoing ruffling, and in spots along the shaft and at the base of filopodia. Immunoelectron microscopy of myosin using postembedment labeling with colloidal gold showed a similar distribution to that seen by immunofluorescence. Label was scattered throughout the growth cone, but present as distinct aggregates in the peripheral region mainly along the border with the central region. Less frequently, aggregates were also seen centrally and along the shaft and at the base of filopodia. This distribution is consistent with myosins involvement in the production of tension and movements of growth cone filopodia and lamellipodia that occur during active neurite elongation.

Association of elastin with pseudoexfoliative material: an immunoelectron microscopic study

Using immunoelectron microscopy, the presence of elastin and tropoelastin was demonstrated in pseudoexfoliative (PSX) material in all its classical sites on the lens capsule, ciliary non-pigment epithelium, iris epithelium and stroma, and conjunctiva. Some variability in binding affinity was seen in different sites, and labelling was more often on the periphery than the center of the PSX fibers. The elastin epitope on PSX material was more sensitive to processing than the remarkably stable epitope on mature elastic fibers. Since neither elastin nor a related component of PSX fibers, elastic microfibrillar protein, is a circulating protein, both are likely to be secreted by local ocular cells. Most of these local cells are not involved in elastogenesis normally, suggesting that an abnormal stimulus or defective regulation of matrix synthesis exists in this disease.

Changes in the organization and antigenic determinants of intermediate filaments of rat hepatocytes after infusion of cytochalasin B in vivo

The changes in cytokeratin intermedial filaments (IFs) after cytochalasin B (CB) infusion of rat liver in vivo were studied by light and electron microscopy, immunofluorescent staining (IMF), and immunoelectron microscopy (IEM). The CB treatment caused a change in the IFs at the cell border associated with a change in the distribution of microfilaments. The IFs at the cell border were partially disrupted. Actin aggregates were localized at points where IFs had condensed together. The pericanalicular sheath was intact but very dilated. These results indicated that the CB treatment caused an irregular distribution of the microfilaments at the cell periphery but spared the actin at the bile canaliculus. Cytokeratin staining by IMF was markedly decreased or absent; however, IEM clearly showed the presence of nonstaining IFs after CB treatment. These results indicated that the antigenic determinant of normal cytokeratin IFs became masked after CB treatment. The results indicate that F-actin disassembly induced by CB affects both the organization and conformation of cytokeratins associated with loss of integrity of the plasma membrane and vesicular uptake of plasma proteins by hepatocytes.

Microfilament association of ASGP-2, the concanavalin A-binding glycoprotein of the cell-surface sialomucin complex of 13,762 rat mammary ascites tumor cells

Microfilament-associated proteins and membrane-microfilament interactions are being investigated in microvilli isolated from 13,762 rat mammary ascites tumor cells. "Phalloidin shift" analyses on velocity sedimentation gradients of Triton X-100 extracts of [3H]-glucosamine-labeled microvilli identified a 120-kDa cell-surface glycoprotein associated with the microvillar microfilament core. The identification was verified by concanavalin A (Con A) blots of one- and two-dimensional (2D) electrophoresis gels of sedimented microfilament cores. By 2D-electrophoresis and lectin analyses the 120-kDa protein appeared to be a fraction of ASGP-2, the major Con A-binding glycoprotein of the sialomucin complex of the 13,762 cells. This identity was confirmed by immunoblot analyses using immunoblot-purified anti-ASGP-2 from anti-membrane serum prepared against microvillar membranes. Proteolysis of the microvilli with subtilisin or trypsin resulted in an increase in the amount of ASGP-2 associated with the microfilament cores. An increase was also observed with sialidase treatment of the microvilli, suggesting that negative charges, probably present on the highly sialated sialomucin ASGP-1 of the ASGP-1/ASGP-2 sialomucin complex, reduce ASGP-2 association with the microfilament core. Proteolysis of isolated microvillar membranes, which contain actin but not microfilaments, also increased the association of ASGP-2 with a Triton-insoluble, actin-containing membrane fraction. Purified ASGP-2 does not bind to microfilaments in sedimentation assays. Since the Triton-insoluble membrane residue is enriched in an actin-containing transmembrane complex, which contains a different glycoprotein, we suggest that the ASGP-2 is binding indirectly via this complex to the microfilament core in the intact microvilli.

Two classes of actin microfilaments are associated with the inner cytoskeleton of axons

The distribution and length of actin microfilaments (MF) was determined in axoplasm extruded from the giant axons of the squid (Loligo pealeii). Extruded axoplasm that was separated from the axonal cortex contains approximately 92% of the total axonal actin, and 60% of this actin is polymerized (Morris, J., and R. Lasek. 1984. J. Cell Biol. 98:2064-2076). Localization of MF with rhodamine-phalloidin indicated that the MF were organized in fine columns oriented longitudinally within the axoplasm. In the electron microscope, MF were surrounded by a dense matrix and they were associated with the microtubule domains of the axoplasm. The surrounding matrix tended to obscure the MF which may explain why MF have rarely been recognized before in the inner regions of the axon. The axoplasmic MF are relatively short (number average length of 0.55 micron). Length measurements of MF prepared either in the presence or absence of the actin-filament stabilizing drug phalloidin indicate that axoplasm contains two populations of MF: stable MF (number average length of 0.79 micron) and metastable MF (number average length of 0.41 micron). Although individual axonal MF are much shorter than axonal microtubules, the combined length of the total MF is twice that of the total microtubules. Apparently, these numerous short MF have an important structural role in the architecture of the inner axonal cytoskeleton.

Actin filament organization and myosin head labelling patterns in vertebrate skeletal muscles in the rigor and weak binding states

The structures of vertebrate skeletal muscles (particularly from frog and fish) in the rigor state are analysed in terms of the concept of target areas on actin filaments. Assuming that 100% of the heads are to be attached to actin in rigor, then satisfactory qualitative low-resolution modelling of observed X-ray diffraction data is obtained if the outer ends of these myosin heads can move axially (total range about 200A) and azimuthally (total range less than 60 degrees) from their original lattice sites on the myosin filament surface to attach in defined target areas on the actin filaments. On this basis, each actin target area comprises about four actin monomers along one of the two long-pitched helical strands of the actin filament (about 200 A) or an azimuthal range of actin binding sites of about 100 degrees around the thin filament axis. If myosin heads simply label in a non-specific way the nearest actin monomers to them, as could occur with non-specific transient attachment in a 'weak binding' state, then the predicted X-ray diffraction pattern would comprise layer lines at the same axial spacings (orders of 429 A) as those seen in patterns from resting muscle. It is shown that actin target areas in vertebrate skeletal muscles are probably arranged on an approximate 62 (right-handed) helix of pitch (P) of about 720 A, subunit translation P/6 and near repeat P/2. Troponin position need not be considered in defining the labelling pattern of cross-bridges on this 62 helix of target areas; the target areas appear to be defined solely by the azimuthal position of the actin binding sites. The distribution of actin filament labelling patterns could be regular in fish muscle which has a 'crystalline' A-band, but will be irregular in higher vertebrate muscles such as frog sartorius muscle.

Cytoskeleton in preimplantation mouse development

This paper reviews the constituents of the cytoskeleton in the cells of the preimplantation mouse embryo and how they change as the development proceeds. The cytoskeleton can be divided into two distinct groups, that in the cytosplasm and that associated with the membrane. The first and better-known group contains microfilaments, microtubules and intermediate filaments, the second such components of the cell and nuclear membrane as spectrin-like protein and nuclear lamin. The filamentous components of the cytoplasmic cytoskeleton adhere to the nuclear and cell membrane at attachment points where specific proteins such as vinculin may mediate the interaction. Each cell of the early embryo has all of these components, but their morphological organization and molecular constitution alter as the embryo develops. These modifications are especially pronounced when the cleavage-stage embryo compacts and when the blastocysts forms and differentiates. These events represent the most critical stages of morphogenesis and cytodifferentiation in the preimplantation embryo. The cytoskeleton may thus have an important role in the control of the early mammalian development.

Three-dimensional reconstruction of an actin bundle

We present the three-dimensional structure of an actin filament bundle from the sperm of Limulus. The bundle is a motile structure which by changing its twist, converts from a coiled to an extended form. The bundle is composed of actin plus two auxiliary proteins of molecular masses 50 and 60 kD. Fraying the bundle with potassium thiocyanate created three classes of filaments: actin, actin plus the 60-kD protein, and actin plus both the auxiliary proteins. We examined these filaments by transmission electron microscopy and scanning transmission electron microscopy (STEM). Three-dimensional reconstructions from electron micrographs allowed us to visualize the actin subunit and the 60- and 50-kD subunits bound to it. The actin subunit appears to be bilobed with dimensions 70 X 40 X 35 A. The inner lobe of the actin subunit, located at 20 A radius, is a prolate ellipsoid, 50 X 25 A; the outer actin lobe, at 30 A radius, is a 35-A-diam spheroid. Attached to the inner lobe of actin is the 60-kD protein, an oblate spheroid, 55 X 40 A, at 50 A radius. The armlike 50-kD protein, at 55 A radius, links the 60-kD protein on one of actin's twin strands to the outer lobe of the actin subunit on the opposite strand. We speculate that the 60-kD protein may be a bundling protein and that the 50-kD protein may be responsible for the change in twist of the filaments which causes extension of the bundle.

Characterization of flagella purified from enterohemorrhagic, vero-cytotoxin-producing Escherichia coli serotype O157:H7

Escherichia coli of the serotype O157:H7 has recently been isolated in human fecal specimens in association with sporadic cases and outbreaks of hemorrhagic colitis and with the hemolytic uremic syndrome. The aim of this study was to characterize the flagellin protein subunit constituents of flagellar filaments from E. coli O157:H7 strain CL-56. Flagellin isolated from a reference Salmonella enteritidis strain was used for comparison. Flagella were dissociated by incubation of bacteria under acidic conditions, centrifugation, and differential ammonium sulfate precipitation. Reconstituted flagellar filaments were demonstrated by three complementary methods: transmission electron microscopy, antigenic reactivity with H7 antiserum by a dot blot immunoassay, and immunogold localization of antiserum raised to the purified antigen to intact flagella on whole E. coli O157:H7. On sodium dodecyl sulfate-polyacrylamide gels flagellin proteins from E. coli O157:H7 demonstrated an apparent Mr of 66,000. The isoelectric point of E. coli O157:H7 flagellin was 5.42. By immunoblotting, H7 flagellin proteins were shown to be immunogenic. They induced a systemic immune response both in rabbits challenged with whole bacteria and in a human previously infected with E. coli O157:H7.

Identification of new actin-associated polypeptides that are modified by viral transformation and changes in cell shape

By using a monoclonal antibody we have identified a new polypeptide doublet (C4h and C4l) of Mr approximately 21 kD and pI 8 and 7, respectively, that is associated with and (at the immunofluorescence level) uniformly distributed on actin filament bundles in rat, mouse, and other vertebrate species. C4 is absent in neurones, erythrocytes, and skeletal muscle but the epitope is evolutionarily conserved as it is present in invertebrates such as molluscs and crustaceans. C4h is not found in cells such as lymphocytes and oncogenically transformed mesenchymal cells where actin stress fiber bundles are reduced in number or absent. C4l, on the other hand, is always present. C4h expression can also be blocked by switching normal nontransformed mesenchymal cells from adherent to suspension culture. Reexpression of C4h occurs 24 h after these cells are returned to normal adherent culture conditions, but can be blocked by either actinomycin D or cycloheximide, suggesting that the expression of this epitope is regulated at the transcriptional level.

Monoclonal antibodies against caldesmon, a Ca++/calmodulin- and actin-binding protein of smooth muscle and nonmuscle cells

Monoclonal antibodies, C2, C9, C18, C21 and C23, against chicken gizzard caldesmon have been prepared and characterized. These antibodies reacted with gizzard caldesmon (150 KDa) by enzyme-linked immunosorbent assay and protein immunoblotting. Immunofluorescence microscopy with these antibodies on cultured gizzard cells showed strong stress fiber and membrane ruffle stainings. Surprisingly, in addition to these cytoplasmic staining patterns, the C23 antibody also stained nuclei in these cells. Preabsorption of C23 with purified caldesmon abolished the staining of stress fibers and membrane ruffles as well as the staining of nuclei, suggesting that a common epitope existed in both gizzard caldesmon and the nuclear protein. Western blot analysis on the cell extract of chicken embryo fibroblast (CEF) showed that antibodies C2, C9, C18 and C21 recognized a nonmuscle caldesmon (66 KDa), whereas C23 reacted with a protein (60 KDa) different from nonmuscle caldesmon. Antibody C21 also crossreacted with a nonmuscle caldesmon (80 KDa) in normal rat kidney (NRK) cells, with a nonmuscle caldesmon (78 KDa) in human cells, and with a nonmuscle caldesmon (72 KDa) in gerbil fibroma cells. This antibody had broad-species specificity. Immunofluorescent staining of CEF cells with antibodies C2, C9, C18 and C21 showed some stress fibers and ruffles, but mostly diffuse staining. Antibody C23 crossreacted with 62 KDa and 55 KDa proteins in NRK cells, 63 KDa and 55 KDa proteins in gerbil fibroma cells and 66 KDa and 56 KDa proteins in human bladder carcinoma cells. These polypeptides were identified as nuclear lamins A and C by an anti-lamin antibody in immunoblots and two-dimensional gel analysis. Like the nuclear lamins, the C23 antigens also underwent a reversible disassembly during mitosis, as detected by double-label immunofluorescence with C23 antibody and a polyclonal anti-tubulin antibody. Tropomyosin-enriched microfilaments isolated from fibroblastic and epithelial types of NRK cells by monoclonal anti-tropomyosin antibody contained an 80 KDa protein, which had the heat-resistant property of caldesmon. The polyclonal antiserum generated against this 80 KDa protein showed a crossreactivity with purified gizzard caldesmon and vice versa. The amount of this nonmuscle caldesmon associated with the microfilaments of Kirstein virus-transformed NRK cells was greatly decreased.

Vanadate-treated baby hamster kidney fibroblasts show cytoskeleton and adhesion patterns similar to their Rous sarcoma virus-transformed counterparts

Rous sarcoma virus-transformed baby hamster kidney fibroblasts (RSV/B4-BHK) adhere to a fibronectin-coated substratum by means of dot-like adhesion sites called podosomes in view of their shape and function as cellular feet (Tarone et al.: Exp Cell Res 159:141, 1985). Podosomes concentrate tyrosine-phosphorylated proteins, including pp60v-src, and appear in many cells transformed by oncogenes coding for tyrosine kinases. In this paper we used orthovanadate, an inhibitor of phosphotyrosine phosphatases, in order to increase the cellular concentration of phosphotyrosine and to study whether this treatment induced the cytoskeleton remodeling leading to the formation of podosomes. Indeed, orthovanadate (10-100 microM) induced in a time- and dose-dependent manner the redistribution of F-actin and the formation of podosomes in BHK cells. Cytoskeleton remodeling occurred along with a marked increase of tyrosine phosphorylated proteins. The vanadate effect on the cytoskeletal phenotype was enhanced by the simultaneous treatment of cells with a phorbol ester. Under the latter conditions almost all BHK cells showed podosomes. The vanadate effect was reversible insofar as podosomes and tyrosine-phosphorylated proteins disappeared. Then, vanadate treatment of normal cells induced the cascade of events leading to the cytoskeletal changes typical of transformation and suggested that the transformed cytoskeletal phenotype may be primarily induced by the tyrosine phosphorylation of unknown target(s) operated by endogenous kinases.

Immunochemical identification of a thrombospondin-like structure in an arterial microfibrillar extract

Arterial microfibrils contain a 128 Kd collagenase and pepsin resistant glycoprotein (GP 128) essential for their ability to induce platelet aggregation. A previous report (Fauvel F. et al, (1984) Biochem. Biophys. Res. Comm., 123, 114-120) showed that GP 128 and thrombospondin (TSP) synthetized by endothelial cells each inhibited the aggregation of platelets by microfibrils and not by collagen. We used a monospecific antiplatelet TSP IgG in an immunoblotting assay for the identification of a TSP-like structure in untreated, collagenase-treated and pepsin-treated arterial microfibrils. The only constituent recognized in the three samples of microfibrils was GP 128. Fab fragments of this IgG provoked a dose dependent inhibition of the microfibril induced platelet aggregation (50% inhibition with 0.25 mg, 100% inhibition with 1 mg); in contrast, they did not affect collagen induced aggregation. The results indicate that a glycoprotein constituent with a thrombospondin-like antigenicity is involved in the thrombogenic properties of arterial microfibrils.

X-ray diffraction measurements of postmortem changes in the myofilament lattice of pork

Postmortem changes in the lateral spacing between filaments of the longissimus muscle in pork were examined by small-angle x-ray diffraction. Samples that were fixed in glutaraldehyde as soon as they were collected showed a rapid decrease in filament spacing from 1 h to 3 h and then a further, slower decrease to 24 h. Samples that were examined immediately or were kept prior to examination in buffered Ringer's solutions at pH values similar to those expected in the carcass showed a rapid decrease in filament spacing from 1 h to 3 h and then little further change to 24 h. In contrast, samples taken at various times postmortem and stored in Ringer's solutions at pH 7.2 for several hours before examination showed little postmortem change in lattice spacing. Fixed samples showed similar changes to those of unfixed samples, but the lattice spacing always was less in fixed than in unfixed samples. These results support the classic theory that much of the water that may be lost by drip and evaporation from meat originates from the spaces between the filaments. The major factor that caused shrinkage of the filament lattice and loss of water from the fibrils was pH.

Mitochondrial binding of a protein affected in mutants resistant to the microtubule inhibitor podophyllotoxin

Specific antibodies to a protein P1 Mr approximately equal to 63,000) from Chinese hamster ovary cells, which is affected in mutants resistant to the microtubule inhibitor, podophyllotoxin, and behaves like a microtubule-related protein by certain criteria [14], have been raised. The antibody reacts specifically with the P1 protein in one- and two-dimensional immunoblots, and a cross-reacting protein of similar molecular mass and electrophoretic mobility is also found in cells from various vertebrate and invertebrate species. The observed similarity in the peptide maps of the cross-reacting protein from human, mouse, Chinese hamster and chicken cells indicates that the structure of this protein should be highly conserved. However, no P1-antibody cross-reacting protein was observed in plants (corn, mung), fungus (Neurospora crassa), yeast (Saccharomyces cerevisiae) and bacteria (Escherichia coli and Salmonella typhimurium). Immunofluorescence studies with the P1-antibody show that, in interphase cells of various cross-reacting species, it bound specifically to mitochondria which were associated and distributed on and along the length of microtubules. Similar association and codistribution of mitochondria and microtubules were not observed in mitotic cells. Some implications of the mitochondrial localization of the protein P1 and the observed association between microtubules and mitochondria are discussed.

Identification of glycoprotein Ib beta as one of the major proteins phosphorylated during exposure of intact platelets to agents that activate cyclic AMP-dependent protein kinase

Platelet function is inhibited by prostaglandin E1, prostaglandin I2, or forskolin, agents that increase the intracellular concentration of cyclic AMP. The inhibition appears to result from cyclic AMP-stimulated phosphorylation of specific intracellular proteins. One of the major increases in phosphorylation occurs in a polypeptide of Mr = 24,000 (P24). In this study, an effort was made to identify P24. Platelets prelabeled with [32P]phosphate were incubated with prostaglandin E1, prostaglandin I2, or forskolin. Proteins that became phosphorylated were detected by autoradiography of sodium dodecyl sulfate-polyacrylamide gels. Several lines of evidence indicated that P24 was the beta-subunit of the plasma membrane glycoprotein (GP) Ib, a glycoprotein that is essential for the adhesion of platelets to damaged subendothelium, for the rapid response of platelets to thrombin, and for the attachment of the membrane skeleton to the cytoplasmic face of the plasma membrane. P24 co-migrated with GP Ib beta on reduced gels (Mr = 24,000) and also on nonreduced gels (when GP Ib beta is disulfide-linked to GP Ib alpha and migrates with Mr = 170,000). Like GP Ib beta, P24 was associated with actin filaments in Triton X-100 lysates. Like GP Ib beta, it was selectively associated with filaments of the membrane skeleton and was released from filaments when the Ca2+-dependent protease was active. Antibodies against GP Ib immunoprecipitated P24 from platelet lysates. Finally, exposure of Bernard-Soulier platelets (which lack GP Ib) to prostaglandin E1 resulted in phosphorylation of other polypeptides, but not of P24. These studies show that P24, one of the major polypeptides phosphorylated when platelets are exposed to agents that inhibit platelet function by increasing the concentration of cyclic AMP, is the beta-subunit of GP Ib.

Identification and purification of a novel Mr 43,000 tropomyosin-binding protein from human erythrocyte membranes

A new Mr 43,000 tropomyosin-binding protein (TMBP) has been identified in erythrocyte membranes by binding of 125I-labeled Bolton-Hunter tropomyosin to nitrocellulose blots of membrane proteins separated by sodium dodecyl sulfate-gel electrophoresis. This protein is not actin, because 125I-tropomyosin does not bind to purified actin on blots. Binding of 125I-tropomyosin to this protein is specific because it is inhibited by excess unlabeled tropomyosin but not by F-actin or muscle troponins. This protein has been purified to 95% homogeneity from a 1 M Tris extract of tropomyosin-depleted erythrocyte membranes by DEAE-cellulose and hydroxylapatite chromatography, followed by gel filtration on Ultrogel AcA 44. The purified protein has a Stokes radius of 3.9 nm and a sedimentation coefficient of 2.8 S, corresponding to a native molecular weight of 43,000. Binding of 125I-tropomyosin to the purified TMBP saturates at one tropomyosin molecule (Mr 60,000) to two Mr 43,000 TMBPs, with an affinity of about 5 X 10(-7) M. The TMBP is associated with the membrane skeleton after extraction of membranes with the non-ionic detergent, Triton X-100, and is present with respect to tropomyosin at a ratio of about one for every two tropomyosin molecules. Because there is enough tropomyosin for two tropomyosin molecules to be associated with each of the short actin filaments in the membrane skeleton, the erythrocyte membrane TMBP, together with tropomyosin, could function to restrict the number of spectrin molecules attached to each of the short actin filaments and thus specify the hexagonal symmetry of the spectrin-actin lattice. Alternatively, this TMBP could be homologous to one of the muscle troponins and might function with tropomyosin to regulate erythrocyte actomyosin-ATPase activity and influence erythrocyte shape.

Distribution by immunofluorescence of viral products and actin-containing cytoskeletal filaments in rubella virus-infected cells

Rubella virus (RV)-host cell interactions were examined by indirect immunofluorescence staining using antibodies to viral products and cytoskeletal components as probes. The patterns of immunofluorescence observed with human convalescent sera indicated that in infected Vero cells RV-specified proteins were distributed throughout the rough endoplasmic reticulum with some possible accumulation in the region of the Golgi complex. Viral RNA synthesis, detected with anti-double stranded RNA, appeared to be confined to small, intensely stained foci irregularly distributed in the cytoplasm. When cells were infected at a higher multiplicity, these foci appeared to aggregate into linear arrays. Infection with RV had a profound effect on the organization of actin in both Vero and BHK 21 cells, as shown by anti-actin antibodies. Actin microfilaments were observed to disintegrate progressively into amorphous aggregates of apparently monomeric actin as the infection proceeded. Because of the role actin microfilaments may play in cell mitosis it is postulated that this effect may be related to the inhibition of cell division reported to be associated with the congenital rubella syndrome.

Aflatoxin B1 and acetaminophen induce different cytoskeletal responses during prelethal hepatocyte injury

Primary monolayer cultures of adult rat hepatocytes exposed to the hepatocarcinogen aflatoxin B1 (AFB1) undergo a characteristic prelethal cytomorphological change that is distinct from their response to the necrogenic noncarcinogenic hepatotoxin, acetaminophen (AAP). Since changes in cell shape are mediated, at least in part, by the F-actin cytoskeleton, we designed experiments to study early prelethal alterations in the distribution of actin microfilaments in monolayer rat hepatocytes exposed to AFB1 (100 microM) or AAP (16 mM). Using rhodamine-phalloidin fluorescence microscopy, we observed that normal hepatocytes showed a submembranous F-actin distribution with focal short microfilaments extending into filopodia along the periphery of the cell. Hepatocytes exposed to AFB1 for several hours exhibited retraction of their cytoplasm within a prominent circumferential peripheral band of F-actin microfilament bundles. Retraction of focal areas of peripheral cytoplasm was associated with an increased prominence of the radial F-actin-containing filopodia. Subsequently, there appeared peripheral blebs containing very little F-actin. Hepatocytes exposed to equivalently lethal concentrations of AAP initially remained structurally normal. After several hours, the cells exhibited a prominent polar aggregate of short microfilament bundles without the formation of blebs. Both the blebbing and the polar aggregation of F-actin bundles occurred prior to cell death as shown by lactate dehydrogenase release and trypan blue exclusion. These studies support the hypothesis that the lethal effects of these two agents may occur by different biological mechanisms that are associated with remarkably distinct prelethal cytoskeletal responses.

Structural relationships of actin, myosin, and tropomyosin revealed by cryo-electron microscopy

We have calculated three-dimensional maps from images of myosin subfragment-1 (S1)-decorated thin filaments and S1-decorated actin filaments preserved in frozen solution. By averaging many data sets we obtained highly reproducible maps that can be interpreted simply to provide a model for the native structure of decorated filaments. From our results we have made the following conclusions. The bulk of the actin monomer is approximately 65 X 40 X 40 A and is composed of two domains. In the filaments the monomers are strongly connected along the genetic helix with weaker connections following the long pitch helix. The long axis of the monomer lies roughly perpendicular to the filament axis. The myosin head (S1) approaches the actin filament tangentially and binds to a single actin, the major interaction being with the outermost domain of actin. In the map the longest chord of S1 is approximately 130 A. The region of S1 closest to actin is of high density, whereas the part furthest away is poorly defined and may be disordered. By comparing maps from decorated thin filaments with those from decorated actin, we demonstrate that tropomyosin is bound to the inner domain of actin just in front of the myosin binding site at a radius of approximately 40 A. A small change in the azimuthal position of tropomyosin, as has been suggested by others to occur during Ca2+-mediated regulation in vertebrate striated muscle, appears to be insufficient to eclipse totally the major site of interaction between actin and myosin.

Cap Z(36/32), a barbed end actin-capping protein, is a component of the Z-line of skeletal muscle

Various biological activities have been attributed to actin-capping proteins based on their in vitro effects on actin filaments. However, there is little direct evidence for their in vivo activities. In this paper, we show that Cap Z(36/32), a barbed end, actin-capping protein isolated from muscle (Casella, J. F., D. J. Maack, and S. Lin, 1986, J. Biol. Chem., 261:10915-10921) is localized to the barbed ends of actin filaments by electron microscopy and to the Z-line of chicken skeletal muscle by indirect immunofluorescence and electron microscopy. Since actin filaments associate with the Z-line at their barbed ends, these findings suggest that Cap Z(36/32) may play a role in regulating length, orienting, or attaching actin filaments to Z-discs.

Possible translocation of actin and alpha-actinin along stress fibers

We have employed fluorescent analogue cytochemistry and fluorescence photobleaching to study the mobility of actin and alpha-actin along stress fibers. Rhodamine-labeled actin or alpha-actinin microinjected into embryonic chick cardiac fibroblasts soon became incorporated into stress fibers. A pulse of a laser microbeam was used to photobleach small spots on the fluorescent stress fibers. Images of the bleached fiber were recorded with an intensified image processing system at 2-3 min intervals. The distance between the bleached spot and the terminus of the stress fiber, which remained stationary throughout the experiment, was then measured in the successive images. Movement of bleached spots was detected along stress fibers located in the apparently trailing processes of polygonal fibroblasts, and only occurred in one direction: away from the distal tip of the stress fiber. The rate of movement calculated for alpha-actinin-injected cells was 0.24 +/- 0.12 micron/min, for actin-injected cells, 0.29 +/- 0.11 micron/min. The rate did not seem to be affected by the location of the spot relative to the distal end of the stress fiber unless the spot was located within the most distal 5 microns of the stress fiber. Anti-myosin antibody staining indicated that stress fibers which demonstrated translocation were relatively depleted of myosin. The apparent translocation of proteins along stress fibers, possibly generated by stretching, may be related to the retraction of cell processes during locomotion.

Improved immunoelectron microscopic method for localizing cytoskeletal proteins in Lowicryl K4M embedded tissues

We have modified the Lowicryl K4M low-temperature dehydration and embedding procedure for immunoelectron microscopy to provide improved ultrastructural detail and facilitate the localization of actin and tubulin in isolated rat adrenocortical cells, chick spinal cord with attached dorsal root ganglia (SC-DRG), and cultured dorsal root ganglia (DRG). Cells and tissues were fixed for immunocytochemistry either in a mixture of 2% paraformaldehyde and 0.25% glutaraldehyde (0.1 M PIPES buffer, pH 7.3) or in a mixture of 0.3% glutaraldehyde and 1.0% ethyldimethylaminopropylcarbodiimide (0.1 M phosphate buffered saline, pH 7.3). Dehydration was in ethanol at progressively lower temperatures to -35 degrees C. Infiltration at -35 degrees C was followed by ultraviolet polymerization at -20 degrees C. Comparable samples were fixed in glutaraldehyde and osmium tetroxide and embedded in Epon 812 or Epon-Araldite. Post-embedding immunostaining of thin sections utilized commercially available monoclonal antibodies to tubulin and actin followed by the protein A-gold technique (Roth et al., Endocrinology 108:247, 1981). Actin immunoreactivity was observed at the periphery of mitochondria and between mitochondria and lipid droplets in rat adrenocortical cells and at the periphery of neuronal cell processes of SC-DRG. Tubulin immunoreactivity was associated with microtubules throughout neurites of cultured DRG. Our modified technique allows preservation of ultrastructural details as well as localization of antigens by immunoelectron microscopy.

In situ localization of F-actin in the normal and injured guinea-pig tympanic membrane

Although cell migration is an important function of the epithelial cells of the tympanic membrane (TM), little is known about the distribution of the F-actin cytoskeleton, a contractile protein important in cell motility. The purpose of this experiment was to study the in situ localization of F-actin in the epithelial cells of the TM. F-actin, localized using Rhodamine-phalloidin, was present as a thin cortical band at the margin of both the mucosal cells on the inner side of the drum, and the suprabasal cells of the epidermis. The basal cells showed diffuse circumferential F-actin staining sometimes appearing as short microfilaments. Following a full thickness injury, changes in the distribution of F-actin could be observed with in situ localization. While the diffuse F-actin staining of the basal cells was reduced, both long F-actin microfilament bundles extending parallel to the long axis of the cell and focal aggregates of F-actin were prominent. The suprabasal cells became elongated, and while the F-actin remained localized to the cell margin, faint central F-actin microfilaments were observed. The staining of the mucosal cells remained unchanged. This study showed that the guinea pig TM is a useful model to study the distribution of epithelial F-actin in situ under normal and repair conditions, and that the basal cell layer may be important in regulating migration in the epidermis.

Rate of treadmilling of actin filaments in vitro

Actin filaments capped at the barbed ends were formed by polymerizing monomeric actin onto a gelsolin-actin complex. The rate of depolymerization and polymerization of the pointed ends was determined by diluting gelsolin-capped actin filaments into various concentrations of monomeric actin. Under the conditions of the experiments (100 mM-KCl, 2 mM-MgCl2 at 37 degrees C) the rate constant of dissociation of subunits both from a shortening and a lengthening filament was found to be 0.21 s-1. As the rate of dissociation of subunits from the slow pointed end determines the rate of treadmilling, it is concluded that actin filaments treadmill with a rate of about 2 micron/h.