Microtubule and microfilament rearrangements during capping of concanavalin A receptors on cultured ovarian granulosa cells

Molecular Functions of Glycoconjugates in Autophagy

Glycoconjugates, glycans, carbohydrates, and sugars: these terms encompass a class of biomolecules that are diverse in both form and function ranging from free oligosaccharides, glycoproteins, and proteoglycans, to glycolipids that make up a complex glycan code that impacts normal physiology and disease. Recent data suggest that one mechanism by which glycoconjugates impact physiology is through the regulation of the process of autophagy. Autophagy is a degradative pathway necessary for differentiation, organism development, and the maintenance of cell and tissue homeostasis. In this review, we will highlight what is known about the regulation of autophagy by glycoconjugates focusing on signaling mechanisms from the extracellular surface and the regulatory roles of intracellular glycans. Glycan signaling from the extracellular matrix converges on "master" regulators of autophagy including AMPK and mTORC1, thus impacting their localization, activity, and/or expression. Within the intracellular milieu, gangliosides are constituents of the autophagosome membrane, a subset of proteins composing the autophagic machinery are regulated by glycosylation, and oligosaccharide exposure in the cytosol triggers an autophagic response. The examples discussed provide some mechanistic insights into glycan regulation of autophagy and reveal areas for future investigation.

Structural differences between the putative carbohydrate-recognition domains of human IL-1 alpha, IL-1 beta and IL-1 receptor antagonist obtained by in silico modeling

In a previous report (Cebo et al. J Biol Chem 276 (2001) 5685-5691), it was established that biologically active recombinant human IL-1alpha and IL-1beta had different carbohydrate-binding properties. IL-1alpha recognized a di-antennary N-glycan with two alpha2-3-linked sialic acid residues, whereas IL-1beta recognized the GM(4), a alpha2-3-linked sialylated glycosphingolipid. These different carbohydrate-binding properties of two interleukins binding to the same receptor (IL-1R) could explain why these molecules had different biological effects and cell specificities. Molecular modeling of the ligands and in silico docking experiments defined putative carbohydrate-recognition domains localized in the same area of the two molecules, a domain different from that defined as the type I IL-1R binding domain. The calculated pattern of hydrogen bonding and of van der Waals interactions fulfilled the essential features observed for calcium-independent lectins (mammalian, viral or bacterial). The analysis of the same domain of the third members of this family of molecules, the IL-1R-antagonist, indicated it did not fulfill the criteria for carbohydrate-recognition domains. It is proposed that its role as a pure antagonist is due to the absence of lectin activity and consequently explained its inability to associate IL-1R with other surface molecular complexes necessary for signaling.

Perfluorocarbon attenuates response of concanavalin A-stimulated mononuclear blood cells without altering ligand-receptor interaction

Intrapulmonary application of perfluorocarbons (PFC) in acute lung injury is associated with anti-inflammatory effects. A direct impact on leukocytic function may be involved. To further elucidate PFC effects on cellular activation, we compared in an in vitro model the response of concanavalin A (ConA)-stimulated lymphocytes and monocytes exposed to perfluorohexane. We hypothesized that perfluorohexane attenuates the action of the lectin ConA by altering stimulant-receptor interaction on the cell surface. Mononuclear blood cells were stimulated by incubation with ConA in the presence of different amounts of perfluorohexane. The response of lymphocytes and monocytes was determined by means of IL-2 secretion and tissue factor (TF) expression, respectively. The influence of perfluorohexane on cell-surface binding of fluorescence-labeled ConA was studied using flow cytofluorometry and fluorescence microscopy. Perfluorohexane itself did not induce a cellular activation but significantly inhibited both monocytic TF expression and, to a far greater extent, IL-2 secretion of ConA-stimulated mononuclear blood cells. The effect of perfluorohexane was due neither to an alteration of cell viability nor to a binding of the stimulant. The amount of cell surface-bound ConA was not altered by perfluorohexane, and the overall pattern of ConA receptor rearrangement did not differ between controls and treated cells. In the present study, we provide further evidence for an anti-inflammatory effect of PFC that might be beneficial in states of pulmonary hyperinflammation. A PFC-induced alteration of stimulant-receptor interaction on the surface membrane does not seem to be the cause of attenuated cell activation.

The effects of dehydroepiandrosterone (DHEA) and its metabolites on the polycystic ovarian condition (PCO): cystogenic changes of rat granulosa cells in vitro

During mammalian folliculogenesis, granulosa cells (GCs) are initially steroidogenically quiescent, later proliferate, and subsequently commence to hormonally differentiate, first producing estrogen and later, in the preovulatory stage, secreting both estrogen and progesterone. In this study and elsewhere, we have used follicle-stimulating hormone with a combination of growth factors in vitro to simulate the above in vivo conditions. In a previous study, we used dehydroepiandrosterone (DHEA) to accomplish the polycystic ovary condition (PCO) in rats. In the latter model, there were high circulating levels of DHEA and its metabolite, androstenedione. In the present study, we investigated the effects of high levels of DHEA (10(-5) M) and its metabolites, androstenedione, androstenediol and dehydroepiandrosterone sulfate on the quiescent, proliferative, and steroidogenically differentiating stages of GCs cultured in a serum-free medium for up to 10 days. In addition to possessing the regularly occurring organelles, when cultured with the aforementioned androgens, the GCs acquired endoplasmic reticulum of the smooth variety which is associated with steroidogenesis. The radioimmunoassay data showed that GCs cultured in the quiescent and proliferative stages in the presence of the androgens, no longer remain in these stages but proceed to differentiate in a preovulatory direction by producing both estrogen and progesterone. This study supports our hypothesis that high circulating levels of DHEA and/or its metabolites have most effect during the quiescent and proliferative stages of granulosa cells, with regard to their structure and their steroidogenic activities.

The use of immuno-gold silver staining in bluetongue virus adsorption and neutralisation studies

The immuno-gold-silver staining (IGSS) technique was used in scanning electron microscopy for the detection and semi-quantitation of low copy antigens on the surface of cells. The methodology was exploited in experiments designed to examine the interaction of small numbers of virus particles with the surface of susceptible host cells. Using bluetongue virus (BTV) as an example, IGSS procedures confirmed that maximum adsorption occurred within 60 min and that adsorbed virus particles were distributed randomly on the surface of the cell. Neutralising antibody did not prevent binding of BTV to the plasma membrane, but abrogated virus uptake. The use of IGSS in the study of virus-cell interactions was validated by transmission electron microscopy and classical biochemical experiments utilising radioactively-labelled virus.

Cell locomotion: new research tests old ideas on membrane and cytoskeletal flow

Recent studies on the mobility of membrane markers on crawling cells indicate that there is no long-range centripetal flow of membrane proteins or lipids during cell locomotion. In this article we reflect on the history of ideas about membrane flow in cells, and we discuss how these new findings will shift the focus of research in cell locomotion away from the cell surface to the molecular interactions and dynamics of the actin cytoskeleton.

Immunolocalization of the cellular src protein in interphase and mitotic NIH c-src overexpresser cells

The mouse mAb, mAb 327, that recognizes specifically both pp60v-src and pp60c-src in a wide variety of cells, has been used to determine precisely the various locations of pp60c-src in NIH c-src overexpresser cells, using the technique of immunofluorescence microscopy. In interphase cells, the protein exhibits two main distributions: one that appears uniform and in association with the cell surface and the other that is patchy and juxtanuclear and coincides with the centrosomes. The juxtanuclear aggregation of pp60c-src-containing patches depends on microtubules and does not seem to occur within the Golgi apparatus and the rough ER. At the G2-to-M-phase transition, a drastic change in the localization patterns of pp60c-src takes place. We also report experiments in which the NIH c-src overexpresser cells were exposed to Con A for various times to induce a redistribution of the cell surface Con A receptors. We show that, at each stage of the Con A-mediated endocytotic process, the Con A-receptor complexes redistribute into structures to which pp60c-src appears also to be associated: at first, into patches that form at the cell surface level and then, into a cap that stands at the cell center in a juxtanuclear position and that coincides with the Golgi apparatus. During this capping process, pp60c-src-containing vesicles continue to accumulate in a centriolar spot, as in interphase, Con A-untreated cells, from which Con A is excluded. The significance of the intracellular locations of pp60c-src to the possible functions of the protein is discussed. Also, the distribution patterns of the cellular protein in the NIH c-src overexpresser cells are compared with those of pp60v-src in RSV-transformed cells. The differences observed are discussed in relation with the differences in transforming capacities of the two proteins. Finally, the possible physiological significance of the association between pp60c-src and the structures generated after the binding of Con A to its surface receptors is addressed.

Interactions of intermediate filaments with cell structures

Intermediate filaments (IF) are unique components of the cytoskeleton of most eukaryotic cells. Also the nuclear lamins are now recognized to be IF-like proteins, providing the nucleus with a putative skeleton for chromatin attachment. Immunofluorescence and whole-mount electron microscopic studies reveal that IF form a cytoplasmic network that surrounds the nucleus and extends to cell surface, as 'mechanical integrators of cellular space'. It seems however unlikely that IF in the cell accomplish a merely structural role, considering the diversity of IF proteins and the complex regulation of their gene expression. In this work we primarily present electron microscopic data that points to the presence of interactions between IF and several cellular components, namely the nucleus, plasma membrane, other cytoskeletal elements, cytoplasmic organelles and ribonucleoproteins. Although the functional significance of such interactions remains to be demonstrated, assumptions like involvement of IF in information transfer or cytoskeleton-dependent control of gene expression represent attractive hypothesis for future research.

In vivo and in vitro effects of androgens on rat ovarian granulosa cells

In this communication attention is called to some features of ovarian cyst formation in rats after exposure to 6 mg/100 gm body weight of dehydroepiandrosterone for 21 days. Cysts were identified by the transformation of mural granulosa cells into epithelial cells. The invaginations of the basal plasma membrane of the epithelial cells of the cyst are thought to be indications of the cells' involvement in active endocytosis. This endocytosis may be one strategy for moving fluid into the cyst. The filamentous actin of granulosa cells cultured in the presence of 10(-5) mol/L dehydroepiandrosterone was found to be deficient when compared with that of controls. This lack of filamentous actin coupled with other organelle degeneration is believed to lead to early atresia of granulosa cells in vitro in the presence of high concentrations of androgens.

Transferrin endocytosis and iron uptake in developing myogenic cells in culture: effects of microtubular and metabolic inhibitors, sulphydryl reagents and lysosomotrophic agents

The experiments described in this study were designed to investigate receptor-mediated endocytosis of transferrin and its role in iron uptake by cultured chick presumptive myoblasts (dividing and non-dividing) and myotubes. The effects of a variety of inhibitors on the internalization of transferrin and iron were investigated and three main effects were found: (i) sulphydryl reagents and microtubular inhibitors reduced the rate of transferrin and iron internalization to similar degrees, (ii) metabolic inhibitors reduced the rate of iron uptake more than that of transferrin endocytosis, and (iii) lysosomotrophic agents almost completely abolished iron accumulation by the cells without any effect on the rate of transferrin internalization. The results suggest that metabolic energy is required not only for the endocytosis of transferrin but also for subsequent steps in the iron uptake process, and that iron release from transferrin occurs in acidified endosomes. Overall, these experiments show that all or virtually all of the iron taken up by developing muscle cells from transferrin occurs as a consequence of receptor-mediated endocytosis of the protein.

Preparation of shadowed nuclear envelopes from Xenopus oocyte germinal vesicles for electron microscopy

Methods for examining the structure of the nuclear envelope of oocytes of Xenopus laevis by electron microscopy using metal shadowing have been developed and evaluated. Minor modifications were made to existing methods for preparing specimens by freeze drying, mainly to eliminate unnecessary steps and a rapid method for examining the structure and arrangement of nuclear envelope components, based on dehydration in an ethanol series followed by amyl acetate and then air drying, was also developed. The preservation of the lamina and connections between the nuclear pore complexes using the rapid air drying method was satisfactory for observing the fibrous components of the envelope and their attachment to the pores. Furthermore, air drying required only simple laboratory apparatus and, moreover, offered several advantages compared to freeze drying when assessing the effect of various disruptive treatments on the nuclear envelope or examining the connections between its components. In specimens prepared by either the more rapid air drying method or by freeze drying, the lamina meshwork beneath the nuclear face of the envelope was clear, but the fine structure of the nuclear pore complexes was superior in freeze dried preparations. In views of the nucleoplasmic face of the envelope, the lamina meshwork was suspended above the support film in freeze dried preparations, but collapsed in most air dried specimens. This collapse was not without its advantages, however, as it facilitated observation of the connections between nuclear pore complexes and lamina fibres, which were often masked in freeze dried preparations.

An in vitro study of the effects of androgens on the cytoskeleton of ovarian granulosa cells with special reference to actin

Ovarian granulosa cells from small antral follicles from immature rats were cultured in a serum-free medium for 1-6 days with or without the presence of 10(-5) M dehydroepiandrosterone (DHEA) or 10(-5) M-androstenedione (delta 4-A). Control cultures reveal that the cells are flattened and contain many filamentous bundles organized as stress fibers, numerous scattered cytoplasmic actin filaments, microtubules and vimentin. Alpha actinin and myosin were shown by immunocytochemistry to have a punctate pattern along the stress fibers. For the most part, cells exposed to androgens did not flatten; however, they assumed a varied shape and contained fewer stress fibers and actin filaments. Many of these cells did not develop stress fibers and those that did develop were fewer in number and displayed--actinin and myosin in a punctate pattern. Microtubules and vimentin filaments remained unaltered when compared to controls. It is believed that the deficiency of actin filaments, coupled with certain other degenerative changes which express themselves in other cellular compartments, leads to an early atresia of the granulosa cell cultured in high concentrations of androgens.

Immunohistochemistry of the cytoskeleton of human prostatic epithelium. Evidence for disturbed organization in neoplasia

An indirect immunoperoxidase technique was used to evaluate keratin, actin, tubulin, and calmodulin immunoreactivity in histologic sections of normal, hyperplastic, and neoplastic human prostate. Polyclonal as well as monoclonal keratin antibodies produced equivalent and intense staining of normal epithelium. The immunoreactivity of normal prostate with keratin antibodies was more pronounced than with antibodies to the other components of the cytoskeleton. Variation in staining for components of the cytoskeleton was minimal. The same findings applied to hyperplastic prostate. The immunoreactivity of prostate tumors with antibodies to these cytoskeletal proteins differed markedly from normal prostate. Prostatic carcinomas showed reduced keratin immunoreactivity with a panepithelial antibody, but unaltered or enhanced immunoreactivity with tubulin, actin, and calmodulin antibodies. Many tumors were unreactive with a monoclonal keratin antibody that was strongly reactive with tissues that contained cytokeratin 18 (45-kd) and which intensely stained normal and hyperplastic prostate. In addition, prostate carcinomas often yielded heterogeneous patterns of staining with actin, tubulin, and calmodulin antibodies in contrast to normal and hyperplastic prostate, which showed uniform staining. The results suggest that a disturbance in the organization of the cytoskeleton may accompany neoplastic transformation of human prostate.

Mechanism of methylmercury cytotoxicity

Although a large number of epidemiological, clinical, and pathological studies on methylmercury intoxication have been published, these investigations have not been able to elucidate the detailed mechanisms by which the metal alkyl causes a wide variety of biological dysfunctions. Thus, the cultured cells which are free from the influence of whole body complexities, such as absorption, distribution, metabolism, etc., which complicate the interpretation of in vivo experimental results, attract the attention of many scientists who are interested in clarifying the mode of toxic action of methylmercury. The aim of this article is to review the recent studies on the toxicity of methylmercury at the cellular level and to outline the mechanisms which have been proposed to be responsible for cell injuries.

Microtubules, microfilaments and the transport of acetylcholine receptors in embryonic myotubes

Both microtubules and microfilaments have been implicated in the exocytotic and endocytotic transport of coated and smooth surfaced membrane vesicles. We have reexamined this question by using specific pharmacological agents to disrupt these filaments and assess the effect on the movement of acetylcholine receptor (AChR) containing membrane vesicles in embryonic chick myotubes. Myotube cultures treated with nocodazole (0.6 microgram/ml) or colcemid (0.5 microgram/ml) (to disrupt microtubules) show only a 20-25% decrease in the number of cell surface AChRs after 48 h. Addition of chick brain extract (CBE) to cultured myotubes causes a significant increase in the total number of cell surface AChRs (measured by [125I]alpha-bungarotoxin (alpha-BGT) binding), thus providing us with a way to manipulate receptor and transport vesicle populations. Cultures treated with CBE plus nocodazole or colcemid show a 1.7-fold increase in AChR number over drug treatment alone, the same increase seen in cultures treated with CBE alone, although the total number remains about 20-25% less than that seen in control cultures. In cultures treated with cytochalasin D (0.2 microgram/ml) or dihydrocytochalasin B (5.0 micrograms/ml) (to disrupt microfilaments), 35 and 65% decreases in cell surface AChR number were seen after 48 h. However, in cultures treated with CBE and cytochalasin D, the same total number of AChRs was found as in cultures treated with CBE alone. No significant effects were seen with any of these drugs on the receptor incorporation rate (the appearance of new alpha-BGT-binding sites) after 6 h. The half-life for AChRs in control cultures was 23.0 h. In cytochalasin D and dihydrocytochalasin B it was 21.9 and 19.0 h, respectively; with colcemid and nocodazole, it increased to 37.1 and 28.1 h. These results suggest that non-myofibrillar microfilament bundles are not involved in the movement of AChR-containing membrane vesicles; further, the small effects seen with microtubule inhibitors tend to rule out a major role for microtubules in this transport.

Analysis of the role of microfilaments and microtubules in acquisition of bipolarity and elongation of fibroblasts in hydrated collagen gels

Fibroblasts in situ reside within a collagenous stroma and are elongate and bipolar in shape. If isolated and grown on glass, they change from elongate to flat shape, lose filopodia, and acquire ruffles. This shape change can be reversed to resemble that in situ by suspending the cells in hydrated collagen gels. In this study of embryonic avian corneal fibroblasts grown in collagen gels, we describe for the first time the steps in the acquisition of the elongate shape and analyze the effect of cytoskeleton-disrupting drugs on filopodial activity, assumption of bipolarity, and cell elongation within extracellular matrix. We have previously shown by immunofluorescence that filopodia contain actin but not myosin and are free of organelles. The cell cortex is rich in actin and the cytosol, in myosin. By using antitubulin, we show in the present study that microtubules are aligned along the long axis of the bipolar cell body. The first step in assumption of the elongate shape is extension of filopodia by the round cells suspended in collagen, and this is not significantly affected by the drugs we used: taxol to stabilize microtubules; nocodazole to disassemble microtubules; and cytochalasin D to disrupt microfilaments. The second step, movement of filopodia to opposite ends of the cell, is disrupted by cytochalasin, but not by taxol or nocodazole. The third step, extension of pseudopodia and acquisition of bipolarity similarly requires intact actin, but not microtubules. If fibroblasts are allowed to become bipolar before drug treatment, moreover, they remain so in the presence of the drugs. To complete the fourth step, extensive elongation of the cell, both intact actin and microtubules are required. Retraction of the already elongated cell occurs on microtubule disruption, but retraction requires an intact actin cytoskeleton. We suggest that the cell interacts with surrounding collagen fibrils via its actin cytoskeleton to become bipolar in shape, and that microtubules interact with the actin cortex to bring about the final elongation of the fibroblast.

Increased rate of capping of concanavalin A receptors during early Xenopus development is related to changes in protein and lipid mobility

The mobility characteristics of plasma membrane constituents were studied in dissociated cells from embryos of Xenopus laevis at various stages of development from early blastula until neurulation. An increased rate of fluorescein isothiocyanate-concanavalin A induced patching and capping of Con A-binding proteins during this period of development was correlated with a threefold increase in the lateral mobility of the receptor molecules, as determined by the fluorescent photobleaching recovery (FPR) method, the major change occurring at the onset of gastrulation. Using the same method, it was demonstrated that the lateral mobility of plasma membrane lipids increases twofold during this period of development. The major change being detectable, however, at the late blastula stage. This is in coincidence with the initiation of cell motility in dissociated Xenopus embryo cells. It is concluded that the lateral mobility of membrane proteins and lipids increases significantly during early Xenopus development, but are at least in part subject to different control mechanisms. The results suggest that the initiation of morphogenetic movements is related to changes in the dynamic properties of plasma membrane constituents.

Inability of newt epidermal cells to migrate over concanavalin A-coated substrates

Pieces of coverslip glass coated with various proteins were implanted under one edge of a fresh skin wound on adult newt hind limbs so that the implant served as wound bed for migrating epidermal cells as they attempted to form a wound epithelium. Despite the fact that concanavalin A (Con A) receptors could be demonstrated on newt epidermal cells with fluorescein isothiocyanate (FITC)-conjugated lectin, Con A-coated implants supported practically no migration, an even poorer response than the modest amount of migration that occurred on uncoated glass. Coomassie blue staining verified that the lectin formed a complete film over the glass, and peroxidase binding assays showed that even after several hours in the wound, the Con A binding sites for mannose were still available. Migration on fibrinogen-coated glass (a good migration substrate) was not affected by placing the implants next to Con A-coated implants. Thus, the failure to migrate on Con A cannot be explained by soluble Con A effects from lectin leaching off the implants. These data suggest that linkages between cell surface mannose and the substrate are not part of the strategy by which newt epidermal cells migrate.

5'-Nucleotidase from bovine caudate nucleus synaptic plasma membranes: specificity for substrates and cations; study of the carbohydrate moiety by glycosidases

We studied 5'-nucleotidase in preparations of synaptic plasma membranes from bovine caudate nucleus. The best substrates for this membrane-bound enzyme were purine nucleotides, particularly 5'AMP. Effects of metal cations and chelating agents suggest that 5'-nucleotidase is a metalloprotein. Optimal conditions for solubilization of the 5'-nucleotidase were found by using a low concentration of the zwitterionic detergent sulfobetaine 14. In contrast, another membrane-bound enzyme, acetylcholinesterase, was not solubilized under these conditions, but only in the presence of Triton X-100. The effects of lectins (concanavalin A, Lens culinaris agglutinin, wheat germ agglutinin, and Limulus polyphemus agglutinin) showed that both enzymes are glycoproteins. Sequential hydrolysis with specific glycosidases produced modifications of the effect of lectins on these enzymes. The results suggest the presence of a complex-type glycosylation, with a fucose residue on the internal N-acetyl-D-glucosamine of the pentasaccharide core.

Microtubule regulation of cell surface receptor topography during granulosa cell differentiation

A possible role for cytoplasmic microtubules in modulating lectin binding site topography has been examined during the hormone-directed differentiation of rat ovarian granulosa cells in vitro. Indirect immunofluorescence staining with anti-tubulin antibodies indicates that undifferentiated cultured granulosa cells contain a network of microtubules which radiate from the cell center to the cell periphery. Cultures induced to differentiate by a three day treatment with 1 microgram/ml prolactin exhibit a marginal distribution of microtubules and a centrally-located primary cilium. Prolactin enhances the incidence of granulosa cells containing a primary cilium from 9% in undifferentiated cultures to 53% in hormone-treated cultures. The pattern of lectin binding site redistribution induced by Concanavalin A (Con A) is also modified by prolactin treatment. In contrast to undifferentiated cells, which randomly endocytose fluorescein Con A, granulosa cells exposed to prolactin respond to fluorescein Con A by forming central surface caps to a greater extent (75%) than undifferentiated controls (25%). Double label fluorescence microscopy and transmission electron microscopy on Con A labeled cells show that caps form at central cell surface sites which contain the primary cilium. Disruption of cytoplasmic microtubules by colchicine, in undifferentiated granulosa cells, results in the formation of cell surface caps upon Con A addition. These data suggest that cytoplasmic microtubules modulate the topography of lectin bindings sites which is subject to hormonal control during the in vitro differentiation of ovarian granulosa cells.

Effects of concanavalin A and cholera toxin on epidermal cAMP and migration rate during wound closure in adult newts

Following removal of a skin patch from each hind limb of a series of adult newts, the limbs were explanted into small dishes of Holtfreter solution containing various combinations of test drugs. Later, the amount of wound epithelium that formed on each limb was determined using a planimeter on wound tracings obtained with the aid of a drawing tube-equipped microscope. Exposure of migrating cells to the plant lectin, concanavalin A (con A), lowered cyclic AMP (cAMP) levels and depressed migration. Exposure to cholera toxin and theophylline (CTX) significantly elevated cAMP levels and significantly depressed migration rate. Exposure of CTX-treated cells to con A tended to lower CTX-elevated cAMP levels while depressing the migration rate well beyond the depression caused by CTX alone. These results provide further evidence that cAMP can regulate the rate of newt epidermal cell migration. They also show that the inhibitory effect of con A on motility in these cells is independent of its effects on cAMP.

A quantitative freeze-etching study on the effects of cytochalasin B, colchicine and concanavalin A on the formation of intercellular contacts in vitro

The effects of concanavalin A (conA), cytochalasin B (CB), and colchicine acting on cell membrane-associated structures, such as surface receptors or submembranal cytoskeletal components, were studied in trypsin-isolated plexus cells reaggregated in vitro. Following reaggregation, these cells form intercellular contacts, i.e. a zonula occludens (ZO) and integrated nexus (gap junctions, GJ). In the control cultures the GJ occupied 4.7%, the tight junctions (TJ) occupied 14 microns/microns2 of the surface of the zonula occludens (ZO). All the drugs/lectins studied inhibit contact formation in a characteristic manner. ConA inhibits the formation of GJ (1% of the ZO) and causes 5 times more defective TJ strands than the controls. CB inhibits the assembly of the TJ (75% of the controls) and the GJ (1% of the ZO) when it is administered during the period of contract formation. Colchicine inhibits the formation of GJ (1% of the ZO) and produces 2.5 times more open-end strands of TJ than the controls. All effects are reversible with the exception that integrated GJ are not re-formed after colchicine administration. It is suggested that an intact interaction between surface receptors, intramembranous particles and associated cytoskeletal structures is necessary for remodelling cellular contacts.

Direct evidence for microfilament-mediated capping of surface receptors on crawling fibroblasts

On moving fibroblasts, cell-surface receptors cross-linked by antibodies or lectins are cleared centripetally from regions of lamellar cytoplasm and collect as a cap over the perinuclear region. Current theories of the mechanism of receptor redistribution on cultured cells variously implicate membrane flow, lipid flow, surface waves and linkage to the cytoskeleton. The last, the anchorage model, is based on observations that ligand-induced clusters of receptors on a variety of cell types either attach to actin or align over structures containing actin, myosin and alpha-actinin. I show here that the capping of antibody receptors on crawling chick embryo fibroblasts is highly coordinated with the apparent centripetal movements of arcs, which are part of a dorsal cortical actin-microfilament sheath (DCMS). This phenomenon can be directly observed in living cells. The data support the anchorage model of membrane receptor mobility and suggest that there is a continuous flow of actin associated with fibroblast locomotion.

A yellow crescent cytoskeletal domain in ascidian eggs and its role in early development

In this investigation, Triton X-100 extraction was utilized to examine the cytoskeleton of ascidian eggs and embryos. The cytoskeleton contained little carbohydrate or lipid and only about 20-25% of the total cellular protein and RNA. It was enriched in polypeptides of molecular weight (Mr)54, 48, and 43 x 10(3) Mr polypeptide was identified as actin based on its Mr, isoelectric point, and affinity for DNase I. Electron microscopy of the detergent-extracted eggs showed that they contained cytoskeletal domains corresponding to colored cytoplasmic regions of specific morphogenetic fate in the living egg. A yellow crescent cytoskeletal domain in the myoplasm was examined and shown to consist of a plasma membrane lamina (PML) and a deeper lattice of filaments which appeared to connect the yellow crescent pigment granules to the PML. The PML probably consists of integral membrane proteins stabilized by an underlying network of actin filaments since NBD-phallacidin stained this area of the egg cortex and the PML was extracted from the cytoskeleton by DNase I treatment. The yellow crescent cytoskeletal domain was found throughout the cortex of the unfertilized egg. During ooplasmic segregation it progressively receded into the vegetal hemisphere and was subsequently partitioned to the presumptive muscle and mesenchyme cells of the 32-cell embryo. It is suggested that contraction of the actin network in the yellow crescent cytoskeletal domain is the motive force for ooplasmic segregation. This structure may also serve as a framework for the positioning of morphogenetic determinants involved in muscle cell development.

Redistribution of fodrin (a component of the cortical cytoplasm) accompanying capping of cell surface molecules

Fodrin, a protein composed of two polypeptides with molecular weights of 250,000 and 240,000, is concentrated in the cortical cytoplasm of neurons, and moves down the axons by the process of axonal transport. We have used immunofluorescence techniques to determine whether fodrin antigens also move in non-neuronal cells when cell surface ligands are induced to redistribute by crosslinking them. A redistribution of fodrin antigens occurred in the following instances: (i) when 3T3 cells were incubated with concanavalin A and anti-concanavalin A, surface concanavalin A receptors formed aggregates and fodrin antigens formed corresponding intracellular aggregates; (ii) when B lymphocytes were incubated with anti-Ig, the surface Ig formed caps and fodrin antigens formed intracellular subcaps; (iii) when T lymphocytes were treated with anti-H-2 followed by a secondary antibody, the H-2 antigen formed caps and fodrin formed corresponding subcaps. These observations show that fodrin antigens can move within non-neuronal cells, as well as in axons, and that their organization can be regulated by interaction between surface proteins and environmental stimuli. They also raise the possibility that fodrin, together with other proteins that form subcaps in lymphocytes (e.g., actin, myosin, and alpha-actinin) is a component of the cellular machinery responsible for the capping process. We consider whether the similarities between the movements of fodrin in lymphocyte capping and axonal transport may indicate that certain aspects of these two processes are related.

Interactions of lectins with CHO cell surface membranes. II. Differential effects of local anesthetics on endocytosis of Con A and WGA binding sites

Using fibroblastic CHO cells, we have examined 1) the internalization and redistribution of surface binding sites for the lectins Concanavalin A and wheat germ agglutinin and 2) the sensitivity of these processes to putative inhibitors of cytoskeletal activity. Following initial exposure to fluorescein conjugated Con A (CAF) or WGA (WGAF) at 4 degrees C, kinetic analysis of internalization and intracellular aggregation of lectin at 37 degrees C indicated more rapid aggregate formation in the case of WGA than in the case of Con A. Treatment with tertiary amine local anesthetics (tetracaine, dibucaine, and xylocaine) or with a lysosomatrophic amine, m-dansyl cadaverine, blocked internalization of Con A but not of WGA. Similar differential effects on redistribution of Con A and WGA were not however observed with the antimicrotubule agents colchicine and nocodazole. Simultaneous treatment of cells with WGAF and with rhodamine labeled Con A (CAR) resulted in the accumulation of both labels in a central perinuclear aggregate; a similar experiment in the presence of local anesthetic resulted in a diffuse peripheral distribution of CAR and a central aggregate of WGAF. These results suggest 1) CHO cells possess at least two distinct pathways for lectin internalization and redistribution, which can be discriminated in terms of drug sensitivity; 2) CHO cells can sort out and independently internalize different populations of lectin binding sites; and 3) different pathways may be a manifestation of biochemically distinct linkages between cytoskeletal elements and various groups of surface glycoproteins. Present findings concur with our previous results concerning the mutual independence of the surface binding sites for Con A and WGA (Emerson and Juliano, 1982).

The intracellular movement of endocytic vesicles in cultured granulosa cells

Ligand binding to cell surface receptors induces rapid internalization of ligand-receptor complexes by receptor mediated endocytosis. We have examined the intracellular movement of endocytic vesicles, induced by the lectin concanavalin A (Con A), in cultured rat ovarian granulosa cells using fluorescence and electron microscopy. Within 20 minutes of ligand treatment at 37 degrees C, numerous Con A-containing endocytic vesicles form, which migrate to the cell center by 60 minutes. Double label fluorescence microscopy, using fluorescein-Con-A and rhodamine immunofluorescent staining of tubulin or vimentin, indicates that during vesicle migration microtubules and 10-nm filaments are altered in their organization. By 30 minutes, vesicles are associated with microtubule bundles, which subsequently collapse around the nucleus. Similarly, 10-nm filaments accumulate around the nucleus in conjunction with the perinuclear aggregation of endocytic vesicles. Electron microscopy of Con A-horseradish peroxidase-labeled cells demonstrates that endocytic vesicles fuse to form large receptosome-like structures during intracellular migration and these structures are associated with cytoplasmic microtubules and 10-nm filaments. Taxol, a drug that stabilizes microtubules, prevents endocytic vesicle translocation to the Golgi region. Nocodazole, which causes microtubule disassembly, results in the collapse of 10-nm filaments and the central aggregation of endocytic vesicles. The data indicate that the cytoskeleton participates in the directed intracellular movement of endocytic vesicles; the possible subcellular basis for this movement is discussed.

Effects of colchicine on gap junction formation during retinal neurogenesis

Colchicine, injected into the amniotic cavity of 3 to 10 day-old chick embryos, has the following effects on the developing retina: 1. Cells in arrested metaphase accumulate in the ventricular portion of the matrix region. If colchicine is applied at the end of the first week of incubation, the kinetic migration of the cell nuclei is inhibited. 2. The cells lose their typical slender, bipolar shape to become ovoid to spherical. These morphological changes are more obvious when colchicine acts during the first week of incubation. 3. The formation of temporary gap junctions between the matrix cells of the retina occurs during the first week of development. When colchicine is administered around the 3rd day of incubation, the assembly of intramembranous particles (IMPs) during gap junctions formation is incomplete. These results suggest that an intact microtubular system within the submembranous cytoskeleton is essential for the assembly of these intercellular contacts.

Cell surface labeling of embryonic neural retina cells exposed to low temperature, energy inhibitors, cytochalasin B and colchicin

Large blebs devoid of receptors for hemocyanin-labeled concanavalin A appear on neural retina cells from 8-day chick embryos exposed to low temperature or to potassium cyanide or 2,4(alpha)-dinitrophenol at room temperature. Labeling with mixed antibodies against a crude retina membrane preparation and with goat anti-rabbit-hemocyanin conjugate showed the same results. Determination of cell ATP content indicated a drop in ATP concentration after exposure to low temperature or to respiration inhibitors. Disruption of microtubules by colchicin inhibited the formation of 'naked' large blebs, whereas cytochalasin B had no such inhibitory effect.

Stimulation of prostaglandin E production by concanavalin A in isolated Graafian follicles

Addition of Concanavalin A (Con A) to isolated rat Graafian follicles induced prostaglandin E (PGE) production after 2 h of incubation. PGE synthesis continued throughout 24 h culture period. Cyclic AMP accumulation was noted after 6 h of incubation with Con A. .Aspirin, indomethacin and flufenamate prevented both the stimulation of PGE production and of cyclic AMP accumulation by Con A; antibodies to PGE prevented the cyclic AMP production. These studies indicate that the interaction of Con A with the follicle results in PGE production. It seems that besides the known pathway for the induction of PGE synthesis in the ovarian follicle, via elevation of intracellular cyclic AMP, an additional pathway, via an external signal which is independent of cyclic AMP exists.

Effects of Ca+2 and Mg+2 deprivation of cell shape in cultured ovarian granulosa cells

After 3 days in culture, rat ovarian granulosa cells assume a flattened epitheloid organization. Ca+2 and Mg+2 deprivation results in cellular rounding which is reversible and was monitored by phase-contrast time-lapse cinematography. Concomitant with the shape change is a dispersion of the structural proteins actin and alpha-actinin. The arrays of large actin-containing bundles (stress fibers) are converted to a diffuse network as observed by electron microscopy. Alpha-actinin, which was observed by immunocytochemistry to be in a periodic array along the actin bundles, is disrupted also and redistributed in the periphery of the cell upon rounding. Measurements made of the culture medium during the rounding process indicate that there is a loss of Ca+2 and Mg+2 from the cell interior. These data led us to speculate that Ca+2 and/or Mg+/ are necessary in order to maintain the integrity of stress fibers and/or restrict the movement of alpha-actinin anchoring sites within the membrane.

Inhibition of the mobility of mouse lymphocyte surface immunoglobulins by locally bound concanavalin A

Fluorescence photobleaching recovery was used to study directly and quantitatively the inhibition of the lateral mobility of surface immunoglobulins (sIg) on mouse lymphocytes by localized binding of concanavalin A (Con A) coupled to platelets. Up to a threshold occupancy of about 10% of the upper cell surface by Con A-platelets, the diffusion coefficient and mobile fraction of sIg remained as in untreated cells (5.3 X 10(-10) cm2/sec and 0.65, respectively). At higher surface occupancy, these values decreased to 8 X 10(-11) cm2/sec and 0.11. The magnitude of the effect was independent of the percentage occupancy above the threshold and of the distance from the bound Con A-platelets, indicating a cooperative and propagated phenomenon. Treatment with colchicine or cytochalasin B separately induced only partial reversal of the Con A-induced modulation. Treatment with both reversal of the Con A-induced modulation. Treatment with both drugs together was synergistic and fully reversed the mobility inhibition. The modulation was unaffected by NaN3 and 2-deoxyglucose, suggesting no dependence on metabolic energy. Con A-platelets did not affect the mobility of a lipid probe. Models for the Con A-induced modulation and the relationship between the effects of Con A on sIg mobility and patch formation are discussed.

Immunohistological localization of ligandin in human tissues

An immunohistochemical localization of ligandin was undertaken in formalin fixed and paraffin wax embedded human tissues using the indirect immunoperoxidase (PAP) method and a monospecific antiligandin serum raised in rabbits. A substance reacting with this antiligandin serum was distributed diffusely in normal liver and selectively in kidney, intestine, testis, ovary and adrenal cortex. Small changes in the distribution and intensity of the reaction product were found in inflammatory conditions such as hepatitis, cholestasis, pyelonephritis and renal allograft rejection. Tissues which normally appear to contain abundant ligandin produce, as a general rule, easily demonstrate amounts of antiligandin reacting substance in the tumors and hyperplasias which arise from them.

Effect of modulators of cytoskeletal function on desensitization and recovery of PGE2-responsive ovarian adenylate cyclase

Exposure of cultured Graafian follicles to PGE2 for 20 h resulted in a loss of the cyclic AMP response to fresh hormone. This desensitization was prevented by addition to the medium of D2O (25--50%) or Li+ (0.6--6 mM), agents believed to stabilize microtubules, as well as by phalloidin (1.0--10 microM), believed to stabilize the polymerized state of actin, in a dose-dependent manner. The spontaneous recovery of responsiveness to PGE2 upon incubation of refractory follicles for 6 h in hormone-free medium was prevented by addition to the medium of cytochalasin B (CB; 3 microgram/ml) or of the actin-binding myosin subfragment HMM S-1 (80 microgram/ml) or of anti-actin serum; viz. by agents likely to interfere with microfilament function. D2O (50%) caused morphological damage to the inner layer of the membrana granulosa and severe depression of protein synthesis. The other drugs used (phalloidin, LiCl and cytochalasin B) had no such effects. Resensitization of refractory follicles was also prevented by cycloheximide (10 micrograms/ml) and by actinomycin D (10 micrograms/ml). It is speculated that the recovery process may involve the insertion of a newly synthesized protein, such as PG-receptor, into the membrane by a mechanism dependent on microfilament action. These findings provide suggestive evidence for the hypothesis that cytoskeletal elements associated with the cell membrane take part in the modulation of the adenylate cyclase response to hormones.

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.

Restriction of patching of bound concanavalin A after incorporation of arachidonic acid into the plasma membrane of virally transformed fibroblasts

Topographical distribution of concanavalin A binding sites (CABS) was studied in two lines of virally transformed fibroblasts as a function of fatty acid composition. Fatty acid composition was manipulated by incubating cells in fatty acid, ATP, CoA, and delipidated fetal calf serum (FCS). VLM cells grown in medium containing 5% FCS have a clustered CABS distribution. Plasma membrane vesicles (PMVs) derived from these cells have an arachidonate content of 1.7%. Elevation of PMV arachidonate to 15.8% results in a marked restriction of CABS patching, while elevation to 6.8% is associated with intermediate restriction of patching. Restriction of patching is associated with increased microviscosity. CABS of Rous sarcoma virus-transformed chicken embryo fibroblasts (RSV-CEF) are also responsive to arachidonate enrichment medium. Whereas untreated cells have a clustered CABS distribution, cells incubated for 24 h in arachidonate enrichment medium have predominantly a dispersed CABS distribution. In both VLM cells and RSV-CEF, ATP, CoA, and delipidated FCS alone have no effect upon CABS mobility. Inhibition of CABS patching is also observed when aspirin is included in the arachidonate enrichment medium but not when the cells are incubated in prostaglandins, thus suggesting that the restriction of CABS mobility is not mediated by prostaglandins. Other fatty acids (palmitate, oleate, nonadecanoate) failed to restrict CABS movement. The inhibition of CABS mobility is independent of cell shape change.

Localization of LH receptors on luteal cells with a ferritin--LH conjugate

In order to study the distribution of LH (HCG) receptors on luteal cells ferritin was coupled to ovine LH with glutaraldehyde and purified by gel chromatography. The conjugate (FELH) competed with 125I-hCG for binding to isolated luteal membranes and stimulated a dose-dependent release of progesterone (P) from isolated luteal cells which was inhibited by PGF2 alpha. FELH was distributed as single molecules or in small clusters at intervals on the surfaces of luteal cells labeled at 37 degrees C, 4 degrees C or with formaldehyde prefixation. Capping or preferential labeling at one site was not observed. The general distribution of LH (hCG) binding sites at 37 degrees C was confirmed by light-microscopic autoradiography. The distribution at 4 degrees C or with prefixation was more diffuse than at 37 degrees C suggesting that FELH binding induces small changes in receptor aggregation. Binding of FELH was specific since excess hCG reduced FELH binding to luteal cells. In cells labeled at 4 degrees C, rinsed and warmed to 37 degrees C FELH was observed along cell surfaces and within some coated vesicles and a few lysosomes within minutes suggesting that receptor internalization is a rapid and possibly continual process.