Sequestration of tetanus toxin in developing neuronal cell cultures

The Molecular Basis of Toxins' Interactions with Intracellular Signaling via Discrete Portals

An understanding of the molecular mechanisms by which microbial, plant or animal-secreted toxins exert their action provides the most important element for assessment of human health risks and opens new insights into therapies addressing a plethora of pathologies, ranging from neurological disorders to cancer, using toxinomimetic agents. Recently, molecular and cellular biology dissecting tools have provided a wealth of information on the action of these diverse toxins, yet, an integrated framework to explain their selective toxicity is still lacking. In this review, specific examples of different toxins are emphasized to illustrate the fundamental mechanisms of toxicity at different biochemical, molecular and cellular- levels with particular consideration for the nervous system. The target of primary action has been highlighted and operationally classified into 13 sub-categories. Selected examples of toxins were assigned to each target category, denominated as portal, and the modulation of the different portal’s signaling was featured. The first portal encompasses the plasma membrane lipid domains, which give rise to pores when challenged for example with pardaxin, a fish toxin, or is subject to degradation when enzymes of lipid metabolism such as phospholipases A₂ (PLA₂) or phospholipase C (PLC) act upon it. Several major portals consist of ion channels, pumps, transporters and ligand gated ionotropic receptors which many toxins act on, disturbing the intracellular ion homeostasis. Another group of portals consists of G-protein-coupled and tyrosine kinase receptors that, upon interaction with discrete toxins, alter second messengers towards pathological levels. Lastly, subcellular organelles such as mitochondria, nucleus, protein- and RNA-synthesis machineries, cytoskeletal networks and exocytic vesicles are also portals targeted and deregulated by other diverse group of toxins. A fundamental concept can be drawn from these seemingly different toxins with respect to the site of action and the secondary messengers and signaling cascades they trigger in the host. While the interaction with the initial portal is largely determined by the chemical nature of the toxin, once inside the cell, several ubiquitous second messengers and protein kinases/ phosphatases pathways are impaired, to attain toxicity. Therefore, toxins represent one of the most promising natural molecules for developing novel therapeutics that selectively target the major cellular portals involved in human physiology and diseases.

Bilirubin-neural cell interaction: characterization of initial cell surface binding leading to toxicity in the neuroblastoma cell line N-115

The toxicity of bilirubin to the nervous system might be due to its effect on several key enzyme reactions occurring in the intracellular compartment as suggested by in vitro studies. The question of how bilirubin, a molecule with poor solubility in water and organic solvents, interacts with the plasma membrane and reaches intracellular targets is unclear. In an attempt to get some insight into this problem, we have measured the uptake of bilirubin from bilirubin-albumin solutions by the murine neuroblastoma cell line N-115. At a constant total concentration of bilirubin, the initial rate, as well as the extent of uptake, increases with increasing bilirubin to albumin molar ratio (B/A). The binding is reversible, at least partially, as indicated by the ability of albumin to extract cell-bound bilirubin. The cellular uptake of bilirubin was found to depend also on the concentration of bilirubin, on temperature and on pH. The results are not consistent with either a carrier-mediated transport or passive diffusion across the plasma membrane. The data, however, seem to fit a multistep binding of bilirubin to the plasma membrane proposed for the interaction of bilirubin with synaptosomal plasma membrane vesicles, erythrocyte ghosts and lipid vesicles. These studies, thus, reveal the complexity of the binding interaction at the level of the plasma membrane and leave open the question of transport across the membrane.

Preparation of affinity-purified, biotinylated tetanus toxin, and characterization and localization of cell surface binding sites on nerve growth factor-treated PC12 cells

Biotinylated derivatives of tetanus toxin were prepared and isolated by chromatofocusing and ganglioside-affinity chromatography. Biotinylation was monitored by the appearance of a 210,00 dalton complex upon SDS-polyacrylamide gel electrophoresis in the presence of avidin, and by selective binding to an avidin-Sepharose gel. At molar biotin:toxin ratios from 1:1 to 20:1 only biotinylated derivatives with low toxicity were obtained; these derivatives, however, retained 60-80% of their specific binding affinity for brain synaptosomes. A biotinylated tetanus toxin derivative purified by ganglioside-affinity chromatography was used to identify and localize tetanus toxin binding sites on PC12 cells. Electron microscopic analysis with streptavidin-gold revealed very low levels of tetanus toxin binding sites on the surface of untreated cells, and the appearance of such binding sites during the second week of nerve growth factor-induced differentiation. Examination of micrographs of the differentiated cells indicated that the tetanus toxin binding sites sites are concentrated on the neurites, with relatively few appearing on the cell bodies. Cognate studies using 125I-labeled, affinity-purified tetanus toxin revealed an increase in PC12 binding capacity from about 0.07 nmol/mg protein in untreated cells to 0.8 nmoles/mg protein in cells treated for 14 days with nerve growth factor. Cells treated in suspension for 2-3 weeks with nerve growth factor do not express tetanus toxin binding sites; upon plating, these cells required one week for the appearance of binding sites, although neurites grew much more rapidly from these "primed" cells. The high binding capacity of these tetanus toxin sites, as well as their sensitivity to neuraminidase, is indicative of a polysialoganglioside structure. The advantages of biotinylated tetanus toxin derivatives are discussed and the significance of nerve growth factor-differentiated PC12 cells grown as monolayers as a model for the study of the development, localization, and function of neuraminidase-sensitive tetanus toxin binding sites is presented.

Characterization of the channel properties of tetanus toxin in planar lipid bilayers

A detailed characterization of the properties of the channel formed by tetanus toxin in planar lipid bilayers is presented. Channel formation proceeds at neutral pH. However, an acidic pH is required to detect the presence of channels in the membrane rapidly and effectively. Acid pH markedly lowers the single-channel conductance, for phosphatidylserine at 0.5 M KCl gamma = 89 pS at pH 7.0 while at pH 4.8, gamma = 30 pS. The toxin channel is cation selective without significant selectivity between potassium and sodium (gamma [K+]/gamma [Na+] greater than or equal to 1.35). In all the lipids studied gamma is larger at positive than at negative voltages. The toxin channel is voltage dependent both at neutral and acidic pH: for phosphatidylserine membranes, the probability of the channel being open is much greater at positive than at negative voltage. In different phospholipids the channel exhibits different voltage dependence. In phosphatidylserine membranes the channel is inactivated at negative voltages, whereas in diphytanoylphosphatidylcholine membranes channels are more active at negative voltages than at positive. The presence of acidic phospholipids in the bilayers increases both the single-channel conductance as well as the probability of the channel being open at positive voltage. A subconductance state is readily identifiable in the single-channel recordings. Accordingly, single-channel conductance histograms are best fitted with a sum of 3 Gaussian distributions corresponding to the closed state, the open subconductance state and the full open state. Channel activity occurs in bursts of openings separated by long closings. Probability density analysis of the open dwell times of the toxin channel indicate the existence of a single open state with a lifetime greater than or equal to 1 ms in all lipids studied. Analysis of intra-bursts closing lifetimes reveals the existence of two components; the slow component is of the order of 1 ms, the fast one is less than or equal to 0.5 ms. The channel activity induced by tetanus toxin in lipid bilayers suggests a mechanism for its neurotoxicity: a voltage dependent, cation selective channel inserted in the postsynaptic membrane would lead to continuous depolarization and, therefore, persistent activation of the postsynaptic cell.

Tetanus toxin interaction with human erythrocytes. II. Kinetic properties of toxin association and evidence for a ganglioside-toxin macromolecular complex formation

The properties of tetanus toxin interaction with human erythrocytes supplemented with disialo- and trisialo-gangliosides have been investigated. Binding of toxin is linear with time for 1 h and is 3-4-fold higher at 37 degrees C than at 4 degrees C during incubation of long duration. It exhibits saturation at toxin concentrations between 0.1 and 1 microgram/ml; however, it is nonsaturable between 1 and up to 50 micrograms/ml. It is effectively prevented by free gangliosides and antibodies or by pretreatment with sialidase but is unaffected by a number of closely related ligands including toxoid and toxin fragments. NaCl (1 M) removes a great portion (86%) of cell-associated toxin while Triton X-100 extracts an additional fraction (30%) of the salt-resistant cell-bound toxin. The residual sequestred toxin after detergent extraction is sensitive to proteolytic degradation. The trypsin-stable fraction (1.5%) is biotoxic and may be indicative of internalization of toxin. A macromolecular complex of about 700 kDa containing toxin and gangliosides has been isolated and characterized by Sephacryl S-300 gel permeation chromatography, SDS-gel electrophoresis, immunoprecipitability and biotoxicity. This complex is obtained only in ganglioside-supplemented cells and not when free 3H-labeled GD1b is reacted with 125I-labeled toxin in solution in the absence of cells. The hydrophobicity properties acquired as a result of ganglioside-toxin interaction, presumably at the cell surface, suggest a conformational change of the toxin which may enable its penetration into the bilayer.

Tetanus toxin interaction with human erythrocytes. I. Properties of polysialoganglioside association with the cell surface

Human erythrocytes in suspension acquire gangliosides containing di- and trisialosyl residues added to the maintenance medium. This is reflected in the increased cell-associated sialic acid content and ability to bind 125I-labeled tetanus toxin. A salt-sensitive and a salt-insensitive ganglioside-mediated toxin-cell surface association is detected which is reduced after sialidase treatment of ganglioside-supplemented cells. The salt-insensitive ganglioside-cell association is saturable after 2 h incubation in 0.3 M mannitol buffer and has an optimum at pH 5. The association process is higher at 37 degrees C than at 4 degrees C, depends on cell density, and is considerably higher in metabolically active cells compared to lysed cells. Pretreatment of cells with trypsin decreases the salt-resistant toxin association with ganglioside-supplemented cells. In contrast, glutaraldehyde-fixed cells treated with trypsin and supplemented with gangliosides bind more toxin which is insensitive to salt. Ganglioside-mediated tetanus toxin binding to the intact erythrocyte membrane can be utilized as a model system for studying the role of glycolipids in membrane function.

Binding of tetanus toxin to somatic neural hybrid cells with varying ganglioside composition

125I-labelled tetanus toxin interaction with several somatic hybrid cell lines was investigated. Binding of toxin is most effective in NCB-20, followed by NBr-10A, NG108-C15, and SB21-B1 cells. Specific binding of toxin to NCB-20 and SB21-B1 cells is 7- and 60-fold lower, respectively, in comparison to enriched rat cerebral neuron cultures. The NCB-20, NBr-10A, and NG108-C15 clones display a complex ganglioside pattern, including the presence of [N-acetyl-neuraminyl]-galactosyl-N-acetylgalactosaminyl[ N-acetylneuraminyl]-galactosylglucosyl-ceramide (GD1a) and two unidentified [14C]galactose-labelled lipid-soluble compounds, while the SB21-B1 is most abundant in [N-acetyl-neuraminyl]-galactosylglucosyl-ceramide (GM3) and N-acetyl-galactosaminyl-[N-acetyl-neuraminyl]-galactosylglucosyl-c eramide (GM2) gangliosides. None of the cells tested contain measurable levels of [14C]galactose-labelled or resorcinol-positive bands of galactosyl-N-acetyl-galactosaminyl-[ N-acetylneuraminyl-N-acetylneuraminyl]-galactosylglucosyl-ceramide (GD1b) and [N-acetylneuraminyl]-galactosyl-N-acetylgalactosaminyl-[ N-acetylneuraminyl-N-acetylneuraminyl]-galactosylglucosyl-ceramide (GT1b) gangliosides. After 2 h at 37 degrees C a near plateau of toxin association with NCB-20 cells is seen. Binding in low-ionic-strength medium is 1.35-fold higher at 37 degrees C than at 4 degrees C, but is reduced by 21 and 51% at 4 degrees C and 37 degrees C, respectively, in physiologic medium. Treatment of NCB-20 cells with neuraminidase causes a partial loss (29%) of toxin-binding sites. Binding to the hybrid cells is significantly different from that of cerebral cultures with respect to temperature, salt effect, and sensitivity to neuraminidase, suggesting perhaps a different class of receptors for the toxin.(ABSTRACT TRUNCATED AT 250 WORDS)

Affinity chromatographic purification and characterization of two iodinated tetanus toxin fractions exhibiting different binding properties

Highly purified iodinated tetanus toxin preparations separate on ganglioside affinity columns into two distinct (A and B) fractions representing about 20% and 75% of the iodinated toxin, respectively. Fraction A, eluted by 1% NaCl, migrates like native tetanus toxin (150,000 mol. wt) on SDS polyacrylamide gel electrophoresis. It forms an aggregate of molecular weight approximately 360,000 on Sephacryl S-300 gel permeation chromatography in the presence of detergent and contains two isoforms on preparative chromatofocusing. Fraction A binds poorly to neurons in tissue culture or to synaptosomal membrane preparations. It retains, however, its antigenicity and biotoxicity. Fraction B, eluted by 6% NaCl, binds effectively to gangliosides and also to neurons or synaptosome preparations. It has a similar molecular weight and chain composition to the native toxin and displays two isoforms, precipitable during chromatofocusing. Fraction B possesses similar binding, immunological and toxic properties to the original iodinated tetanus toxin. Following excessive iodination (4-6 mCi/mg protein), toxicity can be remarkably reduced. Unlabeled toxin shows a similar chromatographic pattern to the iodinated toxin on affinity columns, suggesting that a large portion (30% by protein and 55% by toxicity) of the toxin has a poor affinity for gangliosides. The molecular pharmacokinetics of tetanus toxin with respect to affinity toward ganglioside-dependent and ganglioside-independent receptors needs re-evaluation.

Gangliosides as modulators of the coupling of neurotransmitters to adenylate cyclase

Cultured NCB-20 mouse neuroblastoma X Chinese hamster brain clonal hybrid cells express an adenylate cyclase-coupled receptor for serotonin (5HT) which corresponds pharmacologically to the 5HT1 receptor in whole brain, except for its much lower affinity for serotonin. Studies showed that the affinity of the NCB-20 receptor could be increased to near that of the whole brain receptor and the potency of 5HT in elevating cyclic AMP levels increased by pre-incubating NCB-20 cells for at least 3 hours with submicromolar concentrations of brain gangliosides. Tetrasialoganglioside (GQ1b) was found to be the most potent ganglioside tested, producing a ten-fold increase in affinity. However, the actual 5HT binding site is a protein and we have obtained no evidence that serotonin binds directly to gangliosides at the concentrations at which it labels the receptor. The receptor-mediated inhibition of adenylate cyclase by biogenic amines such as dopamine and clonidine through dopamine (D2) and alpha-adrenoreceptors was unaffected by pre-incubation of the NCB-20 cells with gangliosides. Enkephalin was also found to acutely supress both the ability of 5HT to stimulate adenylate cyclase activity and the synthesis of polysialogangliosides in NCB-20 cells. After 6 hours of exposure, the cells became tolerant to enkephalin and after 36 hours the cells became supersensitive to 5HT in terms of adenylate cyclase activation and 5HT binding. The affinity of the receptor for 5HT increased the same 10-fold magnitude as achieved by GQ1b pre-incubation in comparison with untreated cells. This increase in receptor affinity appeared to coincide chronologically with the increase in ganglioside synthesis observed in enkephalin tolerant cells, further suggesting an important role of polysialogangliosides in the function of the serotonin (5HT1) receptor.

Cells with neuronal properties in permanent cultures of quail embryo neuroretinas infected with Rous sarcoma virus

Neuroretina cells from 7-day quail embryos infected 'in vitro' with the mutant ts NY-68 of Rous sarcoma virus, were established into permanent cultures. An initial stage of cellular proliferation was followed by a period of minimal multiplication. After recovery from this crisis, cell proliferation resumed. About 30% of the cells had binding sites for tetanus toxin and the monoclonal antibody A2B5 which seem to be specific for neurons, and an ultrastructural study suggested the presence of neurons and Müller (astroglial) cells. The specific activity of glutamic acid decarboxylase, the enzyme responsible for the synthesis of the neurotransmitter gamma-aminobutyric acid was high (10-30 nmol CO2/h/mg of protein) and electrophysiology showed that some cells had 'active' membranes. After about 18 months in culture, approximately 20% of the cells were able to respond to electrical stimulation by producing action potentials which were inhibited by 10(-7) M tetrodotoxin. These electrophysiological properties are stable: they have been repeatedly found at regular intervals throughout a 20 months period. Furthermore, a clone in which all tested cells are excitable, has been derived from the mass culture. Quail embryo neuroretina cells with typical neuronal properties can thus be established into permanent cultures after infection with Rous sarcoma virus.

Temperature-mediated interaction of tetanus toxin with cerebral neuron cultures: characterization of a neuraminidase-insensitive toxin-receptor complex

Energy-dependent internalization of 125I-labeled tetanus toxin into cultured neural cells is shown to follow an energy-independent binding process. A three-step model, involving receptor-mediated binding followed by sequestration and internalization is proposed. In the first step, binding of toxin is enhanced in appearance under low ionic strength medium, at 0-4 degrees C; it is suppressed, however, with increasing incubation temperature under physiological salt concentrations. Cell-bound toxin is displaced by approximately 35.5% when high-salt medium (physiological concentrations) is added to cells at 0-4 degrees C; the effect is further amplified at 37 degrees C. Addition of disialoganglioside GD1b (1-5 micrograms/ml) also lowers the amount of cell-associated toxin. The fraction of 125I-labeled toxin retained by the cells after exposure to high-salt medium at 0-4 degrees C or after addition of GD1b is operationally defined as sequestered toxin. This second step, characterized by a stable association of the toxin with the neural cells, is affected by both physiological salt and by 37 degrees C conditions. Lastly, an energy-dependent phenomenon of firm association of tetanus toxin with neural cells, compatible with internalization, is described. The toxin residing in this fraction is bioactive and cannot be removed by salts, gangliosides, or by treatment with protease or neuraminidase. Binding, sequestration, and internalization are mutually dependent, as they are all blocked by pretreatment of cells with neuraminidase and by an enhanced energy-independent sequestration event, which results in enhanced tetanus toxin internalization by an energy-dependent process.

A neuronal clone derived from a Rous sarcoma virus-transformed quail embryo neuroretina established culture

Neuroretina (NR) is an evagination of the central nervous system (CNS) which is composed of photoreceptors, glial (Müller) cells and horizontal, bipolar, amacrine and ganglion neuronal cells. We describe here the usefulness of Rous sarcoma virus (RSV) in the establishment of a neuronal clone from quail embryo neuroretina. When primary cultures of chick and quail embryo neuroretina cells are transformed by RSV, neuronal markers such as ribbon synapses, choline acetyltransferase (CAT) and glutamic acid decarboxylase (GAD) specific activity are present. These RSV-transformed primary cultures can be established into permanent cell lines from which neuronal clones have been isolated. One of them, clone QNR/D, can generate tetrodotoxin(TTX)-inhibitable action potentials on electrical stimulation, has a high GAD activity and binds monoclonal antibodies raised against chick embryo neuroretina. The presence of these neuronal markers suggests that the QNR/D clone is derived from cells of the amacrine or ganglionic lineage. This is the first time that a neuronal cell clone of defined origin has been obtained from the CNS. The neuronal markers of the QNR/D clone are expressed at both the permissive and the non-permissive temperatures for transformation.

Cell surface and cytoskeletal antigens in cerebral cell cultures after chloroform-methanol delipidation

Fixation and in situ total delipidation of nerve cells in monolayer cultures have been used for immunofluorescence visualization of cellular antigens. Cells grown on poly-L-lysine-coated coverslips are exposed to a chloroform/methanol (2/1 by vol) solvent mixture for 15 min at -65 degrees C. This step results in the fixation of cells and is accompanied by the complete extraction of cellular lipids. About 3.2% of the total 35S-methionine-labeled trichloroacetic acid-insoluble cellular material is extracted into the organic solvent mixture, demonstrating that aqueous soluble proteins are not significantly extracted under these conditions. A total loss of tetanus toxin binding sites, presumably reflecting the removal of polysialogangliosides from the cell surface, is observed. The accessibility of antibodies against tubulin and against intermediate filaments after this treatment has been investigated. Visualization of these cytoskeletal elements by indirect immunofluorescence and by phase microscopy suggests that the removal of cellular lipids does not affect the apparent cytoarchitecture and that extraction with these organic solvents does not interfere with the staining of tubulin and intermediate filaments, the latter being exclusively present in nonneuronal cells. The advantage of this method of cell fixation and lipid extraction for immunofluorescence of cells in monolayer culture is discussed.