The response of cultured mammalian cells to diphtheria toxin. I. Amino acid transport, accumulation, and incorporation in normal and intoxicated sensitive cells

Challenges of Diphtheria Toxin Detection

Diphtheria toxin (DT) is the main virulence factor of Corynebacterium diphtheriae, C. ulcerans and C. pseudotuberculosis. Moreover, new Corynebacterium species with the potential to produce diphtheria toxin have also been described. Therefore, the detection of the toxin is the most important test in the microbiological diagnosis of diphtheria and other corynebacteria infections. Since the first demonstration in 1888 that DT is a major virulence factor of C. diphtheriae, responsible for the systemic manifestation of the disease, various methods for DT detection have been developed, but the diagnostic usefulness of most of them has not been confirmed on a sufficiently large group of samples. Despite substantial progress in the science and diagnostics of infectious diseases, the Elek test is still the basic recommended diagnostic test for DT detection. The challenge here is the poor availability of an antitoxin and declining experience even in reference laboratories due to the low prevalence of diphtheria in developed countries. However, recent and very promising assays have been developed with the potential for use as rapid point-of-care testing (POCT), such as ICS and LFIA for toxin detection, LAMP for tox gene detection, and biosensors for both.

Diphtheria pathogenesis in Guinea pig tracheal organ culture

The effect of diphtheria toxin on guinea pig trachea in organ culture was examined to measure the susceptibility of respiratory epithelial cells to toxin action. Exposure of individual tracheal rings to toxin resulted in cessation of protein synthesis as well as the development of cytopathology within a few hours. Continued incubation led to further inhibition of protein synthesis and extensive disorganization of the epithelial layer. Other inhibitors of protein synthesis were monitored for their effect on the structural integrity of tracheal cells but were found incapable of eliciting similar histopathology. Early after its addition, toxin at minute concentrations possessed cytotoxic properties as well as the ability to inhibit protein synthesis. Interpretation of these data is correlated with current information on the structure and activity of diphtheria toxin.

Interactions of diphtheria toxin with lipid vesicles: determinants of ion channel formation

Lipid vesicles have been utilized to study the interactions of diphtheria toxin (DT) with membranes. The assay for DT ion channel formation was fluorescence-detected membrane potential depolarization of vesicles in which valinomycin-induced potassium diffusion gradients had been generated. The following requirements for ion channel formation have been identified: (1) acid pH (less than 5); (2) trans-negative membrane potentials (35-fold increase in channel-forming activity from -6 mV to -59 mV); and (3) negatively charged phospholipid headgroups (about 100-fold more activity using vesicles formed from asolectin compared to soybean phosphatidylcholine). Concentration dependence plots of toxin activity showed a linear response with logarithmic slopes of nearly one for each lipid composition. These results show a close parallel to those obtained previously with planar lipid bilayers and thus provide guidelines for conditions which facilitate functional insertion of the toxin into vesicles.

Diphtheria toxin: mode of action and structure

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Response of cultured mammalian cells to diphtheria toxin. V. Concurrent resistance to ribonucleic acid viruses in diphtheria toxin-resistant KB cell strains

It was determined that KB-R cell strains, isolated from the KB cell line and resistant to diphtheria toxin, also resist infection by poliovirus, Mengo virus, vesicular stomatitis virus, and Newcastle disease virus. This resistance manifests itself by reduction in yields of progeny virus (a reduction of more than 2 logs in some cases), reduced production of viral-specific ribonucleic acid (RNA), and delayed cytopathic effect. In three KB-R strains tested, resistance was related to a step which falls between adsorption of virus and uncoating or release of viral messenger RNA. In two of these three strains, a second resistance mechanism was also active, causing a reduced production of viral-specific RNA. A relationship between the resistance to diphtheria toxin and the resistance to viral infection of the KB-R strains is considered. It has been postulated that the native diphtheria toxin molecule must be "activated" at the surface of a susceptible cell by a proteolytic process before it can enter the cell and inhibit protein synthesis. It is also known that the eclipse of some viruses occurs at or near the cell membrane and involves proteolytic activity. Resistance to viruses and toxin in the KB-R strains may result from the loss or modification of related labilizing or activating principles associated with the surface receptors for these agents.

Response of cultured mammalian cells to diphtheria toxin. IV. Isolation of KB cells resistant to diphtheria toxin

KB cell strains which are resistant to the lethal effects of diphtheria toxin were isolated from the toxin-sensitive KB cell line. For one such strain, KB-R2, the concentration of diphtheria toxin required to cause a 50% reduction of incorporation of amino acids into protein was more than 2,000 times that required in the sensitive KB-S cell. The KB-R2 strain was cloned by a single-cell isolation method to yield uniform strains with an even greater resistance to toxin. Resistance is a stable, inheritable property, providing a good genetic marker, and appears to be related to some alteration in the cell membrane. Preliminary studies showed that these cell strains are resistant to poliovirus and vesicular stomatitis virus as well as to toxin.

Response of interferon-treated cells to diphtheria toxin

It has been reported that treatment with homologous interferon of cells in culture interferes with the process of diphtheria intoxication in these cells. This report both confirms and extends this observation. The most sensitive indicator of diphtheria toxin action in cells, i.e., the inhibition of incorporation of labeled amino acids into protein, was employed, and human, mouse, and chicken interferon-cell systems were studied. Interferon treatment afforded a distinct, reproducible protection from the action of toxin, but in no case could total protection be shown. The "antitoxic" component is stable at pH 2, is species-specific, is not sedimented at 100,000 x g, and is trypsin-sensitive. Human cell preparations are heat-labile. Experiments with cell-free amino acid incorporation systems suggest that the "anti-toxic" effect of interferon is concentrated at the level of the cell membrane and subcellular components are not directly involved.

Action of diphtheria toxin in the guinea pig

The blood clearance and distribution in the tissues of (125)I after intravenous injection of small doses (1.5-5 MLD or 0.08-0.25 microg) of (125)I-labeled diphtheria toxin has been followed in guinea pigs and rabbits and compared with the fate of equivalent amounts of injected (125)I-labeled toxoid and bovine serum albumin. Toxoid disappeared most rapidly from the blood stream and label accumulated and was retained in liver, spleen, and especially in kidney. Both toxin and BSA behaved differently. Label was found widely distributed among all the organs except the nervous system and its rate of disappearance from the tissues paralleled its disappearance from the circulation. There was no evidence for any particular affinity of toxin for muscle tissue or for a "target" organ. Previous reports by others that toxin causes specific and selective impairment of protein synthesis in muscle tissue were not confirmed. On the contrary, both in guinea pigs and rabbits, a reduced rate of protein synthesis was observed in all tissues that had taken up the toxin label. In tissues removed from intoxicated animals of both species there was an associated reduction in aminoacyl transferase 2 content. It is concluded that the primary action of diphtheria toxin in the living animal is to effect the inactivation of aminoacyl transferase 2. The resulting inhibition in rate of protein synthesis leads to morphologic damage in all tissues reached by the toxin and ultimately to death of the animal.

Radioautographic studies of choline incorporation into peripheral nerve myelin

This radioautographic study was designed to localize the cytological sites involved in the incorporation of a lipid precursor into the myelin and the myelin-related cell of the peripheral nervous system. Both myelinating and fully myelinated cultures of rat dorsal root ganglia were exposed to a 30-min pulse of tritiated choline and either fixed immediately or allowed 6 or 48 hr of chase incubation before fixation. After Epon embedding, light and electron microscopic radioautograms were prepared with Ilford L-4 emulsion. Analysis of the pattern of choline incorporation into myelinating cultures indicated that radioactivity appeared all along the length of the internode, without there being a preferential site of initial incorporation. Light microscopic radioautograms of cultures at varying states of maturity were compared in order to determine the relative degree of myelin labeling. This analysis indicated that the myelin-Schwann cell unit in the fully myelinated cultures incorporated choline as actively as did this unit in the myelinating cultures. Because of technical difficulties, it was not possible to determine the precise localization of the incorporated radioactivity within the compact myelin. These data are related to recent biochemical studies indicating that the mature myelin of the central nervous system does incorporate a significant amount of lipid precursor under the appropriate experimental conditions. These observations support the concept that a significant amount of myelin-related metabolic activity occurs in mature tissue; this activity is considered part of an essential and continuous process of myelin maintenance and repair.

Studies on the mode of action of diphtheria toxin. VII. Toxin-stimulated hydrolysis of nicotinamide adenine dinucleotide in mammalian cell extracts

When diphtheria toxin and NAD are added to soluble fractions containing aminoacyl transfer enzymes isolated from rabbit reticulocytes or from HeLa cells, free nicotinamide is released and, simultaneously, an inactive ADP ribose derivative of transferase II is formed. The reaction is reversible, and in the presence of excess nicotinamide, toxin catalyzes the restoration of aminoacyl transfer activity in intoxicated preparations. In living cultures of HeLa cells, the internal NAD concentration is sufficiently high to account for the rapid conversion, catalyzed by a few toxin molecules located in the cell membrane, of the entire cell content of free transferase II to its inactive ADP ribose derivative. Completely inactive ammonium sulfate fractions containing soluble proteins isolated from cells that have been exposed for several hours to excess toxin, can be reactivated to full aminoacyl transfer activity by addition of nicotinamide together with diphtheria toxin. Transferase II appears to be a highly specific substrate for the toxin-stimulated splitting of NAD and thus far no other protein acceptor for the ADP ribose moiety has been found.

Response of cultured mammalian cells to diphtheria toxin. 3. Inhibition of protein synthesis studied at the subcellular level

Diphtheria toxin inhibited protein synthesis in intact KB cells. The action of the toxin upon the cell did not result in disaggregation of polyribosomes, or in impairment of their ability to function in protein synthesis. A reduction in single ribosomes and a concomitant increase in polyribosomes did result from the action of toxin. Nascent peptides were not cleaved from polyribosomes by the action of toxin, but treatment of fully intoxicated cells with puromycin resulted in cleavage of these peptides, and caused accelerated polyribosome breakdown. Our data indicated that the toxin must enter the cell to exert its effect. The component or components sensitive to toxin were localized in the 100,000 x g supernatant fraction of cytoplasmic extracts. When extracts from intoxicated cells were treated with nicotinamide, a significant proportion of their capacity to synthesize protein was restored. The specificity of this reaction suggested that nicotinamide adenine dinucleotide is involved in the action of toxin in the intact cell, and that one component inactivated by toxin is soluble transferase II.

Studies on the mode of action of diphtheria toxin. VI. Site of the action of toxin in living cells

Using the technique of radioautography, it has been shown that a probable maximum of only 25-50 molecules iodine-125-labeled toxin per cell is bound by human HeLa cells treated with approximately 10(7) molecules of toxin per cell, or just under one saturating dose. Radioautographs of sections from labeled cells locate most if not all of the toxin molecules fixed to the outer cell membrane. Under identical conditions far less label is taken up by mouse L cells. It is probable that the resistance of this species to diphtheria toxin can be accounted for in terms of the failure of mouse cells to bind the toxin protein. The irreversible inhibition of protein synthesis in a living cell culture by a few molecules of toxin located at the cell surface is discussed in relation to the known interaction between toxin, NAD, and transferase II in mammalian cell extracts.

The response of cultured mammalian cells to diphtheria toxin. II. The resistant cell: enhancement of toxin action by poly-L-ornithine

We have investigated the response of the resistant mouse L cell to diphtheria toxin. Intact cells and cell-free systems were studied. It was determined that the cell-free system is as sensitive to toxin as those from sensitive reticulocyte, HeLa, and KB cells previously studied. Poly-L-ornithine, reported to stimulate macromolecular uptake, enhanced toxin activity under conditions which also stimulated acid phosphatase activity. Resistance, in the L cell, appears to be linked to the cell membrane and the process of macromolecular uptake, and not to any intrinsic property of either the polyribosomes or associated protein synthesizing factors. Because of the unique way in which diphtheria toxin inhibits protein synthesis, the ability of poly-L-ornthine to enhance this action provides convincing evidence that poly-L-ornithine stimulates a true absorption of macromolecules and not just increased surface adsorption.