Binding and uptake of diphtheria toxin by toxin-resistant Chinese hamster ovary and mouse cells

Adjuvant effect of diphtheria toxin after mucosal administration in both wild type and diphtheria toxin receptor engineered mouse strains

The finding that murine and simian cells have differential susceptibility to diphtheria toxin (DTx) led to the development of genetically engineered mouse strains that express the simian or human diphtheria toxin receptor (DTR) under the control of various mouse gene promoters. Injection of DTx into DTR engineered mice allows for rapid and transient depletion of various cell populations. There are several advantages to this approach over global knockout mice, including normal mouse development and temporal control over when cell depletion occurs. As a result, many DTR engineered mouse strains have been developed, resulting in significant insights into the cell biology of various disease states. We used Foxp3(DTR) mice to attempt local depletion of Foxp3+ cells in the lung in a model of tolerance breakdown. Intratracheal administration of DTx resulted in robust depletion of lung Foxp3+ cells. However, DTx administration was accompanied by significant local inflammation, even in control C57Bl/6 mice. These data suggest that DTx administration to non-transgenic mice is not always an immunologically inert event, and proper controls must be used to assess various DTx-mediated depletion regimens.

Mycoplasma pneumoniae CARDS toxin is internalized via clathrin-mediated endocytosis

Bacterial toxins possess specific mechanisms of binding and uptake by mammalian cells. Mycoplasma pneumoniae CARDS (Community Acquired Respiratory Distress Syndrome) toxin is a 68 kDa protein, which demonstrates high binding affinity to human surfactant protein-A and exhibits specific biological activities including mono-ADP ribosylation and vacuolization. These properties lead to inflammatory processes in the airway and a range of cytopathologies including ciliostasis, loss of tissue integrity and injury, and cell death. However, the process by which CARDS toxin enters target cells is unknown. In this study, we show that CARDS toxin binds to mammalian cell surfaces and is internalized rapidly in a dose and time-dependent manner using a clathrin-mediated pathway, as indicated by inhibition of toxin internalization by monodansylcadaverine but not by methyl-β-cyclodextrin or filipin. Furthermore, the internalization of CARDS toxin was markedly inhibited in clathrin-depleted cells.

Impairment of podocyte function by diphtheria toxin--a new reversible proteinuria model in mice

Diphtheria toxin (DTx) receptor (DTR)-mediated conditional cell ablation in transgenic mice is a powerful tool to analyze cell function in vivo. Transgenic mice with cell-specific expression of the human DTR have been developed that allow conditional depletion of these cells in vivo through administration of the toxin. We have performed a careful analysis of mice after DTx injection and found an unexpected side effect. Treatment of wild-type C57BL/6 mice with DTx leads to a marked transient and completely reversible proteinuria, as a consequence of podocyte dysfunction that is morphologically characterized by foot process fusion and detachment from the glomerular basal membrane. In vitro analysis displayed that DTx-treated podocytes show diminished attachment to basal membrane proteins. Five to 9 days after DTx application the mice recover completely. Glomerular proteinuria is a hallmark of glomerular disease due to dysfunction of the filtration barrier. Rodents have been extensively used experimentally to better define the mechanisms of disease induction and progression. However, nongenetic mouse models of proteinuric glomerular damage are limited and display various shortcomings. We suggest DTx-induced transient kidney dysfunction as a new reversible model of experimental podocyte injury, which could be used as an additional approach to complement studies in human.

Isolation of a fibroblast mutant resistant to Clostridium difficile toxins A and B

A mutant of Chinese hamster lung fibroblasts (Don cells), resistant against Clostridium difficile toxins A and B, was isolated after mutagenization with ethylmethanesulphonate and a two-step selection with toxin B. The mutant, termed CdtR-Q, was 10(4) times more resistant to toxin B than wild-type cells and cross-resistant to toxin A (10(3) times more resistant). The resistance was overcome by increasing the dose of toxin. The resistance has been stable after cultivation for 40 generations in the absence of toxin. The morphology of the mutant was more epithelial-like than that of the fibroblast parental cells. The plating efficiency was about half that of the wild-type, whereas the growth rate was the same. The mutant was significantly less sensitive than the wild-type to the microfilament-interacting cytochalasins B and D. It was as sensitive as the wild-type to endocytosed toxins (diphtheria, pertussis, ricin), to microtubule-interacting agents (colchicine, gossypol, nocodazole, taxol, vinblastine), and to membrane-damaging toxins with different mechanisms of action, with one exception; the mutant was more highly sensitive to the action of phospholipase C (with broad substrate-specificity) than the wild-type. The results suggest that the mutant has a normal endocytosis, and that the mutation does not affect the microtubuli. The results are consistent with a mutation affecting the microfilaments in the cytoskeleton.

Kinetics of alpha 2-macroglobulin endocytosis and degradation in mutant and wild-type Chinese hamster ovary cells

The production of Chinese hamster ovary (CHO) cell mutants which are defective in endocytosis has led to a greater understanding of the process by which cells sort ligands and their receptors. Robbins and coworkers have obtained CHO mutants which are resistant to diphtheria toxin, defective in the delivery of endocytosed lysosomal enzymes to lysosomes, and have a decreased uptake of iron from transferrin (Robbins et al.: J. Cell Biol. 96:1064-1071, 1983). We have previously shown that these CHO mutants are markedly deficient in the acidification of early endocytic compartments (Yamashiro and Maxfield: J. Cell Biol. 105:2713-2721, 1987). In this study we examined the endocytosis of alpha 2-macroglobulin (alpha 2M) to determine whether the defects in early endosome acidification would alter the processing of this ligand. We found that the CHO mutants DTG 1-5-4 and DTF 1-5-1 bind, internalize, and degrade 125I-alpha 2M in a manner similar to the wild-type cells. We also found that the CHO mutants retain the ability to recycle the receptors for alpha 2M. Since the binding of alpha 2M is greatly reduced at mildly acidic pH (approximately 6.8), only slight acidification of the endosomal compartment should be sufficient to achieve sorting of alpha 2M from its receptor. In contrast, lysosomal enzymes require more acidic conditions (pH less than 6.0) for dissociation. The different behavior of the two ligands provides biochemical evidence for a partial (but not complete) defect in early endosome acidification in the mutants. The data also indicate that pH regulation in a relatively narrow range can achieve differential sorting of various ligands.

Three new complementation groups of temperature-sensitive Chinese hamster ovary cell mutants defective in the endocytic pathway

We describe here the results of complementation studies with six mutant Chinese hamster ovary cells expressing temperature-sensitive lesions affecting the endocytic pathway. The mutants were crossed with representatives of the End1 and End2 complementation groups identified previously by Robbins et al. (J. Cell Biol. 99:1296-1308, 1984). Two mutants, G.8.1 and 31.1, were members of the End1 complementation group. One mutant, 25.2, was a member of the End2 complementation group. The other three mutants each defined new complementation groups, which we have designated End3 (mutant G.7.1), End4 (mutant V.24.1), and End5 (mutant 42.2). Previous work on mutants of the End1, End2, and End3 classes had shown that these mutants were defective in endosomal acidification. We prepared postnuclear supernatants from mutants harvested at the nonpermissive temperature and compared their acidification activities, assessed by ATP-stimulated quenching of acridine orange. Members of the End1, End2, and End2 groups had reduced acidification activity, correlating with the acidification defects known to be expressed by these mutants. Strain V.24.1 (End4) also expressed a 40% reduction in acidification activity, while strain 42.2 (End5) had no reduction of acidification activity.

Binding of diphtheria toxin to CHO-K1 and Vero cells is dependent on cell density

We studied the binding of 125I-labeled diphtheria toxin (DTX) to receptors on monolayer cultures of Chinese hamster ovary cells (CHO-K1) and Vero cells. The number of DTX receptors detected on the cell surface was shown to be dependent on the cell density (number of cells per unit area). Cells at low density (less than 23,000 cells per cm2 for CHO-K1 cells; less than 80,000 cells per cm2 for Vero cells) had more receptors for DTX than cells at higher densities. The difference in receptor number between low- and high-density cells was 33-fold for CHO-K1 cells and 19-fold for Vero cells. We estimated the maximum number of DTX receptors on low-density CHO-K1 and Vero cells to be 50,000 and 370,000 per cell, respectively. The cell density at which the binding of DTX was reduced to 50% of maximum was considerably lower for CHO-K1 cells than for Vero cells (33,000 vs. 220,000 cells per cm2, respectively). Vero cells grown on a surface that had been conditioned by high-density cells bound less DTX, suggesting that interaction of these cells with the underlying extracellular matrix might regulate the number of cell surface receptors for DTX. Low-density cells were more sensitive to DTX than high-density cells, suggesting that low-density cells possessed an increased number of functional receptors that actively transported DTX to the cytosol. CHO-K1 and Vero cells were equally protected by SITS (4-Acetamido-4'-Isothiocyano-Stilbene-2,2'-disulfonic Acid), a compound that has been shown to inhibit the binding and entry of DTX in Vero cells, suggesting that intoxication of CHO-K1 and Vero cells is mediated by a similar mechanism. The data illustrate the importance of taking into account the cell density when measuring the number of DTX receptors on adherent cells.

Cellular internalisation of Clostridium difficile toxin A

The cytopathogenic effect of toxin A from Clostridium difficile was studied in cultured human lung fibroblasts. The final effect was dependent on toxin concentration and exposure time. Binding of the toxin to cells occurred at 0 degrees C as well as at 37 degrees C. The latency before appearance of the cytopathogenic effect was dose-dependent with a minimum of 45 min. The appearance of a cytopathogenic effect in toxin-treated cells was prevented by the addition of trypsin, antitoxin, lysosomotropic agents, inhibitors of the energy metabolism, 200 mM KCl, 20 mM benzyl alcohol and by incubation at 18 degrees C. Several inhibitors of lysosomal proteases did not prevent the appearance of the cytopathogenic effect. When the extracellular pH was lowered to 4.5 for 5 min immediately after toxin binding the period of latency was significantly shortened. Likewise, the protective effects of lysosomotropic agents were abolished by lowering the extracellular pH. Chinese hamster ovary cell mutants, defective in acidification of their endosomes, were less sensitive to toxin A than wildtype cells. The results indicate that cellular internalisation of toxin A is necessary for intoxication. Moreover, we postulate that the toxin needs some sort of enzymatic activation which can take place only after exposure of the toxin to a low pH.

Lysosomal involvement in cellular intoxication with Clostridium difficile toxin B

The process of internalisation of Clostridium difficile toxin B into human lung fibroblasts was further studied, with the aim of elucidating the fate of endocytosed toxin. Development of the toxin-induced cytopathogenic effect was reversibly inhibited at 18 degrees C and in the presence of 200 mM KCl or 1-20 mM benzyl alcohol, i.e. at conditions when the fusion between endosomes and lysosomes is prevented. Fibroblasts treated with toxin at 37 degrees C but transferred to 18 degrees C within 10 min were also completely protected, whereas transfer to 18 degrees C later during the latency resulted in only partial protection. KCl was also protective upon addition after the toxin binding step. Inhibitors of lysosomal proteases, such as chymostatin, leupeptin and antipain, prevented the appearance of the cytopathogenic effect, when present during toxin exposure or added after the toxin binding step. Chinese hamster ovary cell mutants, defective in acidification of their endosomes, were resistant to toxin B, whereas wildtype cells were sensitive. The resistance was not overcome by applying a low extracellular pH. The results suggest that exposure to a low pH compartment is necessary but not sufficient for entry of active toxin B to the cytosol. In addition to a low pH, a fusion of toxin-containing endosomes with lysosomes and a further processing of the toxin by lysosomal proteases is required for cellular intoxication.

Reduced temperature alters Pseudomonas exotoxin A entry into the mouse LM cell

The movement of Pseudomonas exotoxin A (PE) into the cytoplasm of mouse LM fibroblasts was followed by using inhibition of protein synthesis as a biochemical index of toxin activity; biotinyl-PE and avidin-gold colloids were used for electron microscopy. At 37 degrees C both specific antitoxin and pronase-trypsin protected cells against PE toxicity when added within seconds of warming cells, whereas methylamine was protective when added during the first 7 min of endocytosis. Lowering the temperature to 19 degrees C afforded protection when the temperature transition was accomplished within 15 min of the original endocytic event. These data suggest that PE enters an acidic compartment before reaching a step blocked by shifting cells from 37 to 19 degrees C. PE expressed toxicity for LM cells at 19 degrees C, but at a concentration 1 order of magnitude higher than that required at 37 degrees C. At 19 degrees C, antitoxin or trypsin-pronase protection was rapidly ablated. In contrast cells were fully protected by methylamine for 90 min. Using electron microscopy we demonstrated that toxin moved normally (30 s) to coated areas at 19 degrees C, but remained at this site for up to 20 min before being internalized. The majority of the toxin internalized at 19 degrees C remained in endosomes or in Golgi-associated vesicles and was not delivered to lysosomes. The results suggest that, under physiological conditions (37 degrees C), PE rapidly enters cells through coated areas, moves to an acidic compartment (i.e., the endosome), and then probably to the Golgi region en route to lysosomes. The evidence suggests that movement of toxin from endosomes or Golgi vesicles to lysosomes is blocked at 19 degrees C. We hypothesize that the active form of PE enters the cytosol, where it expresses its toxicity during fusion of Golgi-derived, toxin-laden vesicles with lysosomes.

Characterization of diphtheria-toxin-resistant mutants lacking receptor function or containing nonribosylatable elongation factor 2

Stable mutants resistant to diphtheria toxin (DT) were isolated from Chinese hamster ovary cells (CHO-K1) by single-step mutations with various mutagens. All the mutants were classified into two major groups as reported by other workers (4-6): toxin-entry mutants (DTrI) and translational mutants (DTRII) at the level of elongation factor 2 (EF-2). These mutants were further characterized by directly measuring the specific uptake of [125I]DT and the content of nonribosylatable EF-2 by two-dimensional gel analysis. DTrI mutants, which showed no cross-resistance to Pseudomonas exotoxin A (PA), had no ability to associate with [125I]DT and contained only ADP-ribosylatable EF-2, like wild-type cells. DTRIIb mutants maintained about 50% of the normal level of cellular protein synthesis in the presence of DT, and two-dimensional gel analysis directly showed that they contained equivalent amounts of ADP-ribosylatable and nonribosylatable EF-2 molecules. Fully toxin-resistant cells, named KEE1 (DTRIIa), were isolated from a DTRIIb mutant (KE1) by two-step mutation. KEE1 cells showed full resistance to DT and PA, the normal level of association with [125I]DT, and produced only nonribosylatable EF-2. Biochemical analysis of somatic cell hybrids indicated that the DT-resistant character of class II behaved codominantly. These results strongly supported the hypothesis that two copies of the gene for EF-2 are functional in CHO-K1 cells.

Entry of diphtheria toxin linked to concanavalin A into primate and murine cells

Diphtheria toxin linked by a disulfide bridge to concanavalin A was highly toxic to HeLa S3 and Vero cells, as well as to murine L cells. The cells could be protected with alpha-methyl mannoside, indicating that the conjugate binds mainly through its concanavalin A moiety. Treatment of Vero cells with phospholipase C, TPA (12-O-tetradecanoylphorbol-13-acetate), and vanadate, which strongly reduce the ability of the cells to bind free diphtheria toxin, had little protective effect against the conjugate, whereas SITS (L-acetamido-4'-isothiocyano-stilbene-2,2'disulfonic acid), which inhibits diphtheria toxin binding, as well as the subsequent entry, protected Vero cells, but not L cells. Both types of cells are protected against the conjugate by NH4Cl and monensin, indicating that an acidified compartment is necessary for entry into the cytosol. Exposure of cells, bound with surface conjugate, to low pH induced entry of the toxin into Vero cells, but not into L Cells. Phospholipase C, TPA, and vanadate did not protect L cells against the conjugate. It is concluded that toxin in the conjugate enters L cells by a route which involves low pH, but which is not identical to that in Vero cells.

Diphtheria toxin resistance in human lymphocytes and lymphoblasts in the in vivo somatic cell mutation test

It has been shown that circulating peripheral blood lymphocytes can be used for the enumeration of 6-thioguanine-resistant cells that presumably arise by mutation in vivo. This somatic cell mutation test has been studied in lymphocytes from human populations exposed to known mutagens and/or carcinogens. The sensitivity of the test could be further enhanced by including other gene markers, since there is evidence for locus-specific differences in response to mutagens. Resistance to diphtheria toxin (Dipr) seemed like a potential marker to incorporate into the test because the mutation acts codominantly, can readily be selected in human diploid fibroblasts and Chinese hamster cells with no evidence for cell density or cross-feeding effects, and can be assayed for in nondividing cells by measuring protein synthesis inhibition. Blood samples were collected from seven individuals, and fresh, cryopreserved, or Epstein-Barr virus (EBV)-transformed lymphocytes were tested for continued DNA synthesis (3H-thymidine, autoradiography) or protein synthesis (35S-methionine, scintillation counting). Both fresh and cryopreserved lymphocytes, stimulated to divide with phytohemagglutinin (PHA), continued to synthesize DNA in the presence of high doses of diphtheria toxin (DT). Similarly, both dividing (PHA-stimulated) and nondividing fresh lymphocytes carried on significant levels of protein synthesis even 68 hr after exposure to 100 flocculating units (LF)/ml DT. The findings were confirmed in cord blood lymphocytes, ruling out the possibility that diphtheria immunization could have led to a selection of Dipr lymphocytes. One lymphoblast line (EBV-transformed lymphocytes) showed a reduction in protein synthesis to 0.2% of controls only at 192 hr after exposure to 100 LF/ml. The results suggest that human T and B lymphocytes may not be as sensitive to DT protein synthesis inhibition as human fibroblast and Chinese hamster cells. For this reason, Dipr may not be a suitable marker for the somatic cell mutation test.

Different pH requirements for entry of the two picornaviruses, human rhinovirus 2 and murine encephalomyocarditis virus

The entry into cells of human rhinovirus 2 (HRV 2) and murine encephalomyocarditis (EMC) virus was studied by the use of light-sensitive virus grown in the presence of acridine orange (HRV 2) and neutral red (EMC). HeLa cells were protected against infection with HRV 2 by NH4Cl, monensin, and other compounds known to increase the pH of intracellular vesicles. Preincubation of the cells with the same compounds reduced the ability of the cells to bind [35S]methionine-labeled HRV 2, apparently due to inhibition of recycling of endocytosed receptors back to the cell surface. The cells were also protected against infection when HRV 2 was bound to cells on ice and the cells were then incubated at 37 degrees with the different compounds. This indicates that low pH is also necessary for some event in the entry process taking place after the virus is bound to the cells. In contrast, compounds which increase the pH in acidic intracellular compartments did not protect mouse L-cells against infection with EMC-virus, and the entry of the virus was inhibited by low pH in the medium. This inhibition was partly overcome by the presence of the ionophore monensin, which elevates the pH in endosomes and lysosomes. Possibly, EMC virus enters the cytosol from vesicles with neutral or slightly alkaline pH.

Increased surface binding sites of insulin in ML236B (compactin)-resistant mutants of Chinese hamster cell line

Mutants resistant to ML236B (compactin) were isolated from the Chinese hamster lung V79 cell line (1). Three ML236B-resistant mutants, MF-1, MF-2 and MF-3, were enhanced in insulin-specific binding activity about 2 to 3 times over the parental V79 cell lines. Compared to V79, endocytosis of insulin was also increased 2 to 3-fold in ML236B-resistant mutants than V79. Scatchard analysis showed that 5,000 insulin binding sites per cell in V79 and 16,000 in a NL236B-resistant clone, MF-2. Insulin receptors in mutant and parental strains are down-regulated to a similar extent in the parental V79 treated with an excess insulin. This is the first somatic cell mutant with increased surface binding sites for insulin.