Response of cultured mammalian cells to the exotoxins of Pseudomonas aeruginosa and Corynebacterium diphtheriae: differential cytotoxicity

Revisiting the virulence hallmarks of Pseudomonas aeruginosa: a chronicle through the perspective of quorum sensing

Pseudomonas aeruginosa is an opportunistic pathogen and the leading cause of mortality among immunocompromised patients in clinical setups. The hallmarks of virulence in P. aeruginosa encompass six biologically competent attributes that cumulatively drive disease progression in a multistep manner. These multifaceted hallmarks lay the principal foundation for rationalizing the complexities of pseudomonal infections. They include factors for host colonization and bacterial motility, biofilm formation, production of destructive enzymes, toxic secondary metabolites, iron-chelating siderophores and toxins. This arsenal of virulence hallmarks is fostered and stringently regulated by the bacterial signalling system called quorum sensing (QS). The central regulatory functions of QS in controlling the timely expression of these virulence hallmarks for adaptation and survival drive the disease outcome. This review describes the intricate mechanisms of QS in P. aeruginosa and its role in shaping bacterial responses, boosting bacterial fitness. We summarize the virulence hallmarks of P. aeruginosa, relating them with the QS circuitry in clinical infections. We also examine the role of QS in the development of drug resistance and propose a novel antivirulence therapy to combat P. aeruginosa infections. This can prove to be a next-generation therapy that may eventually become refractory to the use of conventional antimicrobial treatments.

Circuit and Synaptic Plasticity Mechanisms of Drug Relapse

High rates of relapse to drug use during abstinence is a defining feature of human drug addiction. This clinical scenario has been studied at the preclinical level using different animal models in which relapse to drug seeking is assessed after cessation of operant drug self-administration in rodents and monkeys. In our Society for Neuroscience (SFN) session entitled "Circuit and Synaptic Plasticity Mechanisms of Drug Relapse," we will discuss new developments of our understanding of circuits and synaptic plasticity mechanisms of drug relapse from studies combining established and novel animal models with state-of-the-art cellular, electrophysiology, anatomical, chemogenetic, and optogenetic methods. We will also discuss the translational implications of these new developments. In the mini-review that introduces our SFN session, we summarize results from our laboratories on behavioral, cellular, and circuit mechanisms of drug relapse within the context of our session.

Regulation of Alcohol Extinction and Cue-Induced Reinstatement by Specific Projections among Medial Prefrontal Cortex, Nucleus Accumbens, and Basolateral Amygdala

The ability to inhibit drinking is a significant challenge for recovering alcoholics, especially in the presence of alcohol-associated cues. Previous studies have demonstrated that the regulation of cue-guided alcohol seeking is mediated by the basolateral amygdala (BLA), nucleus accumbens (NAc), and medial prefrontal cortex (mPFC). However, given the high interconnectivity between these structures, it is unclear how mPFC projections to each subcortical structure, as well as projections between BLA and NAc, mediate alcohol-seeking behaviors. Here, we evaluate how cortico-striatal, cortico-amygdalar, and amygdalo-striatal projections control extinction and relapse in a rat model of alcohol seeking. Specifically, we used a combinatorial viral technique to express diphtheria toxin receptors in specific neuron populations based on their projection targets. We then used this strategy to create directionally selective ablations of three distinct pathways after acquisition of ethanol self-administration but before extinction and reinstatement. We demonstrate that ablation of mPFC neurons projecting to NAc, but not BLA, blocks cue-induced reinstatement of alcohol seeking and neither pathway is necessary for extinction of responding. Further, we show that ablating BLA neurons that project to NAc disrupts extinction of alcohol approach behaviors and attenuates reinstatement. Together, these data provide evidence that the mPFC→NAc pathway is necessary for cue-induced reinstatement of alcohol seeking, expand our understanding of how the BLA→NAc pathway regulates alcohol behavior, and introduce a new methodology for the manipulation of target-specific neural projections.SIGNIFICANCE STATEMENT The vast majority of recovering alcoholics will relapse at least once and understanding how the brain regulates relapse will be key to developing more effective behavior and pharmacological therapies for alcoholism. Given the high interconnectivity of cortical, striatal, and limbic structures that regulate alcohol intake, it has been difficult to disentangle how separate projections between them may control different aspects of these complex behaviors. Here, we demonstrate a new approach for noninvasively ablating each of these pathways and testing their necessity for both extinction and relapse. We show that inputs to the nucleus accumbens from medial prefrontal cortex and amygdala regulate alcohol-seeking behaviors differentially, adding to our understanding of the neural control of alcoholism.

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.

Immature doublecortin-positive hippocampal neurons are important for learning but not for remembering

It is now widely accepted that hippocampal neurogenesis underpins critical cognitive functions, such as learning and memory. To assess the behavioral importance of adult-born neurons, we developed a novel knock-in mouse model that allowed us to specifically and reversibly ablate hippocampal neurons at an immature stage. In these mice, the diphtheria toxin receptor (DTR) is expressed under control of the doublecortin (DCX) promoter, which allows for specific ablation of immature DCX-expressing neurons after administration of diphtheria toxin while leaving the neural precursor pool intact. Using a spatially challenging behavioral test (a modified version of the active place avoidance test), we present direct evidence that immature DCX-expressing neurons are required for successful acquisition of spatial learning, as well as reversal learning, but are not necessary for the retrieval of stored long-term memories. Importantly, the observed learning deficits were rescued as newly generated immature neurons repopulated the granule cell layer upon termination of the toxin treatment. Repeat (or cyclic) depletion of immature neurons reinstated behavioral deficits if the mice were challenged with a novel task. Together, these findings highlight the potential of stimulating neurogenesis as a means to enhance learning.

Diphtheria in Europe: current problems and new challenges

Diphtheria, caused by toxigenic strains of Corynebacterium diphtheriae, is an ancient disease with high incidence and mortality that has always been characterized by epidemic waves of occurrence. Whilst towards the beginning of the 1980s, many European countries were progressing towards the elimination of diphtheria, an epidemic re-emergence of diphtheria in the Russian Federation and the Newly Independent States of the former Soviet Union demonstrated a continuous threat of the disease into the 1990s. At present, the epidemic is under control and only sporadic cases are observed in Europe. However, the circulation of toxigenic strains is still observed in all parts of the world, posing a constant threat to the population with low levels of seroprotection. More recently, Corynebacterium ulcerans has been increasingly isolated as emerging zoonotic agent of diphtheria from companion animals such as cats or dogs, indicating the enduring threat of this thought-to-be controlled disease.

Posttraining ablation of adult-generated neurons degrades previously acquired memories

New neurons are continuously generated in the subgranular zone of the adult hippocampus and, once sufficiently mature, are thought to integrate into hippocampal memory circuits. However, whether they play an essential role in subsequent memory expression is not known. Previous studies have shown that suppression of adult neurogenesis often (but not always) impairs subsequent hippocampus-dependent learning (i.e., produces anterograde effects). A major challenge for these studies is that these new neurons represent only a small subpopulation of all dentate granule cells, and so there is large potential for either partial or complete compensation by granule cells generated earlier on during development. A potentially more powerful approach to investigate this question would be to ablate adult-generated neurons after they have already become part of a memory trace (i.e., retrograde effects). Here we developed a diphtheria toxin-based strategy in mice that allowed us to selectively ablate a population of predominantly mature, adult-generated neurons either before or after learning, without affecting ongoing neurogenesis. Removal of these neurons before learning did not prevent the formation of new contextual fear or water maze memories. In contrast, removal of an equivalent population after learning degraded existing contextual fear and water maze memories, without affecting nonhippocampal memory. Ablation of these adult-generated neurons even 1 month after learning produced equivalent memory degradation in the water maze. These retrograde effects suggest that adult-generated neurons form a critical and enduring component of hippocampal memory traces.

Continuing the search for the engram: examining the mechanism of fear memories

The goal of my research is to gain insight using rodent models into the fundamental molecular, cellular and systems that make up the base of memory formation. My work focuses on fear memories. Aberrant fear and/or anxiety may be at the heart of many psychiatric disorders. In this article, I review the results of my research group; these results show that particular neurons in the lateral amygdala, a brain region important for fear, are specifically involved in particular fear memories. We started by showing that the transcription factor CREB (cAMP/Ca(2+) response element binding protein) plays a key role in the formation of fear memories. Next, we used viral vectors to overexpress CREB in a subset of lateral amygdala neurons. This not only facilitated fear memory formation but also "drove" the memory into the neurons with relatively increased CREB function. Finally, we showed that selective ablation of the neurons overexpressing CREB in the lateral amygdala selectively erased the fear memory. These findings are the first to show disruption of a specific memory by disrupting select neurons within a distributed network.

Nitrite reductase NirS is required for type III secretion system expression and virulence in the human monocyte cell line THP-1 by Pseudomonas aeruginosa

The nitrate dissimilation pathway is important for anaerobic growth in Pseudomonas aeruginosa. In addition, this pathway contributes to P. aeruginosa virulence by using the nematode Caenorhabditis elegans as a model host, as well as biofilm formation and motility. We used a set of nitrate dissimilation pathway mutants to evaluate the virulence of P. aeruginosa PA14 in a model of P. aeruginosa-phagocyte interaction by using the human monocytic cell line THP-1. Both membrane nitrate reductase and nitrite reductase enzyme complexes were important for cytotoxicity during the interaction of P. aeruginosa PA14 with THP-1 cells. Furthermore, deletion mutations in genes encoding membrane nitrate reductase (Delta narGH) and nitrite reductase (Delta nirS) produced defects in the expression of type III secretion system (T3SS) components, extracellular protease, and elastase. Interestingly, exotoxin A expression was unaffected in these mutants. Addition of exogenous nitric oxide (NO)-generating compounds to Delta nirS mutant cultures restored the production of T3SS phospholipase ExoU, whereas nitrite addition had no effect. These data suggest that NO generated via nitrite reductase NirS contributes to the regulation of expression of selected virulence factors in P. aeruginosa PA14.

A transgenic mouse model of inducible macrophage depletion: effects of diphtheria toxin-driven lysozyme M-specific cell lineage ablation on wound inflammatory, angiogenic, and contractive processes

Whether the wound macrophage is a key regulatory inflammatory cell type in skin repair has been a matter of debate. A transgenic mouse model mediating inducible macrophage depletion during skin repair has not been used to date to address this question. Here, we specifically rendered the monocyte/macrophage leukocyte lineage sensitive to diphtheria toxin by expressing the lysozyme M promoter-driven, Cre-mediated excision of a transcriptional STOP cassette from the simian DT receptor gene in mice (lysM-Cre/DTR). Application of diphtheria toxin to lysM-Cre/DTR mice led to a rapid reduction in both skin tissue and wound macrophage numbers at sites of injury. Macrophage-depleted mice revealed a severely impaired wound morphology and delayed healing. In the absence of macrophages, wounds were re-populated by large numbers of neutrophils. Accordingly, macrophage-reduced wound tissues exhibited the increased and prolonged persistence of macrophage inflammatory protein-2, macrophage chemoattractant protein-1, interleukin-1beta, and cyclooxygenase-2, paralleled by unaltered levels of bioactive transforming growth factor-beta1. Altered expression patterns of vascular endothelial growth factor on macrophage reduction were associated with a disturbed neo-vascularization at the wound site. Impaired wounds revealed a loss of myofibroblast differentiation and wound contraction. Our data in the use of lysM-Cre/DTR mice emphasize the pivotal function of wound macrophages in the integration of inflammation and cellular movements at the wound site to enable efficient skin repair.

Cholix toxin, a novel ADP-ribosylating factor from Vibrio cholerae

The ADP-ribosyltransferases are a class of enzymes that display activity in a variety of bacterial pathogens responsible for causing diseases in plants and animals, including those affecting mankind, such as diphtheria, cholera, and whooping cough. We report the characterization of a novel toxin from Vibrio cholerae, which we call cholix toxin. The toxin is active against mammalian cells (IC(50) = 4.6 +/- 0.4 ng/ml) and crustaceans (Artemia nauplii LD(50) = 10 +/- 2 mug/ml). Here we show that this toxin is the third member of the diphthamide-specific class of ADP-ribose transferases and that it possesses specific ADP-ribose transferase activity against ribosomal eukaryotic elongation factor 2. We also describe the high resolution crystal structures of the multidomain toxin and its catalytic domain at 2.1- and 1.25-A resolution, respectively. The new structural data show that cholix toxin possesses the necessary molecular features required for infection of eukaryotes by receptor-mediated endocytosis, translocation to the host cytoplasm, and inhibition of protein synthesis by specific modification of elongation factor 2. The crystal structures also provide important insight into the structural basis for activation of toxin ADP-ribosyltransferase activity. These results indicate that cholix toxin may be an important virulence factor of Vibrio cholerae that likely plays a significant role in the survival of the organism in an aquatic environment.

Conditional ablation of mature olfactory sensory neurons mediated by diphtheria toxin receptor

The vertebrate olfactory epithelium provides an excellent model system to study the regulatory mechanisms of neurogenesis and neuronal differentiation due to its unique ability to generate new sensory neurons throughout life. The replacement of olfactory sensory neurons is stimulated when damage occurs in the olfactory epithelium. In this study, transgenic mice, with a transgene containing human diphtheria toxin receptor under the control of the olfactory marker protein promoter (OMP-DTR), were generated in which the mature olfactory sensory neurons could be specifically ablated when exposed to diphtheria toxin. Following diphtheria toxin induced neuronal ablation, we observed increased numbers of newly generated growth associated protein 43 (GAP43)-positive immature olfactory sensory neurons. OMP-positive neurons were continuously produced from the newly generated GAP43-positive cells. The expression of the signal transduction components adenylyl cyclase type III and the G-protein alpha subunit G(alpha olf) was sensitive to diphtheria toxin exposure and their levels decreased dramatically preceding the disappearance of the OMP-positive sensory neurons. These data validate the hypothesis that OMP-DTR mice can be used as a tool to ablate the mature olfactory sensory neurons in a controlled fashion and to study the regulatory mechanisms of the neuronal replacement.

Stimulation of CD8+ T cells following diphtheria toxin-mediated antigen delivery into dendritic cells

Recognition and clearance of many intracellular pathogens requires the activation and subsequent effector functions of CD8+ T lymphocytes. To stimulate CD8+ T cells by immunization, the target antigens must be delivered into the cytosol of host cells. There they can be processed into peptides and presented in the context of major histocompatibility complex class I molecules to antigen-specific CD8+ T cells. One method of delivering antigens into the cytosol is to fuse them to modified bacterial toxins that are able to enter mammalian cells. The expression pattern of the toxin receptors in the host will determine the cell population that the toxin fusion protein targets and will thus restrict antigen-specific T-cell recognition to the same population. In this study we describe the development and characterization of a diphtheria toxin (DT)-based antigen delivery system. Using CD11c-DTR transgenic mice that express the DT receptor in dendritic cells (DC), this system allows for targeted delivery of CD8+ T-cell antigen to DC. We show that antigen-specific CD8+ T cells proliferate in CD11c-DTR mice following immunization with catalytically inactive DT-antigen fusion proteins. We also show that a toxin-based system that restricts antigen delivery to DC results in more robust antigen-specific CD8+ T-cell proliferation than a toxin-based system that does not restrict delivery to a particular cell type. These results have implications for vaccine design, and they suggest that use of a toxin-based vector to target antigen to DC may be an effective way to induce a CD8+ T-cell response.

A Cre-inducible diphtheria toxin receptor mediates cell lineage ablation after toxin administration

A new system for lineage ablation is based on transgenic expression of a diphtheria toxin receptor (DTR) in mouse cells and application of diphtheria toxin (DT). To streamline this approach, we generated Cre-inducible DTR transgenic mice (iDTR) in which Cre-mediated excision of a STOP cassette renders cells sensitive to DT. We tested the iDTR strain by crossing to the T cell- and B cell-specific CD4-Cre and CD19-Cre strains, respectively, and observed efficient ablation of T and B cells after exposure to DT. In MOGi-Cre/iDTR double transgenic mice expressing Cre recombinase in oligodendrocytes, we observed myelin loss after intraperitoneal DT injections. Thus, DT crosses the blood-brain barrier and promotes cell ablation in the central nervous system. Notably, we show that the developing DT-specific antibody response is weak and not neutralizing, and thus does not impede the efficacy of DT. Our results validate the use of iDTR mice as a tool for cell ablation in vivo.

A biologically active lectin of enteroaggregative Escherichia coli

The pathogenesis of enteroaggregative Escherichia coli, a major contributor to paediatric diarrhoea, is still not clearly understood. A complex carbohydrate specific lectin was identified from the culture supernatant of an enteroaggregative E. coli strain. The lectin was purified to 660-fold by a combination of sequential saturated ammonium sulphate precipitation and gel filtration chromatography in the FPLC system. The homogeneity of the purified lectin was established by analytical isoelectrofocusing [pI 6.75]. Hemagglutination of rabbit erythrocytes by the purified lectin was best inhibited by fetuin. The N-terminal sequence of the 41.7 kDa subunit showed homology to the outermembrane porins and the 23.4 kDa subunit showed homology to a hypothetical protein of Yersinia pestis and secreted Hcp protein. This protein could induce extensive morphological changes in HEp-2 cells and significant amount of fluid accumulation in rabbit ileal loop. GM1 showed maximum binding to the lectin among all other gangliosides. This purified protein showed cross-reactivity to the binding subunit of cholera toxin in western immunoblot. The presence of this toxin in some of the clinical isolates of enteroaggregative E. coli was also observed. The structural and functional characteristics of the toxin revealed that it is a novel virulence determinant of aggregative E. coli.

Transgenic mice expressing the diphtheria toxin receptor are sensitive to the toxin

Whereas diphtheria and the mechanism of action of diphtheria toxin, the bacterial molecule that induces the disease, have been studied and understood for some time, the receptor that allows animal cells to bind the toxin escaped identification until recently. The receptor was identified by its ability to confer toxin-sensitivity to mouse cells, which are normally toxin-resistant. Although mice are also naturally resistant, we now demonstrate that transgenic mice expressing the diphtheria toxin receptor are as sensitive to the toxin as are humans and other toxin-sensitive animals. These transgenic mice provide a suitable model for studying modern antidotes for diphtheria.

Morphological and immunological evidence of a unique selective production and endoplasmic reticular accumulation of interleukin-1alpha in rat peritoneal macrophages induced by Pseudomonas aeruginosa exotoxin A

The immunotoxicity of Pseudomonas aeruginosa exotoxin A (ETA) on macrophages was evaluated by incubating rat peritoneal macrophages (RPM) with 1-100 ng/ml ETA for 3-60 h. Although the overall changes in cell viability and DNA, RNA, and protein synthesis of the ETA-treated RPM (E-RPM) were reduced in a dose- and time-dependent manner, there was a transient but evident rebound in RNA and/or protein synthesis at 24-36 h post-incubation (HPI) at 1-50 ng/ml ETA. However, a more apparent enhancement appeared in RNA and protein synthesis at 36-48 HPI in 10 and 50 ng/ml E-RPM after normalized on the basis of viable cell. Most 50-100 ng/ml E-RPM underwent necrosis/apoptosis before 24 HPI. By 36 HPI, 41% of 10 ng/ml E-RPM remained viable but were full of cytoplasmic granules due to the accumulation of glycoprotein in segmentally dilated endoplasmic reticulum. Immunological staining of the granules revealed strong IL-1alpha but weak or no signals for IL-1beta, IL-1 receptor antagonist, IL-6, and TNF-alpha. A time-dependent increase in IL-1alpha but no IL-1beta was detected in cell lysate of 10 ng/ml E-RPM; however, neither IL-1alpha nor IL-1beta was detected in culture supernatant. Thus, besides cytopathic and functional effects, ETA could induce a unique selective production and endoplasmic reticular accumulation of IL-1alpha in RPM.

Early mitochondrial dysfunction, superoxide anion production, and DNA degradation are associated with non-apoptotic death of human airway epithelial cells induced by Pseudomonas aeruginosa exotoxin A

It has been shown that bacterial exoproducts may induce airway epithelium injury. During the epithelial repair process, the respiratory epithelial cells no more establish tight junctional intercellular complexes and may be particularly susceptible to bacterial virulence factors. In this study, we analyzed the effect of Pseudomonas aeruginosa exotoxin A (ETA) at different periods of time and concentrations on 16 HBE 14o(-) human bronchial epithelial cells in culture conditions inducing a phenotype of repairing cells. ETA treatment for 24 and 48 h led to the killing of 40.0 +/- 5.7% and 79.0 +/- 1.4% of the cells, respectively, as determined by the dimethylthiazole 2,5 diphenyl tetrazolium bromide assay. At 1,000 ng/ml, ETA led to the killing of 25.2 +/- 6.6, 59.4 +/- 5.9, and 82.3 +/- 3.7% of the cells, after treatment periods of 7, 24, and 48 h, respectively. Cell death could not be inhibited by z-VAD-fmk, a broad spectrum caspase inhibitor. By transmission electron microscopy, ultrastructural characteristics described in apoptosis were not detected in ETA-treated cells. Instead, the mitochondria of cells treated for 24 and 48 h with ETA at 100 and 1,000 ng/ml were highly condensed. Human nasal polyp epithelial cells in primary culture exposed to ETA at 1,000 ng/ml did not exhibit characteristic features of apoptotic cells either. Cytofluorometric analysis of ETA-treated 16 HBE 14o(-) cells labeled with DiOC(6)(3) and hydroethidine showed a time- and dose-dependent reduction of the mitochondrial transmembrane potential, detected 7 h after ETA treatment, and an increase in superoxide production, detected at 24 h, respectively. By a photometric assay, DNA degradation was also detected 7 h after cell treatment with ETA at 100 and 1,000 ng/ml. Taken together, our results show that ETA-induced death of epithelial respiratory cells was preceded by early mitochondrial dysfunction and superoxide anion production, but was not followed by the classically described apoptotic pathways.

Receptor-based antidote for diphtheria

Although equine diphtheria antitoxin may be an effective therapy for human diphtheria, its use often induces serum sickness. We describe here a strategy for developing an alternative treatment based on the human diphtheria toxin (DT) receptor/heparin-binding epidermal growth factor-like growth factor (HB-EGF) precursor. Recombinant mature human HB-EGF acts as a soluble receptor analog, binding radioiodinated DT and preventing its binding to the cellular DT receptor/HB-EGF precursor. However, the possibility existed that radioiodinated DT-HB-EGF complexes associate with cells due to the binding of the heparin-binding domain of recombinant HB-EGF to cell surface heparan sulfate proteoglycans. This possibility was confirmed by performing DT binding studies in the presence of heparin. A recombinant truncated HB-EGF (residues 106 to 149), which lacks most of the heparin-binding domain, showed an essentially heparin-independent binding of radioiodinated DT to cells. Furthermore, it was a more effective inhibitor of DT binding than was recombinant mature HB-EGF. Since mature HB-EGF is a known ligand for the EGF receptor and is thus highly mitogenic (tumorigenic), we then changed amino acid residues in the EGF-like domain of the recombinant truncated HB-EGF and demonstrated that this DT receptor analog (I117A/L148A) displayed a low mitogenic effect. The truncated (I117A/L148A) HB-EGF protein retained high DT binding affinity, as confirmed by using surface plasmon resonance. Our results suggest that the truncated (I117A/L148A) HB-EGF protein could be an effective, safe antidote for human diphtheria.

Molecular characterization of key diphtheria toxin:receptor interactions

The major amino acids necessary for diphtheria toxin (DT) binding to its receptor have been identified previously. Studies by W. H. Shen et al. (J. Biol. Chem. 269, 29077-29084, 1994) and by J. H. Cha et al. (Mol. Microbiol. 29 (5), 1275-1284, 1998) suggested that the positively charged nature of the single amino acid residue, (516)Lys of DT, is crucial for binding to the DT receptor, whereas the negatively charged (141)Glu of the DT receptor is the most important residue for toxin binding. Here, we hypothesize that key interactions occur between these two oppositely charged amino acid residues. Reciprocal substitution of the residues at these positions between the toxin and the receptor was performed, which resulted in a partial reconstitution of the toxin:receptor interaction. This study provides the first biological data that characterizes the specific interaction of these two key residues with each other and also the additional interactions between other positively charged residues of DT and (141)Glu of the DT receptor.

Cell surface monkey CD9 antigen is a coreceptor that increases diphtheria toxin sensitivity and diphtheria toxin receptor affinity

Monkey (Mk) CD9 antigen has been shown previously to increase the diphtheria toxin (DT) sensitivity of cells when co-expressed with Mk proHB-EGF (DT receptor). We have elucidated here the mechanism whereby Mk CD9 influences Mk proHB-EGF and present evidence that Mk CD9 is a coreceptor for DT. We observed that Mk CD9 not only increased the DT sensitivity but also increased the DT receptor affinity of cells. Furthermore, the higher the Mk CD9/Mk proHB-EGF ratio, the higher the affinity. In contrast, mouse (Ms) CD9 did not increase the toxin sensitivity or receptor affinity of cells when co-expressed with Mk proHB-EGF. Using Mk/Ms chimeric CD9 molecules, we determined that the second extracellular domain of Mk CD9 is responsible for both increased sensitivity and receptor affinity. This domain of Mk CD9 also interacts with Mk proHB-EGF in a yeast two-hybrid system. Our findings thus suggest that Mk CD9 has a direct physical interaction with Mk proHB-EGF to form a DT receptor complex and that this contact may change the conformation of the receptor to increase DT binding affinity and consequently increase toxin sensitivity. We thus propose that Mk CD9 is a coreceptor for DT.

Biology and molecular epidemiology of diphtheria toxin and the tox gene

Diphtheria toxin (DT) is an extracellular protein of Corynebacterium diphtheriae that inhibits protein synthesis and kills susceptible cells. The gene that encodes DT (tox) is present in some corynephages, and DT is only produced by C. diphtheriae isolates that harbor tox+ phages. The diphtheria toxin repressor (DtxR) is a global regulatory protein that uses Fe2+ as co-repressor. Holo-DtxR represses production of DT, corynebacterial siderophore, heme oxygenase, and several other proteins. Diagnostic tests for toxinogenicity of C. diphtheriae are based either on immunoassays or on bioassays for DT. Molecular analysis of tox and dtxR genes in recent clinical isolates of C. diphtheriae revealed several tox alleles that encode identical DT proteins and multiple dtxR alleles that encode five variants of DtxR protein. Therefore, recent clinical isolates of C. diphtheriae produce a single antigenic type of DT, and diphtheria toxoid continues to be an effective vaccine for immunization against diphtheria.

Hamster diphtheria toxin receptor: a naturally occurring chimera of monkey and mouse HB-EGF precursors

The sensitivity of mammalian cell lines to diphtheria toxin (DT) varies between species. Monkey (Mk) Vero cells are highly sensitive to DT, whereas rat and mouse (Ms) cells are resistant; hamster (Hm) cells display moderate DT sensitivity. The precursor of the Mk heparin-binding epidermal growth factor-like growth factor (proHB-EGF) functions as a DT receptor but the Ms proHB-EGF does not. In this study we have cloned, expressed, and characterized the Hm proHB-EGF/DT receptor. The expression of Hm proHB-EGF confers moderate DT sensitivity to normally DT-resistant mouse cells. The amino acid sequence of Hm preproHB-EGF shows that, overall, it more closely resembles the Ms preproHB-EGF sequence, except in the DT-binding region where it more closely resembles the Mk sequence. In the DT-binding region the Hm proHB-EGF sequence differs from the Mk proHB-EGF in only four amino acid residues (124, 126, 133, and 147); one of these residues, Ile133 in Mk proHB-EGF, has been previously reported to be important for DT binding and sensitivity. Analysis of Mk proHB-EGF mutants with residues substituted for Ile133 suggests that Asn133 in Hm proHB-EGF may be responsible for the moderate DT sensitivity of Hm proHB-EGF-expressing cells.

Toxin binding site of the diphtheria toxin receptor: loss and gain of diphtheria toxin binding of monkey and mouse heparin-binding, epidermal growth factor-like growth factor precursors by reciprocal site-directed mutagenesis

The transmembrane precursor of the monkey (Mk) heparin-binding, epidermal growth factor-like growth factor (proHB-EGF) functions as a diphtheria toxin (DT) receptor, whereas the mouse (Ms) precursor does not. Previously, using chimeric Ms/Mk precursors, we have shown that DT resistance of cells bearing Ms proHB-EGF may be accounted for by several amino acid substitutions between residues 122 and 148 within the EGF-like domain and that Glu-141 is an important amino acid residue for DT binding. In this study, reciprocal site-directed mutagenesis was performed on the major non-conserved residues in the region of 122-148, alone or in combination, between Mk and Ms precursors to identify more precisely which amino acid residues are important for DT binding. Two approaches were used. The first, more traditional approach was to destroy DT sensitivity and binding of Mk proHB-EGF by substitution(s) with the corresponding Ms residue(s). From the single mutations, the greatest loss of DT sensitivity was observed with Mk/Glu-141His (approximately 4000-fold) and the next greatest with Mk/Ile-133Lys (approximately fourfold). The double mutations Mk/Leu-127Phe/Glu-141His, Mk/Ile-133Lys/Glu-141His and Mk/His-135Leu/Glu-141His resulted in complete toxin resistance (> 100000-fold). The second approach, both novel and complementary, was to gain DT binding and sensitivity of Ms proHB-EGF by substitution(s) with the corresponding Mk residue(s). Surprisingly, the single mutation Ms/His-141Glu resulted in the gain of moderate DT sensitivity (> 260-fold). The double mutation Ms/Lys-133Ile/His-141Glu and the triple mutation Ms/Lys-133Ile/Leu-135His/His-141Glu resulted in a progressive gain in toxin sensitivity (> 4700-fold and >16000-fold respectively) and affinity. This triple mutant cell line is essentially as sensitive (IC50 = 3.1 ng ml(-1)) as the highly toxin-sensitive monkey Vero cell line (IC50 = 4 ng ml(-1)), indicating that these three Mk residues enable the Ms proHB-EGF to act as a fully functional DT receptor. Taken together, these results indicate that Glu-141 plays the most critical role in DT binding and sensitivity and that two additional amino acid residues, Ile-133 and His-135, also play significant roles.

Diphtheria toxin:receptor interaction: association, dissociation, and effect of pH

Diphtheria toxin (DT) binds to a specific heparin-binding epidermal growth factor-like growth factor (HB-EGF) precursor that is expressed in DT-sensitive cells. DT binds to the cell-surface HB-EGF precursor with an apparent dissociation constant (KD) of approximately 1 x 10(-8) - 10(-9) M at 4 degreesC, a temperature at which toxin binds but is not internalized. The interaction of DT with the cell-surface receptor, however, may be influenced by other cell-surface components. We used a biosensor method to measure the binding of DT to immobilized recombinant human HB-EGF (hHB-EGF) at 25 degreesC with no other cellular components present. We observed that at pH 7.4, using this in vitro two component system, DT binds to hHB-EGF with an apparent KD of 2.7 x 10(-8) M. We also observed that the dissociation of DT from hHB-EGF at pH values that approach those of the endosome occurs at a faster rate as the pH is decreased. These results suggest that the low pH of the endosome is sufficient to allow DT to dissociate from the HB-EGF precursor, prior to the translocation of the enzymatically active fragment of DT into the cytosol.

Generation of neutralizing antipeptide antibodies to the enzymatic domain of Pseudomonas aeruginosa exotoxin A

Burn patients suffer a break in the physical barrier (skin), which, when combined with their generalized state of immunodeficiency, creates an open window for opportunistic infections, mainly with Pseudomonas aeruginosa. Infection of the burn wound has always been a major factor in retardation of wound healing, and sepsis remains the leading cause of death in burn patients. Because studies have shown that topical treatment with antiexotoxin A (ETA) antibodies significantly increases survival in rats infected with toxin-producing strains of P. aeruginosa, we examined 11 synthetic peptides encompassing 12 to 45 amino acid (aa) residues, representing what were predicted by computer analysis to be the most hydrophilic and antigenic regions of ETA. These synthetic peptides were injected into rabbits for antibody production. Different groups of rabbits were immunized with a combination of peptides, with each combination representing one of the three distinct domains of ETA. Animals immunized with various peptide combinations produced peptide-specific antibodies that exhibited cross-reactivity to ETA. Two major epitopes were identified on the ETA molecule by experiments with peptide-specific antibodies in enzyme-linked immunosorbent assay and immunoprecipitation. One of these epitopes was located in the translocation domain (II) (aa 297 to 310), while the other was mapped to the last 13 aa residues at the carboxy-terminal end of the enzymatic domain (III) (aa 626 to 638). Of these two regions, the epitope in the enzymatic domain induced a much higher level of neutralizing antibodies that abrogated the cytotoxic activity of ETA in vitro. Antibodies to this epitope blocked the ADP-ribosyltransferase activity of ETA and appeared to interfere with binding of the substrate elongation factor 2 to the enzymatic active site of the ETA molecule. We conclude that polyclonal, as well as monoclonal, antibodies to short peptides, representing small regions of ETA, may have therapeutic potential in passive immunization or topical treatment of burn patients infected with toxin-producing strains of P. aeruginosa.

Receptor-mediated targeted drug or toxin delivery

The new approach to the treatment of cancer or to immunomodulation is drug targeting. Cellular uptake of drugs bound to a targeting carrier or to a targetable polymeric carrier is mostly restricted to receptor-mediated endocytosis. Factors that influence the efficiency of receptor-mediated uptake of targeted drug conjugate are the affinity of the targeting moieties, the affinity and nature of the target antigen, density of the target antigen, the epitope of the target antigen, the type of cell target, the rate of endocytosis, the route of internalization of the ligand-receptor complex, the ability of the drug or toxin to release from its targeted carrier, the ability of the drug or toxin to escape from a vesicular compartment into the cytosol, the affinity of the carrier to the drug and the concentration of the carrier. Targeted chemotherapy is also significantly influenced by the antigenic modulation and/or immunoselection of tumor cells. The binding of drug (toxin) to targetable polymeric carrier considerably decreases unwanted side toxicity.

Structure-function analysis of the diphtheria toxin receptor toxin binding site by site-directed mutagenesis

Diphtheria toxin (DT) binds to the epidermal growth factor (EGF)-like domain of human membrane-anchored heparin-binding EGF-like growth factor (proHB-EGF), the human DT receptor (DTR). DT does not bind to mouse proHB-EGF because of amino acid substitutions within the EGF-like domain. We made 10 independent mutants, replacing a single amino acid within the EGF-like domain of human DTR/proHB-EGF with the corresponding amino acid residue in mouse proHB-EGF. The mutant proteins were transiently expressed in mouse L cells either expressing or not expressing DRAP27/CD9, and DT binding was measured. DT binding activity of GST fusion proteins containing the mutated EGF-like domain was also determined by a cell-free binding assay. The largest effect was seen with E141H, and second largest effects were seen with F115Y and L127F in all of the assay systems. We conclude that Phe115, Leu127, and Glu141 are critical amino acid residues for DT binding. A computer model of the tertiary structure of the EGF-like domain of human DTR/proHB-EGF was made. The model predicts that three amino acid residues critical for DT binding activity, Phe115, Leu127, and Glu141, are all located on the same face of the EGF-like domain, suggesting that this face of DTR/proHB-EGF interacts with the receptor-binding domain of DT.

Heparin-like molecules on the cell surface potentiate binding of diphtheria toxin to the diphtheria toxin receptor/membrane-anchored heparin-binding epidermal growth factor-like growth factor

Diphtheria toxin receptor (DTR), which is identical to the membrane-anchored form of heparin-binding EGF-like growth factor (proHB-EGF), has a high affinity for heparin. We studied the effect of heparin-like molecules on the binding of diphtheria toxin (DT) to DTR/proHB-EGF. Mutant Chinese hamster ovary (CHO) cells deficient in heparan sulfate (HS) proteoglycans were about 15 times less sensitive to DT than wild type CHO-K1 cells. When free heparan sulfate or heparin was added to the culture medium, DT sensitivity of the mutant cells was fully restored. Studies of binding of 125I-labeled DT to HS-deficient CHO cells transfected with human DTR/proHB-EGF cDNA indicated that the increased sensitivity to DT after addition of heparin is due to increased binding of DT to cells. Vero cells display a relatively large amount of heparan sulfate residues compared to CHO-K1 cells or L cells. Enhancement of DT binding by the addition of heparin was also observed with CHO-K1 cells and L cells that had been transfected with human DTR/proHB-EGF cDNA, but the degree of enhancement was less than that observed with the HS-deficient CHO cells. Addition of heparin did not affect DT binding or DT sensitivity of Vero cells. Heparin-dependent binding was observed when intact Vero cells were treated with heparitinase or when the cell membrane was solubilized with a neutral detergent. Scatchard plot analysis for the binding of DT to a recombinant HB-EGF in vitro and to L cells expressing human DTR/proHB-EGF revealed that heparin increases the affinity of DTR/proHB-EGF for DT but does not change the number of binding sites. Although DRAP27/CD9 is known to enhance DT binding to DTR/proHB-EGF, the results indicate that heparin and DRAP27/CD9 increase DT binding by independent mechanisms. Thus, heparin-like molecules, probably in the form of heparan sulfate proteoglycan on the cell surface, are a third factor required for maximal DT binding activity of cells.

Level of receptor-associated protein moderates cellular susceptibility to pseudomonas exotoxin A

Pseudomonas exotoxin A (PE) enters mammalian cells via a receptor-mediated endocytic pathway. The initial step in this pathway is binding to the multiligand receptor termed the alpha 2-macroglobulin receptor/low-density lipoprotein receptor-related protein (LRP). Binding of toxin, and of the many other ligands that bind to LRP, is blocked by the addition of a 39-kDa receptor-associated protein (RAP). Here we show that approximately 40% of the cell-associated LRP is on the surface of toxin-sensitive mouse LM fibroblasts and thus accessible for toxin internalization. The remainder is located intracellularly, primarily in the Golgi region. Mammalian cells exhibit a wide range of sensitivity to PE. To investigate possible reasons for this, we examined the expression levels of both LRP and RAP. Results from a variety of cell lines indicated that there was a positive correlation between LRP expression and toxin sensitivity. In the absence of LRP, cells were as much as 200-fold more resistant to PE compared with sensitive cells. A second group of resistant cells expressed LRP but had a high level of RAP. Thus, a toxin-resistant phenotype would be expected when cells expressed either low levels of LRP or high levels of LRP in the presence of high levels of RAP. We hypothesize that RAP has a pivotal role in moderating cellular susceptibility to PE.

Induction of toxin sensitivity in insect cells by infection with baculovirus encoding diphtheria toxin receptor

The diphtheria toxin receptor (DTR) has been identified as the precursor of heparin-binding epidermal growth factor-like growth factor, which may interact with other membrane proteins to form the functional receptor. To test if mammalian DTR is able to confer toxin sensitivity onto phylogenetically distant cells, we expressed monkey DTR in the baculovirus system and tested infected insect cells for toxin sensitivity. cDNA encoding an epitope-tagged heparin-binding epidermal growth factor-like growth factor precursor (DTRB3) was inserted into the virus genome by allelic replacement to construct the recombinant virus vAc-DTRB3. SF9 cells infected with vAc-DTRB3 expressed functional DTR, which could be precipitated from the solubilized membrane fraction of infected cells with Sepharose-immobilized diphtheria toxin. The highest level of expression (about 5 x 10(6) receptors/cell) was observed 48 h after infection, at which time the infected cells were highly sensitive to diphtheria toxin. Uninfected SF9 cells and cells infected with the wild type virus were resistant to the toxin. The presence of heparin increased both the binding and the toxin sensitivity of vAc-DTRB3-infected SF9 cells. Translocation of toxin A fragment was induced when cells with surface-bound toxin were exposed to low pH, and the translocation was optimal at pH < or = 5.5. It was approximately 100 times more efficient at 24 degrees C than at 4 degrees C. The data indicate that monkey DTR is fully functional when expressed in insect cells.

Diphtheria toxin binds to the epidermal growth factor (EGF)-like domain of human heparin-binding EGF-like growth factor/diphtheria toxin receptor and inhibits specifically its mitogenic activity

The membrane anchored form of human heparin-binding epidermal growth factor-like growth factor (HB-EGF) acts as the diphtheria toxin (DT) receptor. Transfection of human HB-EGF cDNA into mouse LC cells, L cells stably expressing DRAP27, conferred sensitivity to DT, but transfection of mouse HB-EGF cDNA did not. To define the essential regions of HB-EGF that serve as the functional DT receptor, we examined the sensitivity to DT and DT binding of cells expressing several human/mouse HB-EGF chimeras. It was found that DT binds to the EGF-like domain of the human HB-EGF. However, mouse HB-EGF does not serve as a functional DT receptor due to non-conserved amino acid substitutions in this domain. In addition, CRM197, a non-toxic mutant of DT, inhibited strongly the mitogenic activity of the secreted form of human HB-EGF, but not of mouse HB-EGF and other EGF receptor-binding growth factors. These results confirmed further that DT interacts with the EGF-like domain of HB-EGF and that this interaction is specific for human HB-EGF.

Detection of Pseudomonas aeruginosa from clinical and environmental samples by amplification of the exotoxin A gene using PCR

PCR was used to detect Pseudomonas aeruginosa from water samples by amplifying a 396-bp region of the exotoxin A (ETA) structural gene sequence. The identify of the amplified 396-bp fragment was confirmed by digesting it with PvuI restriction endonuclease, which produced the predicted 246- and 150-bp fragments. Specific primers amplified ETA-positive P. aeruginosa DNA, whereas other species of Pseudomonas and GC-rich bacteria did not yield any 396-bp fragment. The specificity and sensitivity of the assay were 100 and 96%, respectively, which confirms the assay's reliability for diagnostic and epidemiological studies. The assay can detect as few as 5 to 10 cells in a 10-ml water sample or 0.1 pg of P. aeruginosa DNA per reaction mixture (5 microliters) by ethidium bromide staining of an agarose gel. Ten-times-lower concentrations were detected by hybridization with a digoxigenin-labeled oligonucleotide probe internal to the PCR product. With this PCR method, ETA-positive P. aeruginosa was detected in animal cage water samples at a level of 40 cells per ml. This method is rapid and less cumbersome than other diagnostic methods for the identification of P. aeruginosa strains. The method described can be used to detect a low level of P. aeruginosa from environmental and clinical samples without the use of selective media or additional biochemical tests.

Cytotoxic effect of a fusion protein from transforming growth factor alpha and Pseudomonas exotoxin on rat and human bladder carcinoma cells in vitro

A protein formed by fusion of transforming growth factor alpha with Pseudomonas exotoxin (TGF alpha-PE40) has been shown to have the ability to kill or inhibit the growth of several carcinoma cell lines. This study was designed to evaluate the in vitro cytotoxic effects of TGF alpha-PE40 on rat and human bladder carcinoma cell lines with different biological potential, and normal rat urothelial cells. The rat cell lines used were D44c, LMC19, and MYU3L, which were established in our laboratory. Human cell lines used were RT4, T24, and 253J. As a normal control, we used the first-passage culture of normal rat bladder urothelium (RU-P1). We examined the number and affinity of epidermal growth factor receptors (EGFR) in these cells, the ability of TGF alpha-PE40 to bind EGFR, and the cytotoxic effect of TGF alpha-PE40 and PE40. Rat cell lines, D44c, LMC19, and MYU3L (EGFR = 4.9 x 10(3)-11.4 x 10(3)/cell) had ED50 values (the concentration of TGF alpha-PE40 needed to reduce the viable cell population by 50%) of 180 pM, 540 pM and 6000 pM respectively; for cI (the concentration required to achieve complete inhibition of growth under continuous serum stimulation) TGF alpha-PE40 concentrations of 10(4) pM, 10(4) pM and higher than 10(4) pM respectively were required. Human cell lines, RT4, T24, and 253J (EGFR = 32 x 10(3)-126 x 10(3)/cell) had ED50 values of 20 pM, 66 pM, and 330 pM respectively and T24 showed cI values of 10(3) pM. RU-P1 (EGFR = 92.6 x 10(3)/cell) had the highest ED50 value of 8000 pM. These data indicate that the susceptibility to TGF alpha-PE40 does not always depend on the number of EGFR, that cells having a relatively small number of EGFR respond well to TGF alpha-PE40, and that normal urothelial cells are more resistant to TGF alpha-PE40 than are cancer cells. The differential effect of TGF alpha-PE40 on normal and neoplastic cells provides a rational basis for its use in vivo to control tumor growth.

Hypersensitivity to diphtheria toxin by mouse cells expressing both diphtheria toxin receptor and CD9 antigen

DTS-II is a highly diphtheria toxin (DT)-sensitive cell line previously isolated by transfection of wild-type DT-resistant mouse L-M(TK-) cells with the cDNA encoding a monkey Vero cell DT receptor. DTS-II cells are as toxin-sensitive as Vero cells, have approximately 3-fold more receptors than Vero cells, and have approximately 10-fold lower affinity for DT than Vero cells. We now cotransfected DTS-II cells with a plasmid containing the Vero cell cDNA coding for CD9 antigen (pCD9) and with a plasmid containing the gene for hygromycin resistance (pHyg). The stably transfected hygromycin-resistant colonies were screened for DT hypersensitivity employing a replica plate system. A DT-hypersensitive colony was isolated and purified. The purified DT-hypersensitive cells, DTS-III, (i) are approximately 10-fold more toxin-sensitive than DTS-II and Vero cells and (ii) bear approximately 10(6) DT receptors per cell (i.e., approximately 20-fold and approximately 60-fold more receptors than DTS-II and Vero cells, respectively), but their receptor affinity is still approximately 10-fold lower than that of Vero cells. Cross-linking experiments employing 125I-labeled DT demonstrated that DTS-II and DTS-III cells have essentially the same profile of DT-binding cell-surface protein(s), suggesting that CD9 antigen, although expressed on the cell surface of DTS-III cells, may not be in close proximity to the DT-binding domain of the receptor. CD9 may affect DT receptor expression by increasing receptor density at the cell surface. By employing DTS-III cells it should be possible to purify and characterize the DT cell-surface receptor protein(s).

In vivo production of exotoxin A and its role in endogenous Pseudomonas aeruginosa septicemia in mice

We have examined the production of Pseudomonas aeruginosa exotoxin A (ETA) and its role in endogenous bacteremia in mice. Mice given P. aeruginosa D4 orally died of bacteremia between days 10 and 13 following cyclophosphamide-induced leukocytopenia. In this model, serum endotoxin was detected beginning on day 7 by the Limulus assay and P. aeruginosa was cultured from blood beginning on day 9. ETA and tumor necrosis factor alpha (TNF) were also detected in serum by enzyme-linked immunosorbent assay beginning on day 9. Purified ETA did not stimulate the production of TNF in normal mice primed with a synthetic derivative of muramyl dipeptide in the absence of endotoxin. However, ETA enhanced and primed endotoxin-induced TNF production in mice. The mortality rate of mice given ETA mutant PAO-PRI (5.0%) was significantly lower than that of mice given the parent strain (78.8%). These data indicate that ETA may be an important factor in the occurrence of P. aeruginosa bacteremia and/or the death of mice. Also, ETA may be responsible for enhancing the production of a lethal dose of TNF in the presence of endotoxin in P. aeruginosa bacteremia.

Role of iron in regulation of virulence genes

The abilities of bacterial pathogens to adapt to the environment within the host are essential to their virulence. Microorganisms have adapted to the iron limitation present in mammalian hosts by evolving diverse mechanisms for the assimilation of iron sufficient for growth. In addition, many bacterial pathogens have used the low concentration of iron present in the host as an important signal to enhance the expression of a wide variety of bacterial toxins and other virulence determinants. The molecular basis of coordinate regulation by iron has been most thoroughly studied in Escherichia coli. In this organism, coordinate regulation of gene expression by iron depends on the regulatory gene, fur. Regulation of gene expression by iron in a number of pathogenic organisms is coordinated by proteins homologous to the Fur protein of E. coli. Additional regulatory proteins may be superimposed on the Fur repressor to provide the fine-tuning necessary for the precise regulation of individual virulence genes in response to iron and other environmental signals. Studies of the mechanisms of regulation of iron acquisition systems and virulence determinants by iron should lead to a better understanding of the adaptive response of bacteria to the low-iron environment of the host and its importance in virulence.

The 27-kD diphtheria toxin receptor-associated protein (DRAP27) from vero cells is the monkey homologue of human CD9 antigen: expression of DRAP27 elevates the number of diphtheria toxin receptors on toxin-sensitive cells

Diphtheria toxin (DT) receptor associates with a 27-kD membrane protein (DRAP27) in monkey Vero cells. A cDNA encoding DRAP27 was isolated, and its nucleotide sequence was determined. The deduced amino acid sequence revealed that DRAP27 is the monkey homologue of human CD9 antigen. DRAP27 is recognized by CD9 antibodies. A human-mouse hybrid cell line (3279-10) possessing human chromosome 5, sensitive to DT, but not expressing CD9 antigen, was used for transfection experiments with DRAP27. When the cloned cDNA encoding DRAP27 was transiently expressed in 3279-10 cells, the total DT binding capacity was three to four times higher than that of untransfected controls. Transfectants stably expressing DRAP27 have an increased number of DT binding sites on the cell surface. Furthermore, the transfectants are 3-25 times more sensitive to DT than untransfected cells, and the sensitivity of these cells to DT is correlated with the number of DRAP27 molecules on the surface. However, when the cloned cDNA was introduced into mouse cell lines that do not express DT receptors, neither an increased DT binding nor enhancement of DT sensitivity was observed. Hence, we conclude that DRAP27 itself does not bind DT, but serves to increase DT binding and consequently enhances DT sensitivity of cells that have DT receptors. 12 proteins related to DRAP27/CD9 antigen were found through homology search analysis. These proteins appear to belong to a new family of transmembrane proteins.

Expression cloning of a diphtheria toxin receptor: identity with a heparin-binding EGF-like growth factor precursor

A monkey cDNA (pDTS) encoding a diphtheria toxin (DT) sensitivity determinant was isolated by expression cloning in mouse L-M cells. Mouse cells are naturally resistant to DT, because they lack functional cell surface receptors for the toxin. Unlike wild-type L-M cells, pDTS-transfected mouse cells are extremely toxin sensitive and specifically bind radioiodinated DT. Intoxication of the transfected cells requires receptor-mediated endocytosis of the bound toxin. The cDNA is predicted to encode an integral membrane protein that is identical to the precursor of a heparin-binding EGF-like growth factor. The DT sensitivity protein is thus a growth factor precursor that DT exploits as a receptor.

Expression of functional diphtheria toxin receptors on highly toxin-sensitive mouse cells that specifically bind radioiodinated toxin

Diphtheria toxin (DT), a bacterial protein exotoxin, inactivates mammalian cell elongation factor 2 after toxin internalization by receptor-mediated endocytosis. To isolate the DT receptor, we cotransfected DT-resistant wild-type mouse L-M cells with a cDNA library constructed from RNA of highly toxin-sensitive monkey Vero cells and with a neomycin-resistance gene. Stably transfected G418-resistant L-M colonies were screened for DT sensitivity in a replica plate assay. After screening of 8000 colonies, one DT-sensitive (DTS) colony was isolated. The purified DTS mouse cells are highly toxin-sensitive; they are at least 1000-fold more sensitive than wild-type L-M cells and only approximately 10-fold less sensitive than Vero cells. Incubation of the DTS mouse cells with CRM 197, a nontoxic form of DT that competitively inhibits the binding of native DT to the toxin receptor, protected them from DT-mediated toxicity. More important, these DTS mouse cells express receptors on their cell surface that bind radioiodinated DT in a specific fashion, a property hitherto readily demonstrable only with highly toxin-sensitive cells of monkey origin. Furthermore, HA6DT, a DT fragment comprising the Mr 6000 carboxyl-terminal receptor-binding domain, inhibited the binding of radioiodinated toxin to these DTS mouse cells to the same extent as unlabeled DT. With these DTS mouse cells as a source of monkey cDNA, it should be possible to clone the gene encoding the DT receptor.