Mechanism of activation of adenylate cyclase by cholera toxin

Sphingolipids controlling ciliary and microvillar function

Cilia and microvilli are membrane protrusions that extend from the surface of many different mammalian cell types. Motile cilia or flagella are only found on specialized cells, where they control cell movement or the generation of fluid flow, whereas immotile primary cilia protrude from the surface of almost every mammalian cell to detect and transduce extracellular signals. Despite these differences, all cilia consist of a microtubule core called the axoneme. Microvilli instead contain bundled linear actin filaments and are mainly localized on epithelial cells, where they modulate the absorption of nutrients. Cilia and microvilli constitute subcellular compartments with distinctive lipid and protein repertoires and specialized functions. Here, we summarize the role of sphingolipids in defining the identity and controlling the function of cilia and microvilli in mammalian cells.

A whole-genome RNAi screen uncovers a novel role for human potassium channels in cell killing by the parasite Entamoeba histolytica

The parasite Entamoeba histolytica kills human cells resulting in ulceration, inflammation and invasion of the colonic epithelium. We used the cytotoxic properties of ameba to select a genome-wide RNAi library to reveal novel host factors that control susceptibility to amebic killing. We identified 281 candidate susceptibility genes and bioinformatics analyses revealed that ion transporters were significantly enriched among susceptibility genes. Potassium (K⁺) channels were the most common transporter identified. Their importance was further supported by colon biopsy of humans with amebiasis that demonstrated suppressed K⁺ channel expression. Inhibition of human K⁺ channels by genetic silencing, pharmacologic inhibitors and with excess K⁺ protected diverse cell types from E. histolytica-induced death. Contact with E. histolytica parasites triggered K⁺ channel activation and K⁺ efflux by intestinal epithelial cells, which preceded cell killing. Specific inhibition of Ca²⁺-dependent K⁺ channels was highly effective in preventing amebic cytotoxicity in intestinal epithelial cells and macrophages. Blockade of K⁺ efflux also inhibited caspase-1 activation, IL-1β secretion and pyroptotic death in THP-1 macrophages. We concluded that K⁺ channels are host mediators of amebic cytotoxicity in multiple cells types and of inflammasome activation in macrophages.

Picomolar inhibition of cholera toxin by a pentavalent ganglioside GM1os-calix[5]arene

Cholera toxin (CT), the causative agent of cholera, displays a pentavalent binding domain that targets the oligosaccharide of ganglioside GM1 (GM1os) on the periphery of human abdominal epithelial cells. Here, we report the first GM1os-based CT inhibitor that matches the valency of the CT binding domain (CTB). This pentavalent inhibitor contains five GM1os moieties linked to a calix[5]arene scaffold. When evaluated by an inhibition assay, it achieved a picomolar inhibition potency (IC50 = 450 pM) for CTB. This represents a significant multivalency effect, with a relative inhibitory potency of 100,000 compared to a monovalent GM1os derivative, making GM1os-calix[5]arene one of the most potent known CTB inhibitors.

Cholera toxin induces tumor necrosis factor alpha production in human monocytes

Cholera toxin covalently ADP-ribosylates the a subunit of Gs proteins. The modified Gsalpha activates adenylate cyclase and leads to a dramatic increase in intracellular cAMP. The effect of cholera toxin on the production of tumor necrosis factor (TNF-alpha), a critical mediator of toxicity for a number of bacterial and viral infections, has not been examined. Here we show that cholera toxin stimulated human monocytes to secrete TNF-alpha. The subunit A of cholera toxin alone also induced TNF-alpha production, suggesting that TNF-alpha production is mediated through ADP-ribosylation activity of the toxin. Inhibitors of ADP-ribosylation such as 3-aminobenzamide and niacinamide blocked TNF-alpha induction. However, cyclic AMP analogs and adenylate cyclase activator forskolin did not induce TNF-alpha production in monocytes, suggesting that TNF-alpha induction is independent of cAMP. Furthermore, cholera toxin-induced TNF-alpha production was suppressed by protein kinase C inhibitors H7 and sphingosine and by phospholipase C inhibitors U73122 and ET-18-OCH3, suggesting that PLC and PKC mediate TNF-alpha induction. Cholera toxin-mediated induction of TNF-alpha occurs at the transcription level as demonstrated by the time-dependent expression of TNF-alpha mRNA. These results raise the possibility that TNF-alpha may play an important role in cholera toxin-mediated toxicity and demonstrate that cholera toxin activates TNF-alpha production through PLC-dependent and cAMP-independent pathways. The probable mechanisms of signal transduction from cholera toxin to PLC in monocytes will be discussed.

A novel ganglioside with a free amino group in bovine brain

A novel ganglioside which binds cholera-toxin B-subunit was purified from bovine brain by an h.p.l.c. system using an Aquasil column subsequent to Q-Sepharose column chromatography. T.l.c./immunostaining showed that the isolated ganglioside had about 60% of the binding reactivity of the authentic ganglioside GM1 for cholera-toxin B-subunit. On h.p.l.c., this ganglioside migrated between ganglioside GD1a and GD1b, and was found to give positive reactions with ninhydrin and fluorescamine reagents which specifically react with amino groups. The presence of a free amino group was further confirmed by chemical re-N-acetylation. The N-acetylated product had an identical RF value on h.p.l.c. and similar reactivity with cholera-toxin B-subunit as the authentic GM1. H.p.t.l.c., t.l.c./immunostaining, negative-ion fast-atom-bombardment (f.a.b.)-m.s., and 1H-n.m.r. spectroscopy of the novel ganglioside unequivocally demonstrated that it has the basal structure of GM1 with de-N-acetylated neuraminic acid instead of N-acetylneuraminic acid. In the present study we report for the first time that a ganglioside derivative containing de-N-acetylated neuraminic acid, de-N-acetylated GM1, exists in natural brain tissues.

Microinjection of synthetic protein kinase inhibitor into single barnacle muscle fibers before and after cyclic AMP

Single muscle fibers from the barnacle Balanus nubilus have been used as a preparation to see if a synthetic 20-residue PKI (5-24)-peptide is able to block or reverse the stimulatory response of the ouabain-insensitive Na efflux to injected cAMP. The results obtained show that this peptide behaves as a powerful inhibitor of the cAMP-mediated response and is also able to partially reverse the sustained stimulation of the Na efflux observed in ouabain-poisoned fibers following the injection of subunit A of cholera toxin.

Effects of temperature and salinity on Vibrio cholerae growth

Laboratory microecosystems (microcosms) prepared with a chemically defined sea salt solution were used to study effects of selected environmental parameters on growth and activity of Vibrio cholerae. Growth responses under simulated estuarine conditions of 10 strains of V. cholerae, including clinical and environmental isolates as well as serovars O1 and non-O1, were compared, and all strains yielded populations of approximately the same final size. Effects of salinity and temperature on extended survival of V. cholerae demonstrated that, at an estuarine salinity (25%) and a temperature of 10 degrees C, V. cholerae survived (i.e., was culturable) for less than 4 days. Salinity was also found to influence activity, as measured by uptake of 14C-amino acids. Studies on the effect of selected ions on growth and activity of V. cholerae demonstrated that Na+ was required for growth. The results of this study further support the status of V. cholerae as an estuarine bacterium.

Ganglioside-cholera toxin interactions: a binding and lipid monolayer study

On t.l.c. plates 125I-cholera toxin binds to a disialoganglioside tentatively identified as GD1b with about 10 times less capacity then to ganglioside GM1. Binding of labeled toxin to both gangliosides was abolished in presence of excess amounts of unlabeled B subunit. Ganglioside extracts from human or pig intestinal mucosa showed toxin binding to gangliosides GM1 and GD1b. In ganglioside-containing lipid monolayers the penetration of the toxin was independent of the ganglioside binding capacity.

Role of membrane gangliosides in the binding and action of bacterial toxins

Gangliosides are complex glycosphingolipids that contain from one to several residues of sialic acid. They are present in the plasma membrane of vertebrate cells with their oligosaccharide chains exposed to the external environment. They have been implicated as cell surface receptors and several bacterial toxins have been shown to interact with them. Cholera toxin, which mediates its effects on cells by activating adenylate cyclase, bind with high affinity and specificity to ganglioside GM1. Toxin-resistant cells which lack GM1 can be sensitized to cholera toxin by treating them with GM1. Cholera toxin specifically protects GM1 from cell surface labeling procedures and only GM1 is recovered when toxin-receptor complexes are isolated by immunoadsorption. These results clearly demonstrate that GM1 is the specific and only receptor for cholera toxin. Although cholera toxin binds to GM1 on the external side of the plasma membrane, it activates adenylate cyclase on the cytoplasmic side of the membrane by ADP-ribosylation of the regulatory component of the cyclase. GM1 in addition to functioning as a binding site for the toxin appears to facilitate its transmembrane movement. The heat-labile enterotoxin of E. coli is very similar to cholera toxin in both form and function and can also use GM1 as a cell surface receptor. The potent neurotoxin, tetanus toxin, has a high affinity for gangliosides GD1b and GT1b and binds to neurons which contain these gangliosides. It is not yet clear whether these gangliosides are the physiological receptors for tetanus toxin. By applying the techniques that established GM1 as the receptor for cholera toxin, the role of gangliosides as receptors for tetanus toxin as well as physiological effectors may be elucidated.

Specific binding of solubilized adenylate cyclase to the erythrocyte cytoskeleton

Concepts and criteria that have been developed for the study of the molecular organization of membrane-associated proteins are employed here to investigate the interaction of adenylate cyclase [ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1] with other membrane components. Detergent-solubilized adenylate cyclase can be shown to bind to erythrocyte-derived Triton X-100 shells containing cytoskeletal elements. This binding appears to be saturable with respect to adenylate cyclase concentration, and it is enhanced by the presence of divalent cations. Preactivation of the enzyme with 5'-guanylyl imidodiphosphate and isoproterenol, or with NaF, is a prerequisite for effective binding. Two exceptions to this general observation are noted: rat brain adenylate cyclase, which binds without prestimulation, and rat testicular cytosolic adenylate cyclase, which fails to bind under any of the conditions tried. The binding sites of the Triton X-100 shells are inactivated or released by treatment with various concentrations of trypsin or KCl. Moreover, exposure of the Triton X-100 shells to increasing temperatures results in a progressive loss of the adenylate cyclase binding capacity. On the basis of these and other findings, it is suggested that the adenylate cyclase complex possesses two principal domains that allow it to interact with both cytoskeletal elements and the lipid bilayer. The specific modulation of these interactions may be involved in the hormonal regulation of adenylate cyclase activity.

Photolabelling of cholera toxin subunits during membrane penetration

There has been much speculation about the mechanism by which cholera toxin exerts its effect on the cytoplasmic side of the membranes with which it interacts. After the pentamer of B subunits (5B) binds to membrane receptors, particularly the monosialylganglioside GM1, the disulphide-linked dimer A1SSA2 (which together with 5B constitutes the complete toxin) is thought to penetrate the membrane, perhaps through a channel formed by 5B and become reduced so that A1SH units reach the cytoplasm and stimulate adenylate cyclase. Evidence for this mechanism is circumstantial. If it is correct, a compound which will specifically label intramembranous sections of the toxin should label the channel-forming B subunits but not the channel-contained A1 subunit. We have tested this prediction with a photoreactive glycolipid compound and have obtained the opposite result. Therefore, we propose that only the A1 subunit enters the membrane and we provide here data on the kinetics of that process.

Assembly of proteins into membranes

Two pathways for protein assembly into biological membranes have been proposed. The "signal hypothesis" emphasizes the role of specific membrane proteins in binding the growing polypeptide and conducting it into the bilayer during its synthesis. The "membrane-triggered folding" hypothesis emphasizes self-assembly and the role of changing protein conformation during transfer from an aqueous compartment into a membrane. These ideas provide a framework for reviewing recent data on the biogenesis of membrane proteins.

Mechanism of action of choleragen

Choleragen exerts its effect on cells through activation of adenylate cyclase. Choleragen initially interacts with cells through binding of the B subunit of the toxin to the ganglioside GM1 on the cell surface. Subsequent events are less clear. Patching or capping of toxin on the cell surface may be an obligatory step in choleragen action. Studies in cell-free systems have demonstrated that activation of adenylate cyclase by choleragen requires NAD. In addition to NAD, requirements have been observed for ATP, GTP, and calcium-dependent regulatory protein. GTP also is required for the expression of choleragen-activated adenylate cyclase. In preparations from turkey erythrocytes, choleragen appears to inhibit an isoproterenol-stimulated GTPase. It has been postulated that by decreasing the activity of a specific GTPase, choleragen would stabilize a GTP-adenylate cyclase complex and maintain the cyclase in an activated state. Although the holotoxin is most effective in intact cells, with the A subunit having 1/20th of its activity and the B subunit (choleragenoid) being inactive, in cell-free systems the A subunit, specifically the A1 fragment, is required for adenylate cyclase activation. The B protomer is inactive. Choleragen, the A subunit, or A1 fragment under suitable conditions hydrolyzes NAD to ADP-ribose and nicotinamide (NAD glycohydrolase activity) and catalyzes the transfer of the ADP-ribose moiety of NAD to the guandino group of arginine (ADP-ribosyltransferase activity). The NAD glycohydrolase activity is similar to that exhibited by other NAD-dependent bacterial toxins (diphtheria toxin, Pseudomonas exotoxin A), which act by catalyzing the ADP-ribosylation of a specific acceptor protein. If the ADP-ribosylation of arginine is a model for the reaction catalyzed by choleragen in vivo, then arginine is presumably an analog of the amino acid which is ADP-ribosylated in the acceptor protein. It is postulated that choleragen exerts its effects on cells through the NAD-dependent ADP-ribosylation of an arginine or similar amino acid in either the cyclase itself or a regulatory protein of the cyclase system.

Activity of covalently cross-linked cholera toxin with the adenylate cyclase of intact and lysed pigeon erythrocytes

Reaction of cholera toxin with NN'-bis(carboximidomethyl)tartaramide dimethyl ester produced several cross-linked species that had subunit B (which binds to the cell surface) and peptides A1 (which activates adenylate cyclase) and A2 all covalently joined together. This cross-linded material had activity with pigeon erythrocytes that was comparable in all respects with that of native toxin. It activated the adenylate cyclase of whole cells, showing a characteristic lag phase, and this activation was increased if the cells had been preincubated with ganglioside GM1, but abolished if the protein had been preincubated with the ganglioside. It activated the enzyme in lysed cells more strongly and without the lag phase. These results show that the toxin is active even when peptide A1 cannot be released from the rest of the molecule.

Choleragen activation of solubilized adenylate cyclase: requirement for GTP and protein activator for demonstration of enzymatic activity

The requirements for choleragen activation of adenylate cyclase [ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1] were investigated by using an enzyme preparation solubilized with Triton X-100 from an extensively washed brain particulate fraction and partially purified with DEAE-cellulose. Unlike the particulate enzyme, this preparation was not activated after incubation with choleragen plus dithiothreitol, ATP, and NAD. Addition of the purified protein activator of cyclic nucleotide phosphodiesterase and calcium to the partially purified enzyme increased basal activity somewhat, but choleragen activation was minimal. When cyclase was incubated with GTP plus the protein activator (and calcium), choleragen markedly increased the activity 3- to 6-fold. When GppNHp and protein activator were incubated with the cyclase prior to assay, activity was elevated but no effect of choleragen was observed. GTP and GppNHp had relatively small effects on cyclase activity in the absence of protein activator or if they were added directly to the assay. Boiled brain supernatant was consistently more effective than protein activator (plus calcium) and GTP, suggesting that other factors are required for maximal cyclase activity after choleragen treatment. It appears that the cyclase system is dissociable into several components, all of which may be necessary for optimal regulation of activity. It is probable that one of these is the heat-stable calcium-dependent protein activator of cyclic nucleotide phosphodiesterase and adenylate cyclase that we have found is required along with GTP for demonstration of choleragen activation of partially purified brain adenylate cyclase.

Opiate-dependent modulation of adenylate cyclase

Reactions mediated by the opiate receptors that inhibit adenylate cyclase (EC 4.6.1.1) are closely coupled to subsequent reactions that gradually increase adenylate cyclase activity of neuroblastoma X glioma NG108-15 hybrid cells. Opiate-treated cells have higher basal-, prostaglandin E1-, and 2-chloroadenosine-stimulated activities than do control cells. However, NaF or guanosine 5'-(beta, gamma-imido)triphosphate abolishes most of the differences in adenylate cyclase activity observed with homogenates from control and opiate-treated cells. Cycloheximide blocked some, but not all, of the opiate-dependent increase in adenylate cyclase activity. These results suggest that the opiate-dependent increase in adenylate cyclase is due to conversion of adenylate cyclase to a form with altered activity. Protein synthesis also is required for part of the opiate effect. We propose that activity of adenylate cyclase determines the rate of conversion of the enzyme from one form to the other and that opiates, by inhibiting adenylate cyclase, alter the relative abundance of low- and high-activity forms of the enzyme.

Cholera toxin requires oxidized nicotinamide-adenine dinucleotide to activate adenylate cyclase in purified rat liver plasma membranes

Activation of adenylate cyclase in isolated rat liver plasma membranes by cholera toxin was demonstrated. The activation requires the presence of NAD+ and ATP and is irreversible.

Effect of gangliosides and substrate analogues on the hydrolysis of nicotinamide adenine dinucleotide by choleragen

Choleragen and its A protomer catalyzed the hydrolysis of NAD to ADP-ribose and nicotinamide. NADase activity was inhibited by gangliosides GM1 (galactosyl-N-acetylgalactosaminyl-[N-acetylneuraminyl]-galactosylglucosylceramide), GM2 (N-acetylgalactosaminyl-[N-acetylneuraminyl]-galactosylglucosylceramide), GM3 (N-acetylneuraminyl-galactosylglucosylceramide), and GD1a (N-acetylneuraminylgalactosyl-N-acetylgalactosaminyl-E1N-acetylneuraminyl]-galactosylglucosylceramide). These gangliosides also increased the intensity of the tryptophanyl fluorescence of the isolated A protomer (lambda max = 328 nm). GM1 but not GM2, GM3, and GD1a caused a "blue shift" in the fluorescence spectrum of the B protomer. These results are consistent with other evidence that the specificity of GM1 as the choleragen receptor resides in its carbohydrate moiety. The NADase activity of choleragen was similar to that of diphtheria toxin previously described [J. Kandel, R. J. Collier & D. W. Chung (1974) J. Biol. Chem. 249, 2088-2097]. As with diphtheria toxin, analogues of NAD were inhibitory, adenine being the most effective. Significant inhibition was also noted with adenosine, AMP, ADP-ribose, nicotinamide, nicotinamide mononucleotide, and NADP. NADP was hydrolyzed only slowly by choleragen. In the NADase reaction catalyzed by diphtheria toxin, water serves as an acceptor for the ADP-ribose moiety of NAD in lieu of the natural acceptor molecule, which is elongation factor II (Kandel et al., 1974). It seems probable that the natural protein acceptor for ADP-ribose in the reaction catalyzed by choleragen is adenylate cyclase or a protein component of a cyclase complex that regulates enzymatic activity.

Hydrolysis of nicotinamide adenine dinucleotide by choleragen and its A protomer: possible role in the activation of adenylate cyclase

Choleragen and the isolated A protomer catalyzed the hydrolysis of NAD to ADP-ribose and nicotinamide. The protein with NADase activity (NAD nucleosidase; NAD glycohydrolase, EC 3-2-2-5) migrated on polyacrylamide gels with choleragen, and chromatographed on Bio-Gel P-60 columns with the A protomer. The NADase activity of choleragen and of the A protomer was increased markedly in acetate and phosphate buffers, and enhanced over 10-fold by dithiothreitol in high concentration. NAD hydrolysis was proportional to choleragen concentration; the Michaelis constant for NAD was about 4 mM with both choleragen and the A protomer. The demonstration that the A protomer of choleragen catalyzes an enzymatic reaction involving activation of the ribosyl-nicotinamide bond of NAD, a reaction analogols to those catalyzed by diphtheria toxin, supports the hypothesis that activation of adenylate cyclase by choleragen involves the ADP-ribosylation of an appropriate acceptor protein.

Biosynthesis and function of gangliosides

Gangliosides are unique acidic glycolipids that are selectively concentrated in the plasma membrane of cells. Surface labeling studies have demonstrated that at least a portion of the oligosaccharde chain of gangliosides extends beyond the hydrophe) is imbedded in the membrane bilayer. It is becoming increasingly apparent that gangliosides participate in the internalization of environmental signals elicited by cholera toxin and glycoprotein hormones such as thyrotropic hormone and chorionic gonadotropin as well as other substances such as interferon and possibly serotonin. The mechanism by which cholera toxin binds to a specific ganglioside receptor on the celraction of trophic agents with gangliosides. We would predict that analyogous phenomena involving gangliosides will be discovered in brain. The biosynthesis of gangliosides proceeds by the ordered sequential addition of sugars to the lipid moiety. These reactions are catalyzed by a cluster of membrane-bound glycosyltransferases. Any alteration in the activity or specificity of one of these enzymes will result in a dramatic change in the ganglioside pattern of an afflicted cell or organ. The drastic consequences that accompany abnormalities of ganglioside synthesis have been documented in a heritable metabolic disorder in vivo and in tumorigenic transformation of cells in vitro. In this article, we have attempted to unify these observations and to provide a reasonable interpretation of the role of gangliosides in mediating cell surface phenomena.

Choleragen-mediated release of trapped glucose from liposomes containing ganglioside GM1

125I-Labeled choleragen was bound to liposomes containing galactosyl-N-acetylgalactosaminyl-(N-acetylneuraminyl)-galactosylglucosylceramide (GM1), but not in large amounts to ganglioside-free liposomes nor to those containing N-acetylneuraminylgalactosylglucosylceramide (GM3), N-acetylgalactosaminyl-(N-acetylneuraminyl)-galactosylglucosylceramide (GM2), or N-acetylneuraminylgalactosyl-N-acetylgalactosaminyl-(N-acetylneuraminyl)-galactosylglucosylceramide (GD1a). Choleragen released trapped glucose only from GM1-liposomes. This choleragen-induced glucose release from GM1-liposomes was relatively rapid for the first few minutes, then continued more slowly. The amount of glucose released from liposomes in 30 min was dependent on both the GM1 content and choleragen concentration. Prior incubation of GM1-liposomes with anti-GM1 antiserum prevented the choleragen-dependent release of trapped glucose. After incubation of GM1-liposomes with choleragen, addition of anticholeragen antibodies and complement led to more extensive glucose release. Under these latter conditions a much smaller glucose release was observed also from liposomes containing GM1 or N-acetylneuraminylgalactosyl-N-acetylgalactosaminyl-(N-acetylneuraminyl)-galactosylglucosylceramide in the absence of choleragen. These releases were attributed to naturally-occurring antiganglioside antibodies in the antiserum and complement. Ganglioside-free liposomes did not release glucose in response to anticholeragen and complement. It appears that choleragen in the absence of other proteins binds specifically to liposomes containing GM1 and can induce permeability changes.

Activation by cholera toxin of adenylate cyclase solubilized from rat liver

Cholera toxin, or peptide A1 from the toxin, activates adenylate cyclase solubilized from rat liver with Lubrol PX, provided that cell sap, NAD+, ATP and thiol-group-containing compounds are present. The activation is abolished by antisera to whole toxin, but not to subunit B.

Cholera toxin interactions with thyrotropin receptors on thyroid plasma membranes

Unlabeled cholera toxin inhibits [125I]thyrotropin binding to thyrotropin receptors on thyroid plasma membranes. Maximal inhibition by cholera toxin does not exceed 40%, whereas unalbeled thyrotropin completely inhibits [125I]thyrotropin binding to these same membranes. Kinetic analyses of the binding data are compatible with the view that the cholera toxin decreases the number of receptor sites available to thyrotropin and that the mechanism by which the cholera toxin inhibits [125I]thyrotropin binding to these receptor sites involves both competitive and noncompetitive elements.

Functional incorporation of ganglioside into intact cells: induction of choleragen responsiveness

NCTC 2071 cells are unable to synthesize the monosialoganglioside GM1. When grown in chemically defined medium these cells contained no detectable GM1 and did not accumulate 3': 5'-cyclic AMP in response to choleragen. Incubation of the cells with [3H]GM1 permitted quantification of ganglioside uptake which was dependent on time and concentration of [3H]GM1 in the medium. Responsiveness to choleragen was demonstrated with binding of as few as 17,000 molecules of [3H]GM1 per cell; a maximal response was observed with 10(5) molecules per cell. With increasing cellular content of GM1, the rate of rise in intracellular cyclic AMP in response to choleragen was increased. With greater than 1 X 10(5) molecules of GM1 per cell, the delay between addition of choleragen and the cyclic AMP response was inversely proportional to choleragen concentration; less than 250 molecules of choleragen per cell caused a significant increase in cyclic AMP after 8 hr of incubation. Although the responsiveness of intact cells to choleragen was dependent on GM1, choleragen activation of adenylate cyclase in homogenates with 0.6 mM NAD was independent of added ganglioside. These observations are consistent with the view that exogenous ganglioside GM1 can be functionally integrated into the surface membrane of intact cells and serve as the choleragen receptor. Furthermore, although exogenous GM1 is required for choleragen responsiveness in intact cells, the ganglioside does not play an obligatory role in cell homogenates, where the surface receptor can presumably be bypassed.

Immunological probes into the mechanism of cholera toxin action

The use of antibodies to specific cell surface proteins or to ligands which interact with cell surface receptors is a powerful tool for analyzing the properties of membrane proteins and the consequences of specific cell surface ligand-receptor interactions. Two central observations concerning membrane structure and function, - the diffusibility of membrane proteins (1) and ligand-triggered modulation of specific receptors (2), have derived from the use of antibodies to analyze the properties of membrane proteins. In our study of the mechanism of action of cholera toxin, a protein which binds to a specific cell surface receptor and results in the activation of adenyl cyclase, considerable information has been gained through the use of immunological techniques. This review will briefly summarize the data underlying our current concept of cholera toxin action at the cell membrane and will emphasize those observations made through the use of immunological approaches.

Cholera toxin, ganglioside receptors and the immune response

Cholera toxin activates plasma membrane adenylate cyclase in all mammalian cell types. The structure-function relationship of the toxin has recently been clarified, and the cell membrane receptor identified. This information has made cholera toxin the "agent of choice" for studies in many biological systems of the possible regulatory role of adenylate cyclase/cyclic AMP. This article describes briefly our current knowledge about the toxin and its receptor. It then reviews recent research which has revealed that cholera toxin has strong modulating influences on the proliferation of normal and malignant lympocytes as well as on the initiation and expression of immune responses. The toxin has been found to inhibit DNA synthesis of B and T lymphocytes in vitro without inducing cell death and also to inhibit seems to decrease antibody secretion from plasma cells in vitro, and might also interfere with the release of other soluble immunological mediator subtances. In vivo cholera toxin induces a transient involution of the spleen and a more prolonged lymphocyte depletion of the thymus in mice; these effects appear to be mediated through the adrenal glands. The toxin inhibitors allograft rejection, and either stimulates or suppresses antibody formation depending on the timing of the toxin in relation to the antigen administration. It increases the capacity of the spleen cells to induce graft-vs-host reactions and the "allogeneic effect" on antibody production. An inhibitory effect on a normal suppressor population among the spleen cells has been identified. The findings illustrate the complex effects induced on the immune system by the probably most discriminative investigative tool available for stimulation of the adenylate cyclase/cyclic AMP system.