Effect of the A and B protomers of choleragen on release of trapped glucose from liposomes containing or lacking ganglioside GM1

Toxins which activate adenylate cyclase

Cholera toxin and other heat-labile enterotoxins have the same subunit structure (A5B) and all catalyse the mono ADP-ribosylation of Ns, a regulator of adenylate cyclase, probably at an arginine residue. They also ADP-ribosylate a variety of other membrane and soluble proteins at much slower rates. The rates differ from protein to protein but it may be that every arginine residue in every protein is ADP-ribosylated at some slow rate. A guanine nucleotide triphosphate is required for the ADP-ribosylation of the major (Ns) and minor substrates alike. It used to be thought that all the substrates were GTP-binding proteins but this cannot be so. Rather, the GTP is required because it has to bind to some additional site on the membrane, termed 'S', in a cooperative event that involves a soluble protein called cytosolic factor (CF). If we expose erythrocyte membranes to CF and the GTP analogue Gpp(NH)p we can later extract in detergent a factor or complex that confers upon naive erythrocyte membranes the ability to be ADP-ribosylated. Pertussis toxin also has an A5B structure and acts on an intracellular substrate for ADP-ribosylation, namely the negative regulator of adenylate cyclase, called Ni. ADP-ribosylation prevents the reduction of cyclase activity by inhibitory hormones. The ADP-ribosylation of Ns or Ni does not affect the rate of ADP-ribosylation of the other protein.

Fluorescence analysis of the interaction between ganglioside GM1-containing phospholipid vesicles and the B subunit of cholera toxin

Binding of cholera toxin B protomer (CT-B) to a pyrene-labeled analogue of its ganglioside GM1 receptor (pyrene-GM1) in the absence and presence of phosphatidylcholine vesicles was monitored using steady-state fluorescence spectroscopy. CT-B association with pyrene-GM1 micelles induces changes in the fluorescence properties of this ganglioside analogue that are consistent with its conversion from an excimer to a monomer form. Incubation of pyrene-GM1 with preformed vesicles of phosphatidylcholine (PC) results in complete conversion of pyrene-GM1 to its monomer form, however, unlike with CT-B binding, incorporation of pyrene-GM1 into PC vesicles occurs with a concomitant loss of fluorescence quenching by the small polar quenching agent acrylamide. Subsequent binding of CT-B to the PC-GM1 composite vesicles causes no further change in the pyrene fluorescence emission spectrum but does appear to increase acrylamide accessibility. These data lead to the conclusion that cholera toxin binding to a cell membrane alters membrane packing at the site of attachment. Furthermore, this phenomenon appears to be influenced by environmental conditions such as pH. A pH of about 4.0 or less causes acrylamide quenching to decrease to approximately the levels observed in the absence of CT-B. These results may be useful in describing the dynamics of the interaction between cholera toxin and target cell membranes. Moreover, these data could provide clues to the mechanism by which the toxic portion of CT is able to enter the cytoplasm of target cells.

Interaction of a cholera toxin derivative containing a reduced number of receptor binding sites with intact cells in culture

Hybrid CTB (hCTB), having only one or two functional binding sites, has been constructed from two chemically inactivated derivatives of CTB. One inactive derivative consisted of CTB formylated in the lone Trp-88 of each beta-chain (fCTB), whereas the other inactive derivative consisted of CTB specifically succinylated in three amino groups located in or near the receptor binding site (sssCTB). hCTB, fCTB and sssCTB were able to reassociate with CTA and form the corresponding holotoxins hCT, fCT and sssCT as measured by gel filtration chromatography. In contrast to fCT and sssCT, hCT could increase the cAMP content of intact Vero cells in a time- and dose-dependent way: concentrations as low as a few nanograms of hCT per milliliter caused a significant increase in the intracellular cAMP level. The maximal cAMP level induced by hCT (1 microgram/ml) was, however, more than 2-fold lower than that elicited by its native counterpart. At saturating ligand concentrations and at 37 degrees C, the lag periods and rates of CT and hCT induced cAMP accumulation were essentially the same. Treatment of Vero and HeLa cells with GM1 did not affect their difference in response to CT and hCT. When Vero cells treated with hCT were incubated for longer periods of time, a further slow accumulation of cAMP occurred until after about 20 h cAMP levels of cells exposed to CT or hCT were essentially the same. In contrast to Vero and HeLa cells, human skin fibroblasts exhibited an almost identical response to CT as well as to hCT. Acidotropic agents such as chloroquine and monensin affected the CT and hCT induced increase in cAMP content of Vero cells, fibroblasts and GM1 treated Hela cells in a similar way. The results are consistent with the view that CT receptor recognition domains are shared between adjacent beta-chains, that pentavalent binding appears not to be essential for cytotoxicity and that in the cell types studied intracellular processing of CT, hCT is involved.

Structure, stability, and receptor interaction of cholera toxin as studied by Fourier-transform infrared spectroscopy

The structure and thermal stability of isolated B and A subunits of cholera toxin, as well as the interaction of the B subunit with a ganglioside GM1 receptor, were studied by Fourier-transform infrared spectroscopy. The B subunit of the toxin is highly folded; its secondary structure consists predominantly of beta-sheets. The temperature dependence of the infrared spectrum indicates that the B subunit undergoes thermal unfolding in the temperature range between approximately 66 and 78 degrees C. Binding to the ganglioside GM1 receptor or to its oligosaccharide moiety results in only marginal, if any, change in the secondary structure of the B subunit; however, the receptor-associated subunit does show a markedly increased thermal stability. The secondary structure of the enzymatically active A subunit is less ordered and much less stable than that of the B subunit. The relatively loose folding of the A subunit is likely to be of importance for the effective membrane translocation of this subunit.

Interaction of the B subunit of cholera toxin with endogenous ganglioside GM1 causes changes in membrane potential of rat thymocytes

The fluorescent anionic dye, bisoxonol, and flow cytometry have been used to monitor changes in the membrane potential of rat thymocytes exposed to the B subunit of cholera toxin. The B subunit induced a rapid hyperpolarization, which was due to activation of a Ca2+-sensitive K+ channel. Reduction of extracellular Ca2+ to less than 1 microM by the addition of [ethylene-bis(oxyethylenenitrilo)]tetraacetic acid immediately abolished the hyperpolarization caused by the B subunit. Cells treated with quinine and tetraethylammonium lost their ability to respond to the B subunit, whereas 4-aminopyridine did not have any effect. Thus, calcium-sensitive and not voltage-gated K+ channels appeared to be responsible for the hyperpolarization. The results of ion substitution experiments indicated that extracellular Na+ was not essential for changes in membrane potential. Further studies with ouabain, amiloride and furosemide demonstrated that electrogenic Na+/K+ ATPase, Na+/H+ antiporter and Na+/K+/Cl- cotransporter, respectively, were not involved in the hyperpolarization process induced by the B subunit. Thus, crosslinking of several molecules of ganglioside GM1 on the cell surface of rat thymocytes by the pentavalent B subunit of cholera toxin modulated plasma membrane permeability to K+ by triggering the opening of Ca2+-sensitive K+ channels. A role for gangliosides in regulating ion permeability would have important implications for the function of gangliosides in various cellular phenomena.

Three-dimensional structure of cholera toxin penetrating a lipid membrane

Two-dimensional crystals of cholera toxin bound to receptors in a lipid membrane give diffraction extending to 15 A resolution. Three-dimensional structure determination reveals a ring of five B subunits on the membrane surface, with one-third of the A subunit occupying the center of the ring. The remaining mass of the A subunit appears to penetrate the hydrophobic interior of the membrane. Cleavage of a disulfide bond in the A subunit, which activates the toxin, causes a major conformational change, with the A subunit mostly exiting from the B ring.

Subunit arrangement of cholera toxin in solution and bound to receptor-containing model membranes

Quasi-elastic laser light scattering (QLS) is used to study the translational frictional properties of cholera toxin and its complex with ganglioside Gm1 receptor containing phospholipid vesicles. These properties are compared to theoretically calculated values for model structures composed of spherical subunits in order to assess the structural configuration of the toxin and its binding geometry on membrane surfaces. The structure for the toxin that best fits the experimental results consists of the five B subunits arranged radially about an elongated A subunit, which extends well above the plane of the B subunits. Binding of cholera toxin to Gm1-containing model membranes results in a complex in which the B subunits are absorbed on the surface while the A subunit penetrates the membrane bilayer.

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.

Detergent extraction of cholera toxin and gangliosides from cultured cells and isolated membranes

Choleragen, when bound to various cultured cells, resisted extraction by Triton X-100 under conditions which retained the cytoskeletal framework of the cells. The resistance (greater than 75% of the bound toxin) was observed in Friend erythroleukemic, mouse neuroblastoma N18 and NB41A and rat glioma C6 cells even though the different cells varied over 1000-fold in the number of toxin receptors. The extent of extraction did not depend on whether the cells were in monolayer culture of in suspension or whether choleragen was found at 0 or 37 degrees C. A similar resistance to extraction was also observed in membranes isolated from toxin-treated cells. Using more drastic conditions and other non-ionic detergents, 90% of the bound choleragen was solubilized from cells and membranes. When rat glioma C6 cells, which bind only small amounts of choleragen, were incubated with the ganglioside GM1, toxin binding was increased and the bound toxin was also resistant to extraction. When these cells were incubated with [3H]GM1, up to 70% of the cell-associated GM1 was extracted under the mild conditions. When the Gm1-labeled cells were incubated with choleragen or its B (binding) component, there was a significant reduction in the solubilization of GM1. Similar results were obtained with isolated membranes. When choleragen-receptor complexes were isolated from N18 cells labeled with [3H] galactose by immunoadsorption, only labeled GM1 was specifically recovered. These results suggest that it is the choleragen-ganglioside complex that is resistant to detergent extraction.

Actions of cholera toxin and the prevention and treatment of cholera

The drastic intestinal secretion of fluid and electrolytes that is characteristic of cholera is the result of reasonably well understood cellular and biochemical actions of the toxin secreted by Vibrio cholerae. Based on this understanding it is possible to devise new techniques for the treatment and prophylaxis of cholera to complement those based on fluid replacement therapy and sanitation.

Mechanism of action of cholera toxin: effect of receptor density and multivalent binding on activation of adenylate cyclase

Choleragen (cholera toxin) activates adenylate cyclase in HeLa cells, which contain less than 15,000 toxin receptors per cell, in a time- and concentration-dependent manner. Activation is blocked by the addition of the oligosaccharide chain of the ganglioside GM1, the receptor for the toxin. When the cells are preincubated with choleragen at 4 degrees C and then incubated with oligosaccharide at 37 degrees C, adenylate cyclase is activated less than 10%. When the preincubation phase is above 18 degrees C, adenylate cyclase becomes activated and the amount of activation depends on the time of preincubation. This inhibitory effect of the oligosaccharide is also observed with human lymphocytes and rat glial C6 cells but not with Friend erythroleukemic and mouse neuroblastoma N18 cells. The latter two cell lines have large numbers ot toxin receptors, whereas the former two cell lines have few receptors. When the number of toxin receptors in HeLa and C6 cells is increased by treating the cells with GM1, activation of adenylate cyclase by choleragen is no longer blocked by the oligosaccharide. The oligosaccharide has a corresponding effect on the displacement of bound 125I-choleragen. When bound to cells at 4 degrees C, most of the radiotoxin is displaced from HeLa, C6, and lymphocytes but not from Friend, N18, or HeLa cells pretreated with GM1. In untreated HeLa cells, dissociation of toxin-receptor complexes by the oligosaccharide depends on the time and temperature of complex formation; above 18 degrees C, the toxin rapidly becomes stably bound to the cells. The inhibitory effect of GM1 oligosaccharide us reversible, as, once it is removed, the small amount of toxin that remains bound can activate adenylate cyclase. These results are consistent with a model in which choleragen, which is multivalent, must bind to several GM1 molecules on the cell surface in order to subsequently activate adenylate cyclase. Lateral mobility of toxin-receptor complexes may be required only to achieve multivalent binding in cells with few receptors.

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.