Comparison of the effects of enterotoxins of Shigella dysenteriae and Vibrio cholerae on the adenylate cyclase system of the rabbit intestine

Studies on the pathogenesis of enteric infections caused by invasive bacteria

Salmonellae, shigellae and some Escherichia coli must invade the intestinal epithelial cell and multiply within the mucosa to cause disease. Although the bacterial cell most likely possesses several properties essential to this invasive ability, the nature of the cell envelope complex is at present the only characteristic which has been implicated in this process. While a number of pathophysiological events result from invasion, some of our recent efforts have concerned the site and mechanism of intestinal fluid loss in salmonellosis and shigellosis. In both these disorders, bacterial invasion of the colonic mucosa, associated with an acute inflammatory reaction and mucosal damage, is regularly seen and colonic salt and water transport is abnormal. These defects may account for mild diarrhoea in salmonellosis and the dysenteric stools of shigellosis. However, in salmonella-infected animals with severe watery diarrhoea and in shigella-infected animals with diarrhoea alone or in combination with dysentery, the jejunum is in a net secretory state. This secretion occurs in the absence of bacterial invasion or morphological abnormalities. Thus, the diarrhoea caused by invasive bacteria may result from the inability of the colon to reabsorb the increased volume of fluid entering it from the small intestine. Although colonic mucosal damage is a feature of invasive-type diarrhoeas, the permeability of both the colon and small intestine to small molecules, mannitol and erythritol, is not altered. Thus intestinal fluid loss cannot be ascribed to transudation. In addition, the results of our Ussing chamber experiments, employing salmonella-infected rabbit ileum, reveal that salt and water secretion is an active process. Since secretion occurs in the jejunum in the absence of bacterial invasion, this might suggest the participation of an enterotoxin. Shigella dysenteriae I is the best-studied invasive organism in which an enterotoxin has been found, yet mutant strains which do not invade but retain the ability to elaborate enterotoxin fail to cause disease in either monkeys or man. Thus, the physiological relevance of Shiga enterotoxin and the mechanism of jejunal secretion in these disorders remain unclear. Recent data suggest that invasive enteropathogens, like the enterotoxin-producing bacteria, activate the mucosal adenylate cyclase-cyclic AMP system and that this activation may play a role in intestinal fluid secretion.

The activation of adenylate cyclase by cholera toxin: possible interaction with the nucleotide regulatory site

The application of cholera toxin to intact cells causes a stimulation of adenylate cyclase activity. The effect is characterized by a lag period followed by a progressive rise in enzyme activity over several hours. Only a few minutes' exposure to the toxin is required to produce effects lasting over several days. Stimulation of adenylate cyclase by cholera toxin in broken cell preparations requires the presence of nicotinamide-adenine dinucleotide (NAD) and an unidentified component of the cytosol. Guanyl nucleotides and certain non-hydrolysable analogues of guanosine triphosphate also stimulate adenylate cyclase. Stimulation by the analogues results in a highly activated enzyme which has characterisitcs similar to those of adenylate cyclase after stimulation by cholera toxin. Thus the stimulation is irreversible, the enzyme may be "solubilized" by non-ionic detergents in the activated state, and responses to certain hormones are enhanced. Therefore the possibility exists that cholera toxin acts on the guanyl nucleotide regulatory protein of the adenylate cyclase complex. In exploring this possibility it was found pretreatment with cholera toxin not only blocked the stimulatory effect of subsequently added guanylylimidodi-phosphate (GppNHp) but that the latter reduced the stimulation by toxin. Similarly, pretreatment with GppNHp blocked the effect of cholera toxin. The similarities in the effects of cholera toxin and GppNHp, together with the mutual interference of their activities, suggests that cholera toxin acts at the same regulatory site at which guanyl nucleotides exert their effects on adenylate cyclase.

Shigella toxin and the pathogenesis of shigellosis

Shigella dysenteriae 1 produces a periplasmic protein with multiple toxic effects in vivo and in vitro. These include neurotoxicity, cytotoxicity and enterotoxicity, as well as the ability to inhibit cell-free protein synthesis. The purified toxin is a protein of relative molecular mass (Mr) 64 000. It is composed of one catalytically active A subunit (Mr = 32 000) that inhibits protein synthesis, and a complex of five B monomers (Mr approximately 6500 each). Studies using subunit-specific antibodies demonstrate that the B subunit mediates the binding of toxin to toxin receptors in the cell membrane. In a model system in HeLa cell culture, the surface membrane receptor has been shown to be a glycoprotein, most probably asparagine-linked, and to contain oligomeric beta 1----4 linked N-acetyl-D-glucosamine. Studies with metabolic inhibitors and agents that disrupt the cytoskeleton, and/or alter the pH and function of acidic cytoplasmic vesicles, provide indirect evidence that toxin is transported from the cell surface to the cell interior. This process is probably receptor-mediated endocytosis, since it is also inhibited by amines that prevent receptor-mediated uptake of other ligands in well-characterized systems. The toxic action in the HeLa cell is due to the subsequent inhibition of protein synthesis which results from catalytic inactivation of the 60S ribosomal subunit and the cessation of polypeptide chain elongation. Inhibition of protein synthesis by toxin produced subsequent to bacterial invasion of colonic epithelial cells could explain the destructive lesions found in shigellosis. Although toxin can induce jejunal secretion in animal models, there is at present no clear explanation for the secretory response of the gut mucosa in shigella infection.

Pathogenesis of Shigella diarrhea. XVI. Selective targetting of Shiga toxin to villus cells of rabbit jejunum explains the effect of the toxin on intestinal electrolyte transport

To examine the mechanism by which Shiga toxin alters intestinal water and electrolyte transport, ligated loops of rabbit jejunum were incubated in vivo with purified toxin and then studied in vivo by single pass perfusion and in vitro by the Ussing chamber voltage-clamp technique. Toxin exposure led to accumulation of water in the jejunal lumen, associated with decreased active basal NaCl absorption. Glucose- and alanine-stimulated Na absorption were also reduced, while toxin had no effect on either basal short-circuit current or the secretory response to theophylline. These observations suggest that Shiga toxin selectively inhibits NaCl absorption without significantly altering active anion secretion. To localize the cellular site of toxin action, populations of villus and crypt cells from rabbit jejunum were isolated and studied. Villus cells had a greater content of the glycolipid Shiga toxin receptor, Gb3, had more toxin binding sites than did crypt cells, and were much more sensitive than crypt cells to toxin-induced inhibition of protein synthesis. These experiments demonstrate that purified Shiga toxin inhibits jejunal fluid absorption without affecting active fluid secretion by a preferential effect on villus cells. The results suggest that this is due to the differential distribution of toxin receptors on villus compared to crypt cells.

New knowledge on pathogenesis of bacterial enteric infections as applied to vaccine development

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Inhibition of protein synthesis in intact HeLa cells by Shigella dysenteriae 1 toxin

Shiga toxin purified to near homogeneity from cell lysates of Shigella dysenteriae 1 inhibited protein and deoxyribonucle acid syntheses in intact HeLa cells. Inhibition was dependent on toxin concentration and time of incubation. A minimal latent period of 30 min was observed with saturating doses of toxin. Ribonucleic acid synthesis, uptake of alpha-aminoisobutyric acid, and maintenance of intracellular K+ concentrations were not affected until well after maximal inhibition of protein and deoxyribonucleic acid syntheses. The inhibitory effect of toxin was sensitive to heat inactivation and was prevented by antibody neutralization. Several cytotoxic components were separated by polyacrylamide gel electrophoresis of the purified toxin preparations; all inhibited protein and deoxyribonucleic acid syntheses equally.

Studies on the production of enterotoxins by Bacillus cereus

Evidence is presented for the existence of three distinct enterotoxins detected in concentrated cell-free culture filtrates of selected Bacillus cereus strains. The first was a product capable of stimulating the adenylate cyclase-cyclic-AMP system in intestinal epithelial cells and, possibly through this, causing fluid accumulation in ligated ileal sections ("loops") of young rabbits. This was elaborated by a strain isolated from an incident of diarrhoea and which caused diarrhoea in 6 of 10 monkey feedings. The second was tentatively identified as a factor which caused fluid accumulation in rabbit loops but not, apparently, through stimulation of the adenylate cyclase-cyclic-AMP system; this was elaborated by a strain isolated from raw rice which failed to produce symptoms in eight monkey feedings. Together, the behaviour of these two factors indicates that diarrhoea caused by B. cereus enterotoxin may be a cyclic-AMP-mediated event. The third, here referred to as "pyogenic toxin", caused severe tissue damage in the ileal mucosa and was elaborated by a strain isolated from a brain abscess. A factor produced by a strain isolated from an outbreak of vomiting which caused vomiting in 10 of 24 monkey feedings could bot be detected in tests reported here but appears to be a fourth enterotoxin type. Cytopathic effects in tissue cultures, suckling mouse tests, and assays of glycerol production by fat cells were not found to be of value in the detection of any of the enterotoxins.