The activation of adenylate cyclase from small intestinal epithelium by cholera toxin

Engineered bacterial communication prevents Vibrio cholerae virulence in an infant mouse model

To investigate the possibility of using commensal bacteria as signal mediators for inhibiting the disease cholera, we stably transformed Escherichia coli Nissle 1917 (Nissle) to express the autoinducer molecule cholera autoinducer 1 (CAI-1) (shown previously to prevent virulence when present with another signaling molecule, autoinducer 2, at high concentrations) and determined the effect on Vibrio cholerae virulence gene expression and colonization in an infant mouse model. We found that pretreatment of mice for 8 h with Nissle engineered to express CAI-1 (Nissle-cqsA) greatly increased the mice's survival (92%) from ingestion of V. cholerae. Pretreatment with Nissle-cqsA for only 4 h increased survival by 77%, whereas ingesting Nissle-cqsA at the same time as V. cholerae increased survival rates by 27%. Immunostaining revealed an 80% reduction in cholera toxin binding to the intestines of mice pretreated for 8 h with Nissle-cqsA. Further, the numbers of V. cholerae in treated mouse intestines was reduced by 69% after 40 h. This finding points to an easily administered and inexpensive approach where commensal bacteria are engineered to communicate with invasive species and potentially prevent human disease.

ADP-ribosylation factors regulate the development of CT signaling in immature human enterocytes

Diarrheal disease is a major cause of morbidity and mortality in infants and children worldwide. Evidence suggests that the interaction of immature human enterocytes with bacteria and their enterotoxins may account for the increased susceptibility of neonates to diarrheal diseases. However, the precise mechanisms that contribute to the excessive response to cholera toxin by the immature gut are largely unknown. Our aim was to characterize the cellular/molecular changes in Gs(alpha) during gut development. In this study, a colonic human epithelial cell line (T84) was used as representative of a mature enterocyte and a human fetal primary small intestinal cell line (H4) as representative of an immature enterocyte. Using our cell culture model of human intestinal development, we provide consistent evidence that cholera toxin (CT)-mediated Gs(alpha) activation in fetal enterocytes differs from that of mature enterocytes, and the difference may be related to ADP-ribosylation factor (ARF) interaction with the CT-signaling process. Here we demonstrated that ARF1 may play a critical role in clathrin-mediated CT trafficking through the endoplasmic reticulum and Golgi and that ARF6 may facilitate clathrin-mediated CT endocytosis that leads to enhanced Gs(alpha) activation by CT. Collectively, these findings support our hypothesis that there is a developmentally regulated intestinal cellular response to bacterial exotoxins involving complex cellular events that accounts for the increased incidence and severity of toxogenic diarrhea during infancy.

Development of microbial-human enterocyte interaction: cholera toxin

Diarrhea in infants and children is a major health hazard worldwide. Certain toxigenic diarrheas occur more commonly and are manifested more severely during the neonatal period. We have previously studied the regulation of cholera toxin-induced secretion in animal models during development. In those studies we have shown that cholera toxin stimulates a much greater secretion by immature compared with mature small intestine, and the mechanism appears to be an up-regulation of postreceptor signal transduction molecules (adenyl cyclase and Gsalpha) leading to an elevated cAMP level. In this study, using experimental models of human intestinal development (fetal cell lines, a micro-Ussing chamber, organ cultures, and fetal intestinal xenograft transplants), we provide preliminary evidence that cholera toxin induces an enhanced secretion mediated in part by a developmental up-regulation of the cAMP response in immature versus mature human small intestine. Additional studies are needed, however, to further define whether other developmental events (e.g. receptor expression) also regulate cholera toxin-enterocyte-enhanced interaction. Nonetheless, this approach to determining the role of development in the pathophysiology of cholera in infants may help in strategies to prevent and treat this condition and other age-related intestinal infectious diseases.

Membrane traffic and the cellular uptake of cholera toxin

In nature, cholera toxin (CT) and the structurally related E. coli heat labile toxin type I (LTI) must breech the epithelial barrier of the intestine to cause the massive diarrhea seen in cholera. This requires endocytosis of toxin-receptor complexes into the apical endosome, retrograde transport into Golgi cisternae or endoplasmic reticulum (ER), and finally transport of toxin across the cell to its site of action on the basolateral membrane. Targeting into this pathway depends on toxin binding ganglioside GM1 and association with caveolae-like membrane domains. Thus to cause disease, both CT and LTI co-opt the molecular machinery used by the host cell to sort, move, and organize their cellular membranes and substituent components.

Identification of the site of uptake of the E. coli heat-labile enterotoxin, LTB

The results of this study demonstrate that the B subunit of the E. coli heat labile toxin (LTB) binds to the brush border of intestinal epithelial cells in a highly specific, lectin-like manner. Uptake of LTB and transcytosis to the basolateral side of the enterocytes can be observed within 1 h after feeding, and occurs through both the villous epithelial cells and the epithelial cells overlying lymphoid follicles and Peyer's patches. Binding and uptake most probably occur via receptor-mediated endocytosis, with GM1 ganglioside and galactoproteins on the enterocyte cell surface acting as specific ligands to which the LTB binds. Cell ELISA data, together with the observed distribution of immunocompetent cells and the localization of LTB binding, suggest that LTB which is taken up by the villous enterocytes enters the circulation and subsequently generates an IgG immune response in the spleen. At the same time, LTB which is taken up via the patch associated epithelium generates a local IgG and IgA immune response within the Peyer's patches and intestinal lymphoid follicles.

Mechanism of cholera toxin action on a polarized human intestinal epithelial cell line: role of vesicular traffic

The massive secretion of salt and water in cholera-induced diarrhea involves binding of cholera toxin (CT) to ganglioside GM1 in the apical membrane of intestinal epithelial cells, translocation of the enzymatically active A1-peptide across the membrane, and subsequent activation of adenylate cyclase located on the cytoplasmic surface of the basolateral membrane. Studies on nonpolarized cells show that CT is internalized by receptor-mediated endocytosis, and that the A1-subunit may remain membrane associated. To test the hypothesis that toxin action in polarized cells may involve intracellular movement of toxin-containing membranes, monolayers of the polarized intestinal epithelial cell line T84 were mounted in modified Ussing chambers and the response to CT was examined. Apical CT at 37 degrees C elicited a short circuit current (Isc: 48 +/- 2.1 microA/cm2; half-maximal effective dose, ED50 integral of 0.5 nM) after a lag of 33 +/- 2 min which bidirectional 22Na+ and 36Cl- flux studies showed to be due to electrogenic Cl- secretion. The time course of the CT-induced Isc response paralleled the time course of cAMP generation. The dose response to basolateral toxin at 37 degrees C was identical to that of apical CT but lag times (24 +/- 2 min) and initial rates were significantly less. At 20 degrees C, the Isc response to apical CT was more strongly inhibited (30-50%) than the response to basolateral CT, even though translocation occurred in both cases as evidenced by the formation of A1-peptide. A functional rhodamine-labeled CT-analogue applied apically or basolaterally at 20 degrees C was visualized only within endocytic vesicles close to apical or basolateral membranes, whereas movement into deeper apical structures was detected at 37 degrees C. At 15 degrees C, in contrast, reduction to the A1-peptide was completely inhibited and both apical and basolateral CT failed to stimulate Isc although Isc responses to 1 nM vasoactive intestinal peptide, 10 microM forskolin, and 3 mM 8Br-cAMP were intact. Re-warming above 32 degrees C restored CT-induced Isc. Preincubating monolayers for 30 min at 37 degrees C before cooling to 15 degrees C overcame the temperature block of basolateral CT but the response to apical toxin remained completely inhibited. These results identify a temperature-sensitive step essential to apical toxin action on polarized epithelial cells. We suggest that this event involves vesicular transport of toxin-containing membranes beyond the apical endosomal compartment.

Proximal tubular epithelial cells possess a novel 42-kilodalton guanine nucleotide-binding regulatory protein

The proximal tubule of the kidney represents an important location where adenylate cyclase regulates salt and water transport; yet a detailed characterization of the distribution and classification of guanine nucleotide-binding protein (G protein) and adenylate cyclase is lacking. We used purified brush border (20-fold) and basolateral membranes (14-fold) to characterize parathyroid hormone- and G protein-regulated adenylate cyclase and G-protein distribution. Adenylate cyclase was predominantly localized to basolateral membranes, while the 46-kDa alpha subunit of the stimulatory G protein (Gs) was 2-fold higher in brush border membranes than in basolateral membranes. The alpha subunit of the inhibitory G protein (Gi; 41 kDa) was equally distributed on immunoblotting but was 2-fold higher in brush border membranes than in basolateral membranes on radiolabeling with pertussis toxin. A 42-kDa cholera toxin substrate that cross-reacted with antisera to the common alpha subunit of G proteins and to Gs on immunoblotting and that was not immunoprecipitated with two Gi antisera was the most abundant alpha subunit and comprised approximately 1% of the total membrane proteins. These observations suggest that G proteins are important regulators of proximal tubular transport independent of adenylate cyclase.

Polarized subcellular distribution of the 1-, 4- and 5-phosphatase activities that metabolize inositol 1,4,5-trisphosphate in intestinal epithelial cells

In intestinal epithelial cells, Ins(1,4,5)P3 is metabolized both by an intracellular 5-phosphatase and by less specific extracellular phosphatases [Rubiera, Velasco, Michell, Lazo & Shears (1988) Biochem. J. 255, 131-137]. A total of 91% of intracellular Ins(1,4,5)P3 5-phosphatase was particulate, and was preferentially associated with plasma membranes rather than with other subcellular organelles. A soluble Ins(1,4,5)P3 3-kinase activity was also characterized, further supporting the idea that inositol phosphates are important in enterocyte function. We have studied the distribution of Ins(1,4,5)P3 phosphatase activities in basolateral and brush-border domains of the plasma membrane. Compared with homogenates, the extracellular phosphatases were 13-17-fold enriched in brush-border membranes, but only 2-fold enriched in basolateral membranes. The 1- and 4-phosphates of Ins(1,4,5)P3 were hydrolysed at equal rates by the extracellular phosphatases; these enzymes are proposed to have digestive functions. The intracellular particulate 5-phosphatase was 2-fold enriched in brush-border membranes and 13-fold enriched in basolateral membranes, at the same pole of the cell where Ins(1,4,5)P3 is believed to be generated. This is opposite to the polarized distribution of particulate 5-phosphatase in hepatocytes [Shears, Evans, Kirk & Michell (1988) Biochem. J. 256, 363-369]; these differences in subcellular distribution may be important in determining cell-specific metabolism of Ins(1,4,5)P3.

The activation of rabbit intestinal adenylate cyclase by cholera toxin

Brush-border and basal-lateral membranes were prepared from rabbit intestinal epithelial cells by differential centrifugation and MgCl2 precipitation. The ADP-ribosylation of proteins in these fractions when incubated with [adenylate-32P]NAD+ and cholera toxin was investigated. Three proteins of molecular mass 45, 40 and 37 kDa were labelled in a toxin-dependent manner in each membrane fraction. The incorporation of 32P-labelled ADP-ribose was 18-fold greater in brush-border membranes than in basal-lateral membranes, comparable to the enrichment of sucrase (marker enzyme for the brush border) in these membranes. There was a 20% release of the 40 and 45 kDa proteins from the brush-border membrane following this ADP-ribosylation. Activation of adenylate cyclase by both cholera toxin and sodium fluoride was 2.7- and 2.3-fold greater, respectively, in basal-lateral membranes than in brush-border membranes, comparable to the enrichment of Na+/K+-ATPase (marker enzyme for the basal-lateral membrane) in these membranes. The effect of sodium fluoride on membranes pretreated with cholera toxin revealed no increase in adenylate cyclase activity above that due to the toxin. This presumably means that both toxin and fluoride activate adenylate cyclase by the same regulatory protein. The results show that cholera toxin catalyzes the ADP-ribosylation of regulatory proteins in the brush-border membrane, and these proteins then migrate to the basal-lateral membrane where they activate the catalytic component of adenylate cyclase.

Na+/H+ exchange mechanism in the basolateral membrane of the rat enterocyte

Basolateral membrane vesicles from rat jejunal enterocytes, especially purified of brush-border contamination, were used for Na+ uptake. The basolateral membrane vesicles are osmotically active and under our experimental conditions Na+ binding is much lower than transport. An outwardly directed proton gradient stimulates Na+ uptake at both 5 microM and 5 mM concentrations. The proton gradient effect can be inhibited completely by 2 mM amiloride and partially by either FCCP or NH4Cl (NH3 diffusion). Membrane potential effects can be excluded by having valinomycin plus K+ on both sides of the vesicles. These results suggest that there is an Na+/H+ exchanger in the basolateral membrane of rat enterocytes.

Intestinal brush border membranes contain regulatory subunits of adenylyl cyclase

Cholera toxin alters intestinal function by stimulation of adenylyl cyclase [ATP pyrophosphate-lyase (cyclizing) or adenylate cyclase, EC 4.6.1.1]. The mechanism of this activation is unknown and particularly puzzling because adenylyl cyclase is confined to the basal lateral membrane of enterocytes, whereas it is the brush border membrane that binds the toxin and contains proteins that undergo cholera toxin-catalyzed ADP ribosylation. It is shown that cholate extracts from cholera toxin-treated brush border membranes can efficiently reconstitute adenylyl cyclase activity in the guanine nucleotide-binding regulatory component (Gs)-deficient cyc- variant of the S49 mouse lymphoma cell line (cyc- cells lack the alpha subunit of Gs needed to activate the catalytic subunit of adenylyl cyclase). Moreover, NaF (in the presence of Al3+) and guanyl-5'-yl imidodiphosphate mediate strong activation of cyc- adenylyl cyclase provided the cholate extracts of brush border membranes are also present. Therefore, it appears that brush border membranes contain high levels of regulatory subunits of adenylyl cyclase in the absence of catalytic subunits. This represents a previously unrecognized feature of this transduction system that presumably plays an important role in the derangement of intestinal cell function by cholera toxin.

Permeability properties of isolated enterocytes from rat small intestine

Metabolic and permeability properties of enterocytes isolated by treatment of rat small intestine with hyaluronidase or EDTA were compared. No significant difference was observed in the ability of the two types of cell to produce lactate from glucose. However, while cells obtained with hyaluronidase accumulate alpha-methylglucoside, cells obtained with EDTA were unable to accumulate the sugar above the medium concentrations. When resuspended in a medium designed to resemble the intracellular medium, potentiometric measurements showed that cells obtained with hyaluronidase released Ca2+ to the medium while cells obtained with EDTA accumulated it. Using 45Ca transport assays, this was shown to be an ATP-dependent process, the accumulated 45Ca being totally released by the addition of the ionophore A23187. When cells obtained with EDTA were resuspended in a medium containing concentrations of free Ca2+ higher that 10 microM, the uptake was partially inhibited by sodium orthovanadate and also by oligomycin and antimycin. At free Ca2+ concentrations lower than 1 microM, the accumulation was inhibited up to 87% by sodium orthovanadate while mitochondrial inhibitors inhibited only 5%. Thus, it appears that during their preparation cells obtained with hyaluronidase retain their integrity while cells obtained with EDTA become permeable to Ca2+ and other ions. The usefulness of both types of preparation in metabolic and transport studies is discussed.

Protein kinase C from small intestine epithelial cells

Protein kinase C activity has been identified in cytosolic and membrane fractions from rat and rabbit small intestine epithelial cells. The cytosolic fraction comprised about the 75% of total activity. Protein kinase C activity was resolved from other protein kinase activities by ion exchange chromatography. Phosphatidylserine or phosphatidylinositol were required for protein kinase C to be active. In addition, the activity was enhanced by the presence of a diacylglycerol. Diolein and dimyristin were the most effective (13-14 fold activation). In the presence of phosphatidylserine and diolein, the Ka for activation by Ca2+ was 10(-7)M. The phorbol ester TPA substituted for diacylglycerol in activating protein kinase C. Brush border and basolateral membranes contained protein kinase C activity, although the specific activity of the basal lateral membranes was four-fold higher than the specific activity of the brush border membranes. The presence of PKC in small intestine epithelial cells might have important implications in the Ca2+ mediated control of ionic transport in this tissue.

Na+/H+ exchange is present in basolateral membranes from rabbit small intestine

Fluorescence quenching of the pH gradient sensitive dye acridine orange and that of the membrane potential sensitive dye Di-S-C3(5) have been studied in purified basolateral membrane vesicles obtained from rabbit small intestine. Basolateral membranes contain an electroneutral, carrier mediated, Na+/H+ exchange activity. They also appear to contain an electrogenic pathway for H+ movement. Based on the comparison of acridine orange fluorescence quenching in the presence of an outwardly directed Na+ gradient and in the presence of known K+ diffusion gradients it can be estimated that at least 50% of the observed proton fluxes are due to the activity of the exchanger. Acridine orange fluorescence recovery measurements have been used to assess the kinetic properties of the exchanger.

Adenylate cyclase from rabbit small intestine: activation by cholera toxin and interaction with calcium

The stimulation of adenylate cyclase in various fractions of plasma membranes from rabbit small intestinal epithelium has been studied. In crude plasma membranes cholera toxin activated 5-fold at 10 micrograms/ml; vasoactive intestinal peptide (VIP) activated at concentration from 10(-8) to 10(-7) M, the maximal stimulation being 6-fold. Fluoride activated 10-fold at 10 mM. VIP-stimulated enzyme was inhibited by Ca2+ concentrations in the micromolar range. In the presence of calmodulin a biphasic response was obtained. At low Ca2+ concentration (4 x 10(-9)-6 x 10(-8) M) the enzyme was activated. As the Ca2+ concentration was increased the enzyme was concomitantly inhibited. We have investigated the mechanism by which cholera toxin activates intestinal adenylate cyclase. We have found that cholera toxin catalyzed incorporation of 32P into proteins located in the brush-border membrane whose molecular weights are in the range of 40-45kDa. These membranes bind [3H]GTP with a Kd of 1.8 x 10(-7) M. In contrast, basal lateral membranes do not contain any protein which becomes labeled in a toxin-dependent manner when incubated with cholera toxin and [32P]NAD. The modification of brush-border membrane protein occurred in spite of the absence of adenylate cyclase in these membranes. Adenylate cyclase in basal lateral membranes was poorly activated by cholera toxin as compared to crude plasma membranes. On the other hand, the ability of VIP and fluoride to activate the enzyme was enhanced in basal lateral membranes with respect to crude membranes. The results are discussed in relation to the mechanism by which cholera toxin activates adenylate cyclase in intact intestinal cells.