Stimulation of intestinal adenyl cyclase by cholera toxin

Caco-2/HT29-MTX co-cultured cells as a model for studying physiological properties and toxin-induced effects on intestinal cells

Infectious gastrointestinal diseases are frequently caused by toxins secreted by pathogens which may impair physiological functions of the intestines, for instance by cholera toxin or by heat-labile enterotoxin. To obtain a functional model of the human intestinal epithelium for studying toxin-induced disease mechanisms, differentiated enterocyte-like Caco-2 cells were co-cultured with goblet cell-like HT29-MTX cells. These co-cultures formed a functional epithelial barrier, as characterized by a high electrical resistance and the presence of physiological intestinal properties such as glucose transport and chloride secretion which could be demonstrated electrophysiologically and by measuring protein expression. When the tissues were exposed to cholera toxin or heat-labile enterotoxin in the Ussing chamber, cholera toxin incubation resulted in an increase in short-circuit currents, indicating an increase in apical chloride secretion. This is in line with typical cholera toxin-induced secretory diarrhea in humans, while heat-labile enterotoxin only showed an increase in short-circuit-current in Caco-2 cells. This study characterizes for the first time the simultaneous measurement of physiological properties on a functional and structural level combined with the epithelial responses to bacterial toxins. In conclusion, using this model, physiological responses of the intestine to bacterial toxins can be investigated and characterized. Therefore, this model can serve as an alternative to the use of laboratory animals for characterizing pathophysiological mechanisms of enterotoxins at the intestinal level.

Intestinal organoid-based 2D monolayers mimic physiological and pathophysiological properties of the pig intestine

Gastrointestinal infectious diseases remain an important issue for human and animal health. Investigations on gastrointestinal infectious diseases are classically performed in laboratory animals leading to the problem that species-specific models are scarcely available, especially when it comes to farm animals. The 3R principles of Russel and Burch were achieved using intestinal organoids of porcine jejunum. These organoids seem to be a promising tool to generate species-specific in vitro models of intestinal epithelium. 3D Organoids were grown in an extracellular matrix and characterized by qPCR. Organoids were also seeded on permeable filter supports in order to generate 2D epithelial monolayers. The organoid-based 2D monolayers were characterized morphologically and were investigated regarding their potential to study physiological transport properties and pathophysiological processes. They showed a monolayer structure containing different cell types. Moreover, their functional activity was demonstrated by their increasing transepithelial electrical resistance over 18 days and by an active glucose transport and chloride secretion. Furthermore, the organoid-based 2D monolayers were also confronted with cholera toxin derived from Vibrio cholerae as a proof of concept. Incubation with cholera toxin led to an increase of short-circuit current indicating an enhanced epithelial chloride secretion, which is a typical characteristic of cholera infections. Taken this together, our model allows the investigation of physiological and pathophysiological mechanisms focusing on the small intestine of pigs. This is in line with the 3R principle and allows the reduction of classical animal experiments.

Regulatory Hierarchies Controlling Virulence Gene Expression in Shigella flexneri and Vibrio cholerae

Gram-negative enteropathogenic bacteria use a variety of strategies to cause disease in the human host and gene regulation in some form is typically a part of the strategy. This article will compare the toxin-based infection strategy used by the non-invasive pathogen Vibrio cholerae, the etiological agent in human cholera, with the invasive approach used by Shigella flexneri, the cause of bacillary dysentery. Despite the differences in the mechanisms by which the two pathogens cause disease, they use environmentally-responsive regulatory hierarchies to control the expression of genes that have some features, and even some components, in common. The involvement of AraC-like transcription factors, the integration host factor, the Factor for inversion stimulation, small regulatory RNAs, the RNA chaperone Hfq, horizontal gene transfer, variable DNA topology and the need to overcome the pervasive silencing of transcription by H-NS of horizontally acquired genes are all shared features. A comparison of the regulatory hierarchies in these two pathogens illustrates some striking cross-species similarities and differences among mechanisms coordinating virulence gene expression. S. flexneri, with its low infectious dose, appears to use a strategy that is centered on the individual bacterial cell, whereas V. cholerae, with a community-based, quorum-dependent approach and an infectious dose that is several orders of magnitude higher, seems to rely more on the actions of a bacterial collective.

Bacterial RNA as a signal to eukaryotic cells as part of the infection process

The discovery of regulatory RNA has identified an underappreciated area for microbial subversion of the host. There is increasing evidence that RNA can be delivered from bacteria to host cells associated with membrane vesicles or by direct release from intracellular bacteria. Once inside the host cell, RNA can act by activating sequence-independent receptors of the innate immune system, where recent findings suggest this can be more than simple pathogen detection, and may contribute to the subversion of immune responses. Sequence specific effects are also being proposed, with examples from nematode, plant and human models providing support for the proposition that bacteria-to-human RNA signaling and the subversion of host gene expression may occur.

Multi-scale molecular dynamics study of cholera pentamer binding to a GM1-phospholipid membrane

The AB5 type toxin produced by the Vibrio cholerae bacterium is the causative agent of the cholera disease. The cholera toxin (CT) has been shown to bind specifically to GM1 glycolipids on the membrane surface. This binding of CT to the membrane is the initial step in its endocytosis and has been postulated to cause significant disruption to the membrane structure. In this work, we have carried out a combination of coarse-grain and atomistic simulations to study the binding of CT to a membrane modelled as an asymmetrical GM1-DPPC bilayer. Simulation results indicate that the toxin binds to the membrane through only three of its five B subunits, in effect resulting in a tilted bound configuration. Additionally, the binding of the CT can increase the area per lipid of GM1 leaflet, which in turn can cause the membrane regions interacting with the bound subunits to experience significant bilayer thinning and lipid tail disorder across both the leaflets.

Spinal cord regeneration

Three theories of regeneration dominate neuroscience today, all purporting to explain why the adult central nervous system (CNS) cannot regenerate. One theory proposes that Nogo, a molecule expressed by myelin, prevents axonal growth. The second theory emphasizes the role of glial scars. The third theory proposes that chondroitin sulfate proteoglycans (CSPGs) prevent axon growth. Blockade of Nogo, CSPG, and their receptors indeed can stop axon growth in vitro and improve functional recovery in animal spinal cord injury (SCI) models. These therapies also increase sprouting of surviving axons and plasticity. However, many investigators have reported regenerating spinal tracts without eliminating Nogo, glial scar, or CSPG. For example, many motor and sensory axons grow spontaneously in contused spinal cords, crossing gliotic tissue and white matter surrounding the injury site. Sensory axons grow long distances in injured dorsal columns after peripheral nerve lesions. Cell transplants and treatments that increase cAMP and neurotrophins stimulate motor and sensory axons to cross glial scars and to grow long distances in white matter. Genetic studies deleting all members of the Nogo family and even the Nogo receptor do not always improve regeneration in mice. A recent study reported that suppressing the phosphatase and tensin homolog (PTEN) gene promotes prolific corticospinal tract regeneration. These findings cannot be explained by the current theories proposing that Nogo and glial scars prevent regeneration. Spinal axons clearly can and will grow through glial scars and Nogo-expressing tissue under some circumstances. The observation that deleting PTEN allows corticospinal tract regeneration indicates that the PTEN/AKT/mTOR pathway regulates axonal growth. Finally, many other factors stimulate spinal axonal growth, including conditioning lesions, cAMP, glycogen synthetase kinase inhibition, and neurotrophins. To explain these disparate regenerative phenomena, I propose that the spinal cord has evolved regenerative mechanisms that are normally suppressed by multiple extrinsic and intrinsic factors but can be activated by injury, mediated by the PTEN/AKT/mTOR, cAMP, and GSK3b pathways, to stimulate neural growth and proliferation.

Insights into binding of cholera toxin to GM1 containing membrane

Interactions of cholera toxin (CT) with membrane are associated with the massive secretory diarrhea seen in Asiatic cholera. Ganglioside GM1 has been shown to be responsible for the binding of the B subunit of cholera toxin (CT-B), which then helps CT to pass through the membrane, but the exact mechanism remains to be explored. In this work, we have carried out atomistic scale molecular dynamics simulation to investigate the structural changes of CT upon membrane binding and alteration in membrane structure and dynamics. Starting from the initial structure where the five units of B subunit bind with five GM1, only three of five units remain bound and the whole CT is tilted such that the three binding units are deeper in the membrane. The lipids that are in contact with those units of the CT-B behave differently from the rest of the lipids. Altogether, our results demonstrate the atomistic interaction of CT with GM1 containing lipid membrane and provide a probable mechanism of the early stage alteration of lipid structure and dynamics, which can make a passage for penetration of CT on membrane surface.

A defined, controlled culture system for primary bovine chromaffin progenitors reveals novel biomarkers and modulators

We present a method to efficiently culture primary chromaffin progenitors from the adult bovine adrenal medulla in a defined, serum-free monolayer system. Tissue is dissociated and plated for expansion under support by the mitogen basic fibroblast growth factor (bFGF). The cultures, although not homogenous, contain a subpopulation of cells expressing the neural stem cell marker Hes3 that also propagate. In addition, Hes3 is also expressed in the adult adrenal medulla from where the tissue is taken. Differentiation is induced by bFGF withdrawal and switching to Neurobasal medium containing B27. Following differentiation, Hes3 expression is lost, and cells acquire morphologies and biomarker expression patterns of chromaffin cells and dopaminergic neurons. We tested the effect of different treatments that we previously showed regulate Hes3 expression and cell number in cultures of fetal and adult rodent neural stem cells. Treatment of the cultures with a combination of Delta4, Angiopoietin2, and a Janus kinase inhibitor increases cell number during the expansion phase without significantly affecting catecholamine content levels. Treatment with cholera toxin does not significantly affect cell number but reduces the ratio of epinephrine to norepinephrine content and increases the dopamine content relative to total catecholamines. These data suggest that this defined culture system can be used for target identification in drug discovery programs and that the transcription factor Hes3 may serve as a new biomarker of putative adrenomedullary chromaffin progenitor cells.

From Escherichia coli heat-stable enterotoxin to mammalian endogenous guanylin hormones

The isolation of heat-stable enterotoxin (STa) from Escherichia coli and cholera toxin from Vibrio cholerae has increased our knowledge of specific mechanisms of action that could be used as pharmacological tools to understand the guanylyl cyclase-C and the adenylyl cyclase enzymatic systems. These discoveries have also been instrumental in increasing our understanding of the basic mechanisms that control the electrolyte and water balance in the gut, kidney, and urinary tracts under normal conditions and in disease. Herein, we review the evolution of genes of the guanylin family and STa genes from bacteria to fish and mammals. We also describe new developments and perspectives regarding these novel bacterial compounds and peptide hormones that act in electrolyte and water balance. The available data point toward new therapeutic perspectives for pathological features such as functional gastrointestinal disorders associated with constipation, colorectal cancer, cystic fibrosis, asthma, hypertension, gastrointestinal barrier function damage associated with enteropathy, enteric infection, malnutrition, satiety, food preferences, obesity, metabolic syndrome, and effects on behavior and brain disorders such as attention deficit, hyperactivity disorder, and schizophrenia.

Cholera toxin enhances Na(+) absorption across MCF10A human mammary epithelia

Cellular mechanisms to account for the low Na(+) concentration in human milk are poorly defined. MCF10A cells, which were derived from human mammary epithelium and grown on permeable supports, exhibit amiloride- and benzamil-sensitive short-circuit current (Isc; a sensitive indicator of net ion transport), suggesting activity of the epithelial Na(+) channel ENaC. When cultured in the presence of cholera toxin (Ctx), MCF10A cells exhibit greater amiloride-sensitive Isc at all time points tested (2 h to 7 days), an effect that is not reduced with Ctx washout for 12 h. Amiloride-sensitive Isc remains elevated by Ctx in the presence of inhibitors for PKA (H-89, Rp-cAMP), PI3K (LY294002), and protein trafficking (brefeldin A). Additionally, the Ctx B subunit, alone, does not replicate these effects. RT-PCR and Western blot analyses indicate no significant increase in either the mRNA or protein expression for α-, β-, or, γ-ENaC subunits. Ctx increases the abundance of both β- and γ-ENaC in the apical membrane. Additionally, Ctx increases both phosphorylated and nonphosphorylated Nedd4-2 expression. These results demonstrate that human mammary epithelia express ENaC, which can account for the low Na(+) concentration in milk. Importantly, the results suggest that Ctx increases the expression but reduces the activity of the E3 ubiquitin ligase Nedd4-2, which would tend to reduce the ENaC retrieval and increase steady-state membrane residency. The results reveal a novel mechanism in human mammary gland epithelia by which Ctx regulates ENaC-mediated Na(+) transport, which may have inferences for epithelial ion transport regulation in other tissues throughout the body.

Vibrio cholerae as a predator: lessons from evolutionary principles

Diarrheal diseases are the second-most common cause of death among children under the age of five worldwide. Cholera alone, caused by the marine bacterium Vibrio cholerae, is responsible for several million cases and over 120,000 deaths annually. When contaminated water is ingested, V. cholerae passes through the gastric acid barrier, penetrates the mucin layer of the small intestine, and adheres to the underlying epithelial lining. V. cholerae multiplies rapidly, secretes cholera toxin, and exits the human host in vast numbers during diarrheal purges. How V. cholerae rapidly reaches such high numbers during each purge is not clearly understood. We propose that V. cholerae employs its bactericidal type VI secretion system to engage in intraspecies and intraguild predation for nutrient acquisition to support rapid growth and multiplication.

Arabidopsis heterotrimeric G-proteins play a critical role in host and nonhost resistance against Pseudomonas syringae pathogens

Heterotrimeric G-proteins have been proposed to be involved in many aspects of plant disease resistance but their precise role in mediating nonhost disease resistance is not well understood. We evaluated the roles of specific subunits of heterotrimeric G-proteins using knock-out mutants of Arabidopsis Gα, Gβ and Gγ subunits in response to host and nonhost Pseudomonas pathogens. Plants lacking functional Gα, Gβ and Gγ1Gγ2 proteins displayed enhanced bacterial growth and disease susceptibility in response to host and nonhost pathogens. Mutations of single Gγ subunits Gγ1, Gγ2 and Gγ3 did not alter bacterial disease resistance. Some specificity of subunit usage was observed when comparing host pathogen versus nonhost pathogen. Overexpression of both Gα and Gβ led to reduced bacterial multiplication of nonhost pathogen P. syringae pv. tabaci whereas overexpression of Gβ, but not of Gα, resulted in reduced bacterial growth of host pathogen P. syringae pv. maculicola, compared to wild-type Col-0. Moreover, the regulation of stomatal aperture by bacterial pathogens was altered in Gα and Gβ mutants but not in any of the single or double Gγ mutants. Taken together, these data substantiate the critical role of heterotrimeric G-proteins in plant innate immunity and stomatal modulation in response to P. syringae.

Immune responses to cholera in children

Cholera is a severe acute dehydrating diarrheal disease caused by Vibrio cholerae O1 or O139 infection, and is associated with significant mortality and morbidity globally. Although young children bear a high burden of the disease, currently available oral vaccines give a lower efficacy and shorter duration of protection in this group than in adults. According to the studies of natural infection, young children achieve comparable systemic anti-V. cholerae antigen-specific antibody, gut-homing antibody-secreting cell and memory B-cell responses as adults. Studies on innate and cell-mediated immune responses are lacking in children, and may offer important insights into differences in vaccine efficacy. The impact of host factors such as malnutrition, genetics and coinfection with other pathogens also remains to be fully defined.

Memory B cell responses to Vibrio cholerae O1 lipopolysaccharide are associated with protection against infection from household contacts of patients with cholera in Bangladesh

Vibrio cholerae O1 causes cholera, a dehydrating diarrheal disease. We have previously shown that V. cholerae-specific memory B cell responses develop after cholera infection, and we hypothesize that these mediate long-term protective immunity against cholera. We prospectively followed household contacts of cholera patients to determine whether the presence of circulating V. cholerae O1 antigen-specific memory B cells on enrollment was associated with protection against V. cholerae infection over a 30-day period. Two hundred thirty-six household contacts of 122 index patients with cholera were enrolled. The presence of lipopolysaccharide (LPS)-specific IgG memory B cells in peripheral blood on study entry was associated with a 68% decrease in the risk of infection in household contacts (P = 0.032). No protection was associated with cholera toxin B subunit (CtxB)-specific memory B cells or IgA memory B cells specific to LPS. These results suggest that LPS-specific IgG memory B cells may be important in protection against infection with V. cholerae O1.

How ricin and Shiga toxin reach the cytosol of target cells: retrotranslocation from the endoplasmic reticulum

A number of protein toxins bind at the surface of mammalian cells and after endocytosis traffic to the endoplasmic reticulum, where the toxic A chains are liberated from the holotoxin. The free A chains are then dislocated, or retrotranslocated, across the ER membrane into the cytosol. Here, in contrast to ER substrates destined for proteasomal destruction, they undergo folding to a catalytic conformation and subsequently inactivate their cytosolic targets. These toxins therefore provide toxic probes for testing the molecular requirements for retrograde trafficking, the ER processes that prepare the toxic A chains for transmembrane transport, the dislocation step itself and for the post-dislocation folding that results in catalytic activity. We describe here the dislocation of ricin A chain and Shiga toxin A chain, but also consider cholera toxin which bears a superficial structural resemblance to Shiga toxin. Recent studies not only describe how these proteins breach the ER membrane, but also reveal aspects of a fundamental cell biological process, that of ER-cytosol dislocation.

Toxin mediated diarrhea in the 21 century: the pathophysiology of intestinal ion transport in the course of ETEC, V. cholerae and rotavirus infection

An estimated 4 billion episodes of diarrhea occur each year. As a result, 2–3 million children and 0.5–1 million adults succumb to the consequences of this major healthcare concern. The majority of these deaths can be attributed to toxin mediated diarrhea by infectious agents, such as E. coli, V. cholerae or Rotavirus. Our understanding of the pathophysiological processes underlying these infectious diseases has notably improved over the last years. This review will focus on the cellular mechanism of action of the most common enterotoxins and the latest specific therapeutic approaches that have been developed to contain their lethal effects.

Cholera toxin: A paradigm for multi-functional engagement of cellular mechanisms (Review)

Cholera toxin (Ctx) from Vibrio cholerae and its closely related homologue, heat-labile enterotoxin (Etx) from Escherichia coli have become superb tools for illuminating pathways of cellular trafficking and immune cell function. These bacterial protein toxins should be viewed as conglomerates of highly evolved, multi-functional elements equipped to engage the trafficking and signalling machineries of cells. Ctx and Etx are members of a larger family of A-B toxins of bacterial (and plant) origin that are comprised of structurally and functionally distinct enzymatically active A and receptor-binding B sub-units or domains. Intoxication of mammalian cells by Ctx and Etx involves B pentamer-mediated receptor binding and entry into a vesicular pathway, followed by translocation of the enzymatic A1 domain of the A sub-unit into the target cell cytosol, where covalent modification of intracellular targets leads to activation of adenylate cyclase and a sequence of events culminating in life-threatening diarrhoeal disease. Importantly, Ctx and Etx also have the capacity to induce a wide spectrum of remarkable immunological processes. With respect to the latter, it has been found that these toxins activate signalling pathways that modulate the immune system. This review explores the complexities of the cellular interactions that are engaged by these bacterial protein toxins, and highlights some of the new insights to have recently emerged.

Enteric infection meets intestinal function: how bacterial pathogens cause diarrhoea

Infectious diarrhoea is a significant contributor to morbidity and mortality worldwide. In bacterium-induced diarrhoea, rapid loss of fluids and electrolytes results from inhibition of the normal absorptive function of the intestine as well as the activation of secretory processes. Advances in the past 10 years in the fields of gastrointestinal physiology, innate immunity and enteric bacterial virulence mechanisms highlight the multifactorial nature of infectious diarrhoea. This review explores the various mechanisms that contribute to loss of fluids and electrolytes following bacterial infections, and attempts to link these events to specific virulence factors and toxins.

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.

Enterotoxigenic Escherichia coli heat-labile toxin seroconversion in US travelers to Mexico

BACKGROUND AND AIMS

Enterotoxigenic Escherichia coli (ETEC) is the most common bacterial pathogen isolated from travelers suffering of diarrhea. Exposure to heat-labile toxin (LT) produces a high rate of seroconversion. However, the role of LT-producing ETEC (LT-ETEC) as a cause of diarrhea is controversial. We conducted a cohort study in US students traveling to Mexico to assess the ETEC-LT seroconversion rate after natural exposure.

METHODS

Participants provided a serum sample on arrival and departure and a stool sample when ill. ETEC-LT immunoglobulin G antibodies were measured by enzyme-linked immunosorbent assay, and LT-ETEC were detected by means of polymerase chain reaction done on fecal DNA.

RESULTS

A total of 422 participants with a mean age of 34.5 years were followed a mean of 19.9 days; 304 were females (72.0%), and 319 (75.6%) traveled during the summer months. In total, 177 individuals (41.9%) developed travelers' diarrhea and 33.9% had LT-ETEC identified in their stools. Among individuals having an LT-ETEC strain, 74% seroconverted compared to 11% of those not having diarrhea (p < 0.0001). When analyzed with a logistic regression model, the odds of seroconversion were significantly reduced in participants not having LT-ETEC in their stool (odds ratio = 0.1, p < 0.0001) after adjusting for season, length of stay, age, gender, race, and ethnicity.

CONCLUSION

In US young adults traveling to Mexico, ETEC-LT seroconversion reliably identifies individuals naturally exposed to ETEC and correlates with symptomatic illness, length and season of travel.

Order-disorder-order transitions mediate the activation of cholera toxin

Cholera toxin (CT) holotoxin must be activated to intoxicate host cells. This process requires the intracellular dissociation of the enzymatic CTA1 domain from the holotoxin components CTA2 and B5, followed by subsequent interaction with the host factor ADP ribosylation factor 6 (ARF6)-GTP. We report the first NMR-based solution structural data for the CT enzymatic domain (CTA1). We show that this free enzymatic domain partially unfolds at the C-terminus and binds its protein partners at both the beginning and the end of this activation process. Deviations from random coil chemical shifts (Delta delta(coil)) indicate helix formation in the activation loop, which is essential to open the toxin's active site and occurs prior to its association with human protein ARF6. We performed NMR titrations of both free CTA1 and an active CTA1:ARF6-GTP complex with NAD(+), which revealed that the formation of the complex does not significantly enhance NAD(+) binding. Partial unfolding of CTA1 is further illustrated by using 4,4'-bis(1-anilinonaphthalene 8-sulfonate) fluorescence as an indicator of the exposed hydrophobic character of the free enzyme, which is substantially reduced when bound to ARF6-GTP. We propose that the primary role of ARF6's allostery is to induce refolding of the C-terminus of CTA1. Thus, as a folded globular toxin complex, CTA1 escapes the chaperone and proteasomal components of the endoplasmic reticulum associated degradation pathway in the cytosol and then proceeds to ADP ribosylate its target G(s)alpha, triggering the downstream events associated with the pathophysiology of cholera.

The discovery of signal transduction by G proteins: a personal account and an overview of the initial findings and contributions that led to our present understanding

The realization that there existed a G-protein coupled signal transduction mechanism developed gradually and was initially the result of an ill fated quest for uncovering the mechanism of action of insulin, followed by a refocused research in many laboratories, including mine, on how GTP acted to increase hormonal stimulation of adenylyl cyclase. Independent research into how light-activated rhodopsin triggers a response in photoreceptor cells of the retina and the attendant biochemical studies joined midway and, without the left hand knowing well what the right hand was doing, preceded classical G protein research in identifying the molecular players responsible for signal transduction by G proteins.

Gas phase characterization of the noncovalent quaternary structure of cholera toxin and the cholera toxin B subunit pentamer

Cholera toxin (CTx) is an AB5 cytotonic protein that has medical relevance in cholera and as a novel mucosal adjuvant. Here, we report an analysis of the noncovalent homopentameric complex of CTx B chain (CTx B5) using electrospray ionization triple quadrupole mass spectrometry and tandem mass spectrometry and the analysis of the noncovalent hexameric holotoxin usingelectrospray ionization time-of-flight mass spectrometry over a range of pH values that correlate with those encountered by this toxin after cellular uptake. We show that noncovalent interactions within the toxin assemblies were maintained under both acidic and neutral conditions in the gas phase. However, unlike the related Escherichia coli Shiga-like toxin B5 pentamer (SLTx B), the CTx B5 pentamer was stable at low pH, indicating that additional interactions must be present within the latter. Structural comparison of the CTx B monomer interface reveals an additional alpha-helix that is absent in the SLTx B monomer. In silico energy calculations support interactions between this helix and the adjacent monomer. These data provide insight into the apparent stabilization of CTx B relative to SLTx B.

Cholera toxin potentiates influences of IFN-gamma through activation of NF-kappaB and release of tumor necrosis factor-alpha

Cholera toxin (Ctx) is a potent adjuvant in the mucosal immune system. Previous studies have indicated that Ctx induces intestinal interferon-gamma (IFN-gamma) production and that adjuvant properties require activation of the IFN-gamma receptor (IFNGR). Thus, we hypothesized that Ctx potentiates IFN-gamma responses in intestinal epithelia. Initial studies suggested that Ctx enhances IFN-gamma-mediated barrier disruption in cultured intestinal epithelia. This response was attributable to liberation of a soluble mediator into conditioned supernatants, subsequently identified as tumor necrosis factor-alpha (TNF-alpha). Extensions of these findings revealed that the Ctx A subunit induces transcriptional activation of proinflammatory genes in addition to TNF-alpha (interleukin-8 [IL- 8], intracellular adhesion molecule-1 [ICAM-1], and IL-6) and that such transactivation is mediated by the transcriptional regulator NF-kappaB. We conclude that Ctx elicits a proinflammatory phenotype in intestinal epithelia and that potentiation of IFN-gamma-mediated barrier disruption by TNF-alpha may contribute to the overall adjuvant properties of Ctx.

Comparison of two Chinese medical herbs, Huangbai and Qianniuzi, on influence of short circuit current across the rat intestinal epithelia

Huangbai (Phellodendron spec.) and Qianniuzi (Pharbitis spec.) are two traditional Chinese medical herbs used for anti-diarrheal and laxative agents, respectively. Ethanol and water extracts of these two herbs were prepared and effects of the extracts on ion transport of the rat intestinal epithelia were studied. For measuring changes of the short circuit current across the epithelia, the rat intestinal epithelia were mounted in the Ussing chamber and attached with voltage/current clamp. The intestinal epithelia were firstly activated by serosal administration of 5 microM forskolin. As current raised and being stable, extracts of these herbs were added, respectively, and changes in the short circuit current were recorded. Ethanol extract of Huangbai attenuated the current increment; on the contrary, ethanol extract of Qianniuzi augmented the current increment additionally. Water extracts of the two herbs showed minor effects on the current in comparison to ethanol extracts. The results provide evidences to reveal the pharmacological mechanism of the two Chinese medical herbs on the intestinal tissue.

Effect of Ethanol Extracts of Three Chinese Medicinal Plants with Laxative Properties on Ion Transport of the Rat Intestinal Epithelia

The effects of ethanol extracts of three Chinese medicinal plants Dahuang (Rheum palmatum L.), Badou (Croton tiglium L.), and Huomaren (Cannabis sativa L.), on ion transport of the rat intestinal epithelia were studied. Rat intestinal epithelia mounted in an Ussing chamber attached with voltage/current clamp were used for measuring changes of the short-circuit current across the epithelia. The intestinal epithelia were activated with current raised by serosal administration of forskolin 5 microM. Ethanol extracts of the three plants all augmented the current additively when each was added after forskolin. In subsequent experiments, ouabain and bumetanide were added prior to ethanol extracts of these medicinal plants to determine their effect on Na(+) and Cl(-) movement. The results suggest that ethanol extracts of the three medicinal plants may affect the Cl(-) movement more directly than Na(+) movement in the intestinal epithelial cells. The results provide evidence for the pharmacologic mechanism of the three Chinese medicinal plants on the intestinal tract.

Effect of Ethanol Extracts of Three Chinese Medicinal Plants With Anti-diarrheal Properties on Ion Transport of the Rat Intestinal Epithelia

Effects of ethanol extracts of three Chinese medicinal plants, namely, Qinpi (Fraxini cortex), Kushen (Sophora flavescens, AITON), and Huanglian (Coptis teeta, WALLICH), on ion transport of the rat intestinal epithelia were determined in this study. Rat intestinal epithelia mounted in an Ussing chamber attached to a voltage/current clamp were used for measuring changes in the short circuit current across the epithelia. Activation of the intestinal epithelia by serosal administration of 5 microM forskolin resulted in an increase in basal short circuit current. The ethanol extracts of each of the three plants partially reduced the current stimulated by forskolin. In the following experiments, ouabain and bumetanide were added prior to adding the ethanol extract of these plants for revealing their effect on Na(+) and Cl(-) movement. The results suggest that the ethanol extract of the Qinpi would affect Cl(-) transport. On the contrary, the ethanol extract of Kushen would affect Na(+) transport rather than Cl(-) movement. This study provides evidences that reveal the pharmacological mechanism of the Chinese plants with anti-diarrheal properties.

Lipid Rafts and Caveolae as Portals for Endocytosis: New Insights and Common Mechanisms

Clathrin-coated pits and caveolae are two of the most recognizable features of the plasma membrane of mammalian cells. While our understanding of the machinery regulating and driving clathrin-coated pit-mediated endocytosis has progressed dramatically, including the elucidation of the structure of individual components and partial in vitro reconstitution, the role of caveolae as alternative endocytic carriers still remains elusive 50 years after their discovery. However, recent work has started to provide new insights into endocytosis by caveolae and into apparently related pathways involving lipid raft domains. These pathways, distinguished by their exquisite sensitivity to cholesterol-sequestering agents, can involve caveolae but also exist in cells devoid of caveolins and caveolae. This review examines the current evidence for the involvement of rafts and caveolae in endocytosis and the molecular players involved in their regulation.

The cox-2-specific inhibitor celecoxib inhibits adenylyl cyclase

Nonsteroidal anti-inflammatory drugs (NSAIDs) are well-known causes of acute renal insufficiency and gastropathy in patients with chronic inflammatory diseases. This action is presumed to result from nonselective inhibition of both constitutive and inducible forms of prostaglandin H synthases, also known as the cyclooxygenase enzymes (i.e., COX-1 amd COX-2). Celecoxib (Celebrex) is a COX-2 enzyme inhibitor and has emerged as a preferred therapeutic agent for the treatment of rheumatoid arthritis as compared to other NSAIDs. Celecoxib has recently been the subject of criticism for its side effects, mainly arterial thrombosis and renal hemorrhage, although it is considered a superior drug in protecting the gastrointestinal tract. In the present study, we report that celecoxib not only inhibited COX-2, but also exhibited the property of inhibiting adenylyl cyclase, an important enzyme forming the intracellular second messenger 3',5'-adenosine monophosphate (cAMP) from adenosine triphosphate (ATP). Celecoxib also inhibited cholera toxin-stimulated cAMP formation, which indicated its ability to permeate cell membranes in order to reach intracellular adenylyl cyclase. It inhibited in vitro adenylyl cyclase activity in both human colonic epithelial cells and purified adenylyl cyclase from Bordetella pertussis. The IC50 of celecoxib for B. pertussis adenylyl cyclase was calculated to be 0.375 mM. Lineweaver-Burk analysis showed that the type of enzyme inhibition was competitive. The apparent Km and Vm of adenylyl cyclase was calculated as 25.0 nM and 7.14 nmol/min/mg, respectively. Celecoxib changed the Km value to 66.6 nM without affecting the Vmax. The current study suggests that apart from inflammation, celecoxib therapy could be further extended to diseases involving cAMP upregulation either by endogenous reactions or exogenous agents. These new data showing inhibition of adenylyl cyclase should be considered in light of the drug's pathological effects or in patients specifically excluded from treatment (e.g., asthmatics).

Enteric nerves and diarrhoea

This review article discusses the importance of the enteric nervous system for the fluid and electrolyte secretion evoked by luminal secretagogues in the small intestine. The first part of the review summarizes observations on augmented secretion caused by cholera toxin, which has been the subject of extensive studies in the past. The latter part reviews studies of the participation of the enteric nervous system in other secretory states of the gut. The involvement of the enteric nervous system in the pathophysiology of intestinal secretory states opens up potential new sites of actions for drugs in the treatment of diarrhoea. This is discussed in the final part of this review.

Caveolae and caveolin in immune cells: distribution and functions

Caveolae are small, cholesterol-rich, hydrophobic membrane domains, characterized by the presence of the protein caveolin and involved in several cellular processes, including clathrin-independent endocytosis, the regulation and transport of cellular cholesterol, and signal transduction. Recently, caveolae have been identified as providing a novel route by which several pathogens are internalized by antigen-presenting cells and as centers for signal transduction. Here, we review the distribution and role of caveolae and caveolin in mammalian immune cells.

Pathogenesis of rotavirus diarrhea

Rotavirus diarrhea is a major cause of infantile gastroenteritis worldwide. This review is mainly devoted to the effects of Rotavirus on intestinal epithelial transport and to the pathophysiological mechanisms proposed to underlie the intestinal fluid secretion caused by the virus.

Co-option of endocytic functions of cellular caveolae by pathogens

It is increasingly becoming clear that various immune cells are infected by the very pathogens that they are supposed to attack. Although many mechanisms for microbial entry exist, it appears that a common route of entry shared by certain bacteria, viruses and parasites involves cellular lipid-rich microdomains sometimes called caveolae. These cellular entities, which are characterized by their preferential accumulation of glycosylphosphatidylinositol (GPI)-anchored molecules, cholesterol and various glycolipids, and a distinct protein (caveolin), are present in many effector cells of the immune system including neutrophils, macrophages, mast cells and dendritic cells. These structures have an innate capacity to endocytoze various ligands and traffic them to different intracellular sites and sometimes, back to the extracellular cell surface. Because caveolae do not typically fuse with lysosomes, the ligands borne by caveolar vesicles are essentially intact, which is in marked contrast to ligands endocytozed via the classical endosome-lysosome pathway. A number of microbes or their exotoxins co-opt the unique features of caveolae to enter and traffic, without any apparent loss of viability and function, to different sites within immune and other host cells. In spite of their wide disparity in size and other structural attributes, we predict that a common feature among caveolae-utilizing pathogens and toxins is that their cognate receptor(s) are localized within plasmalemmal caveolae of the host cell.

Floating cholera toxin into epithelial cells: functional association with caveolae-like detergent-insoluble membrane microdomains

In polarized cells, signal transduction by cholera toxin (CT) requires apical endocytosis and retrograde transport into Golgi cisternae and likely endoplasmic reticulum (ER) (Lencer et al., J. Cell Biol. 131, 951-962 (1995)). We have recently found that the toxin's apical membrane receptor ganglioside GM1 acts specifically in this signal transduction pathway, likely by coupling CT with caveolae or caveolae-related membrane domains (lipid rafts) (Wolf et al., J. Cell Biol. 141, 917-927 (1998)). Work in progress shows that 1) cholesterol depletion uncouples the CT-GM1 receptor complex from signal transduction, a characteristic of lipid rafts; 2) the GM1 acyl chains rather than the carbohydrate head groups appear to account for the structural basis of ganglioside specificity in toxin trafficking; and 3) intestinal epithelial cells obtained from normal adult humans exhibit lipid rafts which differentiate between CT-GM1 and LTIIb-GD1a complexes and which contain caveolin 1.

Biology of heterotrimeric G-protein signaling

The G proteins are components of a complex membrane signaling system designed to modulate extracellular signals as they are transmitted into the cell. The principal components are the receptor, the G proteins including the alpha, beta, gamma subunits and the effector. Associated with these molecules are several molecular processes by which the signal is transmitted, and regulated including desensitization. Molecules such as arrestin, the RGS (regulators of the G-protein signaling) as well as downstream kinases associated with cyclic AMP are key to regulating the G protein signal. Membrane lipids are key for both anchoring this signal system to the plasma membrane but also in defining the signaling process. Through understanding the biology of the signal system, a number of diseases have been linked to dysfunction of the G protein system. It is clear that this important membrane signal system will become the target for more intense investigation and pharmacologic manipulation to treat critical illness.

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.

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.

Cholera toxin B subunit activates arachidonic acid metabolism

Cholera toxin (CT) increases intestinal secretion of water and electrolytes and modulates the mucosal immune response by stimulating cellular synthesis of arachidonic acid (AA) metabolites (e.g., prostaglandin E2), as well as the intracellular second messenger cyclic AMP (cAMP). While much is known about the mechanism of CT stimulation of adenylate cyclase, the toxin's activation of phospholipase A2, which results in increased hydrolysis of AA from membrane phospholipids, is not well understood. To determine whether CT activation of AA metabolism requires CT's known enzymatic activity (i.e., ADP-ribosylation of GSalpha), we used native CT and a mutant CT protein (CT-2*) lacking ADP-ribose transferase activity in combination with S49 wild-type (WT) and S49 cyc- murine Theta (Th)1.2-positive lymphoma cells deficient in GSalpha. The experimental results showed that native CT stimulated the release of [3H[AA from S49 cyc- cells at a level similar to that for S49 WT cells, indicating that GSalpha is not essential for this process. Further, levels of cAMP in the CT-treated cyc- cells remained the same as those in the untreated control cells. The ADP-ribosyltransferase-deficient CT-2* protein, which was incapable of increasing synthesis of cAMP, displayed about the same capacity as CT to evoke the release of [3H]AA metabolites from both S49 WT and cyc- cells. We concluded that stimulation of arachidonate metabolism in S49 murine lymphoma cells by native CT does not require enzymatically functional CT, capable of catalyzing the ADP-ribosylation reaction. These results demonstrated for the first time that stimulation of adenylate cyclase by CT and stimulation of AA metabolism by CT are not necessarily coregulated. In addition, the B subunits purified from native CT and CT-2* both simulated the release of [3H]AA from S49 cyc- cells and murine monocyte/macrophage cells (RAW 264.7), suggesting a receptor-mediated cell activation process of potential importance in enhancing immune responses to vaccine components.

Patterns of gene expression along the crypt-villus axis in mouse jejunal epithelium

BACKGROUND

There is considerable interest in gene expression along the crypt-villus axis of the small intestinal epithelium, particularly in the identification of genes expressed in intestinal crypts.

METHODS

In an attempt to identify crypt-expressed genes, single-stranded cDNA made from normal mouse jejunal epithelium was used in subtractive hybridization against single-stranded cDNA from epithelium from which crypt cells were depleted by 2,000 rads of gamma irradiation. Partial DNA sequence and in situ hybridization of 72 resulting clones were determined.

RESULTS

The sequence of 45 clones matched previously published genes. Gene expression patterns fell into three categories: expression throughout the crypt-villus axis, expression restricted to the villus, and expression restricted to the crypt. Clones in the first two categories could be further divided into three subgroups: those with uniform expression, those with an increasing gradient of expression, and those with a decreasing gradient of expression along the crypt-villus axis. Twenty two clones showed a stronger expression in crypt and lower villus cells, four of these were differentially localized to the crypt. Two of the crypt localized clones were uniformly expressed throughout the crypt, expression of one was stronger in the lower crypt, and expression of the remaining clone was enhanced Paneth cells. We report the full-length cDNA sequence of the Paneth-cell-enhanced clone.

CONCLUSIONS

The screen isolated crypt-expressed genes that may prove useful tools in the study of crypt biology. In a companion report, we characterize one of the crypt clones.

Regulation of 5-HT release from enterochromaffin cells

Large amounts of 5-HT are present in the mammalian intestine where the amine is concentrated in the enterochromaffin cells (ECs) of the mucosa. ECs have the enzymes to synthesize 5-HT, are endowed with a specific, imipramine-sensitive 5-HT uptake mechanism and can store 5-HT in specific secretory vesicles. ECs can secrete 5-HT in a calcium-dependent manner. In particular, calcium influx through voltage-regulated channels and receptor-mediated liberation of intracellular calcium can evoke 5-HT release. 5-HT secretion from ECs occurs predominantly at the interstitial side and is controlled by a complex pattern of receptor-mediated mechanisms. Stimulatory receptors (beta-adrenoceptors, muscarine, nicotine and 5-HT3 receptors) and inhibitory receptors (alpha 2-adrenoceptors, histamine H3, GABAA- and GABAB-, A2 and P2y alpha purine and 5-HT4 receptors as well as receptors for vasoactive intestinal polypeptide (VIP), pituitary adenylate cyclase stimulating peptide (PACAP) and somatostatin) have been shown to be involved in the control of 5-HT release from the ECs.

Purification and properties of periplasmic 3':5'-cyclic nucleotide phosphodiesterase. A novel zinc-containing enzyme from the marine symbiotic bacterium Vibrio fischeri

The 3':5'-cyclic nucleotide phosphodiesterase (CNP) of Vibrio fischeri, due to its unusual location in the periplasm, allows this symbiotic bacterium to utilize extracellular 3':5'-cyclic nucleotides (e.g. cAMP) as sole sources of carbon and energy, nitrogen, and phosphorus for growth. The enzyme was purified to apparent homogeneity by a four-step procedure: chloroform shock, ammonium sulfate precipitation, and chromotography on DEAE-Sephacel and Cibacron Blue 3GA-agarose. The active enzyme consists of a single polypeptide with a mass of 34 kDa. At 25 degrees C, it has a pH optimum of 8.25, a Km for cAMP of 73 microns, and a Vmax of 3700 mumol of cAMP hydrolyzed/min/mg protein (turnover number of 1.24 x 10(5)/min). The specific activity of the V. fischeri enzyme is approximately 20-fold greater than that of any previously characterized CNP when comparisons of activity are made at the same assay temperature. Activity increases with temperature up to 60 degrees C. The CNP contains 2 atoms of zinc/monomer, and zinc, copper, magnesium, and calcium can restore activity of the apoenzyme to varying degrees. The exceptional specific activity of the enzyme and its unusual location in the periplasm support proposals that the enzyme enables the bacterium to scavenge 3':5'-cyclic nucleotides in seawater and that the enzyme plays a role in cAMP-mediated host-symbiont interactions.