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Toxins and the Gut: Role in Human Disease a Fasano iii9 PAPER Gut: first published as 10.1136/gut.50.suppl_3.iii9 on 1 May 2002. Downloaded from Toxins and the gut: role in human disease A Fasano ............................................................................................................................. Gut 2002;50(Suppl III):iii9–iii14 Bacterial enteric infections exact a heavy toll on the TOXINS THAT ACTIVATE ENTEROCYTE human population, particularly among children. Despite SIGNAL PATHWAYS Intestinal cells operate through three well estab- the explosion of knowledge on the pathogenesis of lished intracellular signal transduction pathways enteric diseases experienced during the past decade, to regulate water and electrolyte fluxes across the the number of diarrhoeal episodes and human deaths intestinal mucosa: cyclic adenosine monophos- phate (cAMP); cyclic guanosine monophosphate reported worldwide remains of apocalyptic dimensions. (cGMP); and calcium dependent pathways (fig However, our better understanding of the pathogenic 1). Recently, a fourth pathway involving nitric mechanisms involved in the onset of diarrhoea is finally oxide (NO) has been described. leading to preventive interventions, such as the cAMP development of enteric vaccines, that may have a An extremely heterogeneous group of microor- significant impact on the magnitude of this human ganisms, Escherichia coli encompass almost all features of possible interaction between intestinal plague. The application of a multidisciplinary approach microflora and the host, ranging from a role of to study bacterial pathogenesis, along with the recent mere harmless presence to that of highly patho- sequencing of entire microbial genomes, have made genic organisms. In fact, the E coli species is made up of many strains that profoundly differ from possible discoveries that are changing the way scientists each other in terms of biological characteristics view the bacterium–host interaction. Today, research on and virulence properties.2 Among the pathogenic the molecular basis of the pathogenesis of infective E coli, enterotoxigenic E coli (ETEC) represent the archetype of toxin producing E coli. The heat labile diarrhoeal diseases of necessity transcends established toxin (LT) produced by some ETEC strains is boundaries between microbiology, cell biology, structurally and functionally similar to cholera intestinal pathophysiology, and immunology. This toxin (CT) produced by Vibrio cholerae, and both http://gut.bmj.com/ activate the adenylate cyclase/cAMP pathway.1 review focuses on the most recent outcomes of this However, while LT induces a mild diarrhoea multidisciplinary effort. known as “travellers” diarrhoea, CT is responsible .......................................................................... for the severe, sometimes fatal, clinical condition typical of cholera. CT and LT share a common structure consisting of an A (active) subunit of icroorganisms represent the first species ∼25 kDa and a ring of 5 B (binding) subunits of of living organisms that populated our ∼11 kDa each (AB5). Recently, Rodighiero et al on September 24, 2021 by guest. Protected copyright. Mplanet and will probably continue to have reported that the differential toxicity of CT survive well beyond the extinction of the human and LT is related to a 10 amino acid segment race. Their distinguishing characteristics (small within the A2 fragment of CT that confers a size, concise deployment of genetic information, higher stability to the CT holotoxin during uptake and ability to survive in highly varied circum- and transport into intestinal epithelia.3 stances) contribute to their acclaimed virtuosity to adapt and to learn fast in order to survive. To cGMP be a successful enteric pathogen, a microorgan- Besides LT, ETEC elaborate a family of heat stable ism has to be a good coloniser, compete for enterotoxins (STs). STIp is a small peptide that nutrients, and to be able to interact with the tar- stimulates guanylate cyclase, causing an in- get eukaryotic cell in order to induce secretion of creased intracellular concentration of cGMP, water and electrolytes. As the basic metabolism which evokes chloride secretion and diarrhoea.1 of enteric pathogens and commensals is the STIp is a typical extracellular toxin consisting of same, it follows that pathogens must possess 18 amino acid residues synthesised as a precursor highly specialised attributes which enable protein. The precursor translocates across the ....................... them to activate one of the eukaryotic intra- Correspondence to: cellular pathways leading to intestinal secretion Dr A Fasano, Division of (for a comprehensive review, see Fasano1). ................................................. Pediatric Gastroenterology This cross talk between enteric pathogens and Abbreviations: and Nutrition, University of ANP, atrial natriuretic peptide; BFT, B Maryland School of intestinal host may be activated by either fragilis enterotoxin; CPE, C perfringens enterotoxin; CT, Medicine, 685 W invasion or elaboration of toxins. This review will cholera toxin; CTC, V cholerae cytolysin; EAggEC, Baltimore St HSF Building, focus on this last mechanism of action by enteroaggregative E coli; ETEC, enterotoxigenic Ecoli; LPS, lipopolysaccharide; GC, guanylate cyclase; LT, heat Room 465, Baltimore, MD reviewing recent reports on toxins elaborated 21201, USA; labile toxin; PET, plasmid encoded protein; PKC, protein [email protected] by the most common enteric pathogens kinase C; ShET, Shigella enterotoxin; ST, heat stable toxin; ....................... (table 1). TDH, thermostable direct haemolysin www.gutjnl.com iii10 Fasano genetically and immunologically distinct from Yersinia enteroco- Table 1 Enteric toxins litica ST I, ST II, and other known enterotoxins.10 Toxins that activate enterocyte signal pathways Gut: first published as 10.1136/gut.50.suppl_3.iii9 on 1 May 2002. Downloaded from Cyclic AMP Calcium Cholera toxin (CT) Several toxins, including ciguatera toxin,11 Clostridium difficile Heat labile Escherichia coli enterotoxin (LT) toxin,12 Cryptosporidium toxin,13 and the Helicobacter pylori vacu- Salmonella enterotoxin 14 Campylobacter jejuni enterotoxin olating toxin seem to act through Ca. However, the involvement Pseudomonas aeruginosa enterotoxin of Ca in the secretory effect of these toxins has been only Shigella dysenteriae enterotoxin indirectly shown. A more definitive proof of the Ca mediated Cyclic GMP secretory effect has been provided by Raimondi et al, who have Heat stable Escherichia coli enterotoxin (ST) shown, using direct [Ca] measurement, that the enterotoxic Yersinia enterocolitica ST I and ST II enterotoxins i Yersinia bercovieri enterotoxin effect of the thermostable direct haemolysin (TDH) elaborated 15 Klebsiella pneumoniae enterotoxin by Vibrio parahaemolyticus is mediated by Ca. This toxin seems Heat stable Vibrio cholerae non-O1 enterotoxin to interact with a polysialoganglioside GT1b surface receptor, Enteroaggregative Escherichia coli heat stable enterotoxin (EAST1) whose physiological function remains to be established.15 Calcium Clostridium difficile enterotoxin Nitric oxide Ciguatera enterotoxin Cryptosporidium enterotoxin The role of NO in intestinal fluid and electrolyte balance varies Helicobacter pylori vacuolating toxin according to the pathophysiological conditions that activate Vibrio parahaemolyticus thermostable direct haemolysin (TDH) this pathway. Under physiological circumstances, NO exerts a Nitric oxide proabsorptive effect that involves the enteric nervous Shigella flexneri 2a Shigella enterotoxin 1 (ShET1) system.16 However, high NO production has been shown in 16–19 20 Pore forming toxins both animal models and humans to contribute to Clostridium perfrigens enterotoxin (CPE) diarrhoea by acting as a secretagogue. Staphylococcus aureus α toxin Our laboratory has described the elaboration by Shigella Vibrio cholerae cytolysin (CTC) flexneri 2a of two novel iron regulated enterotoxins, named Shig- ella enterotoxin 1 (ShET1) and 2 (ShET2), that alter electrolyte Toxins blocking protein synthesis Shigella dysenteriae Shiga toxin and water transport in rabbit small intestine both in vitro and in 21 22 EHEC Shiga like toxin 1 (SLT 1) and 2 (SLT 2) vivo. ShET1 is a chromosomally encoded, 55 kDa complex protein21 that is universally elaborated by Shigella flexneri 2a Toxin inducing protein synthesis strains but only rarely by other serotypes.23 ShET 1 appears to Staphylococcus aureus enterotoxin A exert an irreversible, dose dependent enterotoxic effect that EAggEC toxin does not seem to be mediated by Ca, cAMP, or cGMP. Recent Toxins affecting the enterocyte cytoskeleton studies have shown that, when tested on rabbit, rat, and mouse − Clostridium difficile toxin A and B intestines in vitro, ShET1 induces an increased NO2 concentra- Clostridium sordelli toxin tion in the bathing solution that is partially blocked by the Clostridium botulinum C2 and C3 toxins coadministration of inducible NO synthase (iNOS) inhibitors.24 Escherichia coli cytotoxic necrotising factor 1 (CNF 1) http://gut.bmj.com/ Campylobacter jejuni cytolethal distending toxin The direct effect of the toxin on iNOS expression has been Vibrio cholerae Zonula occludens toxin (Zot) confirmed by RT-PCR, Northern blot analysis, and by EAggEC plasmid encoded protein (PET) experiments conducted on a iNOS knock out mouse model.24 Bacteroides fragilis toxin (BFT) Vibrio parahaemolyticus thermostable direct haemolysin (TDH) PORE FORMING TOXINS Clostridium perfrigens is a common
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