Diarrhea remains a worldwide scourge, and its prevalence and impact is also under-recognized and perhaps increasing even in developed countries. Studies from our laboratory, as well as many others, are beginning to reveal the spectrum of mechanisms that are exploited by invasive pathogens, which complements our understanding of enterotoxigenic diarrheal disease. We may also gain a greater understanding of the benefits of the diarrheal response, which has implications for the treatment of diarrhea in otherwise healthy individuals. Likewise, an improved understanding of epithelial transport physiology may lead to the development of more rational treatments, such as those that target signaling events related to growth factor receptors. Continue reading
Tag Archives: Salmonellosis
I will finally make some points about the potential benefits to the host of a diarrheal response, based in part on some new studies we have initiated with another invasive pathogen, Salmonella typhi. This organism is very closely related to S typhimurium at the genetic level, yet causes a quite different clinical picture . Thus, while S typhimurium produces disease that is essentially limited to the gastrointestinal tract in normal individuals, with significant diarrhea, S typhi usually causes typhoid fever, a disease that is characterized by sepsis due to rapid bacterial dissemination to extraintestinal sites, but with little or no diarrhea . We have shown that, unlike S typhimurium, S typhi fails to elicit a delayed hypersecretory response in infected intestinal epithelial cells, and has qualitatively different effects on epithelial cell signaling. Continue reading
We also examined whether probiotic bacteria, commensals that exert health benefits beyond inherent nutrition if ingested in certain numbers, are capable of altering signaling events and their functional outcomes induced in epithelial cells by invasive pathogens. We undertook these studies because there is significant anecdotal evidence that probiotic preparations alleviate the symptoms of intestinal infections and other digestive disorders like inflammatory bowel diseases, yet there is a paucity of information about their mechanism(s) of action. Our studies have revealed that probiotics are capable of causing a sustained increase in barrier function in intestinal epithelial cell lines, and can also attenuate the decrease in transepithelial resistance that is produced by infection with invasive bacteria. Continue reading
Infected epithelial cells also show heightened secretory responses to a variety of cAMP-dependent and other chloride secretagogues, such as galanin, further exacerbating fluid loss. The increases in both basal and stimulated secretion appear to result from the sequential induction of expression of inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2). In turn, the products of these enzymes not only elevate cyclic nucleotide second messengers that are capable of directly stimulating chloride secretion, but also cause increased expression of CFTR and NKCC1, resulting in a hypersecretory phenotype . Finally, infected cells have impaired barrier function, which also appears to depend on iNOS (but not COX-2) induction . This phenomenon probably further contributes to diarrhea in vivo by rendering the epithelium incapable of sustaining the electrochemical gradients needed for absorptive fluxes. The later effects of Salmonella infection that may account for the diarrheal responses are depicted in Figure 3. Continue reading
We speculate, therefore, that the recruitment of EGF receptor-dependent signaling events that limit calcium-dependent chloride secretion allows the Salmonella infection to become established by preventing the short burst of chloride (and accompanying fluid) secretion from washing away the bacteria before they are able to invade (Figure 2). While these data do not explain why patients infected with S typhimurium experience diarrhea, they may elucidate the mechanisms for initial colonization and evasion of host defenses.
On the other hand, at later times (hours to days) after the addition of S typhimurium to epithelial cell monolayers, there is a significant increase in the rate of spontaneous chloride secretion, which might correlate with the diarrheal response in vivo.
Let us now consider the mechanisms of diarrheal disease caused by invasive bacterial pathogens. Invasive bacteria account for more than half of the mortality due to foodborne illness in the United States, yet we are only beginning to understand how they cause diarrhea . This lack of understanding contrasts with that for diarrhea caused by enterotoxigenic bacteria. The latter release soluble toxins into the intestinal lumen, thereby altering epithelial function without mucosal invasion. Examples include cholera and enterotoxigenic Escherichia coli, the latter being the major causative agent of traveler’s diarrhea . Therefore, we have initiated studies that seek to define the mechanisms of diarrhea produced by invasive bacteria, especially Salmonella typhimurium, which is prevalent in both developed and developing countries. Continue reading
We are beginning to develop an understanding of the complex set of integrated signalling events that cooperate to limit the extent of epithelial chloride secretion in intestinal epithelial cells, details of which are provided in Figure 1. The significance of these findings lies in the knowledge that chloride secretion is demonstrably subject to inhibitory as well as stimulatory regulatory mechanisms, and the former might be exploited for antidiarrheal therapy. Likewise, growth factors and their receptors emerge as critical mediators of antisecretory signaling, and small molecule activators of consequent signals may be suitable as antidiarrheal drugs. Continue reading
Moreover, because EGF receptors and relevant ligands (including EGF itself and transforming growth factor-alpha [TGFa]) may be regulated in the setting of intestinal injury and inflammation, the beneficial effects of EGF on net fluid transport may represent an adaptive response that protects the body from excessive fluid loss . We have shown that EGF inhibits chloride secretion via an intracellular signaling pathway that sequentially recruits the enzymes phosphatidylinositol 3-kinase and protein kinase C to the basolateral membrane of secretory epithelial cells, ultimately limiting chloride secretion by reducing the activity of a baso-lateral potassium channel . If potassium cannot be recycled across this membrane, the driving force for chloride exit through apical chloride channels is lost. Continue reading
Thus, in a disease like cholera, where there is an irreversible increase in cAMP levels in intestinal epithelial cells, diarrhea is likely worsened by the simultaneous stimulation of chloride secretion and inhibition of sodium chloride absorption. Notably, however, sodium-coupled nutrient absorption remains intact, providing a rationale for treatment with oral rehydration solutions.
We and other investigators have explored intracellular messengers that regulate secretory transport, with the hope that we might discover pathways that could be exploited to treat diarrheal symptoms. Cyclic nucleotides induce significantly larger and more prolonged increases in chloride secretion than does elevated cytosolic calcium . Moreover, if epithelial cells are presented with combinations of agonists acting through these two pathways, a synergistic enhancement of chloride secretion occurs, leading us to speculate that calcium-dependent responses represent the ‘fine-tuning’ of the system. Continue reading
Glucose then exits basolaterally via a facilitated diffusion glucose transport pathway (GLUT-2). In between meals, when nutrients are not available, water absorption can alternatively be driven by the coupled uptake of chloride and sodium across the apical membrane via the downregulated in adenoma (DRA) chloride/bicarbonate exchanger and the sodium/hydrogen exchanger (NHE-3), respectively . Again, this process is dependent on the Na+/K+-ATPase and probably also involves basolateral chloride exit across a potassium/chloride cotransporter (putatively identified as KCC1).
Considering fluid secretion, chloride uptake from the bloodstream across the basolateral membrane of crypt epithelial cells is mediated by a sodium/potassium/2 chloride cotransporter, NKCC1, which responds to the low intracellular sodium concentration established by the sodium pump. Continue reading