Involvement of intracellular calcium stores in Giardia lamblia induced diarrhoea in mice

Host parasite interactions and pathophysiology in Giardia infections

Giardia is a protozoan parasite of the small intestine, and a leading cause of diarrhoeal disease worldwide in a variety of animals, including humans. The host-parasite interaction and pathophysiological processes of giardiasis remain incompletely understood. Current research suggests that Giardia-induced diarrhoeal disease is mediated by small intestinal malabsorption and maldigestion, chloride hypersecretion and increased rates of small intestinal transit. Small intestinal malabsorption and maldigestion results from the CD8+ lymphocyte-induced diffuse shortening of brush border microvilli. Activation of CD8+ lymphocytes occurs secondary to small intestinal barrier dysfunction, which results from heightened rates of enterocyte apoptosis and disruption of epithelial tight junctions. Both host and parasite factors contribute to the pathogenesis of giardiasis and ongoing research in this field may elucidate genotype/assemblage-specific pathogenic mechanisms. Giardia infections can result in chronic gastrointestinal disorders such as post-infectious Irritable Bowel Syndrome and symptoms may manifest at extra-intestinal sites, even though the parasite does not disseminate beyond the gastrointestinal tract. The infection can cause failure to thrive in children. Furthermore, there is now evidence suggesting that Giardia symptoms may vary between industrialised and developing areas of the world, for reasons that remain obscure. More research is needed to improve our understanding of this parasitic infection which was recently included in the World Health Organisation "Neglected Disease Initiative".

Coordinated epithelial NHE3 inhibition and barrier dysfunction are required for TNF-mediated diarrhea in vivo

Acute T cell-mediated diarrhea is associated with increased mucosal expression of proinflammatory cytokines, including the TNF superfamily members TNF and LIGHT. While we have previously shown that epithelial barrier dysfunction induced by myosin light chain kinase (MLCK) is required for the development of diarrhea, MLCK inhibition does not completely restore water absorption. In contrast, although TNF-neutralizing antibodies completely restore water absorption after systemic T cell activation, barrier function is only partially corrected. This suggests that, while barrier dysfunction is critical, other processes must be involved in T cell-mediated diarrhea. To define these processes in vivo, we asked whether individual cytokines might regulate different events in T cell-mediated diarrhea. Both TNF and LIGHT caused MLCK-dependent barrier dysfunction. However, while TNF caused diarrhea, LIGHT enhanced intestinal water absorption. Moreover, TNF, but not LIGHT, inhibited Na+ absorption due to TNF-induced internalization of the brush border Na+/H+ exchanger NHE3. LIGHT did not cause NHE3 internalization. PKCalpha activation by TNF was responsible for NHE3 internalization, and pharmacological or genetic PKCalpha inhibition prevented NHE3 internalization, Na+ malabsorption, and diarrhea despite continued barrier dysfunction. These data demonstrate the necessity of coordinated Na+ malabsorption and barrier dysfunction in TNF-induced diarrhea and provide insight into mechanisms of intestinal water transport.

Giardia lamblia rearranges F-actin and alpha-actinin in human colonic and duodenal monolayers and reduces transepithelial electrical resistance

The mechanisms of epithelial injury in giardiasis remain unknown. The effects of live Giardia lamblia on cellular G-actin, F-actin, alpha-actinin, and electrical resistance of human intestinal epithelial monolayers were investigated using SCBN and Caco2 cell lines grown on chamber slides or Transwell filter membranes. In separate experiments, some monolayers were also exposed to sonicated trophozoites, some to supernatant from live G. lamblia cultures, and some with or without the Ca2+ channel blocker verapamil. After 2, 24, or 48 hr of coincubation with G. lamblia, monolayers were assessed for cytoskeletal arrangement under fluorescence and confocal laser microscopy, and transepithelial electrical resistance was measured. Exposure to live G. lamblia trophozoites induced localized condensation of F-actin and loss of perijunctional alpha-actinin while G-actin remained unchanged. Confocal laser microscopy indicated that F-actin rearrangement was not affected by verapamil and was localized within the terminal web area. Coincubation of monolayers with G. lamblia lysates or with spent medium alone similarly rearranged F-actin. Verapamil alone did not alter F-actin. Electrical resistance of SCBN and Caco2 monolayers exposed to G. lamblia was significantly decreased versus controls regardless of whether live or lysed trophozoite samples were used. The results indicate that G. lamblia-induced epithelial injury is associated with F-actin and alpha-actinin rearrangements in the terminal web area via mechanisms independent of extracellular Ca2+. These alterations are associated with reduced transepithelial electrical resistance and are due at least in part to trophozoite products.

Role of intracellular second messengers and reactive oxygen species in the pathophysiology of V. cholera O139 treated rabbit ileum

Vibrio cholerae O139 has pandemic potential and it produces copious amounts of fluid secretion. The levels of various second messengers (intracellular Ca2+, cAMP, IP3, PKC) were measured to determine the cause of fluid secretion produced by this strain of V. cholerae. There was a significant increase in the levels of these second messengers in V. cholerae O139 treated ileum as compared to control ileum (enterocytes). Levels of these second messengers were also assessed in V. cholerae 569B induced fluid secretion in rabbit ileum and it was found that the levels were raised more in V. cholerae O139 treated ileum than in V. cholerae 569B treated rabbit ileum. The intestinal damage was assessed by measuring changes in the extent of lipid peroxidation of the enterocytes. Intracellular second messengers are known to raise the extent of lipid peroxidation. In V. cholerae O139 treated loops calcium ionophore A23187 enhanced the extent of lipid peroxidation whereas l-verapamil could only marginally decrease the lipid peroxidation. Dantrolene and H7 significantly decreased the extent of lipid peroxidation of enterocytes in V. cholerae O139 treated rabbit ileum. However, PMA could not enhance further the extent of lipid peroxidation in V. cholerae O139 treated rabbit ileum. So intracellular calcium and protein kinase C appear to be involved in intestinal damage caused by V. cholerae O139. Reactive oxygen species are responsible for causing tissue damage and the extent of oxidative damage depends on the balance between the pro-oxidants and the anti-oxidants. So the changes in the enterocytes' antioxidant level during V. cholerae O139 mediated intestinal infection was estimated. There was a significant decrease in the enterocyte level of the antioxidant enzymes SOD, catalase, glutathione peroxidase, glutathione reductase, glutathione transferase and glucose-6-phosphate dehydrogenase in V. cholerae O139 mediated intestinal infection. So a significant decrease in the levels of antioxidant defenses and a significant increase in the levels of second messengers appear to be important in mediating V. cholerae O139 induced lipid peroxidation which contributes to the changes in membrane permeability and thus to fluid secretion.

Swelling detection for volume regulation in the primitive eukaryote Giardia intestinalis: a common feature of volume detection in present-day eukaryotes

It is increasingly evident that cell swelling is associated with the triggering of many biological processes, including progression of the cell cycle, hormonal response, and gene expression. However, the mechanism by which cell swelling is initially sensed and converted into intracellular signals is still ill-defined. We report here an early event in the detection of cell swelling and initiation of the volume regulatory response in Giardia intestinalis, an ancient representative of the eukaryotic kingdom. Giardial cell swelling, irrespective of the extent, was sensed at a cell volume of 1.06 x isosmotic volume (the threshold volume), at which the transition of the volume regulatory transport system from the 'resting' to the 'open' state occurred. Irreversible modification by p-chloromercuribenzoate (pCMB) and N-ethylmaleimide (NEM) of reduced thiols affected the threshold volume, but in opposing manners: pCMB increased the threshold volume to 1.14 x and NEM decreased to 0.85 x isosmotic volume. The simple modification of the threshold volume by NEM caused a drastic reduction of giardial cell volume under isosmotic conditions, with a process strikingly similar to the opening of mitochondrial permeability transition pore, a causative event in stress-induced programmed cell death. Substantial evidence supports the hypothesis that modulation of the membrane thiol moieties at the threshold volume, causing the 'all-or-nothing' type of swelling detection, represents the event linking cell swelling to the second messenger systems for volume regulation in present eukaryotes. Pathophysiological implications of alteration of the threshold volume are discussed.

Calcium and protein kinase C play a significant role in response to Shigella toxin in rabbit ileum both in vivo and in vitro

The role of second messengers in Shigella toxin (STx) induced fluid secretion in rabbit ileum was evaluated. In vivo and in vitro studies were carried out in presence or absence of following modulators: Ca2+ ionophore A23187 (15 microM), l-verapamil (200 microM), phorbol-12-myristate-13-acetate (PMA, 200 ng), 1-(5-isoquinolinyl-sulphonyl)-2-methyl-piperazine (H-7, 15 microg) and indomethacin (20 microM). In in vivo studies, the fluid accumulation into rabbit ileal loops in response to STx was measured in presence or absence of these modulators. In in vitro studies, unidirectional fluxes of Na+ and Cl- were carried out in presence or absence of these modulators. The addition of Ca2+ ionophore A23187 along with STx further increases the amount of fluid already induced by STx. Whereas the presence of l-verapamil along with STx did not decrease the amount of fluid induced by STx. In vitro findings were in consonance with the in vivo studies. A significant increase in inositol triphosphate (IP3) levels was observed in enterocytes isolated from STx treated rabbit ileum. The addition of PMA into rabbit ileal loops in presence of STx mimicked the effect of STx while the presence of H-7 reversed the secretion caused by STx to absorption. Similar results were obtained while determining unidirectional fluxes of Na+ and Cl- in presence of PMA and also with H-7. A significant increase in PKC levels was observed in the membrane fraction of enterocytes isolated from STx treated rabbit ileum as compared to control. Further a marked decrease in PKC levels was observed in the presence of H-7 in membrane fraction of enterocytes isolated from STx treated rabbit ileum. The addition of indomethacin into rabbit ileal loops reversed the secretion (caused by STx) to absorption. In vitro findings were in consonance with in vivo studies. Besides, there was a significant increase in PG-E levels in enterocytes isolated from STx treated rabbit ileum as compared to control. These findings suggested that STx induced enteritis involves the role of PKC, intracellular calcium stores and prostaglandins. The extracellular calcium pool probably does not play a significant role in this process.