Orchestration of neutrophil movement by intestinal epithelial cells in response to Salmonella typhimurium can be uncoupled from bacterial internalization

Intestinal epithelial cells respond to Salmonella typhimurium by internalizing this pathogen and secreting, in a polarized manner, an array of chemokines which direct polymorphonuclear leukocyte (PMN) movement. Notably, interleukin-8 (IL-8) is secreted basolaterally and directs PMN through the lamina propria, whereas pathogen-elicited epithelial chemoattractant (PEEC) is secreted apically and directs PMN migration across the epithelial monolayer to the intestinal lumen. While most studies of S. typhimurium pathogenicity have focused on the mechanism by which this bacterium invades its host, the enteritis characteristically associated with salmonellosis appears to be more directly attributable to the PMN movement that occurs in response to this pathogen. Therefore, we sought to better understand the relationship between S. typhimurium invasion and epithelial promotion of PMN movement. First, we investigated whether S. typhimurium becoming intracellular was necessary or sufficient to induce epithelial promotion of PMN movement. Blocking S. typhimurium invasion by preventing, with cytochalasin D, the epithelial cytoskeletal rearrangements which mediate internalization did not reduce the epithelial promotion of PMN movement. Conversely, bacterial attainment of an intracellular position was not sufficient to induce model epithelia to direct PMN transmigration, since neither basolateral invasion by S. typhimurium nor apical internalization of an invasion-deficient mutant (achieved by inducing membrane ruffling with epidermal growth factor) induced this epithelial cell response. These results indicate that specific interactions between the apical surface of epithelial cells and S. typhimurium, rather than simply bacterial invasion, mediate the epithelial direction of PMN transmigration. To further investigate the means by which S. typhimurium induces epithelia to direct PMN movement, we investigated whether the same signaling pathways regulate secretion of IL-8 and PEEC. IL-8 secretion, but not PEEC secretion, was activated by phorbol myristate acetate and blocked by an inhibitor (mg-132) of the proteosome which mediates NF-kappabeta activation. Further, secretion of IL-8, but not PEEC, was activated by an entry-deficient (HilDelta) S. typhimurium mutant or by basolateral invasion of a wild-type strain. Together, these results indicate that distinct signaling pathways mediate S. typhimurium invasion, induction of IL-8 secretion, and induction of PEEC secretion in model intestinal epithelia.     

Ciguatoxins and brevetoxins, neurotoxic polyether compounds active on sodium channels

Ciguatoxins (CTXs) and brevetoxins (PbTxs) modify the activation and inactivation processes of voltage-sensitive sodium channels (VSSC). In this study, the specific binding to rat brain synaptosomes of two commercial PbTxs, five purified CTXs and their derivatives was evaluated in competition with various concentrations of radiolabelled brevetoxin ([3H]PbTx-3). The results indicate that all CTXs bind specifically and with high affinity to sodium channels. Statistical analysis of the calculated inhibition constants identified two classes of toxins: the PbTxs and the less polar CTXs, and a group of CTXs of very high affinity. Relatively small chemical differences between the CTXs gave rise to significant differences in their affinity to the rat brain sodium channels. Cytotoxic effects associated with sodium channel activation were evaluated for the two classes of toxins on murine neuroblastoma cells, and their acute toxicity was determined in mice. CTXs have shown high affinities to VSSC of rat brain membranes and strong cytotoxic effects on neuroblastoma cells which correlate with their very low LD50 in mice. For PbTxs, it is different. Although binding with high affinity to VSSC and giving rise to significant cytotoxic effects, they are known to be poorly toxic intraperitoneally to mice. Furthermore, within the CTXs family, even though the most toxic compound (CTX-1B) has the highest affinity and the less toxic one (CTX-4B) the lowest affinity, a detailed analysis of the data pointed out a complex situation: (i) high affinity and toxicity seem to be related to the hydroxylation of the molecule on the A-ring rather than to the backbone type, (ii) acute toxicity in mice does not follow exactly the sodium-dependent cytotoxicity on neuroblastoma cells. These data suggest that the high toxicity of CTXs is related to sodium-dependent disturbances of the excitable membranes but might also involve other cellular mechanisms.     

Comparison of alveolar ventilation, oxygenation, pressure support, and respiratory system resistance in response to noninvasive versus conventional mechanical ventilation in foals

OBJECTIVE: To compare the efficacy of positive pressure ventilation applied through a mask versus an endotracheal tube, using anesthetized/paralyzed foals as a model for foals with hypoventilation. ANIMALS: Six 1-month-old foals. PROCEDURE: A crossover design was used to compare the physiologic response of foals to 2 ventilatory techniques, noninvasive mask mechanical ventilation (NIMV) versus endotracheal mechanical ventilation (ETMV), during a single period of anesthesia and paralysis. Arterial pH, PaO2, PaCO2, oxygen saturation, end-tidal CO2 tension, airway pressures, total respiratory system resistance, resistance across the upper airways (proximal to the midtracheal region), and positive end-expiratory pressures (PEEP) were measured. Only tidal volume (VT; 10, 12.5, and 15 ml/kg of body weight) or PEEP (7 cm of H2O) varied. RESULTS: Compared with ETMV, use of NIMV at equivalent VT resulted in PaCO2 and pH values that were significantly higher, but PaO2 was only slightly lower. Between the 2 methods, peak airway pressure was similar, but peak expiratory flow was significantly lower and total respiratory resistance higher at each VT for NIMV. Delivery of PEEP (7 cm of H2O) was slightly better for ETMV (7.1 +/- 1.3 cm of H2O) than for NIMV (5.6 +/- 0.6 cm of H2O). CONCLUSION: These data suggest that use of NIMV induces similar physiologic effects as ETMV, but the nasal cavities and mask contribute greater dead space, manifesting in hypercapnia. Increasing the VT used on a per kilogram of body weight basis, or the use of pressure-cycled ventilation might reduce hypercapnia during NIMV. CLINICAL RELEVANCE: Use of NIMV might be applicable in selected foals, such as those with hypoventilation and minimal changes in lung compliance, during weaning from endotracheal mechanical ventilation, or for short-term ventilation in weak foals.