Evidence for a rapid, direct effect on epithelial monolayer integrity and transepithelial transport in response to Salmonella invasion

Abstract

In cultured monolayers of high-resistance Ma-din-Darby Canine Kidney (MDCK) cells, infection with Salmonella typhimurium SL1344 resulted in a dose- and time-dependent increase in transepithelial conductance (G_t) and short-circuit current (I _sc). There was a direct linear relationship between the S. typhimurium- induced increments in I _sc and G_t suggesting that this early change in epithelial parameters is, in part, the result of a cellular conductance change most probably at the apical membrane. An additional wild-type S. typhimurium strain, SR11, and an invasion-deficient isogenic mutant SB 111 carrying a non-polar mutation in invA were used to confirm that the S. typhimurium- induced change in epithelial electrical parameters is directly linked to the invasion process. The 5. typhimurium- induced change in epithelial electrical parameters was markedly attenuated in Na⁺-free choline medium. Addition of piretanide (1⁻⁴ M, basal side) failed to affect the increased epithelial conductance and I_sc after a 40-min incubation with S. typhimurium. NPPB (5×10⁻⁴ M) added to the apical medium reduced the S. typhimurium-stimulated I _sc by 28%, but G_t was not significantly reduced. It is unlikely that the S. typhimurium- induced I _sc is due to Cl⁻ secretion. Staining of S. typhimurium- infected MDCK I monolayers with TRITC-phalloidin revealed marked alterations of F-actin; diffuse intracellular accumulations of F-actin corresponding to the presence of invading bacteria were observed by 15 min. After 60 min, prominent extrusions of the apical membrane corresponding to previously described “membrane ruffles” were noted. Marked accumulations of perijunctional F-actin in infected cells corresponded to contraction of the perijunctional actin ring at the apical pole. In adjacent cells marked distortion and stretch of the apical surface was evident. The invasion-deficient invA mutant SB 111 failed to induce these morphological changes. These data demonstrate that S. typhimurium invasion induces increased transcellular conductance which does not result from stimulation of Cl⁻ secretion but instead appears to be predominantly due to increased Na⁺ permeability. The increased membrane conductance is coincident with increased transepithelial inulin permeability indicating that the increment in G_t has an additional “paracellular” component. The S. typhimurium- induced alterations in epithelial parameters may be related to “membrane ruffling” and/or to the accompanying changes in cell shape.