Cellular mechanisms of interaction between uropathogenic Escherichia coli and renal epithelial cells

Sammanfattning: Urinary tract infections will often, particularly in children, lead to renal scar formation or hypoplastic small kidneys post-infectiously. The most common causative agent is uropathogenic Escherichia coli (E. coli). The severity of the renal injury is considered to be determined by the host immunological response and the bacterial virulence factors. Here, we have examined cellular mechanisms by which the host cells respond to E. coli exposure. We found a novel role for the E. coli exotoxin, a-hemolysin (Hly) as an inducer of a low frequency intracellular calcium (Ca2+i) oscillation that acts as a second messenger to induce release of proinflammatory cytokines in rat proximal tubule (RPT) cells. The Ca2+i oscillation exhibited a constant periodicity of 12 minutes and was generated by a combination of calcium influx through voltage-dependent calcium channels and the release from Ca2+i stores through IP3 receptor activation. We show indicating evidence that Hly can act through a cell- membrane receptor. We also found that Hly exerts a dual function on RPT cells. At high concentrations Hly induces sustained elevation of Ca2+i leading to cell lysis whereas at low concentrations it induces Ca2+i oscillations that may serve as a host-cell defense response. Infant rat renal tissue seems to have a fully developed innate immune defense to bacterial toxins. Cytokine release, LPS signaling pathway through Toll-like receptor-4 and Hly induced Ca2+i oscillations responded similarly in infant and adult renal cells. We could show that E. coli pyelonephritis in the infant rat kidney caused a decrease in cell proliferation and increased apoptosis in the renal cortex distant from the site of infection. The higher vulnerability in infants to post-pyelonephritogenic renal growth retardation is likely due to intrarenal cellular effects in the growing kidney, maybe due to bacterial secreted factors. In conclusion, we show a novel, dual role for the E. coli exotoxin, Hly as a virulence factor in childhood pyelonephritis. The higher susceptibility to pyelonephritis in infants is not likely due to an immature innate response to bacterial toxins but rather to anatomical and epithelial cell immaturity that permits bacteria and toxins to accumulate in the renal tissue. Our results shed a new light on the role of pore-forming toxin in disease. Increased knowledge in this field may have therapeutic implications.