N-methyl d-Aspartate Channels Link Ammonia and Epithelial Cell Death Mechanisms in Helicobacter pylori Infection

Abstract

BACKGROUND & AIMS Helicobacter pylori infection is a risk factor for gastric cancer. Ammonia/ammonium (A/A) is a cytotoxin generated by H pylori that kills gastric epithelial cells. We investigated whether A/A cytotoxicity occurs by activating N-methyl D-aspartate (NMDA) channels, which results in Ca2+ permeation and epithelial cell death. METHODS Gastric epithelial cells were cultured to confluence and then incubated with A/A and NMDA channel or cell signaling antagonists. Cells were incubated with wild-type H pylori or mutant strains that do not produce A/A. Changes in intracellular Ca2+ were examined in living cells by confocal microscopy. Biochemical and histochemical techniques were used to examine the relationship between A/A-induced cell death and intracellular levels of Ca2+. RESULTS A/A increased Ca2+ permeation in gastric epithelial cells; the increase was blocked by NMDA receptor and cell signaling antagonists. Wild-type, but not mutant H pylori, also caused extensive Ca2+ permeation of gastric epithelial cells, which was blocked when NMDA receptor expression was repressed. Ca2+ that entered cells was initially cytoplasmic and activated proteases. Later, the Ca2+ was sequestered to cytoplasmic vacuoles that are dilatations of the endoplasmic reticulum (ER). Inositol -3-phosphate–dependent release of Ca2+ from ER and protease activity damaged mitochondria, reduced levels of ATP, and transcriptionally up-regulated cell death effectors. Expression of the NMDA receptor was altered in stomachs of mice infected with H pylori. CONCLUSIONS A/A affects gastric epithelial cell viability by allowing excessive Ca2+ permeation through NMDA channels. NMDA channels might thereby regulate cell survival and death pathways during development of gastric cancers associated with H pylori infection.

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