The gastric barrier is a collection of defenses that protect the body against luminal acid, as well as defend against exogenous insults to gastric tissue (e.g. food, aspirin, alcohol, etc). The overall objective of this project was to understand how the first line of defense, the pre-epithelial mucus-bicarbonate layer, protects the gastric epithelium from microscopic damage. The overall hypothesis was that efficient epithelial repair of damage in the stomach is regulated by surface pH control, via epithelial acid and bicarbonate secretion. Utilizing a unique gastric damage model through two-photon photodamage, we could observe in vivo how manipulation of these secretory processes affected the fidelity of epithelial repair. Additionally, we were able to observe how the gastric carcinogen Helicobacter pylori (H. pylori) responded to areas of microscopic damage, areas which contain micro-environments of raised pH.
Our results are the first to detail how gastric epithelial acid and bicarbonate transport processes are activated in response to local tissue injury. Genetic (SLC26A9 Cl-/HCO3- exchanger null) and pharmacologic (anion transport inhibitor H2DIDS) tools revealed that damage to the gastric epithelium transiently activates cellular bicarbonate secretion, a process which was found to be not necessary for efficient epithelial repair. Measurements of gastric surface epithelial acid/base status (e.g. intracellular pH) suggested that damaged epithelial cells have altered transport in response to damage, suggesting that this transient increase in bicarbonate secretion may be an intrinsic property of the damaged epithelium, to protect undamaged cells that migrate to repair the wound. While inhibiting bicarbonate secretion did not appear to affect the repair of microscopic damage, we found that omeprazole, a clinically used proton pump inhibitor, significantly enhanced epithelial repair of damage. The effect of omeprazole was also found to occur via a pH-independent mechanism, as epithelial repair was significantly enhanced over control animals, despite high gastric surface pH in both groups. Therefore, these data represent a novel in vivo mechanism of action for omeprazole during acute microscopic injury, a mechanism that is unable to be observed in the clinical setting.
Finally, our experiments have revealed that H. pylori appear to migrate towards damaged tissue in response to two-photon photodamage. Such migration is disrupted when surface/luminal pH gradients are reversed, suggesting that this pathogen can use gastric pH gradients to guide them not only towards the healthy gastric surface during colonization, but towards areas of acute injury. Preliminary studies also show that mutations in the vacA and cagA genes, two major H. pylori virulence factors, also render H. pylori unable to migrate towards damage. In summary, this project has detailed how acid and bicarbonate secretion regulate pH at the gastric surface during microscopic injury, and suggests that these transport processes may be involved in attracting H. pylori towards the injured gastric epithelium.