Defining Gastric Epithelial Cell Population Dynamics At Homeostasis And Following Injury

Gastric diseases affect many people around the world, yet surprisingly little is known about the basic dynamics of gastric epithelial cells. Loss of acid-secreting parietal cells has long been observed to precede pre-cancerous gastric metaplasias like Spasmolytic Polypeptide-Expressing Metaplasia (SPEM), yet no signaling component from dying parietal cells has yet been implicated in initiating the metaplastic responses. Also, experiments pulsing 3H-thymidine and or examining intracellular components suggest that gastric mucous neck cells are short-lived transient intermediates between the gastric stem cell and mature zymogenic “chief” cells, yet specifics about this transition remain elusive. Here, we develop a novel mouse line and new techniques for tracing gastric cell lines to further probe these interactions. To identify the changes in parietal cell signaling upon injury which lead to chief cell dedifferentiation and the appearance of SPEM, we bred mice expressing the human diphtheria toxin receptor solely in parietal cells. Injection of diphtheria toxin specifically kills parietal cells through apoptosis. Surprisingly, while the parietal cells died in similar numbers to those in mice treated with tamoxifen, no SPEM or chief cell dedifferentiation was observed, and proliferation only increased through the neck, with minimal proliferation in the base. We also showed that SPEM can still arise if we inject tamoxifen or DMP-777 after the parietal cells are already killed via diphtheria toxin. These experiments indicate that chief cell dedifferentiation is not simply triggered by the loss of parietal cells, nor are dying parietal cells necessary for acute drugs to initiate metaplasia. However, the signal which initiates the metaplasia remains unknown. Furthermore, we studied the dynamics of long-lived cells in the mouse stomach using a modified BrdU pulse-chase protocol. Published reports describing gastric epithelial cell population dynamics have relied on continuous infusion or relatively short pulse-chases of DNA markers such as 3H-thymidine. Here, we pioneer a new technique, pulsing BrdU throughout our normal tamoxifen injury regimen to label nearly all cells in the unit, allowing us to chase for months and track long-lived label-retaining cells. Following our pulse of tamoxifen and BrdU, we find that nearly two thirds of chief cells retain label even through a 9-month chase, indicating that they are either longer-lived than expected or that chief cells slowly divide to maintain their own population without being replaced by newer cells from higher in the unit. We also find subpopulations of label-retaining neck cells and parietal cells exist after a 9-month chase, shedding more light on their population dynamics. To further test whether neck cells give rise to chief cells, as others have reported, we administered a short BrdU pulse followed by various chase lengths and found that most neck cells do not directly give rise to chief cells, indicating that neck cells likely have a functional, as yet unidentified purpose, other than acting as a precursor to chief cells. Finally, we show through additional tamoxifen and Helicobacter pylori injury that long-lived chief cells give rise to acute and chronic SPEM cells and find that SPEM cells can directly redifferentiate back into chief cells upon recovery from injury. Altogether, we suggest for the first time that chief cells may be a stable population in the gastric unit, largely maintaining their own census at homeostasis and in injury independently of neck cell transitions or parietal cell status