SUMMARY Proliferation of the self-renewing epithelium of the gastric corpus occurs almost exclusively in the isthmus of the glands, from where cells migrate bi-directionally towards pit and base. The isthmus is therefore generally viewed as the stem cell zone. We find that the stem cell marker Troy is expressed at the gland base by a small subpopulation of fully differentiated chief cells. By lineage tracing using a Troy-eGFP-ires-CreERT2 allele, single marked chief cells are shown to generate entirely labeled gastric units over periods of months. This phenomenon accelerates upon tissue damage. Troy+ chief cells can be cultured to generate long-lived gastric organoids. Troy marks a specific subset of chief cells that display plasticity in that they are capable of replenishing entire gastric units, essentially serving as quiescent ‘reserve’ stem cells. These observations challenge the notion that stem cell hierarchies represent a 'one-way street'.

Tamoxifen, a selective estrogen receptor modulator, is widely used in research and clinically in patients. We find that treatment of normal mice with a single ≥ 3mg/20g body weight dose of tamoxifen leads to apoptosis of > 90% of all gastric parietal cells and metaplasia of zymogenic chief cells within 3 days. Remarkably, gastric histology returns to nearly normal by 3 weeks. Tamoxifen toxicity occurs by oral and intraperitoneal administration, in both sexes, in multiple strains, and does not depend on estrogen, though acid secretion inhibition is partially protective. Thus, substantial gastric toxicity is a heretofore unappreciated tamoxifen side effect.

Background & Aims The transition of gastric epithelial mucous neck cells (NCs) to digestive-enzyme–secreting zymogenic cells (ZCs) involves an increase in rough endoplasmic reticulum (rER) and formation of many large secretory vesicles. The transcription factor MIST1 is required for granulogenesis of ZCs. The transcription factor XBP1 binds the Mist1 promoter and induces its expression in vitro and expands the ER in other cell types. We investigated whether XBP1 activates Mist1 to regulate ZC differentiation. Methods Xbp1 was inducibly deleted in mice using a tamoxifen/Cre-loxP system; effects on ZC size and structure (ER and granule formation) and gastric differentiation were studied and quantified for up to 13 months after deletion using morphologic, immunofluorescence, quantitative reverse-transcriptase PCR, and immunoblot analyses. Interactions between XBP1 and the Mist1 promoter were studied by chromatin immunoprecipitation from mouse stomach and in XBP1-transfected gastric cell lines. Results Tamoxifen-induced deletion of Xbp1 (Xbp1Δ ) did not affect survival of ZCs but prevented formation of their structure. Xbp1Δ ZCs shrank 4-fold, compared to those of wild-type mice, with granulogenesis and cell shape abnormalities and disrupted rER. XBP1 was required and sufficient for transcriptional activation of MIST1. Despite severe structural defects, ZCs that developed in the absence of XBP1 expressed ZC markers (intrinsic factor, pepsinogen C) but did not lose expression of progenitor NC markers. Conclusions XBP1 controls the transcriptional regulation of ZC structural development; it expands the lamellar rough ER and induces MIST1 expression to regulate formation of large granules. XBP1 is also required for loss of mucous NC markers as ZCs form.

In the mammalian gastrointestinal tract, the cell fate decisions that specify the development of multiple, diverse lineages are governed in large part by interactions of stem and early lineage progenitor cells with their microenvironment, or niche. Here, we show that the gastric parietal cell (PC) is a key cellular component of the previously undescribed niche for the gastric epithelial neck cell, the progenitor of the digestive enzyme secreting zymogenic (chief) cell (ZC). Genetic ablation of PCs led to failed patterning of the entire zymogenic lineage: progenitors showed premature expression of differentiated cell markers, and fully differentiated ZCs failed to develop. We developed a separate mouse model in which PCs localized not only to the progenitor niche, but also ectopically to the gastric unit base, which is normally occupied by terminally differentiated ZCs. Surprisingly, these mislocalized PCs did not maintain adjacent zymogenic lineage cells in the progenitor state, demonstrating that PCs, though necessary, are not sufficient to define the progenitor niche. We induced this PC mislocalization by knocking out the cytoskeleton-regulating gene Cd2ap in Mist1−/− mice, which led to aberrant E-cadherin localization in ZCs, irregular ZC-ZC junctions, and disruption of the ZC monolayer by PCs. Thus, the characteristic histology of the gastric unit, with PCs in the middle and ZCs in the base, may depend on establishment of an ordered adherens junction network in ZCs as they migrate into the base.

The Epidermal Differentiation Complex (EDC) locus comprises a syntenic and linear cluster of genes whose concomitant expression is a hallmark feature of differentiation in the developing skin epidermis. Many of the EDC proteins are cross-linked together to form the cornified envelope, an essential and discrete unit of the mammalian skin barrier. The mechanism underlying coordinate transcriptional activation of the EDC is unknown. Within the human EDC, we identified an epidermal-specific regulatory enhancer, 923, that responded to the developmental and spatio-temporal cues at the onset of epidermal differentiation in the mouse embryo. Comparative chromosomal conformation capture (3C) assays in proliferating and differentiated primary mouse keratinocytes revealed multiple chromatin interactions that were physiologically sensitive between the 923 enhancer and EDC gene promoters and thus depict the dynamic, chromatin topology of the EDC. We elucidate a mechanistic link between c-Jun/AP-1 and 923, whereby AP-1 and 923-mediated EDC chromatin remodeling is required for functional EDC gene activation. Thus, we identify a critical enhancer/transcription factor axis governing the dynamic regulation of the EDC chromatin architecture and gene expression and provide a framework for future studies towards understanding gene regulation in cutaneous diseases.