Stem cells generate rapidly dividing transit-amplifying cells that have lost the capacity for self-renewal but cycle for a number of times until they exit the cell cycle and undergo terminal differentiation. We know very little of the type of signals that trigger the earliest steps of stem cell differentiation and mediate a stem cell to transit-amplifying cell transition. We show that in normal intestinal epithelium, endoplasmic reticulum (ER) stress and activity of the unfolded protein response (UPR) are induced at the transition from stem cell to transit-amplifying cell. Induction of ER stress causes loss of stemness in a Perk-eIF2α-dependent manner. Inhibition of Perk-eIF2α signaling results in stem cell accumulation in organoid culture of primary intestinal epithelium. Our findings show that the UPR plays an important role in the regulation of intestinal epithelial stem cell differentiation.

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Homeostasis of the rapidly renewing intestinal epithelium depends on a balance between cell proliferation and loss. Indian hedgehog (Ihh) acts as a negative feedback signal in this dynamic equilibrium. We discuss recent evidence that Ihh may be one of the key epithelial signals that indicates epithelial integrity to the underlying mesenchyme.

Rosekrans SL, Baan B, Muncan V, van den Brink GR. Esophageal development and epithelial homeostasis. Am J Physiol Gastrointest Liver Physiol 309: G216 -G228, 2015. First published July 2, 2015; doi:10.1152/ajpgi.00088.2015.-The esophagus is a relatively simple organ that evolved to transport food and liquids through the thoracic cavity. It is the only part of the gastrointestinal tract that lacks any metabolic, digestive, or absorptive function. The mucosa of the adult esophagus is covered by a multilayered squamous epithelium with a remarkable similarity to the epithelium of the skin despite the fact that these tissues originate from two different germ layers. Here we review the developmental pathways involved in the establishment of the esophagus and the way these pathways regulate gut-airway separation. We summarize current knowledge of the mechanisms that maintain homeostasis in esophageal epithelial renewal in the adult and the molecular mechanism of the development of Barrett's metaplasia, the precursor lesion to esophageal adenocarcinoma. Finally, we examine the ongoing debate on the hierarchy of esophageal epithelial precursor cells and on the presence or absence of a specific esophageal stem cell population. Together the recent insights into esophageal development and homeostasis suggest that the pathways that establish the esophagus during development also play a role in the maintenance of the adult epithelium. We are beginning to understand how reflux of gastric content and the resulting chronic inflammation can transform the squamous esophageal epithelium to columnar intestinal type metaplasia in Barrett's esophagus. esophagus; development; homeostasis; stem cell; endoderm THE WORD ESOPHAGUS IS DERIVED from the Greek words ε (oisein, to carry) and ␣␥ε (phagein, to eat). This description fits well with the functional role of the esophagus that mainly serves to "carry food" into the stomach. From the pharyngoesophageal junction, the esophagus passes through the mediastinum and diaphragm and connects to the cardia of the stomach at the gastroesophageal junction or Z-line. The pharyngoesophageal and gastroesophageal junctions anatomically overlap with the upper and lower esophageal sphincters. Both sphincters are closed except during swallowing to assure a unidirectional flow of esophageal content toward the stomach and to prevent reflux of gastric content into the esophagus. The relatively simple histology of the esophageal epithelium corresponds with the fact that the esophagus has no role other than to pass food through the thorax to the stomach. It does not play a known digestive, endocrine, or metabolic role and the epithelium consists of a simple stratified squamous epithelium, which provides a good protective layer against the unmodified food stream on its way to the stomach. Despite the perhaps somewhat prosaic functional role of the esophagus compared with other organs in the body, we feel that it is essential to gain a better understanding of the mechanisms that regulate normal esophageal homeo...

Histone deacetylases (HDACs) are posttranslational modifiers that deacetylate proteins. Despite their crucial role in numerous biological processes, the use of broad-range HDAC inhibitors (HDACi), has shown clinical efficacy. However, undesired side effects highlight the necessity to better understand the biology of different HDACs and target the relevant HDACs. Using a novel mouse model, in which HDAC1 and HDAC2 can be simultaneously deleted in the intestine of adult mice, we show that the simultaneous deletion of HDAC1 and HDAC2 leads to a rapid loss of intestinal homeostasis. Importantly, this deletion cannot be sustained, and 8 days after initial ablation, stem cells that have escaped HDAC1 or HDAC2 deletion swiftly repopulate the intestinal lining. In vitro ablation of HDAC1 and HDAC2 using intestinal organoid cultures resulted in a down-regulation of multiple intestinal stem cell markers and functional loss of clonogenic capacity. Importantly, treatment of wild-type organoids with class I-specific HDACi MS-275 also induced a similar loss of stemness, providing a possible rationale for the gastrointestinal side effects often observed in HDACi-treated patients. In conclusion, these data show that HDAC1 and HDAC2 have a redundant function and are essential to maintain intestinal homeostasis.-Zimberlin, C. D., Lancini, C., Sno, R., Rosekrans, S. L., McLean, C. M., Vlaming, H., van den Brink, G. R., Bots, M., Medema, J. P., Dannenberg, J.-H. HDAC1 and HDAC2 collectively regulate intestinal stem cell homeostasis. FASEB J. 29, 2070-2080 (2015). www.fasebj.org