Credit: Macmillan Australia

Renewal of the epithelial lining in the small intestine relies on a small group of stem cells at the base of the intestinal crypt. These stem cells have been described both as rapidly cycling and quiescent, creating some confusion in the field. Winton and colleagues now resolve this issue, demonstrating that quiescent cells are committed to differentiate into intestinal secretory cells but can also be converted into rapidly cycling stem cells upon injury.

Stem cells give rise to Paneth cells, which remain at the base of the intestinal crypt to form a stem cell niche, and to epithelial cells that proliferate, differentiate and move out of the crypt to form the villus. Crypt stem cells are small proliferative cells adjacent to Paneth cells and express the protein marker LGR5 (Leu-rich repeat-containing G protein-coupled receptor 5). Quiescent or slow-cycling cells, which are defined by the property of label retention (that is, their capacity to retain markers that have been used to label their DNA or chromatin), have also been localized to this region and, confusingly, are LGR5+. This has raised questions about the proliferation state of intestinal stem cells and whether there is more than one precursor pool.

To study the nature and behaviour of quiescent label-retaining cells (LRCs) in the crypt, the authors used a histone H2B–YFP reporter construct under the control of a promoter that specifically drives expression in all cells of the crypt–villus axis with the exception of mature Paneth cells. As expected, rapidly dividing cells lost the YFP label over time. Interestingly, however, the authors observed that LRCs, which were located at the base of the crypt, included Paneth cells as well as a population of non-Paneth cells (which they termed YFP–LRCs). These cells expressed quiescence markers but also Lgr5 and markers of Paneth cells and enteroendocrine lineage cells. By tracking YFP expression, the authors found that YFP–LRCs differentiate into Paneth cells and enteroendocrine cells with minimal cell division. This indicates that LRCs (the quiescent cells of the crypt) are direct precursors of these secretory intestinal cell types.

To further study the behaviour of LRCs, the authors labelled them genetically by fusing one part of a split Cre recombinase to H2B; in reassociation with the other part (expressed in all cells) it forms the functional Cre enzyme within LRCs. This genetic labelling enables marked cells and their progeny to be tracked over time. The authors found that LRCs did not divide in healthy mice. However, when intestinal crypts were injured, the cells divided and generated clones that occupied the whole crypt. This suggests that, upon injury, LRCs behave as self-renewing stem cells that can give rise to all cell types of the crypt and villus.

The results from Winton and colleagues indicate that quiescent cells are committed precursors that directly mature into secretory cells of the intestine. This study thereby explains the counter-intuitive observation of LGR5+ cells in the crypt that express quiescence markers. It seems that these quiescent cells are a subpopulation of LGR5+ cells that will become Paneth cells. Although these quiescent cells do not normally maintain the stem cell pool, they can be induced to become rapidly dividing stem cells upon injury.