Proc Natl Acad Sci U S A
The lining of the small intestine is the most rapidly regenerating tissue in the body as it normally undergoes continuous turnover every 5 days. This self-renewal is attributed to intestinal stem cells (ISCs) that repopulate the intestine. The intestine is an ideal system to study stem cell biology and tissue regeneration because of this rapid self-renewal. Understanding this process is the key to unlocking its potential for controlling intestinal regeneration and growth for the treatment of human diseases such as Short Gut Syndrome and Crohn’s disease. Researchers have recently identified ISCs using gene manipulation in mice to label the ISCs with a fluorescent color to distinguish them from other cells in the intestine. This technique allows researchers to track the fate of these ISCs to show that they replenish all the other cells and ultimately repopulate the small intestine (i.e. they behave as stem cells). Interestingly, two separate molecular identifiers, Lgr5 and Bmi1, were independently shown to mark ISCs using this technique. This intriguing result suggests either that Lgr5 and Bmi1 happen to mark an overlapping/identical population of cells or that they mark two distinct subsets of cells both capable of stem cell behavior. This remains unknown. We directly examined this fundamental question and discovered that Lgr5 and Bmi1 mark two distinct populations of ISCs. Although they share a similar ability to repopulate the intestine, they differ dramatically in their behavior. Lgr5 marks a population of ISCs that constantly replenishes the intestine and is responsible for its day-to-day turnover. The Lgr5(+) population is very sensitive to injury. In contrast, Bmi1 marks a special “reserve” ISC population that is normally dormant, but becomes activated to repopulate the intestine upon injury. Our data suggest that the intestine uses two separate types of stem cells for two different purposes: an “active” ISC for the day-to-day regeneration and a “quiescent” ISC that serves as a backup in case of catastrophic injury. We further examined these two populations, and found that they exhibit different behavior in response to manipulation of the Wnt signaling pathway, an important developmental pathway involved in the development of colon cancer. Whereas the Lgr5(+) ISC population expands when Wnt signaling is turned up and contracts when it is turned down, the Bmi1(+) ISC population is relatively insensitive to these signals. This suggests that these two ISC populations respond differently to the same environmental signals and cues. Furthermore, we demonstrate that a single Bmi1(+) ISC isolated from the small intestine can be grown in vitro to form spheroids that resemble the intestinal lining. Importantly, these spheroids contain Lgr5(+) cells, suggesting that Bmi1(+) ISCs can give rise to Lgr5(+) ISCs in vitro.
The small intestine epithelium undergoes rapid and continuous regeneration supported by crypt intestinal stem cells (ISCs). Bmi1 and Lgr5 have been independently identified to mark long-lived multipotent ISCs by lineage tracing in mice; however, the functional distinctions between these two populations remain undefined. Here, we demonstrate that Bmi1 and Lgr5 mark two functionally distinct ISCs in vivo. Lgr5 marks mitotically active ISCs that exhibit exquisite sensitivity to canonical Wnt modulation, contribute robustly to homeostatic regeneration, and are quantitatively ablated by irradiation. In contrast, Bmi1 marks quiescent ISCs that are insensitive to Wnt perturbations, contribute weakly to homeostatic regeneration, and are resistant to high-dose radiation injury. After irradiation, however, the normally quiescent Bmi1(+) ISCs dramatically proliferate to clonally repopulate multiple contiguous crypts and villi. Clonogenic culture of isolated single Bmi1(+) ISCs yields long-lived self-renewing spheroids of intestinal epithelium that produce Lgr5-expressing cells, thereby establishing a lineage relationship between these two populations in vitro. Taken together, these data provide direct evidence that Bmi1 marks quiescent, injury-inducible reserve ISCs that exhibit striking functional distinctions from Lgr5(+) ISCs and support a model whereby distinct ISC populations facilitate homeostatic vs. injury-induced regeneration.