STEM CELLS
The intestine is essential for processes as diverse as digestion, absorption, secretion, and immunity. Remarkably, the entire 25–foot length of a human intestine undergoes complete epithelial regeneration every 5–7 days. This incredible regenerative ability is enabled by extremely active intestinal stem cell (ISC) populations.
We have helped to define distinct functional classes of ISCs. On one hand, actively cycling Lgr5+ ISC are crypt–based, Wnt–sensitive and radiosensitive and represent the “homeostatic” stem cells that regenerate the intestine on a daily basis. However, Bmi1+ ISC are slowly cycling, positioned higher in the crypt, Wnt–insensitive and radioresistant. Interestingly, while Lgr5+ ISC can be rapidly killed by a single dose of radiation, Bmi1+ ISC not only survive, but are massively induced to divide and regenerate the intestine and are thus represent a “quiescent/reserve/injury–inducible” stem cell (PNAS 2012). Bmi1-GFP cells exhibit enteroendocrine features and co-express markers such as Prox1 which also allow injury-inducible lineage tracing (Cell Stem Cell 2017).
We have also developed powerful tools for the study of stem cells. These include adenoviral reagents allowing in vivo Wnt gain– or loss–of–function (PNAS 2004; Nat Med 2009; PNAS 2012; Science 2013; Nature 2013). We have also developed organoid methods for culture of intestinal stem cells within air-liquid interface cultures containing both epithelial and mesenchymal components as well as stable cell lines expressing R–spondin1 (Nat. Med., 2009). Please see the organoid section for more information on organoid cultures.
Some current projects explore signals regulating ISC fate choices between symmetric division and lineage commitment, and characterization of injury–inducible ISC populations. For instance, we are studying how Wnt proteins and R-spondins (Wnt amplifiers) coordinately regulate Lgr5+ ISCs, with Wnts being priming factors and R-spondins catalyzing stem cell self-renewal (Nature 2017a). This has utilized artificial Wnt molecules developed by Chris Garcia's lab (Nature 2017b; Cell Stem Cell 2020).
