Research by the Epistem co-founder Prof. Chris Potten demonstrated that intestinal crypts contain a hierarchy of epithelial stem cells, with slightly different properties (particularly in response to DNA damage) or degrees of 'stemness'. A population of these stem cells was subsequently shown to express high levels of Lgr5, or leucine‑rich repeat‑containing G‑protein coupled receptor 5 (Barker et al 2007). However, the mechanism by which stem cell homeostasis is maintained, with implications for the control of symmetric or asymmetric cell division (to reduce or expand stem cell number in response to external insult), has been the subject of numerous publications. A recent paper by Qi et al published in Nature Comms in January 2017 (8: 13824; DOI 10.1038/ncomms13824) has revealed that BMP (bone morphogenetic protein) restricts the Lgr5+ stem cell number within a crypt.
The paper demonstrates that BMP acts via Smad to transcriptionally repress a range of stem cell associated genes, including Lgr5, and that this is independent of the Wnt//β‑catenin signalling pathway.
Using Villin-CreERT2 conditional knock out mice, the BMP type 1 receptor (Bmpr1a) was virtually (but not totally) ablated and the Lgr5+ (and Olfm4 and Sox9 positive) cell population expanded.
Bmpr1a ko mice had an expanded proliferative population, as demonstrated by BrdU incorporation, but did not accumulate nuclear β‑catenin. Over time these mice developed hyperplasias and dysplasias.
Bmpr1a ko crypts formed organoids, but the self-renewal impact of the Bmpr1a ko was negated by the presence of noggin.
Bmpr1a ko mice appeared to regenerate crypts more rapidly following 10Gy irradiation, consistent with an increased Lgr5+ population.
Addition of BMP4 inhibited Lgr5+ (and Olfm4, Sox9, Ascl2 positive) cell production in intestinal organoids. Organoid dissociation and FACS followed by RNA-Seq were used to characterise stem cell transcriptional profiles and indicated that the effect was mediated via HDAC1 and Smad4. The crypts of Smad4 conditional ko mice had a similar phenotype to the Bmpr1a ko mice and did not respond to BMP4 addition in vitro.
Whilst this data demonstrates an important means of limiting Lgr5+ cell number, the topography of the crypt must also be considered. Stem cells reside towards the Wnt enriched crypt base, with cells maturing as they move upwards. There will be local changes in the relative levels of available factors such as BMP and Wnt, not just their receptors, as a direct consequence of external factors such as the level of damage by irradiation etc., and hence the overall outcome on the stem cell population will be the combination of all these responses.
Epistem has a range of experimental tools which may be used to assess intestinal epithelial cell proliferation/differentiation and stem cell responses. If you would like to discuss these options for your research project, then do get in touch with our team.