ALL STEM CELLS

Stem cells of the gastrointestinal tract, pancreas, liver and other columnar epithelia collectively resist cloning in their elemental states. Here we demonstrate the cloning and propagation of highly clonogenic, ‘ground state’ stem cells of the human intestine and colon. We show that derived stem-cell pedigrees sustain limited copy number and sequence variation despite extensive serial passaging and display exquisitely precise, cell-autonomous commitment to epithelial differentiation consistent with their origins along the intestinal tract. This developmentally patterned and epigenetically maintained commitment of stem cells is likely to enforce the functional specificity of the adult intestinal tract. Using clonally derived colonic epithelia, we show that toxins A or B of the enteric pathogen Clostridium difficile recapitulate the salient features of pseudomembranous colitis. The stability of the epigenetic commitment programs of these stem cells, coupled with their unlimited replicative expansion and maintained clonogenicity, suggests certain advantages for their use in disease modelling and regenerative medicine.

Discussion

Adult stem cells of the highly regenerative intestinal tract remain largely defined by metabolic, marker profiling, or lineage tracing experiments in vivo or transplantation of cells from intestinal organoids. As stem cells comprise only a minor component of organoids—perhaps less than 1% — the molecular features of stem cells of columnar epithelia such as the intestinal tract have remained unclear. Therefore the selective cloning and proliferative expansion of highly clonogenic, ground state intestinal stem cells described here offers a first glimpse into the molecular properties of these cells. Our inability to convert differentiated cells to clonogenic cells supports the notion that we are cloning resident stem cells rather than somehow ‘reprograming’ differentiated enterocytes.
These resident stem cells possess robust epigenetic programs of commitment to regiospecific intestinal differentiation that are stable despite more than six months of continuous propagation. This cellautonomous regiospecificity of stem cells along the intestinal tract argues against a unitary ‘intestinal stem cell’ or even one each for the histologically recognized segments, but rather a developmentally established spectrum of stem cells that ultimately maintains the histological and functional properties that define these segments. Aheuristic deciphering of the commitment code from the regiospecific expression patterns presented here will guide parallel efforts with iPSCs to achieve appropriate lineage fates. Interestingly, many inductive signalling pathways and transcription factors implicated in embryonic gut formation may act to reinforce commitment codes via continued expression in stem cells of the intestinal tract.
We anticipate that the ability to maintain these stem cells in their elemental state will enable the discovery of epigenetic mechanisms that underlie properties of very long-term self-renewal, exquisitely precise lineage commitment, and the intrinsically directed, selfassembly of differentiated epithelia. Although we demonstrate the potential of clonally-derived colonic epithelia to model the pathogenesis of C. difficile toxins, we anticipate the need to restore complexity in the form of mesenchyme, immune cells, enteric neurons and perhaps components of the microbiome to fully recapitulate disease dynamics. In particular, enteric maladies such as inflammatory bowel disease represent important medical challenges whose aetiologies most likely reside in interactions between the immune system, intestinal mucosa and intestinal flora. Finally, the ability to clone patient-specific, ground state stem cells from columnar epithelia via endoscopic biopsies, coupled with their orders-of-magnitude expansion kinetics over organoids, favours their use in regenerative medicine, pre-clinical trials and disease modelling.

Source : Nature Magazine 2015-06-11