Dual states of Bmi1-expressing intestinal stem cells drive epithelial development and tissue regeneration

Abstract

Intestinal development, response to injury and disease states rely upon balanced stem cell proliferation. Historically, two subtypes of intestinal epithelial stem cells (ISCs)-slow-cycling/label-retaining, and actively-cycling/canonical Wnt-dependent-coordinate to drive proliferation and regulate epithelial renewal during adult tissue homeostasis and injury response. Recent studies focused on Bmi1-expressing cells revealed that differentiated Bmi1+ enteroendocrine cells could dedifferentiate towards a canonical Wnt-dependent stem cell state, calling into question the dogma that a dual stem cell axis regulates epithelial proliferation. Herein, we identify stem cell function in a Bmi1+ cell population in early murine intestinal development prior to the establishment of canonical Wnt-dependent, Lgr5-expressing ISCs. In-depth analyses of developmental Bmi1+ ISCs using lineage-tracing and single cell RNA-sequencing reveal their distinct identity and capacity to differentiate into Lgr5+ ISCs and other differentiated lineages. Further, during in utero development, the Bmi1+ ISCs initially exists in a highly proliferative state then transitions to a slow-cycling state, with the emergence of actively-cycling Lgr5+ ISCs. In adult tissue, Bmi1+ ISCs are a distinct population that re-express developmental gene and protein profiles, and a non-canonical Wnt signaling signature in response to injury and in human colorectal tumors. Further, developmental Bmi1+ ISCs are distinct from Lgr5+ ISCs and the previously identified differentiated Bmi1+ progenitor cells. Re-evaluation of an under-appreciated Bmi1+ ISC population with fundamental importance in intestinal development re-establishes the importance of the dynamic interplay between discrete ISC populations that are regulated by opposing Wnt signaling pathways. This finding opens opportunities and targetable pathways to augment regeneration or inhibit tumorigenesis.