Regulation of Trachebronchial Tissue‐Specific Stem Cell Pool Size

Tissue‐specific stem cell (TSC) number is tightly regulated in normal individuals but can change following severe injury. We previously showed that tracheobronchial epithelial TSC number increased after severe naphthalene (NA) injury and then returned to normal. This study focused on the fate of the supernumerary TSC and the signals that regulate TSC pool size. We used the Keratin 5‐rTA/Histone 2B:green fluorescent protein (GFP) model to purify basal cells that proliferated infrequently (GFP bright ) or frequently (GFP dim ) after NA injury. Both populations contained TSC but TSCs were 8.5‐fold more abundant in the GFP bright population. Interestingly, both populations also contained a unipotential basal progenitor (UPB), a mitotic basal cell subtype whose daughters were terminally differentiated basal cells. The ratio of TSC to UPB was 5:1 in the GFP bright population and 1:5 in the GFP dim population. These data suggested that TSC proliferation in vivo promoted TSC‐to‐UPB differentiation. To evaluate this question, we cloned TSC from the GFP bright and GFP dim populations and passaged the clones seven times. We found that TSC number decreased and UPB number increased at each passage. Reciprocal changes in TSC and UPB frequency were more dramatic in the GFP dim lineage. Gene expression analysis showed that β‐catenin and Notch pathway genes were differentially expressed in freshly isolated TSC derived from GFP bright and GFP dim populations. We conclude that (a) TSC and UPB are members of a single lineage; (b) TSC proliferation in vivo or in vitro promotes TSC‐to‐UPB differentiation; and (c) an interaction between the β‐catenin and Notch pathways regulates the TSC‐to‐UPB differentiation process. S tem C ells 2013;31:2767–2778