Clonal Dynamics Reveal Two Distinct Populations of Basal Cells in Slow-Turnover Airway Epithelium

Abstract

SummaryEpithelial lineages have been studied at cellular resolution in multiple organs that turn over rapidly. However, many epithelia, including those of the lung, liver, pancreas, and prostate, turn over slowly and may be regulated differently. We investigated the mouse tracheal epithelial lineage at homeostasis by using long-term clonal analysis and mathematical modeling. This pseudostratified epithelium contains basal cells and secretory and multiciliated luminal cells. Our analysis revealed that basal cells are heterogeneous, comprising approximately equal numbers of multipotent stem cells and committed precursors, which persist in the basal layer for 11 days before differentiating to luminal fate. We confirmed the molecular and functional differences within the basal population by using single-cell qRT-PCR and further lineage labeling. Additionally, we show that self-renewal of short-lived secretory cells is a feature of homeostasis. We have thus revealed early luminal commitment of cells that are morphologically indistinguishable from stem cells.

Lung organoids: current uses and future promise

“…Lineage tracing and other studies have shown that in the adult mouse lung there is only very slow turnover of the mucociliary epithelium and replacement of luminal cells from basal cells and their immediate progeny (Ghosh et al, 2013(Ghosh et al, , 2011Hong et al, 2004;Rock et al, , 2009Watson et al, 2015). However, following cell damage by agents typically used experimentally -for example naphthalene, which kills secretory Club cells, or SO 2 gas, which kills all luminal cells -or viral infection there are rapid changes in the behavior and proliferation of the basal cells so that they quickly regenerate the epithelium and restore barrier function.…”

“…The luminal cells are connected apically by junctional complexes and play a crucial role in forming a selectively permeable barrier between the external and internal environments of the lung. Genes characteristically expressed by basal cells include those encoding the transcription factor Trp63, the cytokeratin Krt5, integrin alpha 6 (Itga6), podoplanin (Pdpn; also known as T1alpha) and the transmembrane nerve growth factor receptor (Ngfr; also known as p75) (Hackett et al, 2011;Rock et al, 2009;Watson et al, 2015).…”

GRHL2 coordinates regeneration of a polarized mucociliary epithelium from basal stem cells

“…This pattern of expression is consistent with a role in promoting the differentiation of ciliated cells and in suppressing the multipotency of ∼37% of BCs. Significantly, recent mathematical modeling of BC lineagetracing experiments in the uninjured mouse trachea suggests that about half of the Krt5 + BCs are already committed to luminal differentiation (Watson et al, 2015). By analogy, based on studies in the epidermis (Cohen et al, 2012;Sen et al, 2012;Boxer et al, 2014), we speculate that in the pseudostratified mucociliary epithelium, Znf750 marks the BC population that has a high probability of differentiation.…”

“…Krt5 + Trp63 + BCs of the pseudostratified mucociliary epithelium both self-renew and give rise to both Foxj1 + ciliated and secretory cells. This differentiation involves notch signaling and is thought to proceed via Trp63 + progenitors committed to either lineage pathway (yellow or blue nuclei; Mori et al, 2015;Pardo-Saganta et al, 2015;Watson et al, 2015). As progenitors and their descendants integrate into the epithelium, they acquire apical-basal polarity and apical-junctional complexes mediating barrier function.…”

“…At steady state, these basal cells (BCs) are relatively quiescent, but after a loss of luminal cells they proliferate and differentiate into ciliated and secretory cells (Borthwick et al, 2001;Hogan et al, 2014). Considerable progress is being made in understanding the transcriptional circuits and signaling pathways that regulate the self-renewal of BCs and the lineage choices of their descendants (Rock et al, 2011b;Tata et al, 2013;Paul et al, 2014;Tadokoro et al, 2014;Zhao et al, 2014;Danahay et al, 2015;Mori et al, 2015;Pardo-Saganta et al, 2015;Watson et al, 2015). However, less is known about the cellular mechanisms that drive morphological changes in the daughters of the BCs as they acquire apical-basal polarity and integrate seamlessly into the epithelium.…”