Epithelial progenitor cells of the embryonic lung and the role of microRNAs in their proliferation

Abstract

The entire epithelium of the lung is generated from a small pool of undifferentiated progenitor cells. At least during the early stages of development these reside in the distal tips of the embryonic lung. They respond to multiple signals from the surrounding mesenchyme and play a critical role as morphogenetic organizing centers. In addition, they proliferate rapidly and give rise to daughter cells that differentiate into all the specialized epithelial cells types of the newborn lung. Despite the importance of the progenitor cells, we still know relatively little about the mechanisms controlling their proliferation, morphogenesis, and developmental fate. Here, we discuss new data on the potential role of microRNAs in co-coordinately regulating multiple signaling pathways in embryonic progenitor cells. In particular, our recent transgenic experiments suggest that microRNAs encoded by the miR-17-92 cluster positively promote proliferation of epithelial progenitor cells and inhibit their differentiation. We speculate on how this information might be exploited therapeutically in the long term.

Figure 1.

Epithelial progenitor cells of the embryonic lung. (A) Schematic representation of epithelial progenitor cells during early lung development. During the pseudoglandular stage (∼E10.5-16 in the mouse), the lung contains a system of actively branching epithelial tubes. Cells at the distal tip are morphologically unspecialized and are characterized by the expression of a number of genes encoding signaling proteins, transcription factors, and components of downstream signaling pathways. These include Sox9, NMyc, Id2 (Idb2), Pea3, Erm, Bmp4 (see References and 31). There are numerous reciprocal interactions (red arrows) between the epithelium and the surrounding mesenchyme and vasculature that involve multiple secreted signaling factors, including sonic hedgehog, Fgfs, and Wnts (–27). As the tips grow out and branch, they leave behind cells in the proximal stalks that give rise to differentiated cell types. Markers for these phenotypes first appear around E14.5 in the most proximal regions (32), and the process of differentiation proceeds from proximal to distal. Just before birth, the cells in the distal tips give rise to the precursors of the terminal alveoli containing type I and type II cells. It is not known how the switch in fate of the daughter cells—from proximal airway cells to alveolar cells—is regulated. (B) The distal progenitor cells rapidly transit the cell cycle so that after a 1-hour pulse of bromodeoxyuridine (BrdU) in utero at E12.5, about 80% of the distal tip cells are labeled (arrow) (29). Scale bar = 50 μm. (C) Whole mount in situ hybridization of E11.5 mouse lung shows that distal epithelial progenitor cells (arrow) preferentially express the transcription factor, Id2 (also known as Idb2). (D) Immunohistochemistry of section of E14.5 mouse lung shows that distal epithelial cells preferentially express Sox9 (arrow). In the adult lung, Sox9 expression is absent (see also Reference 33). Scale bar = 60 μm.

Figure 2.

Transgenic overexpression of the miR-17-92 cluster in the epithelium of the embryonic mouse lung promotes expansion of the pool of progenitor cells and inhibits their differentiation. (A) Section of wild-type (nontransgenic) E18.5 lung stained with hematoxylin and eosin to show normal structure with multiple terminal sacs that will give rise to the alveoli. The black box indicates typical area, which is shown at higher magnification in A′. Scale bar = 200 μm. (B) Section of transgenic Sftpc–miR-17-92 lung at E18.5 showing the persistence of tubes lined with cuboidal epithelial cells; B′ shows area boxed in black at higher magnification. The cuboidal epithelial cells express Sox9 and NMyc (19). (C) Schematic representation of the effect of miR-17-92 overexpression. At E16.5, the terminal tubes of the lung at the canalicular stage still express Sox9 (see Figure 1D). In the wild-type lung, the terminal tubes give rise to the alveolar cells. These include type I (green) and type II (yellow) epithelial cells, whereas the more proximal cells differentiate into Clara and ciliated cells (orange). BADJ = bronchioalveolar junction; P0 = Postnatal Day 0 (birth). In Sftpc–miR-17-92 transgenic lungs, there is persistence of the progenitor cells expressing Sox9 and absence of differentiated alveolar cells.