Multipotent Myoepithelial Progenitor Cells Are Born Early during Airway Submucosal Gland Development

Airway submucosal glands (SMGs) are facultative stem cell niches for the surface epithelium, but the phenotype of the SMG-derived progenitor cells remains unclear. In other organs, glandular myoepithelial cells (MECs) have been proposed to be multipotent progenitors for luminal cells. We sought to determine the developmental phase during which mouse tracheal glandular MECs are born and whether these MECs are progenitors for other cell phenotypes during SMG morphogenesis. To approach this question, we localized two MEC protein markers (α-smooth muscle actin [αSMA/ACTA2] and smooth muscle myosin heavy chain 11 [SMMHC/MYH11]) during various stages of SMG development (placode, elongation, branching, and differentiation) and used ACTA2-Cre^ERT2 and MYH11-Cre^ERT2 transgenic mice to fate map MEC-derived lineages during SMG morphogenesis. Both αSMA- and SMMHC-expressing cells emerged early after placode formation and during the elongation phase of SMG development. Lineage tracing in newborn mice demonstrated that lineage-positive MECs are born at the tips of invading tubules during the elongation phase of gland development. Lineage-positive MECs born within the first 7 days after birth gave rise to the largest percentage of multipotent progenitors capable of contributing to myoepithelial, serous, mucous, and ductal cell lineages. Serial tamoxifen-induction of both Cre-driver lines demonstrated that lineage-positive multipotent MECs contribute to ∼ 60% of glandular cells by 21 days after birth. In contrast, lineage-traced MECs did not contribute to cell types in the surface airway epithelium. These findings demonstrate that MECs born early during SMG morphogenesis are multipotent progenitors with the capacity to differentiate into other glandular cell types.