SOX21 modulates SOX2-initiated differentiation of epithelial cells in the extrapulmonary airways

Abstract

SOX2 expression levels are crucial for the balance between maintenance and differentiation of airway progenitor cells during development and regeneration. Here, we describe patterning of the mouse proximal airway epithelium by SOX21, which coincides with high levels of SOX2 during development. Airway progenitor cells in this SOX2+/SOX21+ zone show differentiation to basal cells, specifying cells for the extrapulmonary airways. Loss of SOX21 showed an increased differentiation of SOX2+ progenitor cells to basal and ciliated cells during mouse lung development. We propose a mechanism where SOX21 inhibits differentiation of airway progenitors by antagonizing SOX2-induced expression of specific genes involved in airway differentiation. Additionally, in the adult tracheal epithelium SOX21 inhibits basal to ciliated cell differentiation. This suppressing function of SOX21 on differentiation contrasts SOX2, which mainly drives differentiation of epithelial cells during development and regeneration after injury. Furthermore, using human fetal lung organoids and adult bronchial epithelial cells, we show that SOX2+/SOX21+ regionalization is conserved. Lastly, we show that the interplay between SOX2 and SOX21 is context and concentration dependent leading to regulation of differentiation of the airway epithelium.

SOX2 and SOX21 in Lung Epithelial Differentiation and Repair

“…Thus, the growing tips are marked by the expression of SOX9, whereas the non-branching parts express SOX2. However, contrasting mouse lung development where SOX2 marks proximal epithelium and SOX9 distal epithelium, the SOX9+ tip cells in human lung consistently coexpress SOX2 at the pseudoglandular stage [ 10 , 11 , 12 , 13 , 14 ]. These tip cells loose SOX9 expression when they exit the branching tip of the epithelium.…”

“…Besides the importance of SOX2 in airway progenitor differentiation, SOX2 is critical for pluripotency in embryonic stem cells and development of many other organs, e.g., brain, esophagus and intestine [ 19 , 20 , 21 ]. Only in some progenitor cells, SOX2 is co-expressed with another SOX protein, SOX21, e.g., airway, inner ear, neuron, and embryonic stem cells [ 14 , 22 , 23 , 24 , 25 ]. Due to the wide spread role of SOX2, its function is subject of multiple studies, but the interaction of SOX2 with SOX21 on progenitor cell behavior has mostly been ignored.…”

“…Due to the wide spread role of SOX2, its function is subject of multiple studies, but the interaction of SOX2 with SOX21 on progenitor cell behavior has mostly been ignored. SOX2 and SOX21 have opposite effects on airway progenitor cell differentiation: SOX2 stimulates and SOX21 inhibits differentiation to fine-tune the balance in maintenance and turnover of airway progenitor cells [ 14 ]. Furthermore, deficiency in SOX2 or SOX21 showed to have opposite phenotypes in clustering PNECs during development [ 18 ].…”

“…A notable difference among these tissues is the effect of SOX2 expression levels on either proliferation or differentiation. Neural progenitor cells exit the cell cycle upon SOX2 deletion [ 28 ], while trachea epithelium exhibit altered differentiation programs without changes in cell proliferation [ 14 ]. The regulatory function of SOX2 is highly dosage dependent, as was nicely illustrated by the regulation of cyclinD1 (CCND1) expression in neuronal progenitor cells.…”

Lung epithelium development and airway regeneration

“…These tip progenitors, are multipotent and give rise to the SOX2+ progenitor cells which will form the airway epithelium (Gontan et al, 2008;Rawlins et al, 2009a;Que et al, 2009). In contrast to the mouse branching airways, in human lung the tip progenitors express both SOX9 and SOX2 (Figure 1D) (Nikolic et al, 2017;Danopoulos et al, 2018;Eenjes et al, 2021).…”