Human lung branching morphogenesis is orchestrated by the spatiotemporal distribution of ACTA2, SOX2, and SOX9

Abstract

Lung morphogenesis relies on a number of important processes, including proximal-distal patterning, cell proliferation, migration and differentiation, as well as epithelial-mesenchymal interactions. In mouse lung development, SOX2 cells are localized in the proximal epithelium, whereas SOX9 cells are present in the distal epithelium. We show that, in human lung, expression of these transcription factors differs, in that during the pseudoglandular stage distal epithelial progenitors at the tips coexpress SOX2 and SOX9. This double-positive population was no longer present by the canalicular stages of development. As in mouse, the human proximal epithelial progenitors express solely SOX2 and are surrounded by smooth muscle cells (SMCs) both in the proximal airways and at the epithelial clefts. Upon Ras-related C3 botulinum toxin substrate 1 inhibition, we noted decreased branching, as well as increased SMC differentiation, attenuated peristalsis, and a reduction in the distal double-positive SOX2/SOX9 progenitor cell population. Thus, the presence of SOX2/SOX9 double-positive progenitor cells in the distal epithelium during the pseudoglandular stage of human lung development appears to be critical to proximal-distal patterning and lung branching. Moreover, SMCs promote a SOX2 proximal phenotype and seem to suppress the SOX9 population.

Discordant roles for FGF ligands in lung branching morphogenesis between human and mouse

“…We, and others, have recently shown that distal tip buds in the early human lung coexpress SOX2 and SOX9 as opposed to only SOX9 in mice . Moreover, we showed that the maintenance of this double‐positive population is required for branching morphogenesis . Here, our results demonstrated that FGF10 impaired the balance and colocalization of SOX2/SOX9 and impaired branching in the early (10–12 weeks) human lung.…”

“…HTII280 did not always colocalize with pro-SFTPC, supporting previous reports that HTII-280 may also be expressed in undifferentiated epithelium [29]. Furthermore, we had previously established that smooth muscle cells (SMCs) appear to promote SOX2+ cells and suppress the SOX9+ population [16]. Given the expansion of SOX9+ cells in the FGF-treated explants, we assessed the expression of SMCs using immunostaining for ACTA2.…”

“…Next, we wanted to establish the spatial localization of some of the expressed ligands and receptors, especially those that are most commonly studied in the mouse lung. Using fluorescence ISH on human fetal lung sections, at different gestational stages (11,14,16, and 18 weeks), we found that FGF7 was expressed throughout the tissue in both epithelium and mesenchyme at each gestational age ( Figure 1B,C,H,I), whereas FGF9 was mostly expressed in the distal epithelial tips and mesothelial cells at each stage, with minimal expression in the proximal epithelium ( Figure 1D,E,J,K). FGF10 was predominantly expressed in the mesenchyme, both distally and proximally ( Figure 1F,G,L,M).…”

Human lung development: recent progress and new challenges

“…3B). By contrast, in human lungs SOX2 is consistently co-expressed with SOX9 in tip cells throughout the pseudoglandular stage (Danopoulos et al, 2018; Miller et al, 2017; Nikolić et al, 2017). As human cells exit the tip progenitor domain and start to differentiate along airway lineages, they turn off SOX9, but retain SOX2 (Fig.…”

“…Recently, a large study that examined lung structure in >3000 individuals reported that developmental variation in human lung branching, specifically a central airway branch variation, is associated with chronic obstructive pulmonary disease (COPD) (Smith et al, 2018). Moreover, one of these airway branch variations is associated with genetic polymorphisms within the FGF10 gene, which is known to be crucial for branching morphogenesis in the developing lung (Bellusci et al, 1997; Danopoulos et al, 2018; Park et al, 1998; Peters et al, 1994). …”