Early lineage specification defines alveolar epithelial ontogeny in the murine lung

Abstract

During the stepwise specification and differentiation of tissue-specific multipotent progenitors, lineage-specific transcriptional networks are activated or repressed to orchestrate cell specification. The gas-exchange niche in the lung contains two major epithelial cell types, alveolar type 1 (AT1) and AT2 cells, and the timing of lineage specification of these cells is critical for the correct formation of this niche and postnatal survival. Integrating cell-specific lineage tracing studies, spatially specific mRNA transcript and protein expression, and single-cell RNA-sequencing analysis, we demonstrate that specification of alveolar epithelial cell fate begins concomitantly with the proximal–distal specification of epithelial progenitors and branching morphogenesis earlier than previously appreciated. By using a newly developed dual-lineage tracing system, we show that bipotent alveolar cells that give rise to AT1 and AT2 cells are a minor contributor to the alveolar epithelial population. Furthermore, single-cell assessment of the transcriptome identifies specified AT1 and AT2 progenitors rather than bipotent cells during sacculation. These data reveal a paradigm of organ formation whereby lineage specification occurs during the nascent stages of development coincident with broad tissue-patterning processes, including axial patterning of the endoderm and branching morphogenesis.

Redundant and additive functions of the four Lef/Tcf transcription factors in lung epithelial progenitors

“…4G, Table S3, Table S4). Such trajectory bias of AT1 and AT2 genes in this premature alveolar differentiation was reminiscent of their spatial bias during normal alveolar differentiation (27,28). Finally, transcriptomic changes in the quadruple mutant mirrored those induced by Ctnnb1 deletion (11) ( Fig.…”

Cyp26b1 is required for proper airway epithelial differentiation during lung development

“…Although morphologic changes between atRA-treated and Cyp26b1 -/lungs were indistinguishable from one another, cell fate changes in atRA-treated lungs did not fully mirror that in Cyp26b1 -/lungs. One possible explanation for this is that distal epithelial progenitors are specified as early as E13.5 and may not be receptive to changes in fate by the first dose of RA at E15.5 (Frank et al, 2019). Cyp26b1 -/lungs are exposed to higher levels of RA throughout lung development potentially leading to a more severe difference in lineage specification.…”

“…Of note, Cyp26b1 -/lungs contain a higher proportion of AT2 cells after standardizing for the increased number of DAPI + cells per given area. Because proSP-C is not exclusively expressed in AT2 cells during development (Frank et al, 2019), we repeated the analysis utilizing the AT2-cell specific marker, Lamp3 ("Lysosomal associated membrane protein 3" or DC-Lamp). Quantification of AT2 cells using Lamp3 demonstrated a similar increase in Lamp3 + AT2 cells as compared to proSP-C + AT2 cells in Cyp26b1 -/lungs, with the vast majority of AT2 cells expressing both proteins (Supplemental Fig.…”

“…These structures mediate gas exchange in neonates and full alveolarization occurs postnatally (Herriges and Morrisey, 2014;Morrisey and Hogan, 2010). In humans, failure to form early saccules results in respiratory distress syndrome (RDS), with a mortality rate as high as 50% depending on the size of the infant at birth (Gallacher et al, 2016).…”

Understanding Human Lung Development through In Vitro Model Systems

“…[ 94 ] However, this latter study suggested that bipotent progenitors are rare and most likely remnant undifferentiated cells that remain at the end of branching morphogenesis. [ 94 ] It was further demonstrated that commitment to alveolar fates occurs much earlier than previously appreciated, taking place concurrently with branching morphogenesis, rather than afterwards. The majority of mature alveolar cells are the progeny of unipotent alveolar progenitor cells fated towards either an ATI or ATII cell early in development, with ATII cells being specified first at the most distal tip of the lung and ATI cells being specified just after ATII cells.…”