Alveolar cell fate selection and lifelong maintenance of AT2 cells by FGF signaling

Abstract

The lung’s gas exchange surface is comprised of alveolar AT1 and AT2 cells that are corrupted in several common and deadly diseases. They arise from a bipotent progenitor whose differentiation is thought to be dictated by differential mechanical forces. Here we show the critical determinant is FGF signaling. Fgfr2 is expressed in the developing progenitors in mouse then restricts to nascent AT2 cells and remains on throughout life. Its ligands are expressed in surrounding mesenchyme and can, in the absence of exogenous mechanical cues, induce progenitors to form alveolospheres with intermingled AT2 and AT1 cells. FGF signaling directly and cell autonomously specifies AT2 fate; progenitors lacking Fgfr2 in vitro and in vivo exclusively acquire AT1 fate. Fgfr2 loss in AT2 cells perinatally results in reprogramming to AT1 identity, whereas loss or inhibition later in life triggers AT2 apoptosis and compensatory regeneration. We propose that Fgfr2 signaling selects AT2 fate during development, induces a cell non-autonomous AT1 differentiation signal, then continuously maintains AT2 identity and survival throughout life.

Culture impact on the transcriptomic programs of primary and iPSC-derived human alveolar type 2 cells

“…The presence of a few AEC1s (coexpressing all canonical AEC1 markers, including AGER, CLIC5, CAV1, EMP2, and PDPN) in the HTII-280 sorted pre-culture 1 o AEC2 samples (Figure 6A) as well as RAGE positive AEC1s in immunostained control lung sections (Figure 6C) provided positive controls for expression levels of these markers and further supported our interpretation that there was little to no detectable expression of a comparable AEC1 program in any of our cultured 1 o AEC2s or iAEC2s, which is in agreement with previous studies (10,13). Given that Wnt and KGF (12,(49)(50)(51)(52)(53)(54)(55)(56)(57)(58) as well as steroids and cAMP (59,60) are factors known to promote maintenance of the AEC2 phenotype and proliferation of AEC2s in culture (12,(49)(50)(51)(52)(53)(54)(55)(56)(57)(58), we cultured 1 o AEC2s in two different media ("3D" medium and SAGM) and iAEC2s in media without CHIR99021 and/or KGF (CDCI, KDCI, DCI vs CK+DCI) and assessed for the presence of AEC1-like cells by RT-qPCR and immunostaining (Supplemental Figure 5). Robust AGER expression (compared to control lung tissue expression levels) was not detected in any condition.…”

Rational engineering of lung alveolar epithelium

“…Even in the presence of cyclic strain resembling breathing, the presence of Wnt and FGF agonism largely prevented the differentiation to an AEC1-like state in the engineered lung system. This observation is consistent with recent reports that FGFR2 expression restricts alveolar epithelium to the AEC2 fate in neonatal lung 71 , 98 . This apparent relationship of soluble to mechanical signals could also shed some light on previous observations that genetic YAP activation in AEC2s or in alveolar progenitors (in effect, activation of some of the downstream signaling induced by mechanical stretch, via a molecular approach) is ineffective at inducing full differentiation of squamous AEC1s 76 , 77 , 99 , 100 .…”

“…This apparent relationship of soluble to mechanical signals could also shed some light on previous observations that genetic YAP activation in AEC2s or in alveolar progenitors (in effect, activation of some of the downstream signaling induced by mechanical stretch, via a molecular approach) is ineffective at inducing full differentiation of squamous AEC1s 76 , 77 , 99 , 100 . It may be that local variation in strain levels within the alveolus 78 acts in tandem with short-range FB-derived Wnts 25 and AEC2-specific FGFR2 expression 34 , 71 , 98 to determine epithelial differentiation into AEC2 versus AEC1 types.…”

A novel human fetal lung-derived alveolar organoid model reveals mechanisms of surfactant protein C maturation relevant to interstitial lung disease

“…Proliferation of the fdAT2 organoids was greatly reduced when FGF7 was removed from the medium (Extended Data Fig. 1G-I), suggesting that FGF7 is a key mitogen for these cells, consistent with recent mouse data 21 . These culture conditions induce the differentiation of 16-22 pcw lung epithelial progenitors into mature AT2 cells which maintain identity and function during prolonged passaging.…”