Regulation of fibroblast lipid storage and myofibroblast phenotypes during alveolar septation in mice

Abstract

Signaling through platelet-derived growth factor receptor-α (PDGFRα) is required for alveolar septation and participates in alveolar regeneration after pneumonectomy. In both adipose tissue and skeletal muscle, bipotent pdgfrα-expressing progenitors expressing delta-like ligand-1 or sex-determining region Y box 9 (Sox9) may differentiate into either lipid storage cells or myofibroblasts. We analyzed markers of mesenchymal progenitors and differentiation in lung fibroblasts (LF) with different levels (absent, low, or high) of pdgfrα gene expression. A larger proportion of pdgfrα-expressing than nonexpressing LF contained Sox9. Neutral lipids, CD166, and Tcf21 were more abundant in LF with a lower compared with a higher level of pdgfrα gene expression. PDGF-A increased Sox9 in primary LF cultures, suggesting that active signaling through PDGFRα is required to maintain Sox9. As alveolar septation progresses from postnatal day (P) 8 to P12, fewer pdgfrα-expressing LF contain Sox9, whereas more of these LF contain myocardin-like transcription factor-A, showing that Sox9 diminishes as LF become myofibroblasts. At P8, neutral lipid droplets predominate in LF with the lower level of pdgfrα gene expression, whereas transgelin (tagln) was predominantly expressed in LF with higher pdgfrα gene expression. Targeted deletion of pdgfrα in LF, which expressed tagln, reduced Sox9 in α-actin (α-SMA, ACTA2)-containing LF, whereas it increased the abundance of cell surface delta-like protein-1 (as well as peroxisome proliferator-activated receptor-γ and tcf21 mRNA in LF, which also expressed stem cell antigen-1). Thus pdgfrα deletion differentially alters delta-like protein-1 and Sox9, suggesting that targeting different downstream pathways in PDGF-A-responsive LF could identify strategies that promote lung regeneration without initiating fibrosis.

Temporal, spatial, and phenotypical changes of PDGFRα expressing fibroblasts during late lung development

“…As early as E12.5, a population of PDGFRα-expressing, SHH-responsive Gli1+ mesenchymal cells can be traced throughout postnatal lung development and contribute to nearly all of the alveolar myofibroblasts required for septation (Li et al, 2015; Liu et al, 2013). During the canalicular phase, PDGFRα + fibroblasts are associated with proximal airway structures, whereas during saccular and alveolar lung development, PDGFRα + fibroblasts are found distally within the walls of the developing distal airspaces where they contribute to both αSMA + myofibroblasts and αSMA − lipofibroblasts (Kimani et al, 2009; McGowan et al, 2008; Ntokou et al, 2015; McGowan and McCoy, 2011, 2013, 2014). Finally, although PDGFRα + fibroblasts are still present in significant numbers toward the end of alveolarization, at P28, few express αSMA or contain lipid.…”

Comparative analysis of the mechanical signals in lung development and compensatory growth

“…Branching morphogenesis is mediated by a host of transcriptional factors including thyroid transcription factor-1 (TTF-1), Forkhead orthologs (FOX), GATA, SOX, and ETS family members (Maeda, et al, 2007), sonic hedgehog (SHH), fibroblast growth factors (FGFs), bone morphogenetic protein-4 (Bmp4), platelet-derived growth factor-α (PDGF-α), retinoic acid, and Wnt signaling pathways (Kim and Vu, 2006) (McGowan and McCoy, 2014) (Perl and Gale, 2009) (Kadzik, et al, 2014, Shu, et al, 2005), among others. FGF and SHH signaling promotes airway SMC differentiation in proximal airways (Jesudason, et al, 2006, Yi, et al, 2009).…”

“…Platelet-derived growth factor receptor-α (PDGFR-α)-expressing lipofibroblasts mediate epithelial-mesenchymal crosstalk, a critical process shared between development and post-PNX CLG (Chen, et al, 2012, McGowan and McCoy, 2014, Paxson, et al, 2013). Serial post-PNX transcriptomic analysis of the remaining lung in mice has shown prominent early expression of genes related to proliferation (Ccnb1, Bub1, and Cdk1), extracellular matrix (Col1a1, Eln, and Tnc), and proteases (Serpinb2 and MMP9) in a pattern consistent with late stage postnatal lung development rather than embryonic or early alveolization stages (Kho, et al, 2013).…”

A three-dimensional study of alveologenesis in mouse lung

“…In Pdgfa−/− mutant, this migration does not occur and no septae forms (Boström et al, 1996; Lindahl et al, 1997; McGowan et al, 2008). While it is believed that PDGFA signaling is mediated via PDGFRA, a similar failure of myofibroblast precursor positioning phenotype has not been shown in any Pdgfrα mutants that have been analyzed (Boström et al, 2002; McGowan and McCoy, 2014, 2013). During alveologenesis, it is proposed that PDGFRA + myofibroblasts migrate from the base of the saccule towards the lumen (Burri, 1984; Lindahl et al, 1997; Prodhan and Bernard Kinane, 2002).…”