Characterization and role of Tcf21-positive cells during lung alveolarization and neo-alveolarization

Abstract

The transcription factor 21 (Tcf21) has been demonstrated to impact cardiac fibroblast differentiation, and is highly expressed in the lung mesenchyme. Tcf21 has been found to be essential for prenatal lung development, since global deletion of Tcf21 in mice results in abnormal lung morphology and early postnatal lethality. In contrast, formal characterization of the postnatal location, phenotype and function of Tcf21 in the lung had not been performed prior to the present study. Therefore, this study aimed to identify and characterize the phenotype and role of pulmonary Tcf21-expressing cells in mice during homeostasis, late lung development and in a model of pulmonary regrowth. In this work, immunophenotyping of the Tcf21 lineage in lung tissue and lung cell suspensions of inducible reporter Tcf21 iCre/+;R26 tdTomato/tdTomato mice resulted in convincing evidence that the pulmonary Tcf21 lineage of mice during postnatal development and in adult mice represents cells of exclusively mesenchymal character (CD31- CD45- CD326- PDGFR-α+). Tcf21-lineage traced (Tcf21lin) cells mostly held a lipofibroblastic phenotype based on localization, cell marker expression (ADRP+) and neutral lipid droplet content (LipidTOX +). Tcf21lin cells were, in addition, essentially distinct from the myofibroblasts that populate the lung during secondary septation. After induction of lineage tracing in adult animals, Tcf21lin cells were found to have a similar phenotype than postnatal Tcf21lin cells. Moreover, Tcf21lin cells were observed to have a peculiar morphology with multiple slender cytoplasmic processes which contain lipid droplets and contact alveolar epithelial cells type 2. Some of these processes kept a close spatial relationship with the border of alveolar capillary loops. Left pneumonectomy was performed to induce compensatory regrowth in adult Tcf21 iCre/+ ;R26 tdT/DTA mice following partial depletion of Tcf21lin cells and in control Tcf21 +/+ ;R26 tdT/DTA mice. Subsequent stereological analysis conducted 7 days after surgery failed to demonstrate a significant impact of preoperative Tcf21+ cell partial depletion on the architecture of the lung; however, results from in vitro experiments using alveolospheres indicated that Tcf21lin cells contribute to alveolosphere formation, and that the Tcf21lin-dependent support for sphere formation was at least partially mediated via soluble factors.