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Early lineage commitment defines alveolar epithelial ontogeny in the lung

ABSTRACT: During the step-wise specification and differentiation of tissue specific multipotent progenitor cells, lineage-specific transcriptional networks are either activated or repressed to orchestrate progenitor cell commitment. The gas exchange niche in the lung contains two major epithelial cell types, alveolar type 1 (AT1) and type 2 (AT2) cells, and the timing of lineage commitment of these cells is critical for correct formation of this niche and postnatal survival. To define the ontogeny of alveolar cell fate in the lung, we used lineage tracing studies combined with spatially specific mRNA transcript and protein expression combined with single cell RNA-seq analysis. These studies reveal that commitment to alveolar epithelial cell fate occurs far earlier than previously appreciated, concomitant with the proximal-distal specification of epithelial progenitors and branching morphogenesis. Using a novel dual lineage tracing system, we show that a small population of alveolar cells express markers of both AT1 and AT2 cells, whose fate is ultimately restricted to a single lineage. However, these bi-transcriptional cells generate only a minor portion of the mature alveolar epithelium. These data reveal a new paradigm of organ formation where early lineage commitment occurs during the nascent stages of development coincident with broad tissue patterning processes including axial patterning of the endoderm and branching morphogenesis.

ORGANISM(S): Mus musculus

PROVIDER: GSE113320 | GEO | 2019/02/08

REPOSITORIES: GEO

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Project description:Alveoli are thin-walled sacs that serve as the gas exchange units of the lung. They are affected in devastating lung diseases including COPD, Idiopathic Pulmonary Fibrosis, and the major form (adenocarcinoma) of lung cancer, the leading cause of cancer deaths. The alveolar epithelium is composed of two morphologically distinct cell types: alveolar type (AT) 1 cells, exquisitely thin cells across which oxygen diffuses to reach the blood, and AT2 cells, specialized surfactant-secreting cells. Classical studies suggested that AT1 cells arise from AT2 cells during development and following injury, but more recent studies suggest other sources. Here we use histological and marker analysis, lineage tracing, and clonal analysis in mice to identify alveolar progenitor and stem cells and map their locations and potential in vivo. The results show that AT1 and AT2 cells arise independently during development from a bipotential progenitor. After birth, new AT1 cells derive from rare, long-lived, self-renewing AT2 cells, each producing a slowly expanding clonal focus of regenerated alveoli contiguous with the founder AT2 cell. This stem cell function of AT2 cells is broadly activated by diffuse AT1 cell injury, and AT2 self-renewal can be induced in vitro by EGF ligands and permanently activated in vivo by AT2 cell-specific targeting of the oncogenic KrasG12D allele, efficiently transforming AT2 cells into monoclonal adenomatous tumors that rapidly enlarge and prove fatal. Thus, there is a developmental switch in alveolar progenitor cells after birth, when mature AT2 cells function as facultative stem cells that contribute to local alveolar renewal, repair, and cancer. We propose that short-range signals from dying AT1 cells regulate AT2 stem cell activity: a signal transduced by EGFR-KRAS controls AT2 self-renewal and is hijacked during oncogenic transformation, and a separate signal controls reprogramming to AT1 cell fate. To compare expression between ATII and E18 BP populations, RNA was isolated from either population purified by FACS. Two populations are analyzed with 3 biological replicates per population.

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Early lineage commitment defines alveolar epithelial ontogeny in the lung

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