Sentinel p16INK4a+ cells in the basement membrane form a reparative niche in the lung.

We engineered an ultrasensitive reporter of p16INK4a, a biomarker of cellular senescence. Our reporter detected p16INK4a-expressing fibroblasts with certain senescent characteristics that appeared shortly after birth in the basement membrane adjacent to epithelial stem cells in the lung. Furthermore, these p16INK4a+ fibroblasts had enhanced capacity to sense tissue inflammation and respond through their increased secretory capacity to promote epithelial regeneration. In addition, p16INK4a expression was required in fibroblasts to enhance epithelial regeneration. This study highlights a role for p16INK4a+ fibroblasts as tissue-resident sentinels in the stem cell niche that monitor barrier integrity and rapidly respond to inflammation to promote tissue regeneration.

Human alveolar type 2 epithelium transdifferentiates into metaplastic KRT5+ basal cells.

Loss of alveolar type 2 cells (AEC2s) and the ectopic appearance of basal cells in the alveoli characterize severe lung injuries such as idiopathic pulmonary fibrosis (IPF). Here we demonstrate that human alveolar type 2 cells (hAEC2s), unlike murine AEC2s, transdifferentiate into basal cells in response to fibrotic signalling in the lung mesenchyme, in vitro and in vivo. Single-cell analysis of normal hAEC2s and mesenchymal cells in organoid co-cultures revealed the emergence of pathologic fibroblasts and basaloid cells previously described in IPF. Transforming growth factor-ß1 and anti-bone morphogenic protein signalling in the organoids promoted transdifferentiation. Trajectory and histologic analyses of both hAEC2-derived organoids and IPF epithelium indicated that hAEC2s transdifferentiate into basal cells through alveolar-basal intermediates that accumulate in proximity to pathologic CTHRC1hi/TGFB1hi fibroblasts. Our study indicates that hAEC2 loss and expansion of alveolar metaplastic basal cells in severe human lung injuries are causally connected through an hAEC2-basal cell lineage trajectory driven by aberrant mesenchyme.

Gli1+ mesenchymal stromal cells form a pathological niche to promote airway progenitor metaplasia in the fibrotic lung.

Aberrant epithelial reprogramming can induce metaplastic differentiation at sites of tissue injury that culminates in transformed barriers composed of scar and metaplastic epithelium. While the plasticity of epithelial stem cells is well characterized, the identity and role of the niche has not been delineated in metaplasia. Here, we show that Gli1+ mesenchymal stromal cells (MSCs), previously shown to contribute to myofibroblasts during scarring, promote metaplastic differentiation of airway progenitors into KRT5+ basal cells. During fibrotic repair, Gli1+ MSCs integrate hedgehog activation signalling to upregulate BMP antagonism in the progenitor niche that promotes metaplasia. Restoring the balance towards BMP activation attenuated metaplastic KRT5+ differentiation while promoting adaptive alveolar differentiation into SFTPC+ epithelium. Finally, fibrotic human lungs demonstrate altered BMP activation in the metaplastic epithelium. These findings show that Gli1+ MSCs integrate hedgehog signalling as a rheostat to control BMP activation in the progenitor niche to determine regenerative outcome in fibrosis.

The Role of Hedgehog Signaling in Adult Lung Regeneration and Maintenance.

As a secreted morphogen, Sonic Hedgehog (SHH) determines differential cell fates, behaviors, and functions by forming a gradient of Hedgehog (Hh) activation along an axis of Hh-receptive cells during development. Despite clearly delineated roles for Hh during organ morphogenesis, whether Hh continues to regulate cell fate and behavior in the same fashion in adult organs is less understood. Adult organs, particularly barrier organs interfacing with the ambient environment, are exposed to insults that require renewal of cellular populations to maintain structural integrity. Understanding key aspects of Hh's ability to generate an organ could translate into conceptual understanding of Hh's ability to maintain organ homeostasis and stimulate regeneration. In this review, we will summarize the current knowledge about Hh signaling in regulating adult lung regeneration and maintenance, and discuss how alteration of Hh signaling contributes to adult lung diseases.

Expansion of hedgehog disrupts mesenchymal identity and induces emphysema phenotype.

GWAS have repeatedly mapped susceptibility loci for emphysema to genes that modify hedgehog signaling, but the functional relevance of hedgehog signaling to this morbid disease remains unclear. In the current study, we identified a broad population of mesenchymal cells in the adult murine lung receptive to hedgehog signaling, characterized by higher activation of hedgehog surrounding the proximal airway relative to the distal alveoli. Single-cell RNA-sequencing showed that the hedgehog-receptive mesenchyme is composed of mostly fibroblasts with distinct proximal and distal subsets with discrete identities. Ectopic hedgehog activation in the distal fibroblasts promoted expression of proximal fibroblast markers and loss of distal alveoli and airspace enlargement of over 20% compared with controls. We found that hedgehog suppressed mesenchymal-derived mitogens enriched in distal fibroblasts that regulate alveolar stem cell regeneration and airspace size. Finally, single-cell analysis of the human lung mesenchyme showed that segregated proximal-distal identity with preferential hedgehog activation in the proximal fibroblasts was conserved between mice and humans. In conclusion, we showed that differential hedgehog activation segregates mesenchymal identities of distinct fibroblast subsets and that disruption of fibroblast identity can alter the alveolar stem cell niche, leading to emphysematous changes in the murine lung.