It is generally thought that the quiescence of tissue is not actively maintained, but rather a state reflecting the absence of proliferative signal; here the authors find that quiescence is actively maintained by paracrine hedgehog signalling provided by the epithelium in the mouse adult lung, and that hedgehog is dynamically regulated during injury repair and resolution for proper restoration of tissue homeostasis after injury. This study questions the common assumption that the quiescence of tissue is not maintained actively, but is a state reflecting the absence of proliferative signal. Edward Morrisey and colleagues report that quiescence in mesenchymal tissue in the mouse adult lung is actively maintained by paracrine sonic hedgehog signalling provided by the adjacent epithelium. They show that this signal needs to be downregulated after injury to allow proliferation of the lung mesenchyme, which is key for tissue repair. Postnatal tissue quiescence is thought to be a default state in the absence of a proliferative stimulus such as injury. Although previous studies have demonstrated that certain embryonic developmental programs are reactivated aberrantly in adult organs to drive repair and regeneration1,2,3, it is not well understood how quiescence is maintained in organs such as the lung, which displays a remarkably low level of cellular turnover4,5. Here we demonstrate that quiescence in the adult lung is an actively maintained state and is regulated by hedgehog signalling. Epithelial-specific deletion of sonic hedgehog (Shh) during postnatal homeostasis in the murine lung results in a proliferative expansion of the adjacent lung mesenchyme. Hedgehog signalling is initially downregulated during the acute phase of epithelial injury as the mesenchyme proliferates in response, but returns to baseline during injury resolution as quiescence is restored. Activation of hedgehog during acute epithelial injury attenuates the proliferative expansion of the lung mesenchyme, whereas inactivation of hedgehog signalling prevents the restoration of quiescence during injury resolution. Finally, we show that hedgehog also regulates epithelial quiescence and regeneration in response to injury via a mesenchymal feedback mechanism. These results demonstrate that epithelial–mesenchymal interactions coordinated by hedgehog actively maintain postnatal tissue homeostasis, and deregulation of hedgehog during injury leads to aberrant repair and regeneration in the lung.

We engineered an ultrasensitive reporter of p16 ^INK4a , a biomarker of cellular senescence. Our reporter detected p16 ^INK4a -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 p16 ^INK4a+ fibroblasts had enhanced capacity to sense tissue inflammation and respond through their increased secretory capacity to promote epithelial regeneration. In addition, p16 ^INK4a expression was required in fibroblasts to enhance epithelial regeneration. This study highlights a role for p16 ^INK4a+ fibroblasts as tissue-resident sentinels in the stem cell niche that monitor barrier integrity and rapidly respond to inflammation to promote tissue regeneration.

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.