HL-131: The adult mouse lung harbors its basal stem/progenitor cells in an environment protected by the cartilaginous rings of the trachea and mainstem bronchi. After major lung epithelial injuries basal stem/progenitor cells are activated, recruited to sites of injury and rapidly restore lung function. How basal/stem progenitor cells are maintained in and restricted to cartilaginous airways during homeostasis and how they are activated and recruited to sites of injury is not well understood. During embryogenesis the development and maintenance of tracheal basal stem/progenitor cells depends on Fgf10 secreted by the stromal tissue located in between the cartilage rings. However, overexpression of Fgf10 in all lung cells during late embryonic development induces the ectopic development of basal/stem progenitors even in conducting airways, where they normally do not reside. Similarly, overexpression of Yap, the transcriptional effector of the Hippo pathway, in adult tracheal basal stem/progenitor cells results in basal cell hyperplasia and stratification, whereas Yap ablation in basal stem/progenitor cells results in their loss. Recent work has also shown that secretory cell quiescence during homeostasis is maintained by the Hippo tumor suppressor pathway, which is down-regulated shortly after naphthalene-mediated injury in surviving and spread out airway epithelial cells, likely to initiate repair. Interestingly, after airway epithelial injury, surviving differentiated epithelial cells spread out to maintain barrier function and secrete Wnt7b to induce ectopic Fgf10 expression in airway smooth muscle cells (ASMCs), which then stimulates the expansion of secretory transit amplifying cells and inhibits their differentiation into ciliated cells. We therefore hypothesize that tracheal basal stem/progenitor cells during homeostasis or differentiated cells after injury down-regulate Hippo signaling, to activate a localized Fgf10 expressing stromal niche that in turn maintains or amplifies and recruits the stem/progenitor cell population. We hypothesize that when surviving airway epithelial cells spread out, signals are transduced by the integrin machinery to induce Yap-mediated epithelial Wnt7b expression and downstream Fgf10 expression in the ASMC niche. Lastly, we hypothesize that aberrant/chronic activation of the above-described pathways drives the abnormal basal cell accumulation in human lung diseases, such as in squamous metaplasia observed in COPD or honeycomb cysts in IPF.
In specific aim 1, we will test the hypothesis that basal stem/progenitor cells maintain and restrict their cell pool during homeostasis by generating their own localized Fgf10 expressing stromal niche, and in which this niche and stem cell pool expands rapidly upon major injury in response to signals emanating from spreading surviving differentiated progeny.
In specific aim 2, we will test the hypothesis that integrin linked kinase (Ilk) positively regulates the Hippo pathway in differentiated airway epithelial progeny during homeostasis, to inhibit activation of the ASMC niche.
In specific aim 3, we will determine whether abnormal basal stem/progenitor accumulation in the lungs of COPD patients is associated with deregulated Ilk-Hippo-Wnt7b-Fgf10 signaling.
Abnormal basal stem/progenitor cell accumulation occurs in many chronic lung diseases that are leading causes of morbidity and mortality, such as asthma, COPD or IPF. These diseases are thought to involve a chronic injury- repair cycle that leads to improper regeneration and the eventual breakdown of normal airway and/or alveolar structure and function resulting in loss of respiratory function. To fulfill the promise of regenerative medicine in the lung, this study aims to identify the molecular mechanisms that maintain basal stem/progenitor cells during homeostasis or recruit them to sites of injury.
