During lung development, parabronchial SMC (PSMC) progenitors in the distal mesenchyme secrete fibroblast growth factor 10 (Fgf10), which acts on distal epithelial progenitors to promote their proliferation. β-catenin signaling within PSMC progenitors is essential for their maintenance, proliferation, and expression of Fgf10. Here, we report that this Wnt/Fgf10 embryonic signaling cascade is reactivated in mature PSMCs after naphthalene-induced injury to airway epithelium. Furthermore, we found that this paracrine Fgf10 action was essential for activating surviving variant Clara cells (the cells in the airway epithelium from which replacement epithelial cells originate) located at the bronchoalveolar duct junctions and adjacent to neuroendocrine bodies. After naphthalene injury, PSMCs secreted Fgf10 to activate Notch signaling and induce Snai1 expression in surviving variant Clara cells, which subsequently underwent a transient epithelial to mesenchymal transition to initiate the repair process. Epithelial Snai1 expression was important for regeneration after injury. We have therefore identified PSMCs as a stem cell niche for the variant Clara cells in the lung and established that paracrine Fgf10 signaling from the niche is critical for epithelial repair after naphthalene injury. These findings also have implications for understanding the misregulation of lung repair in asthma and cancer.

SUMMARYLocalized Fgf10 expression in the distal mesenchyme adjacent to sites of lung bud formation has long been thought to drive stereotypic branching morphogenesis even though isolated lung epithelium branches in the presence of non-directional exogenous Fgf10 in Matrigel. Here, we show that lung agenesis in Fgf10 knockout mice can be rescued by ubiquitous overexpression of Fgf10, indicating that precisely localized Fgf10 expression is not required for lung branching morphogenesis in vivo. Fgf10 expression in the mesenchyme itself is regulated by Wnt signaling. Nevertheless, we found that during lung initiation simultaneous overexpression of Fgf10 is not sufficient to rescue the absence of primary lung field specification in embryos overexpressing Dkk1, a secreted inhibitor of Wnt signaling. However, after lung initiation, simultaneous overexpression of Fgf10 in lungs overexpressing Dkk1 is able to rescue defects in branching and proximal-distal differentiation. We also show that Fgf10 prevents the differentiation of distal epithelial progenitors into Sox2-expressing airway epithelial cells in part by activating epithelial β-catenin signaling, which negatively regulates Sox2 expression. As such, these findings support a model in which the main function of Fgf10 during lung development is to regulate proximal-distal differentiation. As the lung buds grow out, proximal epithelial cells become further and further displaced from the distal source of Fgf10 and differentiate into bronchial epithelial cells. Interestingly, our data presented here show that once epithelial cells are committed to the Sox2-positive airway epithelial cell fate, Fgf10 prevents ciliated cell differentiation and promotes basal cell differentiation. KEY WORDS: Basal cells, Branching, Dkk1, Fgf10, Lung development, Wnt signaling, MouseLocalized Fgf10 expression is not required for lung branching morphogenesis but prevents differentiation of epithelial progenitors progenitors from differentiating into airway epithelial cells by initially inhibiting Sox2 expression (Park et al., 1998;Que et al., 2007;Ramasamy et al., 2007;Nyeng et al., 2008; Hashimoto et al., 2012). β-Catenin is not only a downstream transcriptional target of epithelial Fgf10 signaling (Lü et al., 2005), but increasing data also indicate that Fgf10 is able to increase nuclear β-catenin directly, via phosphorylation of β-catenin on Ser552 and inhibition of Gsk3β, through the PI3K/AKT pathway (He et al., 2007;Ramasamy et al., 2007;Volckaert et al., 2011). In addition, FGF signaling via Erk/MAPK phosphorylates the Wnt co-receptor Lrp6 on Ser1490 and Thr1572 and phosphorylates β-catenin directly on Tyr142, thereby releasing it from cadherin complexes (Krejci et al., 2012). In turn, epithelial β-catenin activation participates in the induction of Fgfr2b expression to increase Fgf10 signaling further (Shu et al., 2005). Epithelial β-catenin signaling, mediated primarily through Fgf10 signaling, is a regulator of branching morphogenesis and functions to maintain the distal epithelial ...

Idiopathic pulmonary fibrosis (IPF) is a progressive and terminal lung disease with no known cure. IPF is a disease of aging, with median age of diagnosis over 65 years. Median survival is between 3 and 5 years after diagnosis. IPF is characterized primarily by excessive deposition of extracellular matrix (ECM) proteins by activated lung fibroblasts and myofibroblasts, resulting in reduced gas exchange and impaired pulmonary function. Growing evidence supports the concept of a pro-fibrotic environment orchestrated by underlying factors such as genetic predisposition, chronic injury and aging, oxidative stress, and impaired regenerative responses may account for disease development and persistence. Currently, two FDA approved drugs have limited efficacy in the treatment of IPF. Many of the genes and gene networks associated with lung development are induced or activated in IPF. In this review, we analyze current knowledge in the field, gained from both basic and clinical research, to provide new insights into the disease process, and potential approaches to treatment of pulmonary fibrosis.

Summary The lung harbors its basal stem/progenitor cells (BSCs) in the protected environment of the cartilaginous airways. After major lung injuries, BSCs are activated and recruited to sites of injury. Here, we show that during homeostasis, BSCs in cartilaginous airways maintain their stem cell state by down-regulating the Hippo pathway (resulting in increased nuclear Yap), which generates a localized Fgf10 expressing stromal niche; in contrast, differentiated epithelial cells in non-cartilaginous airways maintain quiescence by activating the Hippo pathway and inhibiting Fgf10 expression in airway smooth muscle cells (ASMCs). However, upon injury, surviving differentiated epithelial cells spread to maintain barrier function and recruit integrin linked kinase to adhesion sites, which leads to Merlin degradation, down-regulation of the Hippo pathway, nuclear Yap translocation and expression and secretion of Wnt7b. Epithelial-derived Wnt7b, then in turn, induces Fgf10 expression in ASMCs which extends the BSC niche to promote regeneration.

The adaptation to terrestrial life required the development of an organ capable of efficient air-blood gas exchange. To meet the metabolic load of cellular respiration, the mammalian respiratory system has evolved from a relatively simple structure, similar to the two-tube amphibian lung, to a highly complex tree-like system of branched epithelial airways connected to a vast network of gas exchanging units called alveoli. The development of such an elaborate organ in a relatively short time window is therefore an extraordinary feat and involves an intimate crosstalk between mesodermal and endodermal cell lineages. This review describes the molecular processes governing lung development with an emphasis on the current knowledge on the role of Wnt and Fgf signaling in lung epithelial differentiation.

Summary Idiopathic pulmonary fibrosis is a common form of interstitial lung disease resulting in alveolar remodeling and progressive loss of pulmonary function because of chronic alveolar injury and failure to regenerate the respiratory epithelium. Histologically, fibrotic lesions and honeycomb structures expressing atypical proximal airway epithelial markers replace alveolar structures, the latter normally lined by alveolar type 1 (AT1) and AT2 cells. Bronchial epithelial stem cells (BESCs) can give rise to AT2 and AT1 cells or honeycomb cysts following bleomycin-mediated lung injury. However, little is known about what controls this binary decision or whether this decision can be reversed. Here we report that inactivation of Fgfr2b in BESCs impairs their contribution to both alveolar epithelial regeneration and honeycomb cysts after bleomycin injury. By contrast overexpression of Fgf10 in BESCs enhances fibrosis resolution by favoring the more desirable outcome of alveolar epithelial regeneration over the development of pathologic honeycomb cysts.

The lung is morphologically structured into a complex tree-like network with branched airways ending distally in a large number of alveoli for efficient oxygen exchange. At the cellular level, the adult lung consists of at least 40–60 different cell types which can be broadly classified into epithelial, endothelial, mesenchymal, and immune cells. Fibroblast growth factor 10 (Fgf10) located in the lung mesenchyme is essential to regulate epithelial proliferation and lineage commitment during embryonic development and post-natal life, and to drive epithelial regeneration after injury. The cells that express Fgf10 in the mesenchyme are progenitors for mesenchymal cell lineages during embryonic development. During adult lung homeostasis, Fgf10 is expressed in mesenchymal stromal niches, between cartilage rings in the upper conducting airways where basal cells normally reside, and in the lipofibroblasts adjacent to alveolar type 2 cells. Fgf10 protects and promotes lung epithelial regeneration after different types of lung injuries. An Fgf10-Hippo epithelial-mesenchymal crosstalk ensures maintenance of stemness and quiescence during homeostasis and basal stem cell (BSC) recruitment to further promote regeneration in response to injury. Fgf10 signaling is dysregulated in different human lung diseases including bronchopulmonary dysplasia (BPD), idiopathic pulmonary fibrosis (IPF), and chronic obstructive pulmonary disease (COPD), suggesting that dysregulation of the FGF10 pathway is critical to the pathogenesis of several human lung diseases.

Throughout life adult animals crucially depend on stem cell populations to maintain and repair their tissues to ensure life-long organ function. Stem cells are characterized by their capacity to extensively self-renew and give rise to one or more differentiated cell types. These powerful stem cell properties are key to meet the changing demand for tissue replacement during normal lung homeostasis and regeneration after lung injury. Great strides have been made over the last few years to identify and characterize lung epithelial stem cells as well as their lineage relationships. Unfortunately, knowledge on what regulates the behavior and fate specification of lung epithelial stem cells is still limited, but involves communication with their microenvironment or niche, a local tissue environment that hosts and influences the behaviors or characteristics of stem cells and that comprises other cell types and extracellular matrix. As such, an intimate and dynamic epithelial-mesenchymal cross-talk, which is also essential during lung development, is required for normal homeostasis and to mount an appropriate regenerative response after lung injury. Fibroblast growth factor 10 (Fgf10) signaling in particular seems to be a well-conserved signaling pathway governing epithelial-mesenchymal interactions during lung development as well as between different adult lung epithelial stem cells and their niches. On the other hand, disruption of these reciprocal interactions leads to a dysfunctional epithelial stem cell-niche unit, which may culminate in chronic lung diseases such as chronic obstructive pulmonary disease (COPD), chronic asthma and idiopathic pulmonary fibrosis (IPF).