The alveolus: Our current knowledge of how the gas exchange unit of the lung is constructed and repaired.

The mammalian lung completes its last step of development, alveologenesis, to generate sufficient surface area for gas exchange. In this process, multiple cell types that include alveolar epithelial cells, endothelial cells, and fibroblasts undergo coordinated cell proliferation, cell migration and/or contraction, cell shape changes, and cell-cell and cell-matrix interactions to produce the gas exchange unit: the alveolus. Full functioning of alveoli also involves immune cells and the lymphatic and autonomic nervous system. With the advent of lineage tracing, conditional gene inactivation, transcriptome analysis, live imaging, and lung organoids, our molecular understanding of alveologenesis has advanced significantly. In this review, we summarize the current knowledge of the constituents of the alveolus and the molecular pathways that control alveolar formation. We also discuss how insight into alveolar formation may inform us of alveolar repair/regeneration mechanisms following lung injury and the pathogenic processes that lead to loss of alveoli or tissue fibrosis.

FGFR2b signalling restricts lineage-flexible alveolar progenitors during mouse lung development and converges in mature alveolar type 2 cells.

The specification, characterization, and fate of alveolar type 1 and type 2 (AT1 and AT2) progenitors during embryonic lung development are poorly defined. Current models of distal epithelial lineage formation fail to capture the heterogeneity and dynamic contribution of progenitor pools present during early development. Furthermore, few studies explore the pathways involved in alveolar progenitor specification and fate. In this paper, we build upon our previously published work on the regulation of airway epithelial progenitors by fibroblast growth factor receptor 2b (FGFR2b) signalling during early (E12.5) and mid (E14.5) pseudoglandular stage lung development. Our results suggest that a significant proportion of AT2 and AT1 progenitors are lineage-flexible during late pseudoglandular stage development, and that lineage commitment is regulated in part by FGFR2b signalling. We have characterized a set of direct FGFR2b targets at E16.5 which are likely involved in alveolar lineage formation. These signature genes converge on a subpopulation of AT2 cells later in development and are downregulated in AT2 cells transitioning to the AT1 lineage during repair after injury in adults. Our findings highlight the extensive heterogeneity of pneumocytes by elucidating the role of FGFR2b signalling in these cells during early airway epithelial lineage formation, as well as during repair after injury.

Quest for breathing: proliferation of alveolar type 1 cells.

There is much evidence that the vertebrate lung originated from a progenitor structure which was present in bony fish. However, critical basic elements for the evolution of breathing in tetrapods, such as the central rhythm generator sensitive to CO2/pH and the pulmonary surfactant, were present in the lungless primitive vertebrate. This suggests that the evolution of air breathing in all vertebrates may have evolved through exaptations. It appears that the capability for proliferation of alveolar type 1 (AT1) cells is the "critical factor" which rendered possible the most radical subsequent innovation-the possibility of air breathing. "Epithelial remodeling," which consists in proliferation of alveolar cells-the structural basis for gas diffusion-observed in the alimentary tract of the gut-breathing fishes (GBF) has great potential for application in biomedical research. Such a process probably led to the gradual evolutionary development of lungs in terrestrial vertebrates. Research on the cellular and molecular mechanisms controlling proliferation of squamous epithelial cells in the GBF should contribute to explaining the regeneration-associated phenomena that occur in mammal lungs, and especially to the understanding of signal pathways which govern the process.

Receptor for Advanced Glycation End Products (RAGE) and Its Polymorphic Variants as Predictive Diagnostic and Prognostic Markers of NSCLCs: a Perspective.

PURPOSE OF REVIEW: Non-small cell lung cancers (NSCLCs) account for ~ 85% of all lung cancers, and 5-year survival in Europe and the USA is ~ 13-17%. In this review, we focus on the significance of Receptor for Advanced Glycation End products (RAGE) as a diagnostic or post-therapeutic prognostic marker for various forms of NSCLCs. RECENT FINDINGS: The lungs have the highest levels of basal RAGE expression in mammals. The physiologic RAGE in lungs may be involved in adhesion and spreading of AT-1 cells and maintenance of pulmonary homeostasis. However, high level expression of RAGE complicates various diseases including acute lung injury. In NSCLCs, while a number of studies report decreased RAGE expression, inferring a protective role, others suggest that RAGE expression may contribute to NSCLC pathogenesis. Genetic polymorphisms of RAGE are reportedly associated with NSCLC development and complications. RAGE and its polymorphic variants may be useful diagnostic or post-therapeutic prognostic markers of NSCLCs.

Epithelial Vegfa Specifies a Distinct Endothelial Population in the Mouse Lung.

The lung microvasculature is essential for gas exchange and commonly considered homogeneous. We show that VEGFA from the epithelium is required for a distinct endothelial cell (EC) population in the mouse lung. Vegfa is predominantly expressed by alveolar type 1 (AT1) cells and locally required to specify a subset of ECs. Single-cell RNA sequencing (scRNA-seq) reveals that ∼15% of lung ECs are transcriptionally distinct-marked by Carbonic anhydrase 4 (Car4)-and arise from bulk ECs, as suggested by trajectory analysis. Car4 ECs have extensive cellular projections and are separated from AT1 cells by a limited basement membrane without intervening pericytes. Car4 ECs are specifically lost upon epithelial Vegfa deletion; without Car4 ECs, the alveolar space is aberrantly enlarged despite the normal appearance of myofibroblasts. Lung Car4 ECs and retina tip ECs have common and distinct features. These findings support a signaling role of AT1 cells and shed light on alveologenesis.

Transcriptional control of lung alveolar type 1 cell development and maintenance by NK homeobox 2-1.

The extraordinarily thin alveolar type 1 (AT1) cell constitutes nearly the entire gas exchange surface and allows passive diffusion of oxygen into the blood stream. Despite such an essential role, the transcriptional network controlling AT1 cells remains unclear. Using cell-specific knockout mouse models, genomic profiling, and 3D imaging, we found that NK homeobox 2-1 (Nkx2-1) is expressed in AT1 cells and is required for the development and maintenance of AT1 cells. Without Nkx2-1, developing AT1 cells lose 3 defining features-molecular markers, expansive morphology, and cellular quiescence-leading to alveolar simplification and lethality. NKX2-1 is also cell-autonomously required for the same 3 defining features in mature AT1 cells. Intriguingly, Nkx2-1 mutant AT1 cells activate gastrointestinal (GI) genes and form dense microvilli-like structures apically. Single-cell RNA-seq supports a linear transformation of Nkx2-1 mutant AT1 cells toward a GI fate. Whole lung ChIP-seq shows NKX2-1 binding to 68% of genes that are down-regulated upon Nkx2-1 deletion, including 93% of known AT1 genes, but near-background binding to up-regulated genes. Our results place NKX2-1 at the top of the AT1 cell transcriptional hierarchy and demonstrate remarkable plasticity of an otherwise terminally differentiated cell type.

Expression of histone deacetylase 3 instructs alveolar type I cell differentiation by regulating a Wnt signaling niche in the lung.

The commitment and differentiation of the alveolar type I (AT1) cell lineage is a critical step for the formation of distal lung saccules, which are the primitive alveolar units required for postnatal respiration. How AT1 cells arise from the distal lung epithelial progenitor cells prior to birth and whether this process depends on a developmental niche instructed by mesenchymal cells is poorly understood. We show that mice lacking histone deacetylase 3 specifically in the developing lung mesenchyme display lung hypoplasia including decreased mesenchymal proliferation and a severe impairment of AT1 cell differentiation. This is correlated with a decrease in Wnt/ß-catenin signaling in the lung epithelium. We demonstrate that inhibition of Wnt signaling causes defective AT1 cell lineage differentiation ex vivo. Importantly, systemic activation of Wnt signaling at specific stages of lung development can partially rescue the AT1 cell differentiation defect in vivo. These studies show that histone deacetylase 3 expression generates an important developmental niche in the lung mesenchyme through regulation of Wnt signaling, which is required for proper AT1 cell differentiation and lung sacculation.