Gerner-Mauro, K. N. | Search bioRxiv

bioRxiv2024

Morphogenic, molecular, and cellular adaptations for unidirectional airflow in the chicken lung

Unidirectional airflow in the avian lung enables gas exchange during both inhalation and exhalation. The underlying developmental process and how it deviates from that of the bidirectional mammalian lung are poorly understood. Sampling key developmental stages with multiscale 3D imaging and single-cell transcriptomics, we delineate morphogenic, molecular, and cellular features that accommodate the unidirectional airflow in the chicken lung. Primary termini of hyper-elongated branches are eliminated via proximal-short and distal-long fusions, forming parabronchi. Neoform termini extend radially through parabronchial smooth muscle to form gas-exchanging alveoli. Supporting this radial alveologenesis, branch stalks halt their proximalization, defined by SOX9-SOX2 transition, and become SOX9low parabronchi. Primary and secondary vascular plexi interface with primary and neoform termini, respectively. Single-cell and Stereo-seq spatial transcriptomics reveal a third, chicken-specific alveolar cell type expressing KRT14, hereby named luminal cells. Luminal, alveolar type 2, and alveolar type 1 cells sequentially occupy concentric zones radiating from the parabronchial lumen. Our study explores the evolutionary space of lung diversification and lays the foundation for functional analysis of species-specific genetic determinants.

developmental biology↗

bioRxiv2020

Redundant and additive functions of the four Lef/Tcf transcription factors in lung epithelial progenitors

In multicellular organisms, paralogs from gene duplication survive purifying selection by evolving tissue-specific expression and function. Whether this genetic redundancy is also selected for within a single cell type is unclear for multi-member paralogs, as exemplified by the 4 obligatory Lef/Tcf transcription factors of canonical Wnt signaling, mainly due to the dauntingly complex genetics involved. Using the developing mouse lung as a model system, we generate 2 quadruple conditional knockouts and myriad triple and double combinations, and show that the 4 Lef/Tcf genes function redundantly in the presence of at least 2 Lef/Tcf paralogs, but additively upon losing additional paralogs to specify and maintain lung epithelial progenitors. Pre-lung-specification, pan-epithelial double knockouts have no lung phenotype, triple knockouts have varying phenotypes, including defective branching and tracheoesophageal fistulas, and the quadruple knockout barely forms a lung, resembling the Ctnnb1 mutant. Post-lung-specification deletion of all 4 Lef/Tcf genes leads to branching defects, downregulation of progenitor genes, premature alveolar differentiation, and derepression of gastrointestinal genes, again phenocopying the corresponding Ctnnb1 mutant. Our study supports a monotonic, positive signaling relationship between CTNNB1 and Lef/Tcf in lung epithelial progenitors and represents, to our knowledge, the first in vivo analysis of cell-type-specific genetic redundancy among the 4 Lef/Tcf paralogs. SIGNIFICANCE STATEMENTParalogs represent genetic redundancy and survive purifying selection by evolving overlapping and distinct functions. In multicellular organisms, such functional diversification can manifest as tissue and cell type specific expression, which masks possible selective pressure for genetic redundancy within a single cell type. Using in vivo genetic and genomic analyses, we show that although the 4 mammalian Lef/Tcf transcription factors have evolved organ-specific functions, they function additively and redundantly, depending on gene dosage, to promote lung epithelial progenitors and do so in a monotonic, positive manner with beta-Catenin in the canonical Wnt signaling pathway.

developmental biology↗