Transgenic Ferret Models Define Pulmonary Ionocyte Diversity and Function

Speciation leads to adaptive changes in organ cellular anatomy and physiology. These evolutionary changes create challenges for studying rare cell type functions that diverge between human and mice. Rare CFTR-rich pulmonary ionocytes exist throughout the cartilaginous airways of humans, but limited presence and divergent biology in the proximal trachea of mice has prevented the use of traditional transgenic models to elucidate ionocyte functions in the airway. Here we describe the creation and use of novel conditional genetic ferret models to dissect pulmonary ionocyte biology and function by enabling ionocyte lineage tracing (FOXI1-CreERT2::ROSA-TG), ionocyte ablation (FOXI1-KO), and ionocyte-specific deletion of CFTR (FOXI1-CreERT2::CFTRL/L). By comparing these models to cystic fibrosis (CF) ferrets, we demonstrate that ionocytes control airway surface liquid (ASL) absorption, secretion, pH, and mucus viscosity—leading to reduced ASL volume and impaired mucociliary clearance in CF, FOXI1-KO, and FOXI1-CreERT2::CFTRL/L ferrets. These processes were regulated by CFTR- dependent ionocyte transport of Cl– and HCO3–, as determined electrophysiologically and by single-cell imaging with a conditionally activated halide fluorescent sensor. Single-cell transcriptomics, using pulse-seq of lineage-traced ionocyte-enriched cultures, revealed three subtypes of pulmonary ionocytes with unique biologic functions and a common rare cell progenitor of ionocytes, tuft, and neuroendocrine cells. Thus, rare pulmonary ionocytes perform critical CFTR-dependent functions in the proximal airway that are hallmark features of CF airway disease. These studies provide a road map for using conditional genetics in the first non- rodent mammal to address gene function, cell biology, and disease processes that have greater evolutionary conservation between humans and ferrets. Airway epithelial cells were cultured in air-liquid interface from donor ferrets of 3 different genotypes and then isolated and profiled using single-cell RNA-sequencing. We profiled a total of 94,664 tracheal epithelial cells from WT (n=4), FOXI1-KO (n=4) and FOXI1-CreERT2::ROSA-TG (n=8) differentiated ALI cultures derived from 4 independent donor ferrets using 10X Chromium droplet-based 3’ scRNA-seq.