Regulation of Airway Progenitor Homeostasis and Cell Composition by Tight Junction Protein Claudin‐18

Abstract

RationaleClaudin‐18 (Cldn18) is comprised of two isoforms, Cldn 18.1 and Cldn18.2. The lung‐specific isoform, Cldn18.1, regulates distal lung epithelial progenitor cell homeostasis. Its role in airway epithelium has not been elucidated, although Cldn18 expression is decreased in lungs of patients with chronic airway diseases (e.g., asthma and chronic obstructive pulmonary disease (COPD)). The airway epithelium is comprised of three progenitor populations, basal cells (BC), pulmonary neuroendocrine cells (PNEC) and club cells, and two non‐progenitor cell populations, namely goblet and ciliated cells. Club cells are secretory cells that are susceptible to naphthalene (NAP) injury. Club cell depletion occurs by day 3 post‐NAP, with spreading of ciliated cells to cover the denuded epithelial surface, and full reconstitution of the airway epithelium by day 14–21. Here we investigated the role of Cldn18.1 in airway epithelium using a previously developed global Cldn18 knockout (KO) mouse model.MethodsWild‐type (WT) and Cldn18 KO mice were injected intraperitoneally with NAP (225–275 mg/kg, corn oil as control). Lungs were harvested at baseline and 3‐days (maximal injury), 7‐days (beginning of regeneration) and 21‐days (full regeneration) post‐NAP, inflated with 4% paraformaldehyde and fixed overnight prior to embedding and sectioning. Hematoxylin and eosin, Periodic Acid‐Schiff (PAS) and immunofluorescence staining were performed.ResultsIn WT mice, Cldn18.1 was expressed in a small subset of airway ciliated cells. Cldn18 KO mice showed increased numbers of pulmonary neuroendocrine cells and neuroendocrine bodies, as well as ectopic localization of club cells and localization of keratin 5‐positive cells within the distal parenchyma. At day 3 post‐NAP in WT mice, Cldn18.1 was transiently expressed in all the ciliated cells covering the denuded epithelium, returning to being expressed in only a subset of cells by day 7. Cldn18 KO mice subjected to injury unexpectedly developed marked goblet cell hyperplasia at day 14–21 post‐NAP, with cells expressing markers for both club (CC‐10) and goblet (MUC5AC) cells.ConclusionsOur findings demonstrate an unexpected role for Cldn18.1 in regulating airway progenitor homeostasis and cell composition, implicating Cldn18.1 in regulation of downstream signaling pathways that inhibit goblet cell differentiation. A transient increase in the number of Cldn18.1‐positive ciliated cells in WT mice in the initial stages post‐injury suggests paracrine regulation of these inhibitory effects. Modulation of Cldn18 expression may be beneficial in the treatment of airway diseases characterized by goblet cell metaplasia.Support or Funding InformationFunding: American Lung Association, Hastings Foundation, NIH