Electrolyte transport properties in distal small airways from cystic fibrosis pigs with implications for host defense

Abstract

While pathological and clinical data suggest that small airways are involved in early cystic fibrosis (CF) lung disease development, little is known about how the lack of cystic fibrosis transmembrane conductance regulator (CFTR) function contributes to disease pathogenesis in these small airways. Large and small airway epithelia are exposed to different airflow velocities, temperatures, humidity, and CO2 concentrations. The cellular composition of these two regions is different, and small airways lack submucosal glands. To better understand the ion transport properties and impacts of lack of CFTR function on host defense function in small airways, we adapted a novel protocol to isolate small airway epithelial cells from CF and non-CF pigs and established an organotypic culture model. Compared with non-CF large airways, non-CF small airway epithelia cultures had higher Cl− and bicarbonate (HCO3−) short-circuit currents and higher airway surface liquid (ASL) pH under 5% CO2 conditions. CF small airway epithelia were characterized by minimal Cl− and HCO3− transport and decreased ASL pH, and had impaired bacterial killing compared with non-CF small airways. In addition, CF small airway epithelia had a higher ASL viscosity than non-CF small airways. Thus, the activity of CFTR is higher in the small airways, where it plays a role in alkalinization of ASL, enhancement of antimicrobial activity, and lowering of mucus viscosity. These data provide insight to explain why the small airways are a susceptible site for the bacterial colonization.

Nominal carbonic anhydrase activity minimizes airway-surface liquid pH changes during breathing

“…We first assessed mRNA transcripts of carbonic anhydrase isozymes ( Ca ) predicted to be enzymatically active outside of cells, which include the membrane‐bound and secreted carbonic anhydrase isozymes. Microarray analysis from Li et al (Fig. D) revealed that most mRNA transcripts for these isozymes were present at levels lower than Cftr mRNA transcripts, which is present in low abundance in airway epithelia (Trapnell et al.…”

“…Piglet lungs were excised and proximal large airways, including trachea and main stem bronchi, were dissected out from the airway tree as described (Li et al. ). Next, primary porcine airway epithelia were isolated according to an adapted procedure originally developed for tracheal airway cells.…”

“…Isolated large airway cells were expanded as recently described (Li et al. ) using a method developed to conditionally reprogram airway epithelial cells (Liu et al. ; Suprynowicz et al.…”

Relationships among CFTR expression, HCO 3 − secretion, and host defense may inform gene- and cell-based cystic fibrosis therapies

“…The abnormally acidic ASL pH results from loss of CFTR-mediated HCO 3 − secretion in the presence of continued H + secretion (35). CFTR is also key for HCO 3 − secretion and alkalization of ASL pH in small airways (36).…”

Roadmap for an early gene therapy for cystic fibrosis airway disease

“…An important feature of these models is the presence of submucosal glands (SMGs) throughout the proximal airways. CF airway pathology is largely attributed to defects in the SMGs as CFTR dysfunction is associated with abnormal mucus secretion and defective innate immunity . SMGs are therefore proposed as one of the targets for gene therapy.…”