Mucins and CFTR: Their Close Relationship

Abstract

Mucociliary clearance is a critical defense mechanism for the lungs governed by regionally coordinated epithelial cellular activities, including mucin secretion, cilia beating, and transepithelial ion transport. Cystic fibrosis (CF), an autosomal genetic disorder caused by the dysfunction of the cystic fibrosis transmembrane conductance regulator (CFTR) channel, is characterized by failed mucociliary clearance due to abnormal mucus biophysical properties. In recent years, with the development of highly effective modulator therapies, the quality of life of a significant number of people living with CF has greatly improved; however, further understanding the cellular biology relevant to CFTR and airway mucus biochemical interactions are necessary to develop novel therapies aimed at restoring CFTR gene expression in the lungs. In this article, we discuss recent advances of transcriptome analysis at single-cell levels that revealed a heretofore unanticipated close relationship between secretory MUC5AC and MUC5B mucins and CFTR in the lungs. In addition, we review recent findings on airway mucus biochemical and biophysical properties, focusing on how mucin secretion and CFTR-mediated ion transport are integrated to maintain airway mucus homeostasis in health and how CFTR dysfunction and restoration of function affect mucus properties.

Keywords: CFTR; airway clearance; cystic fibrosis (CF); mucins; mucus; polymeric network; single-cell transcriptomics; viscoelasticity.

Figure 1

Intraregional expression of secretory mucins and CFTR in human airway superficial epithelia. (A) Regional distribution of secretory mucins, ionocytes, and CFTR transcripts in human conducting airway superficial epithelia. In the proximal (large) airways, both MUC5B and MUC5AC are expressed by the superficial epithelium. In the distal (small) airways, MUC5AC expression ceases, while MUC5B expression persists. CFTR is expressed by the superficial epithelium throughout the entire conducting airways, whereas the frequency of ionocytes is reduced in the distal airways. (B) RNA in situ hybridization (RNA-ISH) on isolated small airway epithelial cells. Cells were isolated from freshly excised small airway tissue obtained from a non-diseased transplant donor lung. Using RNA-ISH probes, CFTR colocalized with secretory cell marker transcripts (SCGB1A1) in a non-ciliated cell (arrow) and not with FOXJ1-positive ciliated cells. Left image shows differential interference contrast and fluorescent image overlay. Right image shows fluorescent image only. Scale bar = 10 μm.

Figure 2

Scanning electron microscope (SEM) image of a CF submucosal gland duct obstructed by mucus. Large airways were collected from a CF lung at the time of transplant, stimulated with methacholine (10 µM) for 30 min and processed for SEM. Image was acquired with a Zeiss Supra 25 SEM and colorized using Adobe Photoshop software. Color schemes show mucus in yellow, cilia in pink, gland duct in blue, secretory cells in purple, and a red blood cell in red. Scale bar 10 μm.

Figure 3

Effects of ETI treatment on mucus removal from the cell surfaces. Primary HBE cells homozygous for F508del were treated for 3 days with 0.06% dimethyl sulfoxide (DMSO/vehicle) or 3 µM VX-661, 2 µM VX-445, and 1 µM VX-770 (ETI). Cells were washed for 15 min with PBS, fixed with 4% PFA, and processed for immunohistochemistry. Representative images of histological sections stained with MUC5AC (red), MUC5B (green), DAPI (blue), and DIC overlay.