Distinct Roles of SLC26A3 and CFTR in Surface pH Regulation and Bicarbonate Secretion in Human Intestinal Epithelium

Colonic bicarbonate secretion is mediated by the chloride/bicarbonate exchanger SLC26A3 and the cystic fibrosis transmembrane conductance regulator (CFTR). Dysfunction of either causes luminal acidosis, altered mucus properties, and inflammation. While physical and functional interactions have been demonstrated in heterologous systems, their relationship in native epithelium is not fully established. We investigated the distinct roles of SLC26A3 and CFTR using human intestinal organoids with inducible SLC26A3 overexpression.

Human colonic and rectal organoids from healthy controls and cystic fibrosis patients with F508del mutations were studied in the proliferative state with high endogenous CFTR expression and inducible SLC26A3 overexpression. Real-time surface pH measurements, electrophysiological analysis, forskolin-induced swelling assays, and confocal microscopy were employed.

Steady-state surface pH was lower in CF versus healthy organoids (7.23 ± 0.03 vs. 7.34 ± 0.03). SLC26A3 overexpression normalized surface pH in CF organoids and CFTR-inhibited organoids, equalizing responses between genotypes. SLC26A3 overexpression corrected abnormal morphology and significantly improved intracellular MUC2 distribution in CF organoids. However, SLC26A3 did not restore fluid secretion in CF organoids or enhance CFTR-mediated electrogenic anion secretion in Ussing chambers.

SLC26A3 and CFTR perform distinct yet complementary functions. SLC26A3 dominates surface pH regulation and maintains bicarbonate efflux independently of CFTR, while CFTR drives agonist-stimulated fluid secretion. SLC26A3's ability to restore pH homeostasis and normalize mucin intracellular distribution in CF organoids demonstrates its critical importance for maintaining colonic mucosal health.