Novel human NKCC1 mutations cause defects in goblet cells mucus secretion and chronic inflammation

ABSTRACT Background Infections due to intestinal yeast and bacteria affect a large number of patients with deficits in absorptive or secretory epithelial transport mechanisms. The basolateral Na+-K+-2Cl- cotransporter (NKCC1) has been implicated in intestinal epithelial fluid secretion. Two patients with deleterious heterozygous (NKCC1-DFX) or homozygous (Kilquist) mutations in SLC12A2 (NKCC1) suffer from gastrointestinal deficits. Due to chronic infections, the colon and the small intestine of the NKCC1-DFX patient were surgically resected. Methods To investigate how NKCC1 affects the integrity and function of the gut epithelia, we used a mouse model recapitulating the NKCC1-DFX patient mutation. Electron microscopy and immunostaining were used to analyze the integrity of the colonic mucus layers and immune cells infiltration. Fluorescence in situ hybridization was performed on the distal colon sections to measure bacteria translocation to the mucosa and submucosa. Citrobacter rodentium was used to measure mouse ability to clear enteric infection. Multiplex cytokine assay was used to analyze mouse inflammatory response to infection. Results We show that NKCC1-DFX expression causes defective goblet cells mucus granule exocytosis, leading to secretion of intact granules into the lumen of the large intestine. In addition, NKCC1-DFX colon submucosal glands secrete mucus that remained attached to the epithelium. Importantly, expression of the mutant NKCC1 or complete loss of NKCC1 function leads to aggravated inflammatory response to Citrobacter rodentium infection. Compared to wild-type, NKCC1-DFX mice showed decreased expression of claudin-2, a tight junction protein involved in paracellular Na+ and water transport and enteric infection clearance. Conclusions Our data indicate that NKCC1-DFX impairs gut barrier function by affecting mucus secretion and immune properties.