A truncation mutation in human SLC12A2 leads to abnormal goblet cell mucus secretion

Abstract

The basolateral Na+‐K+‐2Cl− cotransporter (NKCC1) plays a critical role in maintaining the intracellular concentrations of Na+, K+, and Cl− ions, and cell volume. The cotransporter also plays a key role in mediating epithelial fluid secretion. In previous studies, we have described the first known human SLC12A2 (NKCC1) mutation (Cold Spring Harbor Mol. Case Stud. 2(6): a001289, 2016). The now 16‐year‐old patient carries a carboxyl‐terminal truncation mutation (NKCC1‐DFX) in the transporter that leads to complex clinical features including complete gastrointestinal and bladder failure, endocrine insufficiencies, small intestinal dysmotility, seizure‐like episodes, and multi‐organ failure. The 11 bp deletion leads to truncation of the cytosolic COOH‐terminal domain of the cotransporter, making it non‐functional. In polarized epithelial MDCK cells and a mouse model that recapitulates the patient mutation we demonstrated that expression of the truncated Na‐K‐2Cl cotransporter causes mistargeting of the mutant and wild‐type cotransporter to the apical membrane of Cl− secreting epithelia and affects proper lumen formation in MDCK cysts (Am. J. Physiol. Cell Physiol. 315: C258–C276, 2018). Recently due to chronic infections, the patient's colon was surgically resected and lately, the patient has been battling chronic yeast infections in the small intestine. To investigate whether NKCC1‐DFX affect the integrity and function of the gut epithelia, we used MDCK cells expressing the mutant transporter and a mouse model recapitulating the patient mutation. We show that expression of the NKCC1‐DFX mutant impairs tight junction integrity, as indicated by a decrease in trans‐epithelial electrical resistance and inability of MDCK‐I cells expressing NKCC1‐DFX to re‐establish barrier function after a calcium‐switch challenge. Compared to wild‐type mice, NKCC1‐DFX have increased intestinal transit time and displayed an increase in unhealthy mitochondria characterize by dense inclusion bodies. Relative to wild‐type mice, NKCC1‐DFX mice showed decreased expression of Claudin‐2, a tight junction protein involved in paracellular Na+ and water transport. NKCC1‐DFX expression causes defective goblet cells mucus granules exocytosis leading to secretion of intact granules in the lumen of the large intestine. In addition, NKCC1‐DFX colon submucosal glands secrete mucus that remained attached to the epithelium. Taken together, these data indicate that NKCC1‐DFX likely impairs gut barrier function by affecting epithelia tight junction integrity and mucus secretion.Support or Funding InformationSupported by NIH grant GM118944.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.