Question:

We recently identified Slc26a9 as an anion conductance that is upregulated in airway inflammation and prevents bronchial mucus obstruction (Anagnostopoulou et al. JCI 2012). Slc26a9 variants were recently found associated with meconium ileus in cystic fibrosis infants (Sun et al. Nature 2012). The association with meconium ileus raises the question where Slc26a9 is expressed in the gastrointestinal tract and what is its function.

Methods:

Acid, HCO₃^-, short circuit current (Isc) measurements were performed in isolated mucosa and acid, HCO₃^- and fluid absorptive and secretory rates were measured by single pass perfusion and back titration in anesthetized Slc26a9 KO and WT mice by inhalation of 2.0% isoflurane. Slc26a9 cellular expression was studied by laser dissection and qPCR, and quantitative morphometry was performed in the different segments of the murine Gastrointestinal tract.

Results:

Slc26a9 was found highly expressed in the mucosae of the upper gastrointestinal tract, with abrupt decrease of expression levels to virtually undectable levels beyond the duodenum. As previously reported, Slc26a9 KO mice had completely lost the ability to secrete acid in adulthood. However, Slc26a9 was found highly expressed along the whole gastric gland, even in areas without H⁺,K⁺-ATPase expression. Proximal duodenal bicarbonate and fluid secretory rates, which are higher in the proximal than the distal duodenum in WT mice, as well as the ability to stimulate these rates with forskolin, were reduced in the absence of Slc26a9 expression. The gastric antrum, as well as the fundus (after omeprazole treatment to rule out any residual acid secretory capacity) was studied to test whether Slc26a9 transports HCO₃^- itself. Gastric antrum, while expressing high Slc26a9 levels in WT mice, had lower basal and forskolin-stimulated HCO₃^- rate as well as lower Isc response in WT than KO mice, arguing against a role of Slc26a9 as a HCO₃^- transporter. Morphometry revealed strongly elongated fundic as well as antral glands, and slightly elongated proximal duodenal villi as well as crypts.

Conclusions:

Slc26a9 expression is necessary for normal gastric acid and proximal duodenal bicarbonate secretion, but it is not expressed in more distal parts of the gastrointestinal mucosa. The increased risk for meconium ileus may be due to loss of digestive function of the stomach and proximal duodenum.