Slc26a3 deficiency is associated with loss of colonic <scp>HCO</scp>₃⁻ secretion, absence of a firm mucus layer and barrier impairment in mice

<jats:title>Abstract</jats:title><jats:sec><jats:title>Aim</jats:title><jats:p>Downregulated in adenoma (<jats:styled-content style="fixed-case">DRA</jats:styled-content>, Slc26a3) is a member of the solute carrier family 26 (<jats:styled-content style="fixed-case">SLC</jats:styled-content>26), family of anion transporters, which is mutated in familial chloride‐losing diarrhoea (CLD). Besides Cl<jats:sup>−</jats:sup>‐rich diarrhoea, <jats:styled-content style="fixed-case">CLD</jats:styled-content> patients also have a higher‐than‐average incidence of intestinal inflammation. In a search for potential explanations for this clinical finding, we investigated colonic electrolyte transport, the mucus layer and susceptibility against dextran sodium sulphate (<jats:styled-content style="fixed-case">DSS</jats:styled-content>)‐induced colitis in Slc26a3<jats:sup>−/−</jats:sup> mice.</jats:p></jats:sec><jats:sec><jats:title>Methods</jats:title><jats:p>HCO<jats:sub>3</jats:sub><jats:sup>−</jats:sup> secretory (J<jats:sub>HCO3</jats:sub><jats:sup>−</jats:sup>) and fluid absorptive rates were measured by single‐pass perfusion <jats:italic>in vivo</jats:italic> and in isolated mid‐distal colonic mucosa in Ussing chambers <jats:italic>in vitro</jats:italic>. Colonocyte intracellular <jats:styled-content style="fixed-case">pH</jats:styled-content> (<jats:styled-content style="fixed-case">pH</jats:styled-content><jats:sub>i</jats:sub>) was assessed fluorometrically, the mucus layer by immunohistochemistry and colitis susceptibility by the addition of DSS to the drinking water.</jats:p></jats:sec><jats:sec><jats:title>Results</jats:title><jats:p>HCO<jats:sub>3</jats:sub><jats:sup>−</jats:sup> secretory (J<jats:sub>HCO3‐</jats:sub>) and fluid absorptive rates were strongly reduced in Slc26a3<jats:sup>−/−</jats:sup> mice compared to wild‐type (WT) littermates. Despite an increase in sodium/hydrogen exchanger 3 (NHE3) m<jats:styled-content style="fixed-case">RNA</jats:styled-content> and protein expression, and intact acid‐activation of NHE3, the high colonocyte p<jats:styled-content style="fixed-case">H</jats:styled-content> in Slc26a3<jats:sup>−/−</jats:sup> mice prevented Na<jats:sup>+</jats:sup>/H<jats:sup>+</jats:sup> exchange‐mediated fluid absorption <jats:italic>in vivo</jats:italic>. Mucin 2 (MUC2) immunohistochemistry revealed the absence of a firm mucus layer, implying that alkaline secretion and/or an absorptive flux may be necessary for optimal mucus gel formation. Slc26a3<jats:sup>−/−</jats:sup> mice were highly susceptible to DSS damage.</jats:p></jats:sec><jats:sec><jats:title>Conclusions</jats:title><jats:p>Deletion of <jats:styled-content style="fixed-case">DRA</jats:styled-content> results in severely reduced colonic <jats:styled-content style="fixed-case">HCO</jats:styled-content><jats:sub>3</jats:sub><jats:sup>−</jats:sup> secretory rate, a loss of colonic fluid absorption, a lack of a firmly adherent mucus layer and a severely reduced colonic mucosal resistance to <jats:styled-content style="fixed-case">DSS</jats:styled-content> damage. These data provide potential pathophysiological explanations for the increased susceptibility of <jats:styled-content style="fixed-case">CLD</jats:styled-content> patients to intestinal inflammation.</jats:p></jats:sec>