Backround: 

The molecular mechanisms for inflammatory diarrhea are multifactorial and incompletely understood. One important component may be the loss of active salt absorption. The Na⁺/H⁺ exchanger isoform 3 is the major intestinal salt absorptive protein in the intestine, and acute administration of very high TNF-alpha concentrations leads to retrieval of NHE3 from the membrane and diarrhea.

Aim: 

We investigated the expression, brush border membrane (BBM) localization, and transport function of the major intestinal sodium absorptive transporter, the Na⁺/H⁺ exchanger NHE3, as well as the expression of its regulatory PDZ-adapter proteins NHERF1 and PDZK1, TNF-alpha, actin, and a variety of housekeeping genes, in the small and large intestine of mice after induction of a CD45RB^high transfer colitis in the acute phase of diarrhea.

Results: 

After colitis induction, TNF-alpha levels were increased both in the inflamed colon and microscopically normal-appearing small intestine. NHE3 mRNA expression was upregulated in the colon and unaltered in the small intestine, NHE3 protein expression and localization in the brush border membrane (BBM) was not altered, but acid-activated NHE3 transport rates in small intestinal villous and colonic surface cells were severely decreased, and fluid absorption in vivo was decreased in the small intestine. PDZK1 mRNA expression was dowregulated, whereas that of NHERF1 was not altered. Glucocorticoid treatment of colitic mice for 4 days decreased colonic mucosal cytokine expression and increased PDZK1 expression as well as fluid absorption. In order to investigate whether the downregulation of PDZK1 and the disturbed NHE3 transport activity is causally related, we studied PDZK1 protein expression and NHE3 transport activity in colonic surface enterocytes in the intestine of PDZK1 +/­ mice, and found PDZK1 content reduced by 60% and NHE3 transport activity significantly decreased.

Conclusion: 

We suggest that during immune-mediated intestinal inflammation, NHE3 BBM protein abundance is normal, but the regulation of its transport activity is disturbed. PDZK1 downregulation during inflammation may be one factor responsible for inflammation-associated NHE3 dysfunction.