Alterations of expression and function of the Na+/H+ exchanger NHE3 during early in-flammation in IL-10 knockout Mouse

Introduction: Acute flares of Crohn's disease are often accompanied by watery diarrhea, the pathophysiology of which is incompletely understood. In distal ileum and proximal colon, electroneutral salt absorption predominantes, mediated by the Na+/H+ exchanger NHE3 and the anion exchanger SLC26A3.

Aim: This study was undertaken to investigate the NHE3 mRNA and protein expression, NHE3 localization and function in the terminal ileum and proximal colon of IL-10 deficient mice, which develop a proximally predominant inflamma-tion.

Methods: IL-10 +/+ and -/- mice were raised under SPF conditions, then in part trans-ferred to a normal environment. We studied mRNA expression levels of IF-gamma and IL-1beta, as well as NHE3 in the ileal and proximal colonic tissue of IL-10 deficient mice in the early stage of inflammation by semiquantiative PCR. To differentiate between disturbances of NHE3 transport function and leaky tight junctions, NHE3 transport rates were assessed fluorometrically in the NHE3-expressing surface cells of BCECF-loaded murine colonic crypts isolated from IL-10 deficient mice and (wt) littermates. Total NHE exchange activity in the surface cell and cryptal region was also measured, as well as NHE1 plus NHE2 activity in the crypts.

Results: We found that in IL-10 deficient mice in normal but not SPF conditions developed mild diarrhea and a virtual loss of active Na+ absorption in inflamed tissue. IF-gamma and IL-1beta mRNA were upregulated, as expected. Histology showed mild crypt hyperplasia and leukocyte infiltration. Surprisingly, NHE3 mRNA was strongly upregulated, and the surface cell area which stained apically for NHE3 were found to penetrate more into the openings of the hypertrophied crypts. On the other hand, NHE3 activity showed strongly decreased acid-activated transport activity as well as impaired regulation. This reduction in transport activity was seen only for NHE3, not for NHE in general.

Conclusion: The data suggest that in the early stage of immune-mediated intestinal inflammation, mRNA expres-sion of the Na+/H+ exchanger NHE3 is strongly upregulated and NHE3 protein is correctly located in the brush border membrane, but nevertheless transport activity is severely impaired due to as yet unknown intracellular events.