Questions: 

Glucocorticoids increase intestinal sodium absorption and play a major role in the treatment of inflammatory diarrhea. This study was performed to search for a role of the PDZ-adaptors NHERF1-3 in glucocorticoid-induced NHE3-mediated fluid absorption in murine ileum.

Methods and Results: 

Dexamethasone (1mg/kg i.p.) treatment significantly increased fluid absorption after 6h and more strongly after 4d in WT ileum (as well as in NHERF1 KO ileum, which was therefore not further studied). In NHERF2 KO ileum, the 6h but not the 4d increase was lost, and in NHERF3 KO mice, both the 6h and 4d increase was lost. NHE3 protein was unchanged in isolated BBMs of any genotype after 6h, was significantly increased in WT and NHERF2 KO, but not in NHERF3 KO BBM after 4d. qPCR of WT ileum revealed an increase in SGK1 and NHE3 mRNA at 6h, an increase in NHE3 and NHERF3 mRNA at 4d, no significant change of NHERF2 mRNA at 6h and 4d. In NHERF2 KO ileum, this pattern was preserved, whereas it was strongly altered in NHERF3KO ileum, with increased NHE3 as well as SGK1 mRNA expression levels in untreated ileum and no further increase after 4d treatment, explaining the lack of NHE3 protein increase at 4d. In WT ileum, the majority of NHE3 was found in the terminal web region and redistributed into the microvilli after 6h dex treatment, while in NHERF2 KO ileum approx half, and in NHERF3 KO ileum the majority of NHE3 was already in the microvilli in the resting state ileum. 6h dex treatment resulted in minor NHE3 redistribution into the microvilli in NHERF2 KO and no change in NHE3 distribution in NHERF3 KO ileum. NHERF3 KO but not NHERF2KO and WT mice had increased serum aldosterine levels.

Conclusions: 

Loss of NHERF2 reduces the acute effect of glucocorticoids on NHE3 trafficking into the ileal microvilli, but does not interfere with an increase in NHE3 membrane abundance after longer gluococorticoid exposure times. Loss of NHERF3 results in a loss of the early as well as late effects of glucocorticoids on ileal NHE3 synthesis and membrane abundance, possibly because the systems to stimulate NHE3 synthesis and trafficking are already activated. The results also suggest that pharmacologic glucocorticoid doses may loose their effect on intestinal salt absorption in fluid-depleted states.