Defective jejunal and colonic salt absorption and alteredNa+/H+ exchanger 3 (NHE3) activity in NHE regulatory factor 1 (NHERF1) adaptor protein-deficient mice

Abstract

We investigated the role of the Na+/H+ exchanger regulatory factor 1 (NHERF1) on intestinal salt and water absorption, brush border membrane (BBM) morphology, and on the NHE3 mRNA expression, protein abundance, and transport activity in the murine intestine. NHERF1-deficient mice displayed reduced jejunal fluid absorption in vivo, as well as an attenuated in vitro Na+ absorption in isolated jejunal and colonic, but not of ileal, mucosa. However, cAMP-mediated inhibition of both parameters remained intact. Acid-activated NHE3 transport rate was reduced in surface colonocytes, while its inhibition by cAMP and cGMP was normal. Immunodetection of NHE3 revealed normal NHE3 localization in the BBM of NHERF1 null mice, but NHE3 abundance, as measured by Western blot, was significantly reduced in isolated BBM from the small and large intestines. Furthermore, the microvilli in the proximal colon, but not in the small intestine, were significantly shorter in NHERF1 null mice. Additional knockout of PDZK1 (NHERF3), another member of the NHERF family of adaptor proteins, which binds to both NHE3 and NHERF1, further reduced basal NHE3 activity and caused complete loss of cAMP-mediated NHE3 inhibition. An activator of the exchange protein activated by cAMP (EPAC) had no effect on jejunal fluid absorption in vivo, but slightly inhibited NHE3 activity in surface colonocytes in vitro. In conclusion, NHERF1 has segment-specific effects on intestinal salt absorption, NHE3 transport rates, and NHE3 membrane abundance without affecting mRNA levels. However, unlike PDZK1, NHERF1 is not required for NHE3 regulation by cyclic nucleotides.

Restoration of Na+/H+ exchanger NHE3-containing macrocomplexes ameliorates diabetes-associated fluid loss

“…We observed a 31% increase in NHE3 activity in Nherf1 -/-mice compared with a 47% increase in WT mice, indicating that NHE3 regulation by chronic insulin treatment is partially dependent on transcriptional/translational activation of NHE3 (Supplemental Figure 3). However, the basal NHE3 activity was reduced in Nherf1 -/-mice, as previously reported (17), which therein indicates that NHE3 activity in Nherf1 -/-mice after 5 days of insulin treatment was significantly lower compared with that of WT mice.…”

“…The changes in the expression of NHERF1 and NHERF2 but not NHERF3 were reversed by insulin. It has been shown that basal NHE3 activity is reduced in Nherf1 -/-or Nherf3 -/-mouse intestines (17,37), suggesting that NHERF1 and NHERF3 may be involved in the basal expression of NHE3 at the plasma membrane. Loss of NHERF1 in mice or knockdown of NHERF1 in Caco-2bbe cells ablated insulin-mediated NHE3 activation, indicating that NHERF1 is indispensable for NHE3 regulation.…”

Role of epithelial ion transports in inflammatory bowel disease

“…NHERF1 Ϫ/Ϫ mice featured reduced jejunal fluid absorption in vivo, as well as attenuated in vitro Na ϩ absorption in isolated jejunal and colonic, but not ileal, mucosa. Moreover, NHE3 activity and membrane expression were reduced, although mRNA levels were not affected (21). One study found NHERF2 Ϫ/Ϫ mice to have lower ileal NHE3 activity due to impaired trafficking or retention of NHE3 in the apical membrane (99), which is possibly related to NHERF2 association with lipid-raft NHE3 (136).…”

“…1). A direct implication of this decrease in NHE3 activity is a lower Na ϩ absorption [with highest impact in the proximal colon, where electroneutral NaCl uptake is more relevant (122)], which in turn contributes to the IBD-associated diarrhea (21,43,135,156). Some studies reported both lowered activity and expression of NHE3 (13,125,135), whereas others have observed reduced NHE3 activity associated with no changes in expression (43,85,156,157).…”

Effect of Tenapanor on Interdialytic Weight Gain in Patients on Hemodialysis

“…In the gastrointestinal tract, the sodium/hydrogen exchanger isoform 3 (NHE3) plays an important role in sodium and fluid homeostasis (11)(12)(13). Tenapanor (AZD1722 and RDX5791) is a small molecule inhibitor of NHE3 being developed for the treatment of patients with CKD.…”