Na+‐H+ exchanger 3 (NHE3) is present in lipid rafts in the rabbit ileal brush border: a role for rafts in trafficking and rapid stimulation of NHE3

Abstract

1 Rabbit ileal Na⁺‐absorbing cell Na⁺‐H⁺ exchanger 3 (NHE3) was shown to exist in three pools in the brush border (BB), including a population in lipid rafts. Approximately 50 % of BB NHE3 was associated with Triton X‐100‐soluble fractions and the other ∼50 % with Triton X‐100‐insoluble fractions; ∼33 % of the detergent‐insoluble NHE3 was present in cholesterol‐enriched lipid microdomains (rafts). 2 The raft pool of NHE3 was involved in the stimulation of BB NHE3 activity with epidermal growth factor (EGF). Both EGF and clonidine treatments were associated with a rapid increase in the total amount of BB NHE3. This EGF‐ and clonidine‐induced increase of BB NHE3 was associated with an increase in the raft pool of NHE3 and to a smaller extent with an increase in the total detergent‐insoluble fraction, but there was no change in the detergent‐soluble pool. In agreement with the rapid increase in the amount of NHE3 in the BB, EGF also caused a rapid stimulation of BB Na⁺‐H⁺ exchange activity. 3 Disrupting rafts by removal of cholesterol with methyl‐β‐cyclodextrin (MβCD) or destabilizing the actin cytoskeleton with cytochalasin D decreased the amount of NHE3 in early endosomes isolated by OptiPrep gradient fractionation. Specifically, NHE3 was shown to associate with endosomal vesicles immunoisolated by anti‐EEA1 (early endosomal autoantigen 1) antibody‐coated magnetic beads and the endosome‐associated NHE3 was decreased by cytochalasin D and MβCD treatment. 4 We conclude that: (i) a pool of ileal BB NHE3 exists in lipid rafts; (ii) EGF and clonidine increase the amount of BB NHE3; (iii) lipid rafts and to a lesser extent, the cytoskeleton, but not the detergent‐soluble NHE3 pool, are involved in the EGF‐ and clonidine‐induced acute increase in amount of BB NHE3; (iv) lipid rafts and the actin cytoskeleton play important roles in the basal endocytosis of BB NHE3.