The down regulated in adenoma (dra) gene product binds to the second PDZ domain of the NHE3 kinase A regulatory protein (E3KARP), potentially linking intestinal Cl-/HCO3- exchange to Na+/H+ exchange

Intestinal electroneutral NaCl absorption is mediated by parallel operation of Na(+)/H(+) and Cl(-)/HCO(3)(-) exchange in the enterocyte apical membrane. The ion transporters involved are Na(+)/H(+) exchanger 3 (NHE3) and the down regulated in adenoma (dra) gene product. cAMP-mediated inhibition of NHE3 requires the transporter to bind to the second PDZ (PSD95, disk large, ZO1) domain of the adapter protein NHE3 kinase A regulatory protein (E3KARP). Because the C-terminal four amino acids of dra are ETKF (glutamate-threonine-lysine-phenylalanine), resembling a PDZ interaction motif, we hypothesized that dra may also bind to one of the PDZ domains of E3KARP. In vitro the ETKF motif of dra binds to the second PDZ domain of E3KARP, the affinity being comparable to that of the known ligand CFTR. The C-terminal phenylalanine, which is an unconventional residue in PDZ interaction motifs, can only be substituted by the classical residue leucine, but not by other hydrophobic residues (valine, isoleucine). Immunofluorescence colocalizes dra, NHE3, and E3KARP in the apical compartment of human proximal colon. We suggest a model in which both NHE3 and dra bind to the second PDZ domain of E3KARP and that linking of the transporters occurs through dimerization of E3KARP. In such a model, the first PDZ domain would remain available for instance for signal transduction proteins.

Down-regulated in adenoma mediates apical Cl-/HCO3- exchange in rabbit, rat, and human duodenum

BACKGROUND & AIMS

Duodenal bicarbonate secretion is in part mediated by an apical Cl-/HCO3- exchanger of unknown molecular nature. The recently discovered dra (down-regulated in adenoma) gene encodes a transport protein (DRA) for SO4(2-), Cl-, and HCO3-. The aim of this study was to investigate whether DRA may be the duodenal apical Cl-/HCO3- exchanger.

METHODS

DRA, Na+/H+ exchanger (NHE) isoform 3, and anion exchanger isoform (AE) 2 messenger RNA expression levels were studied in rat, rabbit, and human gastrointestinal tract by semiquantitative reverse-transcription polymerase chain reaction and in situ hybridization (DRA in human intestine). The subcellular localization of DRA was determined by Western analysis and immunohistochemistry. Using rabbit and rat duodenal brush border membrane vesicles, anion exchange characteristics were investigated.

RESULTS

DRA expression was high in duodenum and colon of all species, whereas NHE3 messenger RNA expression was low in duodenum and high in colon. Western analysis and immunohistochemistry showed an apical localization for DRA. Rabbit and rat duodenal brush border membrane vesicles showed 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid-sensitive Cl-/Cl-, HCO3-/Cl-, SO4(2-)/Cl-, and Cl-/SO4(2-) exchange, with evidence for one major brush border membrane Cl-/anion exchanger, an affinity for Cl- > HCO3-, and a much higher affinity for SO4(2-) in rat than rabbit. The strong predominance of DRA over NHE3 and NHE2 expression in duodenum was paralleled by much higher Cl-/HCO3- than Na+/H+ exchange rates in brush border membrane vesicles and likely explains the high duodenal HCO3- secretory rates.

CONCLUSIONS

These data suggest that DRA is the major apical anion exchanger in the duodenum as well as the colon and the likely transport protein for duodenal electroneutral HCO3- secretion.