Multiple transport functions of a red blood cell anion exchanger, tAE1: its role in cell volume regulation

Abstract

It was previously shown that expressed in Xenopus oocyte the mouse (mAE1) and the trout (tAE1) anion exchanger behave differently: both elicit anion exchange activity but only tAE1 induces a transport of organic solutes correlated with a chloride channel activity. The present data, obtained by measurement of Xenopus oocyte membrane permeability and conductance, provide evidence that tAE1 also induces a large increase in Na+ and K+ permeability inhibited by several AE1 inhibitors. This inhibition does not result from an effect on the driving force for electrodiffusion but represents a direct effect on the cation pathway. As a control, expression of cystic fibrosis transmembrane conductance regulator (CFTR) having, once stimulated by 3‐isobutyl‐1‐methylxanthine (IBMX), the same anion conductance magnitude as tAE1 did not induce any cation movement. Chloride exchange, channel activity and cation transport induced by anion exchanger expression are inhibited by free or covalently bound H2DIDS as well. This covalent inhibition is reversed by the point mutation of Lys‐522, the covalent binding site of H2DIDS to the protein. These data reveal that tAE1 itself acts both as an anion exchanger and as a channel of broad selectivity. All results obtained by expression of AE1 isoforms in Xenopus oocytes and those obtained in erythrocytes are consistent with the proposal that, in nucleated erythrocytes, tAE1 functions as the swelling‐activated osmolyte anion channel involved in cell volume regulation. In contrast AE1 from mammalian red cells, which do not regulate their volume, lacks swelling‐activated osmolyte channel properties. tAE1 illustrates the ability of a specific transport system to be a multifunctional protein exhibiting other transport functions when submitted to regulation.

Evidence for Up-regulation of the Endogenous Na-K-2Cl Co-transporter by Molecular Interactions with the Anion Exchanger tAE1 Expressed in Xenopus Oocyte

“…All of the AE polypeptides can be divided into two main domains of about the same size: an N-terminal cytoplasmic domain interacting with cytoskeleton and a membrane spanning domain, which is responsible for ion translocation, with a short C-terminal end in the cytoplasm. By expression studies of tAE1 in Xenopus oocytes, we have demonstrated previously that this normally electroneutral anion exchanger is able to form an anion conductive path-way permeable to different charged or neutral osmolytes such as urea, choline, sorbitol, Na ϩ , and K ϩ (13)(14)(15). In physiological conditions, these transport properties of tAE1 are activated by a decrease in intracellular ionic strength in swollen erythrocytes, and they are involved in the cell regulatory volume decrease response (16,17).…”

Leaky Cl − –HCO 3 − exchangers: cation fluxes via modified AE1

“…Xenopus oocyte expression of hAE1 wild-type (wtAE1), the AE1 R760Q mutant and the AE double mutant E681Q-R760Q. (a) cation influx: (i) Li C and (ii) Rb C influx, measured as previously reported (Guizouarn et al 2001) (b) AE1 anion exchange function assessed by intracellular pH measurements, performed as previously reported ). (i) wt hAE1, (ii) R760Q, (iii) E681Q-R760Q.…”

“…In fishes, particularly trout, it has been known for a decade that trout band 3 (tAE1) has important extra transport properties, and does not function strictly in an anion exchange mode (Guizouarn et al 2001). Experiments with oocyte expression have confirmed that tAE1 can function predominantly as an anion channel, but simultaneously as a transport pathway for organic osmolytes, and monovalent cations, indicating a high degree of promiscuity (Fievet et al 1995(Fievet et al , 1998Guizouarn et al 2001). Physiologically, this functional modification is for cell volume regulation, in response to cell swelling: the classical regulatory volume decrease.…”

“…taurine) and even significant Na and K fluxes (Fievet et al 1995(Fievet et al , 1998. Physiologically this multifunctional property of tAE1 plays a role in cell volume regulation in response to swelling (Guizouarn et al 2001).…”

Propriétés inattendues de l’échangeur anionique du globule rouge : la leçon des poissons

“…Deux études indépendantes, l'une sur la régulation de volume cellulaire des globules rouges de truite et l'autre sur la perméabilité membranaire des hématies de patients souffrant d'anémies hémoly-tiques héréditaires, ont permis de démas-quer une fonction de transport des cations tout à fait inattendue pour un AE1. Nous avons montré il y a plusieurs années que, contrairement aux bandes 3 de mammifères, la bande 3 des globules rouges de truite était capable d'adopter une conformation canal anionique permettant le transport à travers la membrane plasmique de solutés tels le Na, le K mais aussi la taurine, principal acide aminé contenu dans ces cellules [4,5]. Cette conformation est induite par la dilution des élec-trolytes intracellulaires lors d'une entrée d'eau importante dans la cellule [6].…”