Direct Sensing of Intracellular pH by the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) Cl− Channel

Abstract

In cystic fibrosis (CF), dysfunction of the cystic fibrosis transmembrane conductance regulator (CFTR) ClThe ATP-binding cassette (ABC) 3 transporter cystic fibrosis transmembrane conductance regulator (CFTR) (1) is a multifunctional protein best known as a regulated Cl Ϫ channel (2). CFTR is assembled from five domains: two membrane-spanning domains (MSDs) that form an anion-selective pore, two nucleotide-binding domains (NBDs) that bind and hydrolyze ATP to control channel gating, and a unique regulatory domain (RD), whose phosphorylation by PKA is critical for CFTR activation (2, 3). CFTR is principally expressed in the apical membrane of epithelia throughout the body where it plays a fundamental role in fluid and electrolyte movements (4). Malfunction of CFTR causes the common genetic disease cystic fibrosis (CF) (4).Previous studies demonstrate that the regulation of intracellular pH (pH i ) is defective in CF epithelial cells (e.g. Ref. 5). They also reveal that expression of recombinant CFTR in heterologous cells modulates pH i (e.g. Ref. 6). Analysis of the literature suggests that CFTR modulates pH i in three main ways. First, CFTR itself directly transports HCO 3 Ϫ ions with a modest permeability (P HCO 3 Ϫ/P Cl Ϫ ϳ0.26 (7)). Second, CFTR regulates the Na ϩ /H ϩ exchanger isoform 3 (NHE3), which contributes to Na ϩ -dependent HCO 3 Ϫ reabsorption in pancreatic duct epithelia. CFTR stabilizes NHE3 expression at the cell surface and inhibits NHE3 activity by a cAMP-dependent mechanism when pancreatic HCO 3 Ϫ secretion is stimulated (8). Of note, the regulation of NHE3 by CFTR involves the association of CFTR and NHE3 with the scaffolding protein EBP50 to form a macromolecular complex (8). Third, CFTR regulates the Cl Ϫ /HCO 3 Ϫ (anion) exchanger (AE), which plays a central role in pancreatic HCO 3 Ϫ secretion. CFTR regulation of AE requires the cell surface expression and cAMP-dependent phosphorylation of CFTR, but not its transport of anions (6). Interestingly, Ko et al. (9) demonstrated that CFTR and members of the SLC26 family of AEs coordinate their activities through the interaction of the, phosphorylated RD of CFTR with the STAS (sulfate transporter and antisigma-factor antagonist) domain of SLC26 transporters. Thus, CFTR modulates pH i through its roles as an ion channel and regulator of transport proteins.A key unresolved question is how CFTR senses changes in pH i . As described above, CFTR might detect pH i changes indirectly through its interactions with NHE3 and SLC26 transporters. Consistent with this idea, Reddy et al. (10)

Modulation of Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) Activity and Genistein Binding by Cytosolic pH

“…Results apparently contrasting with ours have been recently reported in a study about the effects of pH i in excised membrane patches (29). The authors found that the P o of CFTR channels, but not the single channel amplitude nor the number of active channels under the pipette tip, was increased at pH i 6.3 compared with pH i 7.3 and that this effect was, at least in part, mediated by an augmented apparent affinity of MgATP for CFTR.…”

Decreased Soluble Adenylyl Cyclase Activity in Cystic Fibrosis Is Related to Defective Apical Bicarbonate Exchange and Affects Ciliary Beat Frequency Regulation

“…The importance of sAC for CBF and possibly other cellular functions is highlighted by the recent finding that chloride and bicarbonate conductance through CFTR is increased during states of low intracellular pH (42), allowing sAC activation to counteract the negative CBF effect of low pH even more. The fact that CFTR conductance is also increased by sAC itself, at least in airway epithelial cell lines (22), makes this rescue system a powerful, self-enhancing circuit.…”

Mouse Sperm Membrane Potential Hyperpolarization Is Necessary and Sufficient to Prepare Sperm for the Acrosome Reaction

“…There is a complex but crucial interplay between Em and pH i in mammalian sperm. It is worth noting that CFTR's conductance is also pH i dependent (41). Therefore, it is also possible that the regulation of other transporters like ENaCs by CFTR activation is also pH i dependent.…”