CFTR-SLC26 transporter interactions in epithelia

Abstract

Transport mechanisms that mediate the movements of anions must be coordinated tightly in order to respond appropriately to physiological stimuli. This process is of paramount importance in the function of diverse epithelial tissues of the body, such as, for example, the exocrine pancreatic duct and the airway epithelia. Disruption of any of the finely tuned components underlying the transport of anions such as Cl(-), HCO(3) (-), SCN(-), and I(-) may contribute to a plethora of disease conditions. In many anion-secreting epithelia, the interactions between the cystic fibrosis transmembrane conductance regulator (CFTR) and solute carrier family 26 (SLC26) transporters determine the final exit of anions across the apical membrane and into the luminal compartment. The molecular identification of CFTR and many SLC26 members has enabled the acquisition of progressively more detailed structural information about these transport molecules. Studies employing a vast array of increasingly sophisticated approaches have culminated in a current working model which places these key players within an interactive complex, thereby setting the stage for future work.

Fig. 1

Possible means of transepithelial HCO₃ ⁻ secretion. Top Schema outlining cystic fibrosis transmembrane conductance regulator (CFTR)-dependent transport of HCO₃ ⁻ by either direct or indirect pathways. For simplicity, basolateral Cl⁻ and HCO₃ ⁻ uptake are shown with a single generic co-transport graphic element, but it should be noted that the Na⁺/K⁺/2Cl⁻and Na⁺/HCO₃ ⁻ cotransporters are distinct molecular entities. Not shown is basolateral K⁺ conductance that enables the recycling of K⁺ taken up by Na⁺/K⁺ ATPase and the Na⁺/K⁺/2Cl⁻ cotransporter. Left HCO₃ ⁻ enters the cell basolaterally by Na⁺-coupled cotransport and exits into the lumen directly through CFTR. Anions transported by solute carrier family 26 (SLC26) transporters are indicated by A ⁻. Other ions are indicated by standard nomenclature. Right Alternatively, Cl⁻ secretion mediated by CFTR provides a counterion for HCO₃ ⁻ exit via SLC26-mediated exchange pathways. Dotted arrow indicates functional interactions between CFTR and SLC26 transporters. Bottom In the absence of CFTR, both the direct (left) and the indirect (right) pathways are disabled

Fig. 2

a. Depictions of post-synaptic density 95/discs large/zona occludens 1 (PDZ) domain organization and structure. Top Linear cartoon representation showing the predicted secondary structural elements above the relevant sequence (amino acid residues 309–393) from an archetypal PDZ domain, rat PSD-95 PDZ-3. Blue arrows β sheets, purple rectangles α helices. Residues shown in boldface contact the peptide binding partner. Note that the binding loop rests in the region intervening between βA and βB. Bottom Three-dimensional structure of the PDZ-3 of postsynaptic density protein-95 (PSD-95) showing the interacting peptide (orange arrow) wedged between the groove formed by the βB sheet and the αB helix; the carboxylate binding loop hovers above. This image is modified from Doyle et al. (1996), with permission from Elsevier. b Cartoon showing hypothesized apical macromolecular signaling complexes promoted by PDZ protein-facilitated interactions between the CFTR and SLC26 transporters. PDZ domain binding motif of SLC26A3, TKF, is shown in this figure; the counterpart for SLC26A6 is TRL. For CFTR, the motif is TRL as shown. This motif is lacking in SLC26A4, but the potential for regulation by sulfate transporter and anti-sigma factor antagonist (STAS) domain interactions with the R domain of CFTR is preserved

Fig. 3

Secondary structural propensity plot generated by molecular dynamics simulation (DMD) simulations. The plot shows population-weighted averages for residues within the R domain. The positive ordinate values (red; helix) correspond to α-helical probabilities, whereas negative values (black; strand) reflect likelihood for β-sheets. This image is modified from Hegedus et al. (2008), with permission from Elsevier

Fig. 4

Information derived from the rat Slc26a5 high-resolution structure. a Representation of the structure of the rat Slc26a5 STAS domain; green connecting loops joining 5 yellow β-strands and 5 red α-helices. The three-dimensional (3D) image is reprinted from Pasqualetto et al. (2010), with permission from Elsevier. A linear map showing sequential organization of the secondary structural elements is shown beneath the 3D representation. C.L. Conserved loop. b Residues in human SLC26A3-A6 STAS domains that putatively interact with the cytosolic interface of the apical membrane were determined by alignment with the structure rat Slc26a5 (Pasqualetto et al. 2010). Residues are identical in numbering for both rat Slc26a5 and human SLC26A5. For the four human SLC26 transporters compared, note the conservation of the proline residue within the extended N-terminus region (N), as well as the phenylalanine at the end of the C.L. between β-sheet 3 and α-helix 2