Alternative treatment for secretory diarrhea revealed in a new class of CFTR inhibitors

Structural mechanisms of CFTR function and dysfunction

Hwang et al. integrate new structural insights with prior functional studies to reveal the functional anatomy of CFTR chloride channels.

Cystic fibrosis (CF) transmembrane conductance regulator (CFTR) chloride channel plays a critical role in regulating transepithelial movement of water and electrolyte in exocrine tissues. Malfunction of the channel because of mutations of the cftr gene results in CF, the most prevalent lethal genetic disease among Caucasians. Recently, the publication of atomic structures of CFTR in two distinct conformations provides, for the first time, a clear overview of the protein. However, given the highly dynamic nature of the interactions among CFTR’s various domains, better understanding of the functional significance of these structures requires an integration of these new structural insights with previously established biochemical/biophysical studies, which is the goal of this review.

Effects of Small Molecule Calcium-Activated Chloride Channel Inhibitors on Structure and Function of Accessory Cholera Enterotoxin (Ace) of Vibrio cholerae

Cholera pathogenesis occurs due to synergistic pro-secretory effects of several toxins, such as cholera toxin (CTX) and Accessory cholera enterotoxin (Ace) secreted by Vibrio cholerae strains. Ace activates chloride channels stimulating chloride/bicarbonate transport that augments fluid secretion resulting in diarrhea. These channels have been targeted for drug development. However, lesser attention has been paid to the interaction of chloride channel modulators with bacterial toxins. Here we report the modulation of the structure/function of recombinant Ace by small molecule calcium-activated chloride channel (CaCC) inhibitors, namely CaCC_inh-A01, digallic acid (DGA) and tannic acid. Biophysical studies indicate that the unfolding (induced by urea) free energy increases upon binding CaCC_inh-A01 and DGA, compared to native Ace, whereas binding of tannic acid destabilizes the protein. Far-UV CD experiments revealed that the α-helical content of Ace-CaCC_inh-A01 and Ace-DGA complexes increased relative to Ace. In contrast, binding to tannic acid had the opposite effect, indicating the loss of protein secondary structure. The modulation of Ace structure induced by CaCC inhibitors was also analyzed using docking and molecular dynamics (MD) simulation. Functional studies, performed using mouse ileal loops and Ussing chamber experiments, corroborate biophysical data, all pointing to the fact that tannic acid destabilizes Ace, inhibiting its function, whereas DGA stabilizes the toxin with enhanced fluid accumulation in mouse ileal loop. The efficacy of tannic acid in mouse model suggests that the targeted modulation of Ace structure may be of therapeutic benefit for gastrointestinal disorders.

Drug-induced secretory diarrhea: A role for CFTR

Many medications induce diarrhea as a side effect, which can be a major obstacle to therapeutic efficacy and also a life-threatening condition. Secretory diarrhea can be caused by excessive fluid secretion in the intestine under pathological conditions. The cAMP/cGMP-regulated cystic fibrosis transmembrane conductance regulator (CFTR) is the primary chloride channel at the apical membrane of intestinal epithelial cells and plays a major role in intestinal fluid secretion and homeostasis. CFTR forms macromolecular complexes at discreet microdomains at the plasma membrane, and its chloride channel function is regulated spatiotemporally through protein-protein interactions and cAMP/cGMP-mediated signaling. Drugs that perturb CFTR-containing macromolecular complexes in the intestinal epithelium and upregulate intracellular cAMP and/or cGMP levels can hyperactivate the CFTR channel, causing excessive fluid secretion and secretory diarrhea. Inhibition of CFTR chloride-channel activity may represent a novel approach to the management of drug-induced secretory diarrhea.

Convective washout reduces the antidiarrheal efficacy of enterocyte surface-targeted antisecretory drugs

Secretory diarrheas such as cholera are a major cause of morbidity and mortality in developing countries. We previously introduced the concept of antisecretory therapy for diarrhea using chloride channel inhibitors targeting the cystic fibrosis transmembrane conductance regulator channel pore on the extracellular surface of enterocytes. However, a concern with this strategy is that rapid fluid secretion could cause convective drug washout that would limit the efficacy of extracellularly targeted inhibitors. Here, we developed a convection-diffusion model of washout in an anatomically accurate three-dimensional model of human intestine comprising cylindrical crypts and villi secreting fluid into a central lumen. Input parameters included initial lumen flow and inhibitor concentration, inhibitor dissociation constant (K(d)), crypt/villus secretion, and inhibitor diffusion. We modeled both membrane-impermeant and permeable inhibitors. The model predicted greatly reduced inhibitor efficacy for high crypt fluid secretion as occurs in cholera. We conclude that the antisecretory efficacy of an orally administered membrane-impermeant, surface-targeted inhibitor requires both (a) high inhibitor affinity (low nanomolar K(d)) to obtain sufficiently high luminal inhibitor concentration (>100-fold K(d)), and (b) sustained high luminal inhibitor concentration or slow inhibitor dissociation compared with oral administration frequency. Efficacy of a surface-targeted permeable inhibitor delivered from the blood requires high inhibitor permeability and blood concentration (relative to K(d)).

Mucosal potassium efflux mediated via Kcnn4 channels provides the driving force for electrogenic anion secretion in colon

Intermediate conductance K(+) (Kcnn4) channels are present in both mucosal and serosal membranes of colon. However, only serosal Kcnn4 channels have been shown to be essential for agonist-induced (cAMP and Ca(2+)) anion secretion. The present study sought to determine whether mucosal Kcnn4 channels also play a role in colonic anion secretion. Mucosal-to-serosal and serosal-to-mucosal unidirectional (86)Rb (K(+) surrogate) fluxes as well as short-circuit current (I(sc); a measure of anion secretion) were measured under voltage-clamp conditions in distal colon from rats fed either a standard or K(+)-free diet. 5,6-Dichloro-1-ethyl-1,3-dihydro-2H-benzimidazole-2-one (DC-EBIO) was used to activate Kcnn4 channels. Mucosal DC-EBIO both induced K(+) secretion and enhanced anion secretion in normal rat distal colon. The DC-EBIO-induced K(+) secretion was completely blocked by nonspecific (Ba(2+)) and Kcnn4-specific (TRAM-34) inhibitors, but was not blocked by the large-conductance K(+) (iberiotoxin), small-conductance K(+) (apamin), or KCNQ1 (chromanol 293B) specific blockers. Ba(2+) and TRAM-34 also inhibited DC-EBIO-enhanced anion secretion. The DC-EBIO-enhanced anion secretion was completely inhibited by the nonspecific anion channel blocker 5-nitro-2-(3-phenylpropyl-amino)benzoic acid, whereas it was only partially inhibited by CFTR [CFTR(inh)-172, glibenclamide]- and CaCC (niflumic acid)-specific Cl(-) channel blockers. In contrast, mucosal DC-EBIO-enhanced K(+) and anion secretion was not present in distal colon of dietary K-depleted rats, indicating absence of mucosal Kcnn4 channels. These observations indicate that mucosal Kcnn4 channels are capable of driving agonist-induced anion secretion mediated via CFTR and CaCC and likely contribute to stool K(+) losses that accompany diarrheal illnesses.

Phenylalanine-508 mediates a cytoplasmic-membrane domain contact in the CFTR 3D structure crucial to assembly and channel function

Deletion of phenylalanine-508 (Phe-508) from the N-terminal nucleotide-binding domain (NBD1) of the cystic fibrosis transmembrane conductance regulator (CFTR), a member of the ATP-binding cassette (ABC) transporter family, disrupts both its folding and function and causes most cystic fibrosis. Most mutant nascent chains do not pass quality control in the ER, and those that do remain thermally unstable, only partially functional, and are rapidly endocytosed and degraded. Although the lack of the Phe-508 peptide backbone diminishes the NBD1 folding yield, the absence of the aromatic side chain is primarily responsible for defective CFTR assembly and channel gating. However, the site of interdomain contact by the side chain is unknown as is the high-resolution 3D structure of the complete protein. Here we present a 3D structure of CFTR, constructed by molecular modeling and supported biochemically, in which Phe-508 mediates a tertiary interaction between the surface of NBD1 and a cytoplasmic loop (CL4) in the C-terminal membrane-spanning domain (MSD2). This crucial cytoplasmic membrane interface, which is dynamically involved in regulation of channel gating, explains the known sensitivity of CFTR assembly to many disease-associated mutations in CL4 as well as NBD1 and provides a sharply focused target for small molecules to treat CF. In addition to identifying a key intramolecular site to be repaired therapeutically, our findings advance understanding of CFTR structure and function and provide a platform for focused biochemical studies of other features of this unique ABC ion channel.