CFTR transmembrane segments are impaired in their conformational adaptability by a pathogenic loop mutation and dynamically stabilized by Lumacaftor.

The cystic fibrosis transmembrane conductance regulator (CFTR) is an ion channel protein that is defective in individuals with cystic fibrosis (CF). To advance the rational design of CF therapies, it is important to elucidate how mutational defects in CFTR lead to its impairment and how pharmacological compounds interact with and alter CFTR. Here, using a helical-hairpin construct derived from CFTR's transmembrane (TM) helices 3 and 4 (TM3/4) and their intervening loop, we investigated the structural effects of a patient-derived CF-phenotypic mutation, E217G, located in the loop region of CFTR's membrane-spanning domain. Employing a single-molecule FRET assay to probe the folding status of reconstituted hairpins in lipid bilayers, we found that the E217G hairpin exhibits an altered adaptive packing behavior stemming from an additional GXXXG helix-helix interaction motif created in the mutant hairpin. This observation suggested that the misfolding and functional defects caused by the E217G mutation arise from an impaired conformational adaptability of TM helical segments in CFTR. The addition of the small-molecule corrector Lumacaftor exerts a helix stabilization effect not only on the E217G mutant hairpin, but also on WT TM3/4 and other mutations in the hairpin. This finding suggests a general mode of action for Lumacaftor through which this corrector efficiently improves maturation of various CFTR mutants.

Impact of cholesterol and Lumacaftor on the folding of CFTR helical hairpins.

Cystic fibrosis (CF) is caused by mutations in the gene that codes for the chloride channel cystic fibrosis transmembrane conductance regulator (CFTR). Recent advances in CF treatment have included use of small-molecule drugs known as modulators, such as Lumacaftor (VX-809), but their detailed mechanism of action and interplay with the surrounding lipid membranes, including cholesterol, remain largely unknown. To examine these phenomena and guide future modulator development, we prepared a set of wild type (WT) and mutant helical hairpin constructs consisting of CFTR transmembrane (TM) segments 3 and 4 and the intervening extracellular loop (termed TM3/4 hairpins) that represent minimal membrane protein tertiary folding units. These hairpin variants, including CF-phenotypic loop mutants E217G and Q220R, and membrane-buried mutant V232D, were reconstituted into large unilamellar phosphatidylcholine (POPC) vesicles, and into corresponding vesicles containing 70 mol% POPC +30 mol% cholesterol, and studied by single-molecule FRET and circular dichroism experiments. We found that the presence of 30 mol% cholesterol induced an increase in helicity of all TM3/4 hairpins, suggesting an increase in bilayer cross-section and hence an increase in the depth of membrane insertion compared to pure POPC vesicles. Importantly, when we added the corrector VX-809, regardless of the presence or absence of cholesterol, all mutants displayed folding and helicity largely indistinguishable from the WT hairpin. Fluorescence spectroscopy measurements suggest that the corrector alters lipid packing and water accessibility. We propose a model whereby VX-809 shields the protein from the lipid environment in a mutant-independent manner such that the WT scaffold prevails. Such 'normalization' to WT conformation is consistent with the action of VX-809 as a protein-folding chaperone.

Positive Charge Patterning and Hydrophobicity of Membrane-Active Antimicrobial Peptides as Determinants of Activity, Toxicity, and Pharmacokinetic Stability.

Natural α-helical cationic antimicrobial peptide (CAP) sequences are predominantly amphipathic, with only ca. 2% containing four or more consecutive positively charged amino acids (Lys/Arg). We have designed synthetic CAPs that deviate from these natural sequences, as typified by the charge-clustered peptide KKKKKKAAFAAWAAFAA-NH2, (termed 6K-F17), which displays high antimicrobial activity with no toxicity to mammalian cells. We created a series of peptides varying in charge patterning, increasing the amphipathic character of 6K-F17 to mimic the design of natural CAPs (e.g., KAAKKFAKAWAKAFAA-NH2). Amphipathic sequences displayed increased antimicrobial activity against bacteria but were significantly more toxic to mammalian cells and more susceptible to protease degradation than their corresponding charge-clustered variants, suggesting that amphipathic sequences may be desirable in nature to allow for more versatile functions (i.e., antibacterial, antifungal, antipredator) and rapid clearance from vulnerable host cells. Our approach to clustering of charges may therefore allow for specialization against bacteria, in concert with prolonged peptide half-life.

Thiamin supplementation does not improve left ventricular ejection fraction in ambulatory heart failure patients: a randomized controlled trial.

BACKGROUND: Thiamin, a water-soluble B-complex vitamin, functions as a coenzyme in macronutrient oxidation and in the production of cellular ATP. Data suggest that thiamin depletion occurs in heart failure (HF). Therefore, thiamin supplementation in HF patients may improve cardiac function. OBJECTIVE: We sought to determine whether oral thiamin supplementation improves left ventricular ejection fraction (LVEF), exercise tolerance, and quality of life among patients with HF and reduced LVEF. METHODS: In this prospective, multicenter, double-blind, placebo-controlled randomized trial, eligible ambulatory patients with HF and reduced LVEF were recruited from 4 academic and community hospitals between 2010 and 2015. Participants were randomly assigned to receive either 200 mg oral thiamin mononitrate per day or placebo for 6 mo. RESULTS: Sixty-nine patients (mean ± SD age: 64 ± 12 y; 83% men; LVEF: 37% ± 11%) were randomly assigned: 34 received placebo and 35 received thiamin supplementation. Erythrocyte thiamin pyrophosphate and urine thiamin concentrations were significantly higher in the supplemented group than in the placebo group at 6 mo (P = 0.02 and <0.001, respectively). At 6 mo, LVEF was significantly higher in the placebo group than in the thiamin group (38%; 95% CI: 36%, 39% compared with 35%; 95% CI: 33%, 37%, P = 0.047) after adjusting for baseline measurements. There were no significant differences in Minnesota Living with Heart Failure score, distance walked in 6 min, and N-terminal prohormone of brain natriuretic peptide concentrations between the 2 groups. One patient (2.9%) in the thiamin-supplemented group and none in the control group died at 6 mo. CONCLUSIONS: In ambulatory patients with HF and reduced LVEF, thiamin supplementation for 6 mo did not improve LVEF, quality of life, or exercise capacity, despite increases in thiamin concentrations. These findings do not support routine thiamin supplementation in the treatment of HF and reduced LVEF.This trial was registered at clinicaltrials.gov as NCT00959075.