Abstract
Cystic fibrosis, a multi-organ genetic disease, is characterized by abnormal function of the cystic fibrosis transmembrane conductance regulator (CFTR) protein, a chloride channel at the apical membrane of several epithelia. In recent years, therapeutic strategies have been developed to correct the CFTR defect. To evaluate CFTR function at baseline for diagnosis, or the efficacy of CFTR-restoring therapy, reliable tests are needed to measure CFTR function, in vitro, ex vivo and in vivo. In vitro techniques either directly or indirectly measure ion fluxes; direct measurement of ion fluxes and quenching of fluorescence in cell-based assays, change in transmembrane voltage or current in patch clamp or Ussing chamber, swelling of CFTR-containing organoids by secondary water influx upon CFTR activation. Several cell or tissue types can be used. Ex vivo and in vivo assays similarly evaluate current (intestinal current measurement) and membrane potential differences (nasal potential difference), on tissues from individual patients. In the sweat test, the most frequently used in vivo evaluation of CFTR function, chloride concentration or stimulated sweat rate can be directly measured. Here, we will describe the currently available bio-assays for quantitative evaluation of CFTR function, their indications, advantages and disadvantages, and correlation with clinical outcome measures.
Highlights
Cystic fibrosis (CF) is a multi-organ recessive genetic disorder caused by mutations in the CFTR (CF transmembrane conductance regulator) gene
The CFTR protein is an anion channel expressed at the apical cell membrane of many tissues and its dysfunction is the cause of the wide range of disease manifestations seen in people with CF (PwCF)
The in vitro assays using ion fluxes are most adapted for high throughput screening of large libraries of compounds, whereas those that employ electrophysiological methods are more suitable for a detailed understanding of the function of wild type or mutant protein and its correction by modulators preselected by high throughput screening
Summary
Cystic fibrosis (CF) is a multi-organ recessive genetic disorder caused by mutations in the CFTR (CF transmembrane conductance regulator) gene. The CFTR protein is expressed at the apical epithelial cell membrane of most organs, where it functions as an ion channel. CFTR function (net anion efflux) depends on the number of CFTR channels expressed at the apical membrane and on the function of each of these channels The latter relates to the single-channel conductance (rate of ion transport across the apical membrane), which in turn depends on the electrochemical gradient (i.e., membrane potential and chloride concentration) across the membrane and on the open probability (Po ) of the channels [9]. Depending on the mutations of the specific target population, the aim will be to improve the number of CFTR channels at the cell membrane, the gating or conductance of the channel, or a combination of those. We describe the main principle of the technique, cover its main indication, highlight its advantages and disadvantages, and, if applicable, future developments
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