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Cystic fibrosis (CF) is a debilitating disease caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene, which codes for the CFTR anion channel. ΔF508, the most common CF-associated mutation, causes both a gating and a trafficking defect in the CFTR protein. This thesis describes the optimisation and use of two fluorescence assays, capable of measuring the two defects caused by the ΔF508 mutation. Yellow fluorescent protein (YFP)-CFTR, in which halide sensitive YFP is tagged to the Nterminal of the CFTR coding sequence, is a functional assay, used to test for ΔF508-CFTR potentiator activity. CFTR-pHTomato, in which the pH sensor is tagged to the fourth extracellular loop of CFTR, is used to measure membrane density and internal CFTR expression. Human embryonic kidney cells (HEK293 cells), expressing YFP-ΔF508-CFTR were used to screen a pilot library of compounds with some structural similarity to known potentiator VX- 770. Ligand-based virtual screening was then used to construct two further libraries, based on VX-770 and the lead hit compound from the pilot screen. A number of novel ΔF508-CFTR potentiators were identified in each of the screens. Recently published studies suggest that chronic treatment with VX-770 decreases ΔF508- CFTR density at the plasma membrane, potentially limiting its clinical effectiveness. ΔF508- CFTR-pHTomato was used to show that a number of hit compounds from our screens did not decrease membrane density of ΔF508-CFTR. The YFP-CFTR assay was also used for studies investigating the mechanism of potentiation by VX-770. We provide evidence that WT-CFTR does not require phosphorylation for potentiation by VX-770, which has not been reported previously. Additionally, studies using gating mutations do not support the current hypothesis that VX-770 stabilises the posthydrolytic open state in the CFTR gating cycle. |
