Fluorescence assay for simultaneous quantification of CFTR ion-channel function and plasma membrane proximity.

The cystic fibrosis transmembrane conductance regulator (CFTR) is a plasma membrane anion channel that plays a key role in controlling transepithelial fluid movement. Excessive activation results in intestinal fluid loss during secretory diarrheas, whereas CFTR mutations underlie cystic fibrosis (CF). Anion permeability depends both on how well CFTR channels work (permeation/gating) and on how many are present at the membrane. Recently, treatments with two drug classes targeting CFTR-one boosting ion-channel function (potentiators) and the other increasing plasma membrane density (correctors)-have provided significant health benefits to CF patients. Here, we present an image-based fluorescence assay that can rapidly and simultaneously estimate both CFTR ion-channel function and the protein's proximity to the membrane. We monitor F508del-CFTR, the most common CF-causing variant, and confirm rescue by low temperature, CFTR-targeting drugs and second-site revertant mutation R1070W. In addition, we characterize a panel of 62 CF-causing mutations. Our measurements correlate well with published data (electrophysiology and biochemistry), further confirming validity of the assay. Finally, we profile effects of acute treatment with approved potentiator drug VX-770 on the rare-mutation panel. Mapping the potentiation profile on CFTR structures raises mechanistic hypotheses on drug action, suggesting that VX-770 might allow an open-channel conformation with an alternative arrangement of domain interfaces. The assay is a valuable tool for investigation of CFTR molecular mechanisms, allowing accurate inferences on gating/permeation. In addition, by providing a two-dimensional characterization of the CFTR protein, it could better inform development of single-drug and precision therapies addressing the root cause of CF disease.

Multitarget CFTR Modulators Endowed with Multiple Beneficial Side Effects for Cystic Fibrosis Patients: Toward a Simplified Therapeutic Approach †.

Cystic fibrosis (CF) is a multiorgan disease caused by mutations of the cystic fibrosis transmembrane conductance regulator (CFTR). In addition to respiratory impairment due to mucus accumulation, viruses and bacteria trigger acute pulmonary exacerbations, accelerating disease progression and mortality rate. Treatment complexity increases with patients' age, and simplifying the therapeutic regimen represents one of the key priorities in CF. We have recently reported the discovery of multitarget compounds able to "kill two birds with one stone" by targeting F508del-CFTR and PI4KIIIß and thus acting simultaneously as CFTR correctors and broad-spectrum enterovirus (EV) inhibitors. Starting from these preliminary results, we report herein a hit-to-lead optimization and multidimensional structure-activity relationship (SAR) study that led to compound 23a. This compound showed good antiviral and F508del-CFTR correction potency, additivity/synergy with lumacaftor, and a promising in vitro absorption, distribution, metabolism, and excretion (ADME) profile. It was well tolerated in vivo with no sign of acute toxicity and histological alterations in key biodistribution organs.

Potentiation of the cystic fibrosis transmembrane conductance regulator by VX-770 involves stabilization of the pre-hydrolytic, O1 state.

BACKGROUND AND PURPOSE: Cystic fibrosis (CF) is a debilitating hereditary disease caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene, which encodes an anion channel. Wild type-CFTR gating is a non-equilibrium process. After ATP binding, CFTR enters a stable open state (O1 ). ATP hydrolysis leads it to a short-lived post-hydrolytic open state (O2 ), from which channels close. Here, we use mutations to probe the mechanism of VX-770, the first compound directly targeting the CFTR protein approved for treatment of CF. D1370N and K1250R mutations reduce or abolish catalytic activity, simplifying the gating scheme to an equilibrium (C↔O1 ); K464A-CFTR has a destabilized O1 state and rarely closes via hydrolysis. EXPERIMENTAL APPROACH: Potentiation by VX-770 was measured using microscopic imaging of HEK293 cells expressing an anion-sensitive YFP-CFTR. A simple mathematical model was used to predict fluorescence quenching following extracellular iodide addition and estimate CFTR conductance. Membrane density of CFTR channels was measured in a parallel assay, using CFTR-pHTomato. KEY RESULTS: VX-770 strongly potentiated WT-CFTR, D1370N-CFTR and K1250R-CFTR. K464A-CFTR was also strongly potentiated, regardless of whether it retained catalytic activity or not. CONCLUSIONS AND IMPLICATIONS: Similar potentiation of hydrolytic and non-hydrolytic mutants suggests that VX-770 increases CFTR open probability mainly by stabilizing pre-hydrolytic O1 states with respect to closed states. Potentiation of K464A-CFTR channels suggests action of VX-770 did not strongly alter conformational dynamics at site 1. Understanding potentiator mechanism could help develop improved treatment for CF patients. The fluorescence assay presented here is a robust tool for such investigations.

Improved fluorescence assays to measure the defects associated with F508del-CFTR allow identification of new active compounds.

BACKGROUND AND PURPOSE: Cystic fibrosis (CF) is a debilitating disease caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene, which codes for a Cl-/HCO3 - channel. F508del, the most common CF-associated mutation, causes both gating and biogenesis defects in the CFTR protein. This paper describes the optimization of two fluorescence assays, capable of measuring CFTR function and cellular localization, and their use in a pilot drug screen. EXPERIMENTAL APPROACH: HEK293 cells expressing YFP-F508del-CFTR, in which halide sensitive YFP is tagged to the N-terminal of CFTR, were used to screen a small library of compounds based on the VX-770 scaffold. Cells expressing F508del-CFTR-pHTomato, in which a pH sensor is tagged to the fourth extracellular loop of CFTR, were used to measure CFTR plasma membrane exposure following chronic treatment with the novel potentiators. KEY RESULTS: Active compounds with efficacy ~50% of VX-770, micromolar potency, and structurally distinct from VX-770 were identified in the screen. The F508del-CFTR-pHTomato assay suggests that the hit compound MS131A, unlike VX-770, does not decrease membrane exposure of F508del-CFTR. CONCLUSIONS AND IMPLICATIONS: Most known potentiators have a negative influence on F508del-CFTR biogenesis/stability, which means membrane exposure needs to be monitored early during the development of drugs targeting CFTR. The combined use of the two fluorescence assays described here provides a useful tool for the identification of improved potentiators and correctors. The assays could also prove useful for basic scientific investigations on F508del-CFTR, and other CF-causing mutations.

Discovery of Multitarget Agents Active as Broad-Spectrum Antivirals and Correctors of Cystic Fibrosis Transmembrane Conductance Regulator for Associated Pulmonary Diseases.

Enteroviruses (EVs) are among the most frequent infectious agents in humans worldwide and represent the leading cause of upper respiratory tract infections. No drugs for the treatment of EV infections are currently available. Recent studies have also linked EV infection with pulmonary exacerbations, especially in cystic fibrosis (CF) patients, and the importance of this link is probably underestimated. The aim of this work was to develop a new class of multitarget agents active both as broad-spectrum antivirals and as correctors of the F508del-cystic fibrosis transmembrane conductance regulator (CFTR) folding defect responsible for >90% of CF cases. We report herein the discovery of the first small molecules able to simultaneously act as correctors of the F508del-CFTR folding defect and as broad-spectrum antivirals against a panel of EVs representative of all major species.

Gain and loss of function of P2X7 receptors: mechanisms, pharmacology and relevance to diabetic neuropathic pain.

BACKGROUND: Genetic causes of exaggerated or reduced pain sensitivity in humans are well known. Recently, single nucleotide polymorphisms (SNPs) in the gene P2RX7, coding for the ATP-gated ion channel P2X7, have been described that cause gain-of-function (GOF) and loss-of-function (LOF), respectively of this channel. Importantly, P2RX7 SNPs have been associated with more or less severe pain scores in patient suffering of post-mastectomy pain and osteoarthritis. RESULTS: The functional consequences of some P2RX7 SNPs (rs208294 (His155Tyr), rs1718119 (Ala348Thr) and rs3751143 (Glu496Ala)) were studied in recombinant cells in vitro. Our findings suggest a correlation between GOF and LOF of P2X7 and actual channel protein expression. Both channel and pore function for these mutant P2X7 receptors changed in parallel to protein levels. On the other hand, the mutant receptors did not differ in their sensitivity to known P2X7 agonists and antagonists. We further demonstrated that in patients with diabetic peripheral neuropathic pain (DPNP), the presence of the GOF SNPs rs208294 (His155Tyr) and rs1718119 (Ala348Thr) is associated, in females, with higher pain intensity scores. CONCLUSIONS: Our present results confirm the physiological relevance of some of the SNPs in the P2RX7 gene and show that the presence of these genetic variants correlates with pain sensitivity also in a diabetic neuropathic pain patient population.

Pharmacological characterisation of ligand- and voltage-gated ion channels expressed in human iPSC-derived forebrain neurons.

INTRODUCTION: Genetic causes, or predisposition, are increasingly accepted to be part of the ethiopathogenesis of many neuropsychiatric diseases. While genes can be studied in any type of cells, their physiological function in human brain cells is difficult to evaluate, particularly in living subjects. METHODS: As a first step towards the characterisation of human inducible pluripotent stem cell (iPSC)-derived neurons from autism spectrum disorder (ASD) patients, we used gene expression and functional studies to define the regional identity of the typical forebrain differentiation, demonstrate expression patterns of genes of interest in ASD and understand the properties of 'control' iPSC-derived neurons (iCell-Neurons™), with a focus on receptors and ion channels that play a central role in synaptic physio-pathology. RESULTS AND DISCUSSION: The gene expression profile of the iCell-Neurons™ closely resembled that observed in neonatal prefrontal cortex tissues. Functional studies, performed mainly using calcium flux assays, demonstrated the presence of ionotropic glutamate (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid and N-methyl-D-aspartate) and gamma-aminobutyric acid type A receptors. Voltage-gated sodium and calcium channels were also identified using similar techniques. CONCLUSIONS: Overall, the results reported here suggest that iCell-Neurons™ are a good cellular model of a relatively immature forebrain human neuron population that can be used both as a control in comparison to patients cells, and as host cells in which mutations, insertions and deletions can be used in order to study the molecular mechanisms of ASD and other neurological disorders in an isogenic cellular background.