High-throughput screening of libraries of compounds to identify CFTR modulators

Expression of gain-of-function CFTR in cystic fibrosis airway cells restores epithelial function better than wild-type or codon-optimized CFTR

Class Ia/b cystic fibrosis transmembrane regulator (CFTR) variants cause severe lung disease in 10% of cystic fibrosis (CF) patients and are untreatable with small-molecule pharmaceuticals. Genetic replacement of CFTR offers a cure, but its effectiveness is limited in vivo. We hypothesized that enhancing protein levels (using codon optimization) and/or activity (using gain-of-function variants) of CFTR would more effectively restore function to CF bronchial epithelial cells. Three different variants of the CFTR protein were tested: codon optimized (high codon adaptation index [hCAI]), a gain-of-function (GOF) variant (K978C), and a combination of both (hˆK978C). In human embryonic kidney (HEK293T) cells, initial results showed that hCAI and hˆK978C produced greater than 10-fold more CFTR protein and displayed ∼4-fold greater activity than wild-type (WT) CFTR. However, functionality was profoundly different in CF bronchial epithelial cells. Here, K978C CFTR more potently restored essential epithelial functions (anion transport, airway surface liquid height, and pH) than WT CFTR. hCAI and hˆK978C CFTRs had limited impact because of mislocalization in the cell. These data provide a proof of principle showing that GOF variants may be more effective than codon-optimized forms of CFTR for CF gene therapy.

Video abstract

Additive Potentiation of R334W-CFTR Function by Novel Small Molecules

The R334W (c.1000C>T, p.Arg334Trp) is a rare cystic fibrosis (CF)-causing mutation for which no causal therapy is currently approved. This mutation leads to a significant reduction of CF transmembrane conductance regulator (CFTR) channel conductance that still allows for residual function. Potentiators are small molecules that interact with CFTR protein at the plasma membrane to enhance CFTR-dependent chloride secretion, representing thus pharmacotherapies targeting the root cause of the disease. Here, we generated a new CF bronchial epithelial (CFBE) cell line to screen a collection of compounds and identify novel potentiators for R334W-CFTR. The active compounds were then validated by electrophysiological assays and their additive effects in combination with VX-770, genistein, or VX-445 were exploited in this cell line and further confirmed in intestinal organoids. Four compounds (LSO-24, LSO-25, LSO-38, and LSO-77) were active in the functional primary screen and their ability to enhance R334W-CFTR-dependent chloride secretion was confirmed using electrophysiological measurements. In silico ADME analyses demonstrated that these compounds follow Lipinski’s rule of five and are thus suggested to be orally bioavailable. Dose−response relationships revealed nevertheless suboptimal efficacy and weak potency exerted by these compounds. VX-770 and genistein also displayed a small potentiation of R334W-CFTR function, while VX-445 demonstrated no potentiator activity for this mutation. In the R334W-expressing cell line, CFTR function was further enhanced by the combination of LSO-24, LSO-25, LSO-38, or LSO-77 with VX-770, but not with genistein. The efficacy of potentiator VX-770 combined with active LSO compounds was further confirmed in intestinal organoids (R334W/R334W genotype). Taken together, these molecules were demonstrated to potentiate R334W-CFTR function by a different mechanism than that of VX-770. They may provide a feasible starting point for the design of analogs with improved CFTR-potentiator activity.

Improved Trimethylangelicin Analogs for Cystic Fibrosis: Design, Synthesis and Preliminary Screening

A small library of new angelicin derivatives was designed and synthesized with the aim of bypassing the side effects of trimethylangelicin (TMA), a promising agent for the treatment of cystic fibrosis. To prevent photoreactions with DNA, hindered substituents were inserted at the 4 and/or 6 positions. Unlike the parent TMA, none of the new derivatives exhibited significant cytotoxicity or mutagenic effects. Among the synthesized compounds, the 4-phenylderivative 12 and the 6-phenylderivative 25 exerted a promising F508del CFTR rescue ability. On these compounds, preliminary in vivo pharmacokinetic (PK) studies were carried out, evidencing a favorable PK profile per se or after incorporation into lipid formulations. Therefore, the selected compounds are good candidates for future extensive investigation to evaluate and develop novel CFTR correctors based on the angelicin structure.

KDM2A and KDM3B as Potential Targets for the Rescue of F508del-CFTR

Cystic fibrosis (CF) is caused by mutations in the gene encoding of the cystic fibrosis transmembrane conductance regulator (CFTR), an anion-selective plasma membrane channel that mainly regulates chloride transport in a variety of epithelia. More than 2000 mutations, most of which presumed to be disease-relevant, have been identified in the CFTR gene. The single CFTR mutation F508del (deletion of phenylalanine in position 508) is present in about 90% of global CF patients in at least one allele. F508del is responsible for the defective folding and processing of CFTR, failing to traffic to the plasma membrane and undergoing premature degradation via the ubiquitin–proteasome system. CFTR is subjected to different post-translational modifications (PTMs), and the possibility to modulate these PTMs has been suggested as a potential therapeutic strategy for the functional recovery of the disease-associated mutants. Recently, the PTM mapping of CFTR has identified some lysine residues that may undergo methylation or ubiquitination, suggesting a competition between these two PTMs. Our work hypothesis moves from the idea that favors methylation over ubiquitination, e.g., inhibiting demethylation could be a successful strategy for preventing the premature degradation of unstable CFTR mutants. Here, by using a siRNA library against all the human demethylases, we identified the enzymes whose downregulation increases F508del-CFTR stability and channel function. Our results show that KDM2A and KDM3B downregulation increases the stability of F508del-CFTR and boosts the functional rescue of the channel induced by CFTR correctors.

Targeting the E1 ubiquitin-activating enzyme (UBA1) improves elexacaftor/tezacaftor/ivacaftor efficacy towards F508del and rare misfolded CFTR mutants

The advent of Trikafta (Kaftrio in Europe) (a triple-combination therapy based on two correctors—elexacaftor/tezacaftor—and the potentiator ivacaftor) has represented a revolution for the treatment of patients with cystic fibrosis (CF) carrying the most common misfolding mutation, F508del-CFTR. This therapy has proved to be of great efficacy in people homozygous for F508del-CFTR and is also useful in individuals with a single F508del allele. Nevertheless, the efficacy of this therapy needs to be improved, especially in light of the extent of its use in patients with rare class II CFTR mutations. Using CFBE41o- cells expressing F508del-CFTR, we provide mechanistic evidence that targeting the E1 ubiquitin-activating enzyme (UBA1) by TAK-243, a small molecule in clinical trials for other diseases, boosts the rescue of F508del-CFTR induced by CFTR correctors. Moreover, TAK-243 significantly increases the F508del-CFTR short-circuit current induced by elexacaftor/tezacaftor/ivacaftor in differentiated human primary airway epithelial cells, a gold standard for the pre-clinical evaluation of patients’ responsiveness to pharmacological treatments. This new combinatory approach also leads to an improvement in CFTR conductance on cells expressing other rare CF-causing mutations, including N1303K, for which Trikafta is not approved. These findings show that Trikafta therapy can be improved by the addition of a drug targeting the misfolding detection machinery at the beginning of the ubiquitination cascade and may pave the way for an extension of Trikafta to low/non-responding rare misfolded CFTR mutants.

Comprehensive Analysis of Chemical Structures That Have Been Tested as CFTR Activating Substances in a Publicly Available Database CandActCFTR

Background: Cystic fibrosis (CF) is a genetic disease caused by mutations in CFTR, which encodes a chloride and bicarbonate transporter expressed in exocrine epithelia throughout the body. Recently, some therapeutics became available that directly target dysfunctional CFTR, yet research for more effective substances is ongoing. The database CandActCFTR aims to provide detailed and comprehensive information on candidate therapeutics for the activation of CFTR-mediated ion conductance aiding systems-biology approaches to identify substances that will synergistically activate CFTR-mediated ion conductance based on published data. Results: Until 10/2020, we derived data from 108 publications on 3,109 CFTR-relevant substances via the literature database PubMed and further 666 substances via ChEMBL; only 19 substances were shared between these sources. One hundred and forty-five molecules do not have a corresponding entry in PubChem or ChemSpider, which indicates that there currently is no single comprehensive database on chemical substances in the public domain. Apart from basic data on all compounds, we have visualized the chemical space derived from their chemical descriptors via a principal component analysis annotated for CFTR-relevant biological categories. Our online query tools enable the search for most similar compounds and provide the relevant annotations in a structured way. The integration of the KNIME software environment in the back-end facilitates a fast and user-friendly maintenance of the provided data sets and a quick extension with new functionalities, e.g., new analysis routines. CandActBase automatically integrates information from other online sources, such as synonyms from PubChem and provides links to other resources like ChEMBL or the source publications. Conclusion: CandActCFTR aims to establish a database model of candidate cystic fibrosis therapeutics for the activation of CFTR-mediated ion conductance to merge data from publicly available sources. Using CandActBase, our strategy to represent data from several internet resources in a merged and organized form can also be applied to other use cases. For substances tested as CFTR activating compounds, the search function allows users to check if a specific compound or a closely related substance was already tested in the CF field. The acquired information on tested substances will assist in the identification of the most promising candidates for future therapeutics.

On the Corner of Models and Cure: Gene Editing in Cystic Fibrosis

Cystic fibrosis (CF) is a severe genetic disease for which curative treatment is still lacking. Next generation biotechnologies and more efficient cell-based and in vivo disease models are accelerating the development of novel therapies for CF. Gene editing tools, like CRISPR-based systems, can be used to make targeted modifications in the genome, allowing to correct mutations directly in the Cystic Fibrosis Transmembrane conductance Regulator (CFTR) gene. Alternatively, with these tools more relevant disease models can be generated, which in turn will be invaluable to evaluate novel gene editing-based therapies for CF. This critical review offers a comprehensive description of currently available tools for genome editing, and the cell and animal models which are available to evaluate them. Next, we will give an extensive overview of proof-of-concept applications of gene editing in the field of CF. Finally, we will touch upon the challenges that need to be addressed before these proof-of-concept studies can be translated towards a therapy for people with CF.

Phosphorylation of the Chaperone-Like HspB5 Rescues Trafficking and Function of F508del-CFTR

Cystic Fibrosis is a lethal monogenic autosomal recessive disease linked to mutations in Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) protein. The most frequent mutation is the deletion of phenylalanine at position 508 of the protein. This F508del-CFTR mutation leads to misfolded protein that is detected by the quality control machinery within the endoplasmic reticulum and targeted for destruction by the proteasome. Modulating quality control proteins as molecular chaperones is a promising strategy for attenuating the degradation and stabilizing the mutant CFTR at the plasma membrane. Among the molecular chaperones, the small heat shock protein HspB1 and HspB4 were shown to promote degradation of F508del-CFTR. Here, we investigated the impact of HspB5 expression and phosphorylation on transport to the plasma membrane, function and stability of F508del-CFTR. We show that a phosphomimetic form of HspB5 increases the transport to the plasma membrane, function and stability of F508del-CFTR. These activities are further enhanced in presence of therapeutic drugs currently used for the treatment of cystic fibrosis (VX-770/Ivacaftor, VX-770+VX-809/Orkambi). Overall, this study highlights the beneficial effects of a phosphorylated form of HspB5 on F508del-CFTR rescue and its therapeutic potential in cystic fibrosis.

Cystic Fibrosis: Translating Molecular Mechanisms into Effective Therapies

Cystic fibrosis is a genetic disease that affects approximately 75,000 individuals around the world. Long regarded as a lethal and life-limiting disease, with the most severe manifestations expressed in the progressive decline of lung function, treatment advances focusing on airway clearance and management of chronic lung infection have resulted in improved outcomes for individuals with cystic fibrosis. These advances have been realized in conjunction with an improved understanding of the genetic basis of this disease, dating back to the discovery of the cystic fibrosis gene in 1989. The identification of the cystic fibrosis gene and the advancement of our understanding of the resultant cystic fibrosis transmembrane conductance regulator protein have led to the development of a new class of cystic fibrosis therapies designed to directly impact the function of this protein. These therapeutic developments have progressed, targeting the various mutations that can cause cystic fibrosis. These new medications, known as cystic fibrosis transmembrane conductance regulator modulators, have changed the landscape of cystic fibrosis care and cystic fibrosis research. Their demonstrated effect in patients with specific cystic fibrosis mutations has ignited the hope that such therapies will soon be available to more individuals with this disease, moving the cystic fibrosis community significantly closer to the ultimate goal of curing this disease.

High-Throughput Surface Liquid Absorption and Secretion Assays to Identify F508del CFTR Correctors Using Patient Primary Airway Epithelial Cultures

High-throughput screening for drug discovery is increasingly utilizing cellular systems of high physiological relevance, such as patient primary cells and organoid cultures. We used 3D-cultured cystic fibrosis patient bronchial epithelial cells to screen for new small-molecule correctors of the disease-causing F508del mutation in CFTR. Impaired mucociliary clearance due to insufficient airway hydration is a hallmark of cystic fibrosis and we used a simple measure of surface liquid levels to quantify F508del CFTR correction in cultured bronchial epithelial cells. Two robust assay formats were configured and used to screen more than 100,000 compounds as mixtures or individual compounds in 96-well format. The corrector discovery success rate, as measured by the number of hits confirmed by an electrophysiology assay on patient primary bronchial epithelial cells, was superior to screens in cell lines expressing recombinant F508del CFTR. Several novel corrector scaffolds were discovered that when combined with the clinical corrector VX-809 delivered combination responses greater than double that of VX-809 alone. This work exemplifies the advantages of a disease-relevant readout and 3D-cultured patient primary cells for the discovery of new drug candidates.

High-Content Surface and Total Expression siRNA Kinase Library Screen with VX-809 Treatment Reveals Kinase Targets that Enhance F508del-CFTR Rescue

The most promising F508del-CFTR corrector, VX-809, has been unsuccessful as an effective, stand-alone treatment for CF patients, but the rescue effect in combination with other drugs may confer an acceptable level of therapeutic benefit. Targeting cellular factors that modify trafficking may act to enhance the cell surface density of F508-CFTR with VX-809 correction. Our goal is to identify druggable kinases that enhance F508del-CFTR rescue and stabilization at the cell surface beyond that achievable with the VX-809 corrector alone. To achieve this goal, we implemented a new high-throughput screening paradigm that quickly and quantitatively measures surface density and total protein in the same cells. This allowed for rapid screening for increased surface targeting and proteostatic regulation. The assay utilizes fluorogen-activating-protein (FAP) technology with cell excluded and cell permeant fluorogenic dyes in a quick, wash-free fluorescent plate reader format on live cells to first measure F508del-CFTR expressed on the surface and then the total amount of F508del-CFTR protein present. To screen for kinase targets, we used Dharmacon’s ON-TARGETplus SMARTpool siRNA Kinase library (715 target kinases) with and without 10 μM VX-809 treatment in triplicate at 37 °C. We identified several targets that had a significant interaction with VX-809 treatment in enhancing surface density with siRNA knockdown. Select small-molecule inhibitors of the kinase targets demonstrated augmented surface expression with VX-809 treatment.

Personalized medicine in CF: from modulator development to therapy for cystic fibrosis patients with rare CFTR mutations

Cystic fibrosis (CF) is the most common life-shortening genetic disease affecting ~1 in 3,500 of the Caucasian population. CF is caused by mutations in the CF transmembrane conductance regulator (CFTR) gene. To date, more than 2,000 CFTR mutations have been identified, which produce a wide range of phenotypes. The CFTR protein, a chloride channel, is normally expressed on epithelial cells lining the lung, gut, and exocrine glands. Mutations in CFTR have led to pleiotropic effects in CF patients and have resulted in early morbidity and mortality. Research has focused on identifying small molecules, or modulators, that can restore CFTR function. In recent years, two modulators, ivacaftor (Kalydeco) and lumacaftor/ivacaftor (Orkambi), have been approved by the U.S. Food and Drug Administration to treat CF patients with certain CFTR mutations. The development of these modulators has served as proof-of-concept that targeting CFTR by modulators is a viable therapeutic option. Efforts to discover new modulators that could deliver a wider and greater clinical benefit are still ongoing. However, traditional randomized controlled trials (RCTs) require large numbers of patients and become impracticable to test the modulators' efficacy in CF patients with CFTR mutations at frequencies much lower than 1%, suggesting the need for personalized medicine in these CF patients.

Development of Automated Patch Clamp Technique to Investigate CFTR Chloride Channel Function

The chloride (Cl^-) channel cystic fibrosis transmembrane conductance regulator (CFTR) is defective in cystic fibrosis (CF), and mutation of its encoding gene leads to various defects such as retention of the misfolded protein in the endoplasmic reticulum, reduced stability at the plasma membrane, abnormal channel gating with low open probability, and thermal instability, which leads to inactivation of the channel at physiological temperature. Pharmacotherapy is one major therapeutic approach in the CF field and needs sensible and fast tools to identify promising compounds. The high throughput screening assays available are often fast and sensible techniques but with lack of specificity. Few works used automated patch clamp (APC) for CFTR recording, and none have compared conventional and planar techniques and demonstrated their capabilities for different types of experiments. In this study, we evaluated the use of planar parallel APC technique for pharmacological search of CFTR-trafficking correctors and CFTR function modulators. Using optimized conditions, we recorded both wt- and corrected F508del-CFTR Cl^- currents with automated whole-cell patch clamp and compared the data to results obtained with conventional manual whole-cell patch clamp. We found no significant difference in patch clamp parameters such as cell capacitance and series resistance between automated and manual patch clamp. Also, the results showed good similarities of CFTR currents recording between the two methods. We showed that similar stimulation protocols could be used in both manual and automatic techniques allowing precise control of temperature, classic I/V relationship, and monitoring of current stability in time. In conclusion, parallel patch-clamp recording allows rapid and efficient investigation of CFTR currents with a variety of tests available and could be considered as new tool for medium throughput screening in CF pharmacotherapy.

CFTR pharmacology

CFTR protein is an ion channel regulated by cAMP-dependent phosphorylation and expressed in many types of epithelial cells. CFTR-mediated chloride and bicarbonate secretion play an important role in the respiratory and gastrointestinal systems. Pharmacological modulators of CFTR represent promising drugs for a variety of diseases. In particular, correctors and potentiators may restore the activity of CFTR in cystic fibrosis patients. Potentiators are also potentially useful to improve mucociliary clearance in patients with chronic obstructive pulmonary disease. On the other hand, CFTR inhibitors may be useful to block fluid and electrolyte loss in secretory diarrhea and slow down the progression of polycystic kidney disease.

The extracellular calcium-sensing receptor regulates human fetal lung development via CFTR

Optimal fetal lung growth requires anion-driven fluid secretion into the lumen of the developing organ. The fetus is hypercalcemic compared to the mother and here we show that in the developing human lung this hypercalcaemia acts on the extracellular calcium-sensing receptor, CaSR, to promote fluid-driven lung expansion through activation of the cystic fibrosis transmembrane conductance regulator, CFTR. Several chloride channels including TMEM16, bestrophin, CFTR, CLCN2 and CLCA1, are also expressed in the developing human fetal lung at gestational stages when CaSR expression is maximal. Measurements of Cl⁻-driven fluid secretion in organ explant cultures show that pharmacological CaSR activation by calcimimetics stimulates lung fluid secretion through CFTR, an effect which in humans, but not mice, was also mimicked by fetal hypercalcemic conditions, demonstrating that the physiological relevance of such a mechanism appears to be species-specific. Calcimimetics promote CFTR opening by activating adenylate cyclase and we show that Ca²⁺-stimulated type I adenylate cyclase is expressed in the developing human lung. Together, these observations suggest that physiological fetal hypercalcemia, acting on the CaSR, promotes human fetal lung development via cAMP-dependent opening of CFTR. Disturbances in this process would be expected to permanently impact lung structure and might predispose to certain postnatal respiratory diseases.

Simple image-based no-wash method for quantitative detection of surface expressed CFTR

Cystic fibrosis (CF) is the most common lethal genetic disease among Caucasians. It is caused by mutations in the CF Transmembrane Conductance Regulator (CFTR) gene, which encodes an apical membrane anion channel that is required for regulating the volume and composition of epithelial secretions. The most common CFTR mutation, present on at least one allele in >90% of CF patients, deletes phenylalanine at position 508 (F508del), which causes the protein to misfold. Endoplasmic reticulum (ER) quality control elicits the degradation of mutant CFTR, compromising its trafficking to the epithelial cell apical membrane. The absence of functional CFTR leads to depletion of airway surface liquid, impaired clearance of mucus and bacteria from the lung, and predisposes to recurrent infections. Ultimately, respiratory failure results from inflammation and bronchiectasis. Although high throughput screening has identified small molecules that can restore the anion transport function of F508del CFTR, they correct less than 15% of WT CFTR activity, yielding insufficient clinical benefit. To date, most primary CF drug discovery assays have employed measurements of CFTR's anion transport function, a method that depends on the recruitment of a functional CFTR to the cell surface, involves multiple wash steps, and relies on a signal that saturates rapidly. Screening efforts have also included assays for detection of extracellularly HA-tagged or HRP-tagged CFTR, which require multiple washing steps. We have recently developed tools and cell lines that report the correction of mutant CFTR trafficking by currently available small molecules, and have extended this assay to the 96-well format. This new and simple no-wash assay of F508del CFTR at the cell surface may permit the discovery of more efficacious drugs, and hopefully thereby prevent the catastrophic effects of this disease. In addition, the modular design of this platform should make it useful for other diseases where loss-of-function results from folding and/or trafficking defects in membrane proteins.

Protein traffic disorders: an effective high-throughput fluorescence microscopy pipeline for drug discovery

Plasma membrane proteins are essential molecules in the cell which mediate interactions with the exterior milieu, thus representing key drug targets for present pharma. Not surprisingly, protein traffic disorders include a large range of diseases sharing the common mechanism of failure in the respective protein to reach the plasma membrane. However, specific therapies for these diseases are remarkably lacking. Herein, we report a robust platform for drug discovery applied to a paradigmatic genetic disorder affecting intracellular trafficking – Cystic Fibrosis. This platform includes (i) two original respiratory epithelial cellular models incorporating an inducible double-tagged traffic reporter; (ii) a plasma membrane protein traffic assay for high-throughput microscopy screening; and (iii) open-source image analysis software to quantify plasma membrane protein traffic. By allowing direct scoring of compounds rescuing the basic traffic defect, this platform enables an effective drug development pipeline, which can be promptly adapted to any traffic disorder-associated protein and leverage therapy development efforts.

Activation of 3-phosphoinositide-dependent kinase 1 (PDK1) and serum- and glucocorticoid-induced protein kinase 1 (SGK1) by short-chain sphingolipid C4-ceramide rescues the trafficking defect of ΔF508-cystic fibrosis transmembrane conductance regulator (ΔF508-CFTR)

Cystic fibrosis (CF) is due to a folding defect in the CF transmembrane conductance regulator (CFTR) protein. The most common mutation, ΔF508, prevents CFTR from trafficking to the apical plasma membrane. Here we show that activation of the PDK1/SGK1 signaling pathway with C4-ceramide (C4-CER), a non-toxic small molecule, functionally corrects the trafficking defect in both cultured CF cells and primary epithelial cell explants from CF patients. The mechanism of C4-CER action involves a series of mutual autophosphorylation and phosphorylation events between PDK1 and SGK1. Detailed mechanistic studies indicate that C4-CER initially induces autophosphorylation of SGK1 at Ser(422). SGK1[Ser(P)(422)] and C4-CER coincidently bind PDK1 and permit PDK1 to autophosphorylate at Ser(241). Then PDK1[Ser(P)(241)] phosphorylates SGK1[Ser(P)(422)] at Thr(256) to generate fully activated SGK1[Ser(422), Thr(P)(256)]. SGK1[Ser(P)(422),Thr(P)(256)] phosphorylates and inactivates the E3 ubiquitin ligase Nedd4-2. ΔF508-CFTR is thus free to traffic to the plasma membrane. Importantly, C4-CER-mediated activation of both PDK1 and SGK1 is independent of the PI3K/Akt/mammalian target of rapamycin signaling pathway. Physiologically, C4-CER significantly increases maturation and stability of ΔF508-CFTR (t½ ∼10 h), enhances cAMP-activated chloride secretion, and suppresses hypersecretion of interleukin-8 (IL-8). We suggest that candidate drugs for CF directed against the PDK1/SGK1 signaling pathway, such as C4-CER, provide a novel therapeutic strategy for a life-limiting disorder that affects one child, on average, each day.

A rapid membrane potential assay to monitor CFTR function and inhibition

The cystic fibrosis transmembrane conductance regulator (CFTR) protein is an important regulator of ion transport and fluid secretion in humans. Mutations to CFTR cause cystic fibrosis, which is a common recessive genetic disorder in Caucasians. Involvement of CFTR has been noted in other important diseases, such as secretory diarrhea and polycystic kidney disease. The assays to monitor CFTR function that have been described to date either are complicated or require specialized instrumentation and training for execution. In this report, we describe a rapid FlexStation-based membrane potential assay to monitor CFTR function. In this assay, agonist-mediated activation of CFTR results in membrane depolarization that can be monitored using a fluorescent membrane potential probe. Availability of a simple mix-and-read assay to monitor the function of this important protein might accelerate the discovery of CFTR ligands to study a variety of conditions.

Functional Rescue of F508del-CFTR Using Small Molecule Correctors

High-throughput screens for small molecules that are effective in “correcting” the functional expression of F508del-CFTR have yielded several promising hits. Two such compounds are currently in clinical trial. Despite this success, it is clear that further advances will be required in order to restore 50% or greater of wild-type CFTR function to the airways of patients harboring the F508del-CFTR protein. Progress will be enhanced by our better understanding of the molecular and cellular defects caused by the F508del mutation, present in 90% of CF patients. The goal of this chapter is to review the current understanding of defects caused by F508del in the CFTR protein and in CFTR-mediated interactions important for its biosynthesis, trafficking, channel function, and stability at the cell surface. Finally, we will discuss the gaps in our knowledge regarding the mechanism of action of existing correctors, the unmet need to discover compounds which restore proper CFTR structure and function in CF affected tissues and new strategies for therapy development.

Ubiquitin and membrane protein turnover: from cradle to grave

From the moment of cotranslational insertion into the lipid bilayer of the endoplasmic reticulum (ER), newly synthesized integral membrane proteins are subject to a complex series of sorting, trafficking, quality control, and quality maintenance systems. Many of these processes are intimately controlled by ubiquitination, a posttranslational modification that directs trafficking decisions related to both the biosynthetic delivery of proteins to the plasma membrane (PM) via the secretory pathway and the removal of proteins from the PM via the endocytic pathway. Ubiquitin modification of integral membrane proteins (or "cargoes") generally acts as a sorting signal, which is recognized, captured, and delivered to a specific cellular destination via specialized trafficking events. By affecting the quality, quantity, and localization of integral membrane proteins in the cell, defects in these processes contribute to human diseases, including cystic fibrosis, circulatory diseases, and various neuropathies. This review summarizes our current understanding of how ubiquitin modification influences cargo trafficking, with a special emphasis on mechanisms of quality control and quality maintenance in the secretory and endocytic pathways.