Conformational and temperature-sensitive stability defects of the delta F508 cystic fibrosis transmembrane conductance regulator in post-endoplasmic reticulum compartments

Introduction of Ivacaftor/Lumacaftor in Children With Cystic Fibrosis Homozygous for F508del in the Netherlands: A Nationwide Real-Life Study

INTRODUCTION

Lumacaftor/ivacaftor (lum/iva) was introduced in the Netherlands in 2017. We investigated 1-year efficacy of lum/iva on lung function and small airway and structural lung disease evaluated by multiple breath nitrogen washout and CT scan. Additionally, we investigated effects of lum/iva on exacerbations, anthropometry, sweat chloride and safety in children with CF in the Netherlands.

METHODS

Children with CF aged 6-18 years and homozygous for F508del treated in one of the seven Dutch CF centers for at least 12 months were eligible. Data were extracted from the Dutch CF Registry and electronic patient records. Primary outcome was change in percent predicted FEV1 (ppFEV1) after 12 months.

RESULTS

Nationwide, 247 children with CF were eligible for lum/iva. Eight patients discontinued lum/iva due to side effects. In total, 223/247 children (90.3%) were evaluated. Mean (range) age at baseline was 11.0 (6.0-17.1) years. There was no change in FEV1 after 12 months of lum/iva. In a subgroup, markers of small airway function and structural lung disease, such as LCI (n = 28), mean change (SD) -10.0% (15.8), and bronchus-artery (BA) analysis on CT scan (n = 81), showed significant improvement (p < 0.01). Moreover, BMI (n = 192), exacerbations (n = 219) and sweat chloride measurements (n = 105) improved.

CONCLUSION

Lum/iva was generally well tolerated in a real-life, nationwide pediatric cohort. The efficacy of lum/iva was comparable to phase 3 studies in children. LCI and BA analysis as markers of small airway and structural lung disease showed significant improvement which indicates the importance of these parameters to evaluate treatment effects of CF modulators in children.

Trikafta rescues F508del-CFTR by tightening specific phosphorylation-dependent interdomain interactions

Trikafta is well-known for correcting the thermal and gating defects caused by the most common cystic fibrosis mutation F508del in the human cystic fibrosis transmembrane conductance regulator even at physiological temperature. However, the exact pathway is still unclear. Here, the noncovalent interactions among two transmembrane domains (TMD 1 and TMD2), the regulatory (R) domain and two nucleotide binding domains (NBD1 and NBD2), along with the thermoring structures of NBD1, were analyzed around the active gating center. The results demonstrated that Trikafta binding to TMD1 and TMD2 rearranged their interactions with the R domain, releasing the C-terminal region from NBD1 for its tight ATP-dependent dimerization with NBD2, stabilizing NBD1. Taken together, although the deletion of F508 induces the primary thermal defect of NBD1 and then the gating defect at the TMD1-TMD2 interface, Trikafta rescued them in a reverse manner allosterically. Thus, the thermoring structure can be used to uncover the pathway of a drug to correct the thermal defect of health-related protein.

Significance

Trikafta modulators have been approved by the FDA to treat the most common cystic fibrosis- causing mutation F508del CFTR. However, the molecular action mechanisms of these modulators are still unknown. Following the identification of the gating center in CFTR, this study further revealed that the specific noncovalent interactions of the phosphorylated S813 site with cytoplasmic loops 1 and 4 and N-/C- terminal tails of TMD1 upon Trikafta-triggered tight TMD1- TMD2 interactions at the gating center play a pivotal role in rescuing the primary gating defect and then the thermal defect of F508del CFTR.

Highlights

Trikafta strengthened TMD1-TMD2 interactions at the gating center of ΔF508-CFTR Tight TMD1-TMD2 interactions allowed specific interactions of the R domain with the ICL1- ICL4 interface and the N-/C- terminal tails of TMD1 Subsequently, the C-terminal region was released from NBD1 for tight ATP-dependent NBD1-NBD2 dimerization, stabilizing NBD1 of ΔF508-CFTR.

Elexacaftor-Tezacaftor-Ivacaftor Therapy for Chronic Sinus Disease in Cystic Fibrosis

Importance

Cystic fibrosis (CF) is a multiorgan genetic disease with progressive upper and lower airway involvement. The effects of CF transmembrane conductance regulator (CFTR) modifier therapies on CF-related upper airway disease, specifically chronic rhinosinusitis (CRS), are not characterized.

Objective

To determine the outcome of elexacaftor-tezacaftor-ivacaftor (ETI) on CRS as measured by changes in sinus computed tomography (CT) metrics and on clinical parameters in individuals with CF.

Design, Setting, and Participants

This prospective longitudinal cohort study was conducted at the CF center of a tertiary care hospital between October 1, 2019, and July 31, 2021. A total of 64 participants with CF were included in the analysis.

Intervention

Sinus CT was obtained within 1 month of initiation of ETI therapy (baseline), and within 1 month of 1 year of ETI therapy. Images were independently analyzed by pulmonology, radiology, and otolaryngology physicians, using the Lund-Mackay and Sheikh-Lind scoring systems. Percent predicted forced expiratory volume in 1 second (ppFEV1), body mass index (BMI), and microbiologic data collected at initiation of ETI therapy and 3-month intervals for 1 year were also measured.

Main Outcomes and Measures

The study hypothesis was that ETI therapy will improve CRS as measured by changes in sinus CT at initiation and 1 year after ETI therapy and clinical parameters in individuals with CF.

Results

Among the 64 participants (39 [60.9%] female; median age, 18.5 [IQR, 16.0-28.5] years; 64 [100%] White), improvement in CRS was noted by improvements in sinus CT scans using both sinus CT scoring systems after 1 year of ETI therapy. The reduction in the median total score using the Lund-Mackay sinus CT scoring system (from 5.8 [IQR, 5.0-7.0] to 3.3 [IQR, 2.6-4.2]) and the Sheikh-Lind scoring system (from 3.8 [IQR, 3.0-5.0] to 2.2 [IQR, 2.0-2.5]) was noted. Increases in ppFEV1 and BMI were also observed by 3 months of ETI therapy with persistent improvement through 1 year of treatment. Similarly, after 1 year of ETI therapy, participants with CF had reductions in positivity for Pseudomonas aeruginosa and Staphylococcus aureus in oropharyngeal cultures.

Conclusion and Relevance

This cohort study found that use of ETI therapy was associated with improved CRS outcomes in participants with CF as quantified by improved sinus CT scans measured by 2 radiographic scoring systems and was also associated with improved clinical outcomes. Despite improvement in CT scan scores, most people with CF continue to have scores that indicate severe sinus disease.

Transmembrane Helices 7 and 8 Confer Aggregation Sensitivity to the Cystic Fibrosis Transmembrane Conductance Regulator

The Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) is a large multi-spanning membrane protein that is susceptible to misfolding and aggregation. We have identified here the region responsible for this instability. Temperature-induced aggregation of C-terminally truncated versions of CFTR demonstrated that all truncations up to the second transmembrane domain (TMD2), including the R region, largely resisted aggregation. Limited proteolysis identified a folded structure that was prone to aggregation and consisted of TMD2 and at least part of the Regulatory Region R. Only when both TM7 (TransMembrane helix 7) and TM8 were present, TMD2 fragments became as aggregation-sensitive as wild-type CFTR, in line with increased thermo-instability of late CFTR nascent chains and in silico prediction of aggregation propensity. In accord, isolated TMD2 was degraded faster in cells than isolated TMD1. We conclude that TMD2 extended at its N-terminus with part of the R region forms a protease-resistant structure that induces heat instability in CFTR and may be responsible for its limited intracellular stability.

Innovative Strategy toward Mutant CFTR Rescue in Cystic Fibrosis: Design and Synthesis of Thiadiazole Inhibitors of the E3 Ligase RNF5

In cystic fibrosis (CF), deletion of phenylalanine 508 (F508del) in the CF transmembrane conductance regulator (CFTR) is associated to misfolding and defective gating of the mutant channel. One of the most promising CF drug targets is the ubiquitin ligase RNF5, which promotes F508del-CFTR degradation. Recently, the first ever reported inhibitor of RNF5 was discovered, i.e., the 1,2,4-thiadiazol-5-ylidene inh-2. Here, we designed and synthesized a series of new analogues to explore the structure-activity relationships (SAR) of this class of compounds. SAR efforts ultimately led to compound 16, which showed a greater F508del-CFTR corrector activity than inh-2, good tolerability, and no toxic side effects. Analogue 16 increased the basal level of autophagy similar to what has been described with RNF5 silencing. Furthermore, co-treatment with 16 significantly improved the F508del-CFTR rescue induced by the triple combination elexacaftor/tezacaftor/ivacaftor in CFBE41o^- cells. These findings validate the 1,2,4-thiadiazolylidene scaffold for the discovery of novel RNF5 inhibitors and provide evidence to pursue this unprecedented strategy for the treatment of CF.

Identification of novel F508del-CFTR traffic correctors among triazole derivatives

The most prevalent cystic fibrosis (CF)-causing mutation - F508del - impairs the folding of CFTR protein, resulting in its defective trafficking and premature degradation. Small molecules termed correctors may rescue F508del-CFTR and therefore constitute promising pharmacotherapies acting on the fundamental cause of the disease. Here, we screened a collection of triazole compounds to identify novel F508del-CFTR correctors. The functional primary screen identified four hit compounds (LSO-18, LSO-24, LSO-28, and LSO-39), which were further validated and demonstrated to rescue F508del-CFTR processing, plasma membrane trafficking, and function. To interrogate their mechanism of action (MoA), we examined their additivity to the clinically approved drugs VX-661 and VX-445, low temperature, and genetic revertants of F508del-CFTR. Rescue of F508del-CFTR processing and function by LSO-18, LSO-24, and LSO-28, but not by LSO-39, was additive to VX-661, whereas LSO-28 and LSO-39, but not LSO-18 nor LSO-24, were additive to VX-445. All compounds under investigation demonstrated additive rescue of F508del-CFTR processing and function to low temperature as well as to rescue by genetic revertants G550E and 4RK. Nevertheless, none of these compounds was able to rescue processing nor function of DD/AA-CFTR, and LSO-39 (similarly to VX-661) exhibited no additivity to genetic revertant R1070W. From these findings, we suggest that LSO-39 (like VX-661) has a putative binding site at the NBD1:ICL4 interface, LSO-18 and LSO-24 seem to share the MoA with VX-445, and LSO-28 appears to act by a different MoA. Altogether, these findings represent an encouraging starting point to further exploit this chemical series for the development of novel CFTR correctors.

Molecular structures reveal synergistic rescue of Δ508 CFTR by Trikafta modulators

The predominant mutation causing cystic fibrosis, a deletion of phenylalanine 508 (Δ508) in the cystic fibrosis transmembrane conductance regulator (CFTR), leads to severe defects in CFTR biogenesis and function. The advanced therapy Trikafta combines the folding corrector tezacaftor (VX-661), the channel potentiator ivacaftor (VX-770), and the dual-function modulator elexacaftor (VX-445). However, it is unclear how elexacaftor exerts its effects, in part because the structure of Δ508 CFTR is unknown. Here, we present cryo-electron microscopy structures of Δ508 CFTR in the absence and presence of CFTR modulators. When used alone, elexacaftor partially rectified interdomain assembly defects in Δ508 CFTR, but when combined with a type I corrector, did so fully. These data illustrate how the different modulators in Trikafta synergistically rescue Δ508 CFTR structure and function.

Near-digital amplification in paper improves sensitivity and speed in biplexed reactions

The simplest point-of-care assays are usually paper and plastic devices that detect proteins or nucleic acids at low cost and minimal user steps, albeit with poor limits of detection. Digital assays improve limits of detection and analyte quantification by splitting a sample across many wells (or droplets), preventing diffusion, and performing analyte amplification and detection in multiple small wells. However, truly digital nucleic acid amplification tests (NAATs) require costly consumable cartridges that are precisely manufactured, aligned, and operated to enable low detection limits. In this study, we demonstrate how to implement near-digital NAATs in low-cost porous media while approaching the low limits of detection of digital assays. The near-digital NAAT was enabled by a paper membrane containing lyophilized amplification reagents that automatically, passively meters and distributes a sample over a wide area. Performing a NAAT in the paper membrane while allowing diffusion captures many of the benefits of digital NAATs if the pad is imaged at a high spatial resolution during amplification. We show that the near-digital NAAT is compatible with a low-cost paper and plastic disposable cartridge coupled to a 2-layer rigid printed circuit board heater (the MD NAAT platform). We also demonstrate compatibility with biplexing and imaging with mobile phones with different camera sensors. We show that the near-digital NAAT increased signal-to-noise ratios by ~ 10×, improved limits of detection from above 10³ copies of methicillin-resistant Staphylococcus aureus genomic DNA to between 100 and 316 copies in a biplexed reaction containing 10⁵ copies of co-amplifying internal amplification control DNA, and reduced time-to-result from 45 min of amplification to 15-20 min for the positive samples.

Thermo-sensitive mitochondrial trifunctional protein deficiency presenting with episodic myopathy

Mitochondrial trifunctional protein (MTP) is involved in long-chain fatty acid β-oxidation (lcFAO). Deficiency of one or more of the enzyme activities as catalyzed by MTP causes generalized MTP deficiency (MTPD), long-chain hydroxyacyl-CoA dehydrogenase deficiency (LCHADD), or long-chain ketoacyl-CoA thiolase deficiency (LCKATD). When genetic variants result in thermo-sensitive enzymes, increased body temperature (e.g. fever) can reduce enzyme activity and be a risk factor for clinical decompensation. This is the first description of five patients with a thermo-sensitive MTP deficiency. Clinical and genetic information was obtained from clinical files. Measurement of LCHAD and LCKAT activities, lcFAO-flux studies and palmitate loading tests were performed in skin fibroblasts cultured at 37°C and 40°C. In all patients (four MTPD, one LCKATD), disease manifested during childhood (manifestation age: 2-10 years) with myopathic symptoms triggered by fever or exercise. In four patients, signs of retinopathy or neuropathy were present. Plasma long-chain acylcarnitines were normal or slightly increased. HADHB variants were identified (at age: 6-18 years) by whole exome sequencing or gene panel analyses. At 37°C, LCHAD and LCKAT activities were mildly impaired and lcFAO-fluxes were normal. Remarkably, enzyme activities and lcFAO-fluxes were markedly diminished at 40°C. Preventive (dietary) measures improved symptoms for most. In conclusion, all patients with thermo-sensitive MTP deficiency had a long diagnostic trajectory and both genetic and enzymatic testing were required for diagnosis. The frequent absence of characteristic acylcarnitine abnormalities poses a risk for a diagnostic delay. Given the positive treatment effects, upfront genetic screening may be beneficial to enhance early recognition.

One Size Does Not Fit All: The Past, Present and Future of Cystic Fibrosis Causal Therapies

Cystic fibrosis (CF) is the most common monogenic disorder, caused by mutations in the CF transmembrane conductance regulator (CFTR) gene. Over the last 30 years, tremendous progress has been made in understanding the molecular basis of CF and the development of treatments that target the underlying defects in CF. Currently, a highly effective CFTR modulator treatment (Kalydeco™/Trikafta™) is available for 90% of people with CF. In this review, we will give an extensive overview of past and ongoing efforts in the development of therapies targeting the molecular defects in CF. We will discuss strategies targeting the CFTR protein (i.e., CFTR modulators such as correctors and potentiators), its cellular environment (i.e., proteostasis modulation, stabilization at the plasma membrane), the CFTR mRNA (i.e., amplifiers, nonsense mediated mRNA decay suppressors, translational readthrough inducing drugs) or the CFTR gene (gene therapies). Finally, we will focus on how these efforts can be applied to the 15% of people with CF for whom no causal therapy is available yet.

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.

Rescue of Mutant CFTR Trafficking Defect by the Investigational Compound MCG1516A

Although some therapeutic progress has been achieved in developing small molecules that correct F508del-CFTR defects, the mechanism of action (MoA) of these compounds remain poorly elucidated. Here, we investigated the effects and MoA of MCG1516A, a newly developed F508del-CFTR corrector. MCG1516A effects on wild-type (WT) and F508del-CFTR were assessed by immunofluorescence microscopy, and biochemical and functional assays both in cell lines and in intestinal organoids. To shed light on the MoA of MCG1516A, we evaluated its additivity to the FDA-approved corrector VX-661, low temperature, genetic revertants of F508del-CFTR (G550E, R1070W, and 4RK), and the traffic-null variant DD/AA. Finally, we explored the ability of MCG1516A to rescue trafficking and function of other CF-causing mutations. We found that MCG1516A rescues F508del-CFTR with additive effects to VX-661. A similar behavior was observed for WT-CFTR. Under low temperature incubation, F508del-CFTR demonstrated an additivity in processing and function with VX-661, but not with MCG1516A. In contrast, both compounds promoted additional effects to low temperature to WT-CFTR. MCG1516A demonstrated additivity to genetic revertant R1070W, while VX-661 was additive to G550E and 4RK. Nevertheless, none of these compounds rescued DD/AA trafficking. Both MCG1516A and VX-661 rescued CFTR processing of L206W- and R334W-CFTR with greater effects when these compounds were combined. In summary, the absence of additivity of MCG1516A to genetic revertant G550E suggests a putative binding site for this compound on NBD1:NBD2 interface. Therefore, a combination of MCG1516A with compounds able to rescue DD/AA traffic, or mimicking the actions of revertant R1070W (e.g., VX-661), could enhance correction of F508del-CFTR defects.

Familial Temperature-Sensitive Auditory Neuropathy: Distinctive Clinical Courses Caused by Variants of the OTOF Gene

Objective: To investigate the clinical course and genetic etiology of familial temperature-sensitive auditory neuropathy (TSAN), which is a very rare subtype of auditory neuropathy (AN) that involves an elevation of hearing thresholds due to an increase in the core body temperature, and to evaluate the genotype-phenotype correlations in a family with TSAN. Methods: Six members of a non-consanguineous Chinese family, including four siblings complaining of communication difficulties when febrile, were enrolled in this study. The clinical and audiological profiles of the four siblings were fully evaluated during both febrile and afebrile episodes, and the genetic etiology of hearing loss (HL) was explored using next-generation sequencing (NGS) technology. Their parents, who had no complaints of fluctuating HL due to body temperature variation, were enrolled for the genetics portion only. Results: Audiological tests during the patients' febrile episodes met the classical diagnostic criteria for AN, including mild HL, poor speech discrimination, preserved cochlear microphonics (CMs), and absent auditory brainstem responses (ABRs). Importantly, unlike the pattern observed in previously reported cases of TSAN, the ABRs and electrocochleography (ECochG) signals of our patients improved to normal during afebrile periods. Genetic analysis identified a compound heterozygous variant of the OTOF gene (which encodes the otoferlin protein), including one previously reported pathogenic variant, c.5098G > C (p.Glu1700Gln), and one novel variant, c.4882C > A (p.Pro1628Thr). Neither of the identified variants affected the C2 domains related to the main function of otoferlin. Both variants faithfully cosegregated with TSAN within the pedigree, suggesting that OTOF is the causative gene of the autosomal recessive trait segregation in this family. Conclusion: The presence of CMs with absent (or markedly abnormal) ABRs is a reliable criterion for diagnosing AN. The severity of the phenotype caused by dysfunctional neurotransmitter release in TSAN may reflect variants that alter the C2 domains of otoferlin. The observations from this study enrich the current understanding of the phenotype and genotype of TSAN and may lay a foundation for further research on its pathogenesis.

A topological switch in CFTR modulates channel activity and sensitivity to unfolding

The cystic fibrosis transmembrane conductance regulator (CFTR) anion channel is essential to maintain fluid homeostasis in key organs. Functional impairment of CFTR due to mutations in the cftr gene leads to cystic fibrosis. Here, we show that the first nucleotide-binding domain (NBD1) of CFTR can spontaneously adopt an alternate conformation that departs from the canonical NBD fold previously observed. Crystallography reveals that this conformation involves a topological reorganization of NBD1. Single-molecule fluorescence resonance energy transfer microscopy shows that the equilibrium between the conformations is regulated by adenosine triphosphate binding. However, under destabilizing conditions, such as the disease-causing mutation F508del, this conformational flexibility enables unfolding of the β-subdomain. Our data indicate that, in wild-type CFTR, this conformational transition of NBD1 regulates channel function, but, in the presence of the F508del mutation, it allows domain misfolding and subsequent protein degradation. Our work provides a framework to design conformation-specific therapeutics to prevent noxious transitions.

Pharmacological Modulation of Ion Channels for the Treatment of Cystic Fibrosis

Cystic fibrosis (CF) is a life-shortening monogenic disease caused by mutations in the gene encoding the CF transmembrane conductance regulator (CFTR) protein, an anion channel that transports chloride and bicarbonate across epithelia. Despite clinical progress in delaying disease progression with symptomatic therapies, these individuals still develop various chronic complications in lungs and other organs, which significantly restricts their life expectancy and quality of life. The development of high-throughput assays to screen drug-like compound libraries have enabled the discovery of highly effective CFTR modulator therapies. These novel therapies target the primary defect underlying CF and are now approved for clinical use for individuals with specific CF genotypes. However, the clinically approved modulators only partially reverse CFTR dysfunction and there is still a considerable number of individuals with CF carrying rare CFTR mutations who remain without any effective CFTR modulator therapy. Accordingly, additional efforts have been pursued to identify novel and more potent CFTR modulators that may benefit a larger CF population. The use of ex vivo individual-derived specimens has also become a powerful tool to evaluate novel drugs and predict their effectiveness in a personalized medicine approach. In addition to CFTR modulators, pro-drugs aiming at modulating alternative ion channels/transporters are under development to compensate for the lack of CFTR function. These therapies may restore normal mucociliary clearance through a mutation-agnostic approach (ie, independent of CFTR mutation) and include inhibitors of the epithelial sodium channel (ENaC), modulators of the calcium-activated channel transmembrane 16A (TMEM16, or anoctamin 1) or of the solute carrier family 26A member 9 (SLC26A9), and anionophores. The present review focuses on recent progress and challenges for the development of ion channel/transporter-modulating drugs for the treatment of CF.

Therapeutic Uses of Bacterial Subunit Toxins

The B subunit pentamer verotoxin (VT aka Shiga toxin-Stx) binding to its cellular glycosphingolipid (GSL) receptor, globotriaosyl ceramide (Gb₃) mediates internalization and the subsequent receptor mediated retrograde intracellular traffic of the AB5 subunit holotoxin to the endoplasmic reticulum. Subunit separation and cytosolic A subunit transit via the ER retrotranslocon as a misfolded protein mimic, then inhibits protein synthesis to kill cells, which can cause hemolytic uremic syndrome clinically. This represents one of the most studied systems of prokaryotic hijacking of eukaryotic biology. Similarly, the interaction of cholera AB5 toxin with its GSL receptor, GM1 ganglioside, is the key component of the gastrointestinal pathogenesis of cholera and follows the same retrograde transport pathway for A subunit cytosol access. Although both VT and CT are the cause of major pathology worldwide, the toxin–receptor interaction is itself being manipulated to generate new approaches to control, rather than cause, disease. This arena comprises two areas: anti neoplasia, and protein misfolding diseases. CT/CTB subunit immunomodulatory function and anti-cancer toxin immunoconjugates will not be considered here. In the verotoxin case, it is clear that Gb₃ (and VT targeting) is upregulated in many human cancers and that there is a relationship between GSL expression and cancer drug resistance. While both verotoxin and cholera toxin similarly hijack the intracellular ERAD quality control system of nascent protein folding, the more widespread cell expression of GM1 makes cholera the toxin of choice as the means to more widely utilise ERAD targeting to ameliorate genetic diseases of protein misfolding. Gb₃ is primarily expressed in human renal tissue. Glomerular endothelial cells are the primary VT target but Gb₃ is expressed in other endothelial beds, notably brain endothelial cells which can mediate the encephalopathy primarily associated with VT2-producing E. coli infection. The Gb₃ levels can be regulated by cytokines released during EHEC infection, which complicate pathogenesis. Significantly Gb₃ is upregulated in the neovasculature of many tumours, irrespective of tumour Gb₃ status. Gb₃ is markedly increased in pancreatic, ovarian, breast, testicular, renal, astrocytic, gastric, colorectal, cervical, sarcoma and meningeal cancer relative to the normal tissue. VT has been shown to be effective in mouse xenograft models of renal, astrocytoma, ovarian, colorectal, meningioma, and breast cancer. These studies are herein reviewed. Both CT and VT (and several other bacterial toxins) access the cell cytosol via cell surface ->ER transport. Once in the ER they interface with the protein folding homeostatic quality control pathway of the cell -ERAD, (ER associated degradation), which ensures that only correctly folded nascent proteins are allowed to progress to their cellular destinations. Misfolded proteins are translocated through the ER membrane and degraded by cytosolic proteosome. VT and CT A subunits have a C terminal misfolded protein mimic sequence to hijack this transporter to enter the cytosol. This interface between exogenous toxin and genetically encoded endogenous mutant misfolded proteins, provides a new therapeutic basis for the treatment of such genetic diseases, e.g., Cystic fibrosis, Gaucher disease, Krabbe disease, Fabry disease, Tay-Sachs disease and many more. Studies showing the efficacy of this approach in animal models of such diseases are presented.

Partial Rescue of F508del-CFTR Stability and Trafficking Defects by Double Corrector Treatment

Deletion of phenylalanine at position 508 (F508del) in the CFTR chloride channel is the most frequent mutation in cystic fibrosis (CF) patients. F508del impairs the stability and folding of the CFTR protein, thus resulting in mistrafficking and premature degradation. F508del-CFTR defects can be overcome with small molecules termed correctors. We investigated the efficacy and properties of VX-445, a newly developed corrector, which is one of the three active principles present in a drug (Trikafta^®/Kaftrio^®) recently approved for the treatment of CF patients with F508del mutation. We found that VX-445, particularly in combination with type I (VX-809, VX-661) and type II (corr-4a) correctors, elicits a large rescue of F508del-CFTR function. In particular, in primary bronchial epithelial cells of CF patients, the maximal rescue obtained with corrector combinations including VX-445 was close to 60–70% of CFTR function in non-CF cells. Despite this high efficacy, analysis of ubiquitylation, resistance to thermoaggregation, protein half-life, and subcellular localization revealed that corrector combinations did not fully normalize F508del-CFTR behavior. Our study indicates that it is still possible to further improve mutant CFTR rescue with the development of corrector combinations having maximal effects on mutant CFTR structural and functional properties.

The CFTR P67L variant reveals a key role for N-terminal lasso helices in channel folding, maturation, and pharmacologic rescue

Patients with cystic fibrosis (CF) harboring the P67L variant in the cystic fibrosis transmembrane conductance regulator (CFTR) often exhibit a typical CF phenotype, including severe respiratory compromise. This rare mutation (reported in <300 patients worldwide) responds robustly to CFTR correctors, such as lumacaftor and tezacaftor, with rescue in model systems that far exceed what can be achieved for the archetypical CFTR mutant F508del. However, the specific molecular consequences of the P67L mutation are poorly characterized. In this study, we conducted biochemical measurements following low-temperature growth and/or intragenic suppression, which suggest a mechanism underlying P67L that (1) shares key pathogenic features with F508del, including off-pathway (non-native) folding intermediates, (2) is linked to folding stability of nucleotide-binding domains 1 and 2, and (3) demonstrates pharmacologic rescue that requires domains in the carboxyl half of the protein. We also investigated the "lasso" helices 1 and 2, which occur immediately upstream of P67. Based on limited proteolysis, pulse chase, and molecular dynamics analysis of full-length CFTR and a series of deletion constructs, we argue that P67L and other maturational processing (class 2) defects impair the integrity of the lasso motif and confer misfolding of downstream domains. Thus, amino-terminal missense variants elicit a conformational change throughout CFTR that abrogates maturation while providing a robust substrate for pharmacologic repair.

How to determine the mechanism of action of CFTR modulator compounds: A gateway to theranostics

The greatest challenge of 21st century biology is to fully understand mechanisms of disease to drive new approaches and medical innovation. Parallel to this is the huge biomedical endeavour of treating people through personalized medicine. Until now all CFTR modulator drugs that have entered clinical trials have been genotype-dependent. An emerging alternative is personalized/precision medicine in CF, i.e., to determine whether rare CFTR mutations respond to existing (or novel) CFTR modulator drugs by pre-assessing them directly on patient's tissues ex vivo, an approach also now termed theranostics. To administer the right drug to the right person it is essential to understand how drugs work, i.e., to know their mechanism of action (MoA), so as to predict their applicability, not just in certain mutations but also possibly in other diseases that share the same defect/defective pathway. Moreover, an understanding the MoA of a drug before it is tested in clinical trials is the logical path to drug discovery and can increase its chance for success and hence also approval. In conclusion, the most powerful approach to determine the MoA of a compound is to understand the underlying biology. Novel large datasets of intervenients in most biological processes, namely those emerging from the post-genomic era tools, are available and should be used to help in this task.

Ubiquitination of disease-causing CFTR variants in a microsome-based assay

Soluble secreted proteins and membrane proteins are subjected to protein quality control pathways during their synthesis in the endoplasmic reticulum (ER) and delivery to other destinations. Foremost among these quality control pathways is the selection of misfolded proteins for ER-associated degradation (ERAD). A growing number of diseases, including Cystic Fibrosis, are linked to the ERAD pathway. In most cases, a membrane protein known as the Cystic Fibrosis Transmembrane Conductance Regulator, or CFTR, is prematurely degraded by ERAD. Cell-based assays and in vitro studies have elucidated factors required for the recognition and degradation of CFTR, yet mechanistic details on how these factors target specific disease-causing variants is limited. Given the possibility that variants might exhibit unique susceptibilities to ubiquitin modification, which is required for proteasome-mediated degradation, we devised an assay that recapitulates this event. Here, we demonstrate that ER-enriched membranes from transfected human cells support CFTR ubiquitination when combined with radiolabeled ubiquitin and isolated enzymes in the ubiquitination cascade. We also show that select disease-causing variants are ubiquitinated more extensively than wild-type channels and to varying degrees. Our system provides a platform to examine how other purified factors impact CFTR ubiquitination and the ubiquitination of additional disease-associated membrane proteins.

Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) Ubiquitylation as a Novel Pharmaceutical Target for Cystic Fibrosis

Mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene decrease the structural stability and function of the CFTR protein, resulting in cystic fibrosis. Recently, the effect of CFTR-targeting combination therapy has dramatically increased, and it is expected that add-on drugs that modulate the CFTR surrounding environment will further enhance their effectiveness. Various interacting proteins have been implicated in the structural stability of CFTR and, among them, molecules involved in CFTR ubiquitylation are promising therapeutic targets as regulators of CFTR degradation. This review focuses on the ubiquitylation mechanism that contributes to the stability of mutant CFTR at the endoplasmic reticulum (ER) and post-ER compartments and discusses the possibility as a pharmacological target for cystic fibrosis (CF).

The preclinical discovery and development of the combination of ivacaftor + tezacaftor used to treat cystic fibrosis

INTRODUCTION

Cystic Fibrosis (CF) is caused by mutations in the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) gene. The most common mutation, F508del, induces protein misprocessing and loss of CFTR function. The discovery through in vitro studies of the CFTR correctors (i.e. lumacaftor, tezacaftor) that partially rescue the misprocessing of F508del-CFTR with the potentiator ivacaftor is promising in giving an unprecedented clinical benefit in affected patients.

AREAS COVERED

Online databases were searched using key phrases for CF and CFTR modulators. Tezacaftor-ivacaftor treatment has proved to be safer than lumacaftor-ivacaftor, although clinical efficacy is similar. Further clinical efficacy has ensued with the introduction of triple therapy, i.e. applying second-generation correctors, such as VX-569 and VX-445 (elexacaftor) to tezacaftor-ivacaftor. The triple combinations will herald the availability of etiologic therapies for patients for whom no CFTR modulators are currently applied (i.e. F508del/minimal function mutations) and enhance CFTR modulator therapy for patients homozygous for F508del.

EXPERT OPINION

CF patient-derived tissue models are being explored to determine donor-specific response to current approved and future novel CFTR modulators for F508del and other rare mutations. The discovery and validation of biomarkers of CFTR modulation will complement these studies in the long term and in real-life world.

Discovery of a picomolar potency pharmacological corrector of the mutant CFTR chloride channel

F508del, the most frequent mutation causing cystic fibrosis (CF), results in mistrafficking and premature degradation of the CFTR chloride channel. Small molecules named correctors may rescue F508del-CFTR and therefore represent promising drugs to target the basic defect in CF. We screened a carefully designed chemical library to find F508del-CFTR correctors. The initial active compound resulting from the primary screening underwent extensive chemical optimization. The final compound, ARN23765, showed an extremely high potency in bronchial epithelial cells from F508del homozygous patients, with an EC₅₀ of 38 picomolar, which is more than 5000-fold lower compared to presently available corrector drugs. ARN23765 also showed high efficacy, synergy with other types of correctors, and compatibility with chronic VX-770 potentiator. Besides being a promising drug, particularly suited for drug combinations, ARN23765 represents a high-affinity probe for CFTR structure-function studies.

Regulation of CFTR Biogenesis by the Proteostatic Network and Pharmacological Modulators

Cystic fibrosis (CF) is the most common lethal inherited disease among Caucasians in North America and a significant portion of Europe. The disease arises from one of many mutations in the gene encoding the cystic fibrosis transmembrane conductance regulator, or CFTR. The most common disease-associated allele, F508del, along with several other mutations affect the folding, transport, and stability of CFTR as it transits from the endoplasmic reticulum (ER) to the plasma membrane, where it functions primarily as a chloride channel. Early data demonstrated that F508del CFTR is selected for ER associated degradation (ERAD), a pathway in which misfolded proteins are recognized by ER-associated molecular chaperones, ubiquitinated, and delivered to the proteasome for degradation. Later studies showed that F508del CFTR that is rescued from ERAD and folds can alternatively be selected for enhanced endocytosis and lysosomal degradation. A number of other disease-causing mutations in CFTR also undergo these events. Fortunately, pharmacological modulators of CFTR biogenesis can repair CFTR, permitting its folding, escape from ERAD, and function at the cell surface. In this article, we review the many cellular checkpoints that monitor CFTR biogenesis, discuss the emergence of effective treatments for CF, and highlight future areas of research on the proteostatic control of CFTR.

Activity of lumacaftor is not conserved in zebrafish Cftr bearing the major cystic fibrosis-causing mutation

F508del-cystic fibrosis transmembrane conductance regulator (CFTR) is the major mutant responsible for cystic fibrosis (CF). ORKAMBI®, approved for patients bearing this mutant, contains lumacaftor (VX-809) that partially corrects F508del-CFTR's processing defect and ivacaftor (VX-770) that potentiates its defective channel activity. Unfortunately, the clinical efficacy of ORKAMBI® is modest, highlighting the need to understand how the small molecules work so that superior compounds can be developed. Because, human CFTR (hCFTR) and zebrafish Cftr (zCftr) are structurally conserved as determined in recent cryo-EM structural models, we hypothesized that the consequences of the major mutation and small molecule modulators would be similar for the two species of protein. As expected, like the F508del mutation in hCFTR, the homologous mutation in zCftr (F507del) is misprocessed, yet not as severely as the human mutant and this defect was restored by low-temperature (27°C) culture conditions. After rescue to the cell surface, F507del-zCftr exhibited regulated channel activity that was potentiated by ivacaftor. Surprisingly, lumacaftor failed to rescue misprocessing of the F507del-zCftr at either 37 or 27°C suggesting that future comparative studies with F508del-hCFTR would provide insight into its structure: function relationships. Interestingly, the robust rescue of F508del-zCftr at 27°C and availability of methods for in vivo screening in zebrafish present the opportunity to define the cellular pathways underlying rescue.

Short-term consequences of F508del-CFTR thermal instability on CFTR-dependent transepithelial currents in human airway epithelial cells

Loss-of-function mutations in the Cystic Fibrosis Transmembrane conductance Regulator (CFTR) channel in human airway epithelial cells are responsible for Cystic Fibrosis. A deleterious impact of physiological temperature on CFTR plasma membrane expression, residence and channel activity is characteristic of the most common and severe CF mutation, F508del. Using primary human F508del-airway epithelial cells and CF bronchial epithelial CFBE41o- cell lines expressing F508del- or WT-CFTR, we examined the effects of temperature (29 °C-39 °C) on the amplitude and stability of short-circuit CFTR-dependent currents over time and the efficiency of pharmacological strategies to stably restore F508del-CFTR function. We show that F508del-CFTR functional instability at 37 °C is not prevented by low temperature or VX-809 correction, genistein and VX-770 potentiators, nor by the combination VX-809/VX-770. Moreover, F508del-CFTR-dependent currents 30 minutes after CFTR activation at 37 °C did not significantly differ whether a potentiator was used or not. We demonstrate that F508del-CFTR function loss is aggravated at temperatures above 37 °C while limited by a small decrease of temperature and show that the more F508del-CFTR is stimulated, the faster the current loss happens. Our study highlights the existence of a temperature-dependent process inhibiting the function of F508del-CFTR, possibly explaining the low efficacy of pharmacological drugs in clinic.

Innovative Therapies for Cystic Fibrosis: The Road from Treatment to Cure

Cystic fibrosis (CF), a life-threatening multiorgan genetic disease, is facing a new era of research and development using innovative gene-directed personalized therapies. The priority organ to cure is the lung, which suffers recurrent and chronic bacterial infection and inflammation since infancy, representing the main cause of morbidity and precocious mortality of these individuals. After the disappointing failure of gene-replacement approaches using gene therapy vectors, no single drug is presently available to repair all the CF gene defects. The impressive number of different CF gene mutations is now tackled with different chemical and biotechnological tools tailored to the specific molecular derangements, thanks to the extensive knowledge acquired over many years on the mechanisms of CF cell and organ pathology. This review provides an overview and recalls both the successes and limitations of the different experimental approaches, such as high-throughput screening on chemical libraries to discover CF gene correctors and potentiators, dual-acting compounds, read-through molecules, splicing defect repairing tools, cystic fibrosis transmembrane conductance regulator (CFTR) "amplifiers," CFTR interactome modulators and the first gene editing attempts.

F508del disturbs the dynamics of the nucleotide binding domains of CFTR before and after ATP hydrolysis

The cystic fibrosis transmembrane conductance regulator (CFTR) channel is an ion channel responsible for chloride transport in epithelia and it belongs to the class of ABC transporters. The deletion of phenylalanine 508 (F508del) in CFTR is the most common mutation responsible for cystic fibrosis. Little is known about the effect of the mutation in the isolated nucleotide binding domains (NBDs), on dimer dynamics, ATP hydrolysis and even on nucleotide binding. Using molecular dynamics simulations of the human CFTR NBD dimer, we showed that F508del increases, in the prehydrolysis state, the inter-motif distance in both ATP binding sites (ABP) when ATP is bound. Additionally, a decrease in the number of catalytically competent conformations was observed in the presence of F508del. We used the subtraction technique to study the first 300 ps after ATP hydrolysis in the catalytic competent site and found that the F508del dimer evidences lower conformational changes than the wild type. Using longer simulation times, the magnitude of the conformational changes in both forms increases. Nonetheless, the F508del dimer shows lower C-α RMS values in comparison to the wild-type, on the F508del loop, on the residues surrounding the catalytic site and the portion of NBD2 adjacent to ABP1. These results provide evidence that F508del interferes with the NBD dynamics before and after ATP hydrolysis. These findings shed a new light on the effect of F508del on NBD dynamics and reveal a novel mechanism for the influence of F508del on CFTR.

Pharmacological analysis of CFTR variants of cystic fibrosis using stem cell-derived organoids

Cystic fibrosis (CF) is a life-shortening genetic disease caused by mutations of CFTR, the gene encoding cystic fibrosis transmembrane conductance regulator. Despite considerable progress in CF therapies, targeting specific CFTR genotypes based on small molecules has been hindered because of the substantial genetic heterogeneity of CFTR mutations in patients with CF, which is difficult to assess by animal models in vivo. There are broadly four classes (e.g., II, III, and IV) of CF genotypes that differentially respond to current CF drugs (e.g., VX-770 and VX-809). In this review, we shed light on the pharmacogenomics of diverse CFTR mutations and the emerging role of stem cell-based organoids in predicting the CF drug response. We discuss mechanisms that underlie differential CF drug responses both in organoid-based assays and in CF clinical trials, thereby facilitating the precision design of safer and more effective therapies for individual patients with CF.

Tezacaftor/Ivacaftor in Subjects with Cystic Fibrosis and F508del/F508del-CFTR or F508del/G551D-CFTR

RATIONALE

Tezacaftor (formerly VX-661) is an investigational small molecule that improves processing and trafficking of the cystic fibrosis transmembrane conductance regulator (CFTR) in vitro, and improves CFTR function alone and in combination with ivacaftor.

OBJECTIVES

To evaluate the safety and efficacy of tezacaftor monotherapy and of tezacaftor/ivacaftor combination therapy in subjects with cystic fibrosis homozygous for F508del or compound heterozygous for F508del and G551D.

METHODS

This was a randomized, placebo-controlled, double-blind, multicenter, phase 2 study (NCT01531673). Subjects homozygous for F508del received tezacaftor (10 to 150 mg) every day alone or in combination with ivacaftor (150 mg every 12 h) in a dose escalation phase, as well as in a dosage regimen testing phase. Subjects compound heterozygous for F508del and G551D, taking physician-prescribed ivacaftor, received tezacaftor (100 mg every day).

MEASUREMENTS AND MAIN RESULTS

Primary endpoints were safety through Day 56 and change in sweat chloride from baseline through Day 28. Secondary endpoints included change in percent predicted FEV₁ (ppFEV₁) from baseline through Day 28 and pharmacokinetics. The incidence of adverse events was similar across treatment arms. Tezacaftor (100 mg every day)/ivacaftor (150 mg every 12 h) resulted in a 6.04 mmol/L decrease in sweat chloride and 3.75 percentage point increase in ppFEV₁ in subjects homozygous for F508del, and a 7.02 mmol/L decrease in sweat chloride and 4.60 percentage point increase in ppFEV₁ in subjects compound heterozygous for F508del and G551D from baseline through Day 28 (P < 0.05 for all).

CONCLUSIONS

These results support continued clinical development of tezacaftor (100 mg every day) in combination with ivacaftor (150 mg every 12 h) in subjects with cystic fibrosis. Clinical trial registered with www.clinicaltrials.gov (NCT01531673).

Folding-function relationship of the most common cystic fibrosis-causing CFTR conductance mutants

The tight correlation between folding and function in cystic fibrosis patients with CFTR mutations of the altered-conductance CFTR class provides an attractive paradigm for characterizing mode of action of novel therapeutics.

Cystic fibrosis is caused by mutations in the CFTR gene, which are subdivided into six classes. Mutants of classes III and IV reach the cell surface but have limited function. Most class-III and class-IV mutants respond well to the recently approved potentiator VX-770, which opens the channel. We here revisited function and folding of some class-IV mutants and discovered that R347P is the only one that leads to major defects in folding. By this criterion and by its functional response to corrector drug VX-809, R347P qualifies also as a class-II mutation. Other class-IV mutants folded like wild-type CFTR and responded similarly to VX-809, demonstrating how function and folding are connected. Studies on both types of defects complement each other in understanding how compounds improve mutant CFTR function. This provides an attractive unbiased approach for characterizing mode of action of novel therapeutic compounds and helps address which drugs are efficacious for each cystic fibrosis disease variant.

The Autophagy Inhibitor Spautin-1 Antagonizes Rescue of Mutant CFTR Through an Autophagy-Independent and USP13-Mediated Mechanism

The mutation F508del, responsible for a majority of cystic fibrosis cases, provokes the instability and misfolding of the CFTR chloride channel. Pharmacological recovery of F508del-CFTR may be obtained with small molecules called correctors. However, treatment with a single corrector in vivo and in vitro only leads to a partial rescue, a consequence of cell quality control systems that still detect F508del-CFTR as a defective protein causing its degradation. We tested the effect of spautin-1 on F508del-CFTR since it is an inhibitor of USP10 deubiquitinase and of autophagy, a target and a biological process that have been associated with cystic fibrosis and mutant CFTR. We found that short-term treatment of cells with spautin-1 downregulates the function and expression of F508del-CFTR despite the presence of corrector VX-809, a finding obtained in multiple cell models and assays. In contrast, spautin-1 was ineffective on wild type CFTR. Silencing and upregulation of USP13 (another target of spautin-1) but not of USP10, had opposite effects on F508del-CFTR expression/function. In contrast, modulation of autophagy with known activators or inhibitors did not affect F508del-CFTR. Our results identify spautin-1 as a novel chemical probe to investigate the molecular mechanisms that prevent full rescue of mutant CFTR.

Structural mechanisms for defective CFTR gating caused by the Q1412X mutation, a severe Class VI pathogenic mutation in cystic fibrosis

KEY POINTS

Electrophysiological characterization of Q1412X-CFTR, a C-terminal truncation mutation of cystic fibrosis transmembrane conductance regulator (CFTR) associated with the severe form of cystic fibrosis (CF), reveals a gating defect that has not been reported previously. Mechanistic investigations of the gating deficit in Q1412X-CFTR suggest that the reduced open probability in Q1412X-CFTR is the result of a disruption of the function of the second ATP binding site (or site 2) in the nucleotide binding domains (NBDs). Detailed comparisons of several mutations with different degrees of truncation in the C-terminal region of NBD2 reveal the importance of the last two beta-strands in NBD2 for maintaining proper gating functions. The results of the present study also show that the application of clinically-approved drugs (VX-770 and VX-809) can greatly enhance the function of Q1412X, providing in vitro evidence for a therapeutic strategy employing both reagents for patients bearing Q1412X or similar truncation mutations.

ABSTRACT

Cystic fibrosis (CF) is caused by loss-of-function mutations of cystic fibrosis transmembrane conductance regulator (CFTR), a phosphorylation-activated but ATP-gated chloride channel. Based on the molecular mechanism of CF pathogenesis, disease-associated mutations are categorized into six classes. Among them, Class VI, whose members include some of the C-terminal truncation mutations such as Q1412X, is defined as decreased membrane expression because of a faster turnover rate. In the present study, we characterized the functional properties of Q1412X-CFTR, a severe-form premature stop codon mutation. We confirmed previous findings of a ∼90% decrease in membrane expression but found a ∼95% reduction in the open probability (P_o ). Detailed kinetic studies support the idea that the gating defect is the result of a dysfunctional ATP-binding site 2 in the nucleotide binding domains (NBDs). Because the Q1412X mutation results in a deletion of the last two beta-strands in NBD2 and the whole C-terminal region, we further characterized truncation mutations with different degrees of deletion in this segment. Mutations that completely or partially remove the C-terminus of CFTR at the same time as keeping an intact NBD2 (i.e. D1425X and S1455X) assume gating function almost identical to that of wild-type channels. However, the deletion of the last beta-strand in the NBD2 (i.e. N1419X) causes gating dysfunction that is milder than that of Q1412X. Thus, normal CFTR gating requires structural integrity of NBD2. Moreover, our observation that clinically-approved VX-809 (Lumacaftor, Vertex Pharmaceuticals, Boston, MA, USA) and VX-770 (Ivacaftor, Vertex Pharmaceuticals, Boston, MA, USA) significantly enhance the overall function of Q1412X-CFTR provides the conceptual basis for the treatment of patients carrying this mutation.

Thermal-Enhanced bri1-301 Instability Reveals a Plasma Membrane Protein Quality Control System in Plants

Brassinosteroids (BRs) are essential phytohormones mainly perceived by a single-pass transmembrane receptor-like protein kinase (RLK), BRASSINOSTEROID INSENSITIVE 1 (BRI1). bri1-5 and bri1-9, two distinct mutants with point mutations in the extracellular domain of BRI1, show weak defective phenotypes. Previous studies indicated that bri1-5 and bri1-9 mutated proteins can be recognized and eliminated via an endoplasmic reticulum quality control (ERQC) mechanism. Most of these two proteins, therefore, cannot reach their destination, plasma membrane. Here, we report our functional characterization of bri1-301, another BRI1 mutant protein with an amino acid substitution in the cytoplasmic kinase domain. bri1-301 is a partially functional BR receptor with significantly decreased protein abundance. Interestingly, protein stability and subcellular localization of bri1-301 are temperature-sensitive. At 22°C, an optimal temperature for indoor Arabidopsis growth, bri1-301 shows a weak defective phenotype. At a lower temperature condition such as 18°C, bri1-301 exhibits subtle morphological defects. At a higher temperature condition such as 28°C, on the other hand, bri1-301 displays an extremely severe phenotype reminiscent to that of a null bri1 mutant due to greatly increased bri1-301 internalization and degradation. Our detailed analyses suggest that bri1-301 stability is controlled by ERQC and plasma membrane quality control (PMQC) systems. Since PMQC has not been well studied in plants, bri1-301 can be used as a model mutant for future genetic dissection of this critical process.

Molecular basis of cystic fibrosis: from bench to bedside

Cystic fibrosis (CF), is an autosomal recessive disease affecting different organs. The lung disease, characterized by recurrent and chronic bacterial infection and inflammation since infancy, is the main cause of morbidity and precocious mortality of these individuals. The innovative therapies directed to repair the defective CF gene should account for the presence of more than 200 disease-causing mutations of the CF transmembrane conductance regulator (CFTR) gene. The review will recall the different experimental approaches in discovering CFTR protein targeted molecules, such as the high throughput screening on chemical libraries to discover correctors and potentiators of CFTR protein, dual-acting compounds, read-through molecules, splicing defects repairing tools, CFTR "amplifiers".

Correcting CFTR folding defects by small-molecule correctors to cure cystic fibrosis

Pharmacological intervention to treat the lethal genetic disease cystic fibrosis has become reality, even for the severe, most common folding mutant F508del CFTR. CFTR defects range from absence of the protein, misfolding that leads to degradation rather than cell-surface localization (such as F508del), to functional chloride-channel defects on the cell surface. Corrector and potentiator drugs improve cell-surface location and channel activity, respectively, and combination therapy of two correctors and a potentiator have shown synergy. Several combinations are in the drug-development pipeline and although the primary defect is not repaired, rescue levels are reaching those resembling a cure for CF. Combination therapy with correctors may also improve functional CFTR mutants and benefit patients on potentiator therapy.

Substitution of Yor1p NBD1 residues improves the thermal stability of Human Cystic Fibrosis Transmembrane Conductance Regulator

The Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) is a plasma membrane chloride channel protein that regulates vertebrate fluid homeostasis. The inefficiency of wild type human CFTR protein folding/trafficking is exacerbated by genetic mutations that can cause protein misfolding in the endoplasmic reticulum (ER) and subsequent degradation. This project investigates small changes in protein sequence that can alter the thermal stability of the large multi-domain CFTR protein. We target a conserved 70-residue α-subdomain located in the first nucleotide-binding domain that hosts the common misfolding mutation ∆F508. To investigate substitutions that can stabilize this domain, we constructed chimeras between human CFTR and its closest yeast homolog Yor1p. The α-subdomain of Yor1p was replaced with that of CFTR in Saccharomyces cerevisiae. Cellular localization of green fluorescence protein-tagged Yor1p-CFTR chimeras was analyzed by fluorescence microscopy and quantitative multispectral imaging flow cytometry, steady-state protein levels were compared by SDS-PAGE and protein function probed by a phenotypic oligomycin resistance assay. The chimeras exhibited ER retention in yeast characteristic of defective protein folding/processing. Substitution of seven CFTR α-subdomain residues that are highly conserved in Yor1p and other transporters but differ in CFTR (S495P/R516K/F533L/A534P/K536G/I539T/R553K) improved Yor1p-CFTR chimera localization to the yeast plasma membrane. When introduced into human CFTR expressed in mammalian cells, the same substitutions improve the purified protein thermal stability. This stabilized human CFTR protein will be directly useful for structural and biophysical studies that have been limited by the thermal sensitivity of wild type CFTR. The insights into critical structural residues within CFTR could facilitate development of effective therapeutics for CF-causing mutations.

Alteration of protein function by a silent polymorphism linked to tRNA abundance

Synonymous single nucleotide polymorphisms (sSNPs) are considered neutral for protein function, as by definition they exchange only codons, not amino acids. We identified an sSNP that modifies the local translation speed of the cystic fibrosis transmembrane conductance regulator (CFTR), leading to detrimental changes to protein stability and function. This sSNP introduces a codon pairing to a low-abundance tRNA that is particularly rare in human bronchial epithelia, but not in other human tissues, suggesting tissue-specific effects of this sSNP. Up-regulation of the tRNA cognate to the mutated codon counteracts the effects of the sSNP and rescues protein conformation and function. Our results highlight the wide-ranging impact of sSNPs, which invert the programmed local speed of mRNA translation and provide direct evidence for the central role of cellular tRNA levels in mediating the actions of sSNPs in a tissue-specific manner.

Synonymous single nucleotide polymorphisms (sSNPs) occur at high frequency in the human genome and are associated with ~50 diseases in humans; the responsible molecular mechanisms remain enigmatic. Here, we investigate the impact of the common sSNP, T2562G, on cystic fibrosis transmembrane conductance regulator (CFTR). Although this sSNP, by itself, does not cause cystic fibrosis (CF), it is prevalent in patients with CFTR-related disorders. T2562G sSNP modifies the local translation speed at the Thr854 codon, leading to changes in CFTR stability and channel function. This sSNP introduces a codon pairing to a low-abundance tRNA, which is particularly rare in human bronchial epithelia, but not in other human tissues, suggesting a tissue-specific effect of this sSNP. Enhancement of the cellular concentration of the tRNA cognate to the mutant ACG codon rescues the stability and conduction defects of T2562G-CFTR. These findings reveal an unanticipated mechanism—inverting the programmed local speed of mRNA translation in a tRNA-dependent manner—for sSNP-associated diseases.

Atomic Structure of the Cystic Fibrosis Transmembrane Conductance Regulator

The cystic fibrosis transmembrane conductance regulator (CFTR) is an anion channel evolved from the ATP-binding cassette (ABC) transporter family. In this study, we determined the structure of zebrafish CFTR in the absence of ATP by electron cryo-microscopy to 3.7 Å resolution. Human and zebrafish CFTR share 55% sequence identity, and 42 of the 46 cystic-fibrosis-causing missense mutational sites are identical. In CFTR, we observe a large anion conduction pathway lined by numerous positively charged residues. A single gate near the extracellular surface closes the channel. The regulatory domain, dephosphorylated, is located in the intracellular opening between the two nucleotide-binding domains (NBDs), preventing NBD dimerization and channel opening. The structure also reveals why many cystic-fibrosis-causing mutations would lead to defects either in folding, ion conduction, or gating and suggests new avenues for therapeutic intervention.

Combination of Correctors Rescues CFTR Transmembrane-Domain Mutants by Mitigating their Interactions with Proteostasis

BACKGROUND/AIMS

Premature degradation of mutated cystic fibrosis transmembrane conductance regulator (CFTR) protein causes cystic fibrosis (CF), the commonest Mendelian disease in Caucasians. Despite recent advances in precision medicines for CF patients, many CFTR mutants have not been characterized and the effects of these new therapeutic approaches are still unclear for those mutants.

METHODS

Cells transfected or stably expressing four CFTR transmembrane-domain mutants (G85E, E92K, L1077P, and M1101K) were used to: 1) characterize the mutants according to their protein expression, thermal sensitivity, and degradation pathways; 2) evaluate the effects of correctors in rescuing them; and 3) explore the effects of correctors on CFTR interactions with proteostasis components.

RESULTS

All four mutants exhibited lower protein expression than did wild type-CFTR, and they were degraded by proteasomes and aggresomes. At low temperature, only cells expressing the mutants L1077P and M1101K exhibited increased CFTR maturation. Co-administration of C4 and C18 showed the greatest effect, restoring functional expression and partial stability of CFTR bearing E92K, L1077P, or M1101K at the cell surface. However, this treatment was inefficient in rectifying the defect of CFTR bearing G85E. Correctors rescued CFTR mutants by reducing their interactions with proteostasis components associated with protein retention in the endoplasmic reticulum and ubiquitination.

CONCLUSION

Co-administration of C4 and C18 rescued CFTR transmembrane-domain mutants by remodeling the CFTR interactome.

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.

F508del genotype in endoscopic sinus surgery: do differences in outcomes exist between genotypic subgroups?

BACKGROUND

The impact of endoscopic sinus surgery (ESS) on pulmonary function in cystic fibrosis (CF) patients with chronic rhinosinusitis remains unclear, as studies have demonstrated conflicting results. To date, no study has looked specifically at the impact of CF genotype on lung function after ESS. In this study, we reviewed changes in pulmonary function test (PFT) results after ESS in F508del homozygotes and heterozygotes.

METHODS

The charts of 25 patients with CF without prior lung transplant, who underwent ESS performed by the same surgeon between the period of July 2005 to July 2015, were retrospectively reviewed. Data including genotype and PFT results were collected. Patients were grouped based on genotype. Pre- and postoperative PFTs were compared.

RESULTS

Some differences in PFT outcomes after ESS could be seen on subgroup analyses. For example, when considered as a whole group, the overall cohort showed a significant improvement from preoperative FEV₁ levels at 6 months after surgery (p = 0.0127). Interestingly, on subgroup analysis, the heterozygous group saw significant improvements from preoperative FEV₁ levels at 6 and 12 months (p = 0.0155 and p = 0.0333, respectively). No significant improvements were seen from "baseline" FEV₁ in either group at any timepoint.

CONCLUSION

Prior studies investigating the impact of ESS on pulmonary function in CF patients have shown conflicting results. To our knowledge, those earlier udies did not separate and compare different genotypes, which may have introduced heterogeneity in their patient populations. Our study suggests that grouping CF patients more strictly according to genotype or disease severity when investigating outcomes may reveal differences among various subgroups.

Two Small Molecules Restore Stability to a Subpopulation of the Cystic Fibrosis Transmembrane Conductance Regulator with the Predominant Disease-causing Mutation

Cystic fibrosis (CF) is caused by mutations that disrupt the plasma membrane expression, stability, and function of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl^- channel. Two small molecules, the CFTR corrector lumacaftor and the potentiator ivacaftor, are now used clinically to treat CF, although some studies suggest that they have counteracting effects on CFTR stability. Here, we investigated the impact of these compounds on the instability of F508del-CFTR, the most common CF mutation. To study individual CFTR Cl^- channels, we performed single-channel recording, whereas to assess entire CFTR populations, we used purified CFTR proteins and macroscopic CFTR Cl^- currents. At 37 °C, low temperature-rescued F508del-CFTR more rapidly lost function in cell-free membrane patches and showed altered channel gating and current flow through open channels. Compared with purified wild-type CFTR, the full-length F508del-CFTR was about 10 °C less thermostable. Lumacaftor partially stabilized purified full-length F508del-CFTR and slightly delayed deactivation of individual F508del-CFTR Cl^- channels. By contrast, ivacaftor further destabilized full-length F508del-CFTR and accelerated channel deactivation. Chronic (prolonged) co-incubation of F508del-CFTR-expressing cells with lumacaftor and ivacaftor deactivated macroscopic F508del-CFTR Cl^- currents. However, at the single-channel level, chronic co-incubation greatly increased F508del-CFTR channel activity and temporal stability in most, but not all, cell-free membrane patches. We conclude that chronic lumacaftor and ivacaftor co-treatment restores stability in a small subpopulation of F508del-CFTR Cl^- channels but that the majority remain destabilized. A fuller understanding of these effects and the characterization of the small F508del-CFTR subpopulation might be crucial for CF therapy development.

SYVN1, NEDD8, and FBXO2 Proteins Regulate ΔF508 Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) Ubiquitin-mediated Proteasomal Degradation

We previously reported that delivery of a microRNA-138 mimic or siRNA against SIN3A to cultured cystic fibrosis (ΔF508/ΔF508) airway epithelia partially restored ΔF508-cystic fibrosis transmembrane conductance regulator (CFTR)-mediated cAMP-stimulated Cl^- conductance. We hypothesized that dissecting this microRNA-138/SIN3A-regulated gene network would identify individual proteins contributing to the rescue of ΔF508-CFTR function. Among the genes in the network, we rigorously validated candidates using functional CFTR maturation and electrolyte transport assays in polarized airway epithelia. We found that depletion of the ubiquitin ligase SYVN1, the ubiquitin/proteasome system regulator NEDD8, or the F-box protein FBXO2 partially restored ΔF508-CFTR-mediated Cl^- transport in primary cultures of human cystic fibrosis airway epithelia. Moreover, knockdown of SYVN1, NEDD8, or FBXO2 in combination with corrector compound 18 further potentiated rescue of ΔF508-CFTR-mediated Cl^- conductance. This study provides new knowledge of the CFTR biosynthetic pathway. It suggests that SYVN1 and FBXO2 represent two distinct multiprotein complexes that may degrade ΔF508-CFTR in airway epithelia and identifies a new role for NEDD8 in regulating ΔF508-CFTR ubiquitination.

CFTR Modulators: Shedding Light on Precision Medicine for Cystic Fibrosis

Cystic fibrosis (CF) is the most common life-threatening monogenic disease afflicting Caucasian people. It affects the respiratory, gastrointestinal, glandular and reproductive systems. The major cause of morbidity and mortality in CF is the respiratory disorder caused by a vicious cycle of obstruction of the airways, inflammation and infection that leads to epithelial damage, tissue remodeling and end-stage lung disease. Over the past decades, life expectancy of CF patients has increased due to early diagnosis and improved treatments; however, these patients still present limited quality of life. Many attempts have been made to rescue CF transmembrane conductance regulator (CFTR) expression, function and stability, thereby overcoming the molecular basis of CF. Gene and protein variances caused by CFTR mutants lead to different CF phenotypes, which then require different treatments to quell the patients' debilitating symptoms. In order to seek better approaches to treat CF patients and maximize therapeutic effects, CFTR mutants have been stratified into six groups (although several of these mutations present pleiotropic defects). The research with CFTR modulators (read-through agents, correctors, potentiators, stabilizers and amplifiers) has achieved remarkable progress, and these drugs are translating into pharmaceuticals and personalized treatments for CF patients. This review summarizes the main molecular and clinical features of CF, emphasizes the latest clinical trials using CFTR modulators, sheds light on the molecular mechanisms underlying these new and emerging treatments, and discusses the major breakthroughs and challenges to treating all CF patients.

Development and characterization of synthetic antibodies binding to the cystic fibrosis conductance regulator

Cystic fibrosis transmembrane conductance regulator (CFTR) is a chloride channel in the apical surface of epithelial cells in the airway and gastrointestinal tract, and mutation of CFTR is the underlying cause of cystic fibrosis. However, the precise molecular details of the structure and function of CFTR in native and disease states remains elusive and cystic fibrosis researchers are hindered by a lack of high specificity, high affinity binding reagents for use in structural and biological studies. Here, we describe a panel of synthetic antigen-binding fragments (Fabs) isolated from a phage-displayed library that are specific for intracellular domains of CFTR that include the nucleotide-binding domains (NBD1 and NBD2), the R-region, and the regulatory insertion loop of NBD1. Binding assays performed under conditions that promote the native fold of the protein demonstrated that all Fabs recognized full-length CFTR. However, only the NBD1-specific Fab recognized denatured CFTR by western blot, suggesting a conformational epitope requirement for the other Fabs. Surface plasmon resonance experiments showed that the R-region Fab binds with high affinity to both the phosphorylated and unphosphorylated R-region. In addition, NMR analysis of bound versus unbound R-region revealed a distinct conformational effect upon Fab binding. We further defined residues involved with antibody recognition using an overlapping peptide array. In summary, we describe methodology complementary to previous hybridoma-based efforts to develop antibody reagents to CFTR, and introduce a synthetic antibody panel to aid structural and biological studies.

Ribosomal Stalk Protein Silencing Partially Corrects the ΔF508-CFTR Functional Expression Defect

The most common cystic fibrosis (CF) causing mutation, deletion of phenylalanine 508 (ΔF508 or Phe508del), results in functional expression defect of the CF transmembrane conductance regulator (CFTR) at the apical plasma membrane (PM) of secretory epithelia, which is attributed to the degradation of the misfolded channel at the endoplasmic reticulum (ER). Deletion of phenylalanine 670 (ΔF670) in the yeast oligomycin resistance 1 gene (YOR1, an ABC transporter) of Saccharomyces cerevisiae phenocopies the ΔF508-CFTR folding and trafficking defects. Genome-wide phenotypic (phenomic) analysis of the Yor1-ΔF670 biogenesis identified several modifier genes of mRNA processing and translation, which conferred oligomycin resistance to yeast. Silencing of orthologues of these candidate genes enhanced the ΔF508-CFTR functional expression at the apical PM in human CF bronchial epithelia. Although knockdown of RPL12, a component of the ribosomal stalk, attenuated the translational elongation rate, it increased the folding efficiency as well as the conformational stability of the ΔF508-CFTR, manifesting in 3-fold augmented PM density and function of the mutant. Combination of RPL12 knockdown with the corrector drug, VX-809 (lumacaftor) restored the mutant function to ~50% of the wild-type channel in primary CFTR^{ΔF508/ΔF508} human bronchial epithelia. These results and the observation that silencing of other ribosomal stalk proteins partially rescue the loss-of-function phenotype of ΔF508-CFTR suggest that the ribosomal stalk modulates the folding efficiency of the mutant and is a potential therapeutic target for correction of the ΔF508-CFTR folding defect.

Reducing the rate of translational elongation by silencing ribosomal stalk proteins ameliorates the folding and stability defect of the cystic fibrosis mutant protein ΔF508-CFTR, partially restoring the plasma membrane chloride conductance.

Cystic fibrosis (CF) is one of the most common autosomal recessive diseases in Caucasians. It is caused by mutations in the CF transmembrane conductance regulator (CFTR), which functions as an anion channel at the apical plasma membrane of secretory epithelia. The most common CF mutation, a deletion of the phenylalanine residue at position 508 (ΔF508), results in the channel misfolding and subsequent intracellular degradation. Our previous genome-wide phenotypic screens, using a yeast variant, have predicted modifier genes for ΔF508-CFTR biogenesis. Here, we show that silencing of one of these candidate genes, RPL12, a component of the ribosomal stalk, increased the folding and stabilization of ΔF508-CFTR, resulting in its increased plasma membrane expression and function. Our data suggest that reducing the translational elongation rate via RPL12 silencing can partially reverse the ΔF508-CFTR folding defect. Importantly, RPL12 silencing in combination with the corrector drug VX-809 (lumacaftor), increased the mutant function to 50% of the wild-type CFTR channel, suggesting that the ribosomal stalk perturbation may represent a therapeutic target for rescuing the ΔF508-CFTR biogenesis defect.

Cycloheximide Chase Analysis of Protein Degradation in Saccharomyces cerevisiae

Regulation of protein abundance is crucial to virtually every cellular process. Protein abundance reflects the integration of the rates of protein synthesis and protein degradation. Many assays reporting on protein abundance (e.g., single-time point western blotting, flow cytometry, fluorescence microscopy, or growth-based reporter assays) do not allow discrimination of the relative effects of translation and proteolysis on protein levels. This article describes the use of cycloheximide chase followed by western blotting to specifically analyze protein degradation in the model unicellular eukaryote, Saccharomyces cerevisiae (budding yeast). In this procedure, yeast cells are incubated in the presence of the translational inhibitor cycloheximide. Aliquots of cells are collected immediately after and at specific time points following addition of cycloheximide. Cells are lysed, and the lysates are separated by polyacrylamide gel electrophoresis for western blot analysis of protein abundance at each time point. The cycloheximide chase procedure permits visualization of the degradation kinetics of the steady state population of a variety of cellular proteins. The procedure may be used to investigate the genetic requirements for and environmental influences on protein degradation.

Novel Hits in the Correction of ΔF508-Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) Protein: Synthesis, Pharmacological, and ADME Evaluation of Tetrahydropyrido[4,3-d]pyrimidines for the Potential Treatment of Cystic Fibrosis

Cystic fibrosis (CF) is a lethal genetic disease caused by mutations of the gene encoding the cystic fibrosis transmembrane conductance regulator (CFTR) with a prevalence of the ΔF508 mutation. Whereas the detailed mechanisms underlying disease have yet to be fully elucidated, recent breakthroughs in clinical trials have demonstrated that CFTR dysfunction can be corrected by drug-like molecules. On the basis of this success, a screening campaign was carried out, seeking new drug-like compounds able to rescue ΔF508-CFTR that led to the discovery of a novel series of correctors based on a tetrahydropyrido[4,3-d]pyrimidine core. These molecules proved to be soluble, cell-permeable, and active in a disease relevant functional-assay. The series was then further optimized with emphasis on biological data from multiple cell systems while keeping physicochemical properties under strict control. The pharmacological and ADME profile of this corrector series hold promise for the development of more efficacious compounds to be explored for therapeutic use in CF.