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.

The CFTR Corrector, VX-809 (Lumacaftor), Rescues ABCA4 Trafficking Mutants: a Potential Treatment for Stargardt Disease

BACKGROUND/AIMS

Mutations in ABCA4 cause Stargardt macular degeneration, which invariably ends in legal blindness. We studied two common mutants, A1038V (in NBD1) and G1961E (in NBD2), with the purpose of exploring how they interact with the cell's quality control mechanism. The study was designed to determine how these mutants can be rescued.

METHODS

We expressed wt and mutant ABCA4 in HEK293 cells and studied the effect of the mutations on trafficking and processing and the ability of correctors to rescue them. We used a combination of western blotting, confocal microscopy and surface biotinylation coupled with pulldown of plasma membrane proteins.

RESULTS

G1961E is sensitive to inhibitors of the aggresome, tubacin and the lysosome, bafilomycin A. Both mutants cause a reduction in heat shock protein, Hsp27. Incubation of HEK293 cells expressing the mutants with VX-809, an FDA approved drug for the treatment of cystic fibrosis, increased the levels of A1038V and G1961E by 2- to 3-fold. Importantly, VX-809 increased the levels of both mutants at the plasma membrane suggesting that trafficking had been restored. Transfecting additional Hsp27 to the cells also increased the steady state levels of both mutants. However, in combination with VX-809 the addition of Hsp27 caused a dramatic increase in the protein expression particularly in the G1961 mutant which increased approximately 5-fold.

CONCLUSION

Our results provide a new mechanism for the rescue of ABCA4 trafficking mutants based on the restoration of Hsp27. Our results provide a pathway for the treatment of Stargardt disease.

Rescue of CFTR NBD2 mutants N1303K and S1235R is influenced by the functioning of the autophagosome

The missing phenylalanine at position 508, located in nucleotide-binding domain (NBD1) of the cystic fibrosis transmembrane regulator (CFTR), is the most common cystic fibrosis mutation. Severe disease-causing mutations also occur in NBD2. To provide information on potential therapeutic strategies for mutations in NBD2, we used a combination of biochemical, cell biological and electrophysiological approaches and newly created cell lines to study two disease-causing NBD2 mutants, N1303K and S1235R. We observed that neither was sensitive to E64, a cysteine protease inhibitor. However, further investigation showed that when treated with a combination of correctors, C4 + C18, both mutants also responded to E64. Further exploration to assess aggresome throughput using the autophagy regulator LC3 as a marker showed that, in the absence of correctors, N1303K showed a stalled throughput of LC3-II to the aggresome. The throughput became active again after treatment with the corrector combination C4 + C18. Confocal microscopic studies showed that the N1303K and S1235R mutant proteins both co-localized with LC3, but this co-localization was abolished by the corrector combination and, to a lesser extent, by VX-809. Both the corrector combination and VX-809 increased the CFTR chloride channel function of both mutants. We conclude that correctors have a dual effect, particularly on N1303K: they improve trafficking and function at the plasma membrane and reduce the association with autophagosomes. After treatment with correctors persistent degradation by the autophagosome may limit restoration of function. Thus, mutations in NBD2 of CFTR, in contrast to ΔF508-CFTR, may require additional personalized strategies to rescue them.

Wilson’s disease: Prospective developments towards new therapies

Wilson’s disease (WD) is an autosomal recessive disorder of copper metabolism, caused by mutations in the ATP7B gene. A clear demand for novel WD treatment strategies has emerged. Although therapies using zinc salts and copper chelators can effectively cure WD, these drugs exhibit limitations in a substantial pool of WD patients who develop intolerance and/or severe side effects. Several lines of research have indicated intriguing potential for novel strategies and targets for development of new therapies. Here, we review these new approaches, which comprise correction of ATP7B mutants and discovery of new compounds that circumvent ATP7B-deficiency, as well as cell and gene therapies. We also discuss whether and when these new therapeutic strategies will be translated into clinical use, according to the key requirements for clinical trials that remain to be met. Finally, we discuss the hope for the current rapidly developing research on molecular mechanisms underlying WD pathogenesis and for the related potential therapeutic targets to provide a solid foundation for the next generation of WD therapies that may lead to an effective, tolerable and safe cure.

Characterization of mitochondrial function in cells with impaired cystic fibrosis transmembrane conductance regulator (CFTR) function

Evidence supporting the occurrence of oxidative stress in Cystic Fibrosis (CF) is well established and the literature suggests that oxidative stress is inseparably linked to mitochondrial dysfunction. Here, we have characterized mitochondrial function, in particular as it regards the steps of oxidative phosphorylation and ROS production, in airway cells either homozygous for the F508del-CFTR allele or stably expressing wt-CFTR. We find that oxygen consumption, ΔΨ generation, adenine nucleotide translocator-dependent ADP/ATP exchange and both mitochondrial Complex I and IV activities are impaired in CF cells, while both mitochondrial ROS production and membrane lipid peroxidation increase. Importantly, treatment of CF cells with the small molecules VX-809 and 4,6,4'-trimethylangelicin, which act as "correctors" for F508del CFTR by rescuing the F508del CFTR-dependent chloride secretion, while having no effect per sè on mitochondrial function in wt-CFTR cells, significantly improved all the above mitochondrial parameters towards values found in the airway cells expressing wt-CFTR. This novel study on mitochondrial bioenergetics provides a springboard for future research to further understand the molecular mechanisms responsible for the involvement of mitochondria in CF and identify the proteins primarily responsible for the F508del-CFTR-dependent mitochondrial impairment and thus reveal potential novel targets for CF therapy.

Correctors of mutant CFTR enhance subcortical cAMP-PKA signaling through modulating ezrin phosphorylation and cytoskeleton organization

The most common mutation of the cystic fibrosis transmembrane regulator (CFTR) gene, F508del, produces a misfolded protein resulting in its defective trafficking to the cell surface and an impaired chloride secretion. Pharmacological treatments partially rescue F508del CFTR activity either directly by interacting with the mutant protein and/or indirectly by altering the cellular protein homeostasis. Here, we show that the phosphorylation of ezrin together with its binding to phosphatidylinositol-4,5-bisphosphate (PIP2) tethers the F508del CFTR to the actin cytoskeleton, stabilizing it on the apical membrane and rescuing the sub-membrane compartmentalization of cAMP and activated PKA. Both the small molecules trimethylangelicin (TMA) and VX-809, which act as 'correctors' for F508del CFTR by rescuing F508del-CFTR-dependent chloride secretion, also restore the apical expression of phosphorylated ezrin and actin organization and increase cAMP and activated PKA submembrane compartmentalization in both primary and secondary cystic fibrosis airway cells. Latrunculin B treatment or expression of the inactive ezrin mutant T567A reverse the TMA and VX-809-induced effects highlighting the role of corrector-dependent ezrin activation and actin re-organization in creating the conditions to generate a sub-cortical cAMP pool of adequate amplitude to activate the F508del-CFTR-dependent chloride secretion.

ΔF508-CFTR correctors: synthesis and evaluation of thiazole-tethered imidazolones, oxazoles, oxadiazoles, and thiadiazoles

The most common mutation causing cystic fibrosis (CF) is deletion of phenylalanine residue 508 in the cystic fibrosis transmembrane regulator conductance (CFTR) protein. Small molecules that are able to correct the misfolding of defective ΔF508-CFTR have considerable promise for therapy. Reported here are the design, preparation, and evaluation of five more hydrophilic bisazole analogs of previously identified bithiazole CF corrector 1. Interestingly, bisazole ΔF508-CFTR corrector activity was not increased by incorporation of more H-bond acceptors (O or N), but correlated best with the overall bisazole molecular geometry. The structure activity data, together with molecular modeling, suggested that active bisazole correctors adopt a U-shaped conformation, and that corrector activity depends on the molecule's ability to access this molecular geometry.

The cystic fibrosis V232D mutation inhibits CFTR maturation by disrupting a hydrophobic pocket rather than formation of aberrant interhelical hydrogen bonds

Processing mutations that inhibit folding and trafficking of CFTR are the main cause of cystic fibrosis. Repair of CFTR mutants requires an understanding of the mechanisms of misfolding caused by processing mutations. Previous studies on helix-loop-helix fragments of the V232D processing mutation suggested that its mechanism was to lock transmembrane (TM) segments 3 and 4 together by a non-native hydrogen bond (Asp232(TM4)/Gln207(TM3)). Here, we performed mutational analysis to test for Asp232/Gln207 interactions in full-length CFTR. The rationale was that a V232N mutation should mimic V232D and a V232D/Q207A mutant should mature if the processing defect was caused by hydrogen bonds. We report that only Val232 mutations to charged amino acids severely blocked CFTR maturation. The V232N mutation did not mimic V232D as V232N showed 40% maturation compared to 2% for V232D. Mutation of Val232 to large nonpolar residues (Leu, Phe) had little effect. The Q207L mutation did not rescue V232D because Q207L showed about 50% maturation in the presence of corrector VX-809 while V232D/Q207A could no longer be rescued. These results suggest that V232D inhibits maturation by disrupting a hydrophobic pocket between TM segments rather than forming a non-native hydrogen bond. Disulfide cross-linking analysis of cysteines W356C(TM6) and W1145C(TM12) suggest that the V232D mutation inhibits maturation by trapping CFTR as a partially folded intermediate. Since correctors can efficiently rescue V232D CFTR, the results suggest that hydrophilic processing mutations facing a hydrophobic pocket are good candidates for rescue with pharmacological chaperones.

Regulated recycling of mutant CFTR is partially restored by pharmacological treatment

Efficient trafficking of the cystic fibrosis transmembrane conductance regulator (CFTR) to and from the cell surface is essential for maintaining channel density at the plasma membrane (PM) and ensuring proper physiological activity. The most common mutation, F508del, exhibits reduced surface expression and impaired function despite treatment with currently available pharmacological small molecules, called correctors. To gain more detailed insight into whether CFTR enters compartments that allow corrector stabilization in the cell periphery, we investigated the peripheral trafficking itineraries and kinetics of wild type (WT) and F508del in living cells using high-speed fluorescence microscopy together with fluorogen activating protein detection. We directly visualized internalization and accumulation of CFTR WT from the PM to a perinuclear compartment that colocalized with the endosomal recycling compartment (ERC) markers Rab11 and EHD1, reaching steady-state distribution by 25 minutes. Stimulation by protein kinase A (PKA) depleted this intracellular pool and redistributed CFTR channels to the cell surface, elicited by reduced endocytosis and active translocation to the PM. Corrector or temperature rescue of F508del also resulted in targeting to the ERC and exhibited subsequent PKA-stimulated trafficking to the PM. Corrector treatment (24 hours) led to persistent residence of F508del in the ERC, while thermally destabilized F508del was targeted to lysosomal compartments by 3 hours. Acute addition of individual correctors, C4 or C18, acted on peripheral trafficking steps to partially block lysosomal targeting of thermally destabilized F508del. Taken together, corrector treatment redirects F508del trafficking from a degradative pathway to a regulated recycling route, and proteins that mediate this process become potential targets for improving the efficacy of current and future correctors.