Establishment and characterization of a novel polarized MDCK epithelial cellular model for CFTR studies

Polarized trafficking and copper transport activity of ATP7B: a mutational approach to establish genotype-phenotype correlation in Wilson disease

Mutation in ATP7B gene causes Wilson disease (WD) that is characterized by severe hepatic and neurological symptoms. ATP7B localizes at the trans-Golgi Network (TGN) transporting copper to copper-dependent enzymes and traffics in apically targeted vesicles upon intracellular copper elevation. To decode the cellular underpinnings of WD manifestation we investigated copper-responsive polarized trafficking and copper transport activity of fifteen WD causing point mutations in ATP7B. Amino-terminal mutations Gly85Val, Leu168Pro and Gly591Asp displayed TGN and sub-apical localization whereas, Leu492Ser mislocalized at the basolateral region. The actuator domain mutation Gly875Arg shows retention in the endoplasmic reticulum (ER), Ala874Val and Leu795Phe show partial targeting to TGN and post-Golgi vesicles. The Nucleotide-Binding Domain mutations His1069Gln and Leu1083Phe also display impaired targeting. The C-terminal mutations Leu1373Pro/Arg is arrested at ER but Ser1423Asn shows TGN localization. Transmembrane mutant Arg778Leu resides in ER and TGN while Arg969Gln is exclusively ER localized. Cellular Cu level does not alter the targeting of any of the studied mutations. Mutants that traffic to TGN exhibits biosynthetic function. Finally, we correlated cellular phenotypes with the clinical manifestation of the two most prevalent mutations; the early onset and more aggressive WD caused by Arg778Leu and the milder form of WD caused by mutation His1069Gln. This article is protected by copyright. All rights reserved.

Loss of PKD1 and PKD2 share common effects on intracellular Ca2+ signaling

In polycystic kidney disease (PKD) multiple bilateral renal cysts gradually enlarge causing a decline in renal function. Transepithelial chloride secretion through cystic fibrosis transmembrane conductance regulator (CFTR) and TMEM16A (anoctamin 1) drive cyst enlargement. We demonstrated recently that a loss of PKD1 increases expression and function of TMEM16A in murine kidneys and in mouse M1 collecting duct cells. The data demonstrated that TMEM16A contributes essentially to cyst growth by upregulating intracellular Ca2+ signaling. Enhanced expression of TMEM16A and Ca2+ signaling increased both cell proliferation and fluid secretion, which suggested inhibition of TMEM16A as a novel therapy in ADPKD. About 15 % of all ADPKD cases are caused by mutations in PKD2. To analyze the effects of loss of function of PKD2 on Ca2+ signaling, we knocked-down Pkd2 in mouse primary renal epithelial cells in the present study, using viral transfection of shRNA. Unlike in Pkd1-/- cells, knockdown of PKD2 lowered basal Ca2+ and augmented store-operated Ca2+ entry, which was both independent of TMEM16A. However, disease causing purinergic Ca2+ store release was enhanced, similar to that observed in Pkd1-/- renal epithelial cells. The present data suggest pharmacological inhibition of TMEM16A as a treatment in ADPKD caused by mutations in both PKD1 and PKD2.

TMEM16A drives renal cyst growth by augmenting Ca2+ signaling in M1 cells

Abstract

Polycystic kidney disease (PKD) leads to continuous decline of renal function by growth of renal cysts. Enhanced proliferation and transepithelial chloride secretion through cystic fibrosis transmembrane conductance regulator (CFTR) and Ca²⁺-activated TMEM16A Cl⁻ channels is thought to cause an increase in cyst volume. Recent work shows the pro-proliferative role of the Ca²⁺ activated Cl⁻ channel TMEM16A (anoctamin 1), and demonstrates the essential contribution of TMEM16A to CFTR-dependent Cl⁻ secretion. The present data demonstrate an increase in intracellular Ca²⁺ ([Ca²⁺]i) signals and Cl⁻ secretion by TMEM16A, in renal collecting duct principle cells from dog (MDCK) and mouse (M1) as well as primary tubular epithelial cells from PKD1−/− knockout mice. M1 organoids proliferated, increased expression of TMEM16A, and secreted Cl⁻ upon knockdown of endogenous polycystin 1 or 2 (PKD1,2), by retroviral transfection with shPKD1 and shPKD2, respectively. Knockdown of PKD1 or PKD2 increased basal intracellular Ca²⁺ levels and enhanced purinergic Ca²⁺ release from endoplasmic reticulum. In contrast, ryanodine receptors were found not to be expressed in mouse renal epithelial cells and caffeine had no effects on [Ca²⁺]i. Ca²⁺ signals, proliferation, and Cl⁻ secretion were largely reduced by knockdown or blockade of TMEM16A. TMEM16A may be therefore important for enhanced Ca²⁺ release from IP₃-sensitive Ca²⁺ stores in polycystic kidney disease.

Key messages

• ADPKD leads to continuous decline of renal function by growth of renal cysts.

• Knockdown of PKD1 or PKD2 increases TMEM16A expression.

• TMEM16A enhanced intracellular Ca²⁺ signals, Cl⁻ secretion, and proliferation.

• TMEM16A contributes to cyst growth in ADPKD.

Electronic supplementary material

The online version of this article (10.1007/s00109-020-01894-y) contains supplementary material, which is available to authorized users.

Capitalizing on the heterogeneous effects of CFTR nonsense and frameshift variants to inform therapeutic strategy for cystic fibrosis

CFTR modulators have revolutionized the treatment of individuals with cystic fibrosis (CF) by improving the function of existing protein. Unfortunately, almost half of the disease-causing variants in CFTR are predicted to introduce premature termination codons (PTC) thereby causing absence of full-length CFTR protein. We hypothesized that a subset of nonsense and frameshift variants in CFTR allow expression of truncated protein that might respond to FDA-approved CFTR modulators. To address this concept, we selected 26 PTC-generating variants from four regions of CFTR and determined their consequences on CFTR mRNA, protein and function using intron-containing minigenes expressed in 3 cell lines (HEK293, MDCK and CFBE41o-) and patient-derived conditionally reprogrammed primary nasal epithelial cells. The PTC-generating variants fell into five groups based on RNA and protein effects. Group A (reduced mRNA, immature (core glycosylated) protein, function <1% (n = 5)) and Group B (normal mRNA, immature protein, function <1% (n = 10)) variants were unresponsive to modulator treatment. However, Group C (normal mRNA, mature (fully glycosylated) protein, function >1% (n = 5)), Group D (reduced mRNA, mature protein, function >1% (n = 5)) and Group E (aberrant RNA splicing, mature protein, function > 1% (n = 1)) variants responded to modulators. Increasing mRNA level by inhibition of NMD led to a significant amplification of modulator effect upon a Group D variant while response of a Group A variant was unaltered. Our work shows that PTC-generating variants should not be generalized as genetic 'nulls' as some may allow generation of protein that can be targeted to achieve clinical benefit.

Targeting of the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) Protein with a Technetium-99m Imaging Probe

Cystic fibrosis (CF) is caused by mutations in the gene that encodes the CF transmembrane conductance regulator (CFTR) protein. The most common mutation, F508del, leads to almost total absence of CFTR at the plasma membrane, a defect potentially corrected via drug-based therapies. Herein, we report the first proof-of-principle study of a noninvasive imaging probe able to detect CFTR at the plasma membrane. We radiolabeled the CFTR inhibitor, CFTR_inh -172a, with technetium-99m via a pyrazolyl-diamine chelating unit, yielding a novel ^99m Tc(CO)₃ complex. A non-radioactive surrogate showed that the structural modifications introduced in the inhibitor did not affect its activity. The radioactive complex was able to detect plasma membrane CFTR, shown by its significantly higher uptake in wild-type versus mutated cells. Furthermore, assessment of F508del CFTR pharmacological correction in human cells using the radioactive complex revealed differences in corrector versus control uptake, recapitulating the biochemical correction observed for the protein.

Improved plasma membrane expression of the trafficking defective P344R mutant of muscle, skeletal, receptor tyrosine kinase (MuSK) causing congenital myasthenic syndrome

Muscle, skeletal, receptor tyrosine kinase (MuSK) is a key organizer at the postsynaptic membrane and critical for proper development and maintenance of the neuromuscular junction. Mutations in MUSK result in congenital myasthenic syndrome (CMS). We hypothesized that the CMS-causing missense mutation (P344R), found within the cysteine-rich domain of the protein, will affect its conformational tertiary structure. Consequently, the protein will misfold, get retained in the endoplasmic reticulum (ER) and lose its biological function through degradation by the highly conserved ER associated degradation (ERAD) machinery. We report that P344R-MuSK mutant is trafficking-deficient when expressed at 37°C in HeLa, COS-7 and HEK293 cell lines. It colocalized with the ER marker calnexin in contrast to wild-type MuSK which localized to the plasma membrane. The N-glycosylation status of P344R-MuSK is that of an immature and not properly post-translationally modified protein. Inhibition of protein synthesis showed that the P344R mutant's half-life is shorter than wild-type MuSK protein. Proteasomal inhibition resulted in the stabilization of the mutant protein. The mutant protein is highly ubiquitinated compared to wild-type confirming targeting for proteasomal degradation. The mutant showed around 50% of its in vivo autophosphorylation activity. P344R-MuSK mutant's trafficking defect is correctable by culturing the expressing cells at 27°C. Moreover, chemical compounds namely 2.5% glycerol, 1% dimethyl sulfoxide, 10 μM thapsigargin and 1 μM curcumin improved the maturation and exit of the mutant protein from the ER. These findings open perspectives for potential therapeutic intervention for patients with CMS harboring the P344R-MuSK mutation.

Impact of the cystic fibrosis mutation F508del-CFTR on renal cyst formation and growth

In autosomal dominant polycystic kidney disease (ADPKD), cystic fibrosis transmembrane conductance regulator (CFTR), the protein product of the gene defective in cystic fibrosis (CF), plays a crucial role in fluid accumulation, which promotes cyst swelling. Several studies have identified individuals afflicted by both ADPKD and CF. Two studies suggested that CF mutations might attenuate the severity of ADPKD, whereas a third found no evidence of a protective effect. In this study, we investigated the impact of the commonest CF mutation F508del-CFTR on the formation and growth of renal cysts. As a model system, we used Madin-Darby canine kidney (MDCK) epithelial cells engineered to express wild-type and F508del human CFTR. We grew MDCK cysts in collagen gels in the presence of the cAMP agonist forskolin and measured transepithelial resistance and Cl(-) secretion with the Ussing chamber technique and assayed cell proliferation using nonpolarized MDCK cells. When compared with untransfected MDCK cells, cells expressing wild-type CFTR generated substantial numbers of large cysts, which grew markedly over time. By contrast, MDCK cells expressing F508del-CFTR formed very few tiny cysts that failed to enlarge. Interestingly, treatment of F508del-CFTR cysts with the CFTR corrector VRT-325 and the CFTR corrector-potentiator VRT-532 increased the number, but not size, of F508del-CFTR cysts, possibly because VRT-325 inhibited strongly cell proliferation. Based on its effects on transepithelial resistance, Cl(-) secretion, and cell proliferation, we conclude that the F508del-CFTR mutation disrupts cyst formation and growth by perturbing strongly fluid accumulation within the cyst lumen without compromising epithelial integrity.

Type IV(B) pili are required for invasion but not for adhesion of Salmonella enterica serovar Typhi into BHK epithelial cells in a cystic fibrosis transmembrane conductance regulator-independent manner

The cystic fibrosis transmembrane conductance regulator (CFTR) has been proposed as an epithelial cell receptor for the entry of Salmonella Typhi but not Salmonella Typhimurium. The bacterial ligand recognized by CFTR is thought to reside either in the S. Typhi lipopolysaccharide core region or in the type IV pili. Here, we assessed the ability of virulent strains of S. Typhi and S. Typhimurium to adhere to and invade BHK epithelial cells expressing either the wild-type CFTR protein or the ∆F508 CFTR mutant. Both S. Typhi and S. Typhimurium invaded the epithelial cells in a CFTR-independent fashion. Furthermore and also in a CFTR-independent manner, a S. Typhi pilS mutant adhered normally to BHK cells but displayed a 50% reduction in invasion as compared to wild-type bacteria. Immunofluorescence microscopy revealed that bacteria and CFTR do not colocalize at the epithelial cell surface. Together, our results strongly argue against the established dogma that CFTR is a receptor for entry of Salmonella to epithelial cells.

Folding and rescue of a cystic fibrosis transmembrane conductance regulator trafficking mutant identified using human-murine chimeric proteins

Impairment of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel causes cystic fibrosis, a fatal genetic disease. Here, to gain insight into CFTR structure and function, we exploited interspecies differences between CFTR homologues using human (h)-murine (m) CFTR chimeras containing murine nucleotide-binding domains (NBDs) or regulatory domain on an hCFTR backbone. Among 15 hmCFTR chimeras analyzed, all but two were correctly processed, one containing part of mNBD1 and another containing part of mNBD2. Based on physicochemical distance analysis of divergent residues between human and murine CFTR in the two misprocessed hmCFTR chimeras, we generated point mutations for analysis of respective CFTR processing and functional properties. We identified one amino acid substitution (K584E-CFTR) that disrupts CFTR processing in NBD1. No single mutation was identified in NBD2 that disrupts protein processing. However, a number of NBD2 mutants altered channel function. Analysis of structural models of CFTR identified that although Lys(584) interacts with residue Leu(581) in human CFTR Glu(584) interacts with Phe(581) in mouse CFTR. Introduction of the murine residue (Phe(581)) in cis with K584E in human CFTR rescued the processing and trafficking defects of K584E-CFTR. Our data demonstrate that human-murine CFTR chimeras may be used to validate structural models of full-length CFTR. We also conclude that hmCFTR chimeras are a valuable tool to elucidate interactions between different domains of CFTR.

Missense mutations in a retinal pigment epithelium protein, bestrophin-1, cause retinitis pigmentosa

Bestrophin-1 is preferentially expressed at the basolateral membrane of the retinal pigmented epithelium (RPE) of the retina. Mutations in the BEST1 gene cause the retinal dystrophies vitelliform macular dystrophy, autosomal-dominant vitreochoroidopathy, and autosomal-recessive bestrophinopathy. Here, we describe four missense mutations in bestrophin-1, three that we believe are previously unreported, in patients diagnosed with autosomal-dominant and -recessive forms of retinitis pigmentosa (RP). The physiological function of bestrophin-1 remains poorly understood although its heterologous expression induces a Cl--specific current. We tested the effect of RP-causing variants on Cl- channel activity and cellular localization of bestrophin-1. Two (p.L140V and p.I205T) produced significantly decreased chloride-selective whole-cell currents in comparison to those of wild-type protein. In a model system of a polarized epithelium, two of three mutations (p.L140V and p.D228N) caused mislocalization of bestrophin-1 from the basolateral membrane to the cytoplasm. Mutations in bestrophin-1 are increasingly recognized as an important cause of inherited retinal dystrophy.

Localization studies of rare missense mutations in cystic fibrosis transmembrane conductance regulator (CFTR) facilitate interpretation of genotype-phenotype relationships

We have been investigating the functional consequences of rare disease-associated amino acid substitutions in the cystic fibrosis transmembrane conductance regulator (CFTR). Mutations of the arginine residue at codon 1070 have been associated with different disease consequences; R1070P and R1070Q with "severe" pancreatic insufficient cystic fibrosis (CF) and R1070W with "mild" pancreatic sufficient CF or congenital bilateral absence of the vas deferens. Intriguingly, CFTR bearing each of these mutations is functional when expressed in nonpolarized cells. To determine whether R1070 mutations cause disease by affecting CFTR localization, we created polarized Madin Darby canine kidney (MDCK) cell lines that express either wild-type or mutant CFTR from the same genomic integration site. Confocal microscopy and biotinylation studies revealed that R1070P was not inserted into the apical membrane, R1070W was inserted at levels reduced from wild-type while R1070Q was present in the apical membrane at levels comparable to wild-type. The abnormal localization of CFTR bearing R1070P and R1070W was consistent with deleterious consequences in patients; however, the profile of CFTR R1070Q was inconsistent with a "severe" phenotype. Reanalysis of 16 patients with the R1070Q mutation revealed that 11 carried an in cis nonsense mutation, S466X. All 11 patients carrying the complex allele R1070Q-S466X had severe disease, while 4 out of 5 patients with R1070Q had "mild" disease, thereby reconciling the apparent discrepancy between the localization studies of R1070Q and the phenotype of patients bearing this mutation. Our results emphasize that localization studies in relevant model systems can greatly assist the interpretation of the disease-causing potential of rare missense mutations.

Prolonged treatment of cells with genistein modulates the expression and function of the cystic fibrosis transmembrane conductance regulator

BACKGROUND AND PURPOSE

Cystic fibrosis (CF) is caused by dysfunction of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel. In the search for new CF therapies, small molecules have been identified that rescue the defective channel gating of CF mutants (termed CFTR potentiators). Here, we investigate the long-term effects of genistein, the best-studied CFTR potentiator, on the expression and function of CFTR.

EXPERIMENTAL APPROACH

We pre-treated baby hamster kidney (BHK) cells expressing wild-type or F508del-CFTR (the most common CF mutant) with concentrations of genistein that potentiate (30 microM) or inhibit (100 microM) CFTR function for 2 or 24 h at 37 degrees C before examining CFTR maturation, expression and single-channel activity.

KEY RESULTS

Using the iodide efflux technique, we found that genistein pre-treatment failed to restore function to F508del-CFTR, but altered that of wild-type CFTR. Pre-treatment of cells with genistein for 2 h had little effect on CFTR processing, whereas pre-treatment for 24 h either augmented (30 microM genistein) or impaired (100 microM genistein) CFTR maturation. Using immunocytochemistry, we found that all genistein pre-treatments increased the localization of CFTR protein to the cell surface. However, following the incubation of cells with genistein (100 microM) for 2 h, individual CFTR Cl(-) channels exhibited characteristics of channel block upon channel activation.

CONCLUSIONS AND IMPLICATIONS

Genistein pre-treatment alters the maturation, cell surface expression and single-channel function of CFTR in ways distinct from its acute effects. Thus, CFTR potentiators have the potential to influence CFTR by mechanisms distinct from their effects on channel gating.

Coupling cystic fibrosis to endoplasmic reticulum stress: Differential role of Grp78 and ATF6

Cystic fibrosis (CF) is the most common Caucasian autosomal recessive disease. It is due to mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene encoding the CFTR protein, which is a chloride (Cl(-)) channel. The most common mutation leads to a missing phenylalanine at position 508 (DeltaF508). The DeltaF508-CFTR protein is misfolded and retained in the endoplasmic reticulum and may trigger the unfolded protein response (UPR). Furthermore, CF is accompanied by inflammation and infection, which are also involved in the UPR. To date, the UPR transducer ATF6 and ER stress sensor Grp78 have been used as UPR markers. Therefore, our aim was to study the activation of ATF6 and Grp78 in transfected human epithelial cells expressing the DeltaF508-CFTR protein, and we showed that they are activated in these cells. We investigated the effect of exogenous UPR inducers thapsigargin (Tg) and tunicamycin (Tu) on Grp78 and ATF6 expression. Whereas the cells reacted to the UPR induction, we show a difference in the electrophoretic pattern of ATF6. The Grp78/ATF6 complex was previously described, but its stability during UPR is controversial. Using co-immunoprecipitation we show that it is stable in DeltaF508-CFTR-expressing cells and is maintained under UPR conditions. Finally, using siRNA, we show that decreased ATF6 expression induces increased cAMP-dependent halide flux through DeltaF508-CFTR due to its increased membrane localization. Therefore, our results suggest that UPR may be triggered in CF and that ATF6 may be a therapeutic target.

Therapy through chaperones: sense or antisense? Cystic fibrosis as a model disease

Massive production and accumulation of a single abnormal protein may constitute a major toxic burden for the cell and even compromise the organism's long-term viability. Consequently, adaptation and survival have forced evolution to create 'quality control' mechanisms that detect, monitor, and often degrade such abnormally folded gene products, in which molecular chaperones are key players. Notwithstanding this, there are numerous examples of misfolded proteins which, in spite of being recognized as aberrant and efficiently discarded by cellular quality control, still retain some of the functional properties of their wild-type counterparts, so that their maintenance in the cell would be beneficial for the organism. Herein are described the cellular roles of molecular chaperones and some new insights on the mechanisms by which they influence the development of human diseases caused by mutations that lead to protein misfolding. A special emphasis is given to cystic fibrosis, a classical genetic disorder resulting from the retention and degradation of a mutant, albeit functional, protein by the endoplasmic reticulum quality control. This particular system has been a good example to describe the mechanisms that are likely to be shared by a number of protein substrates, to define the common characteristics of the mutants, as well as to identify the mechanistic intervenients in their retention and degradation. Finally, new approaches aimed at correcting protein folding defects are discussed, including the potential of molecular chaperones (e.g., through RNA interference) as novel therapeutic targets, and the usage of chemical or pharmacological chaperones as new therapeutic agents.