Regulatory Crosstalk by Protein Kinases on CFTR Trafficking and Activity

Cell type-specific regulation of CFTR trafficking-on the verge of progress

Trafficking of the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) protein is a complex process that starts with its biosynthesis and folding in the endoplasmic reticulum. Exit from the endoplasmic reticulum (ER) is coupled with the acquisition of a compact structure that can be processed and traffic through the secretory pathway. Once reaching its final destination-the plasma membrane, CFTR stability is regulated through interaction with multiple protein partners that are involved in its post-translation modification, connecting the channel to several signaling pathways. The complexity of the process is further boosted when analyzed in the context of the airway epithelium. Recent advances have characterized in detail the different cell types that compose the surface epithelium and shifted the paradigm on which cells express CFTR and on their individual and combined contribution to the total expression (and function) of this chloride/bicarbonate channel. Here we review CFTR trafficking and its relationship with the knowledge on the different cell types of the airway epithelia. We explore the crosstalk between these two areas and discuss what is still to be clarified and how this can be used to develop more targeted therapies for CF.

SUMOylation Inhibition Enhances Protein Transcription under CMV Promoter: A Lesson from a Study with the F508del-CFTR Mutant

Cystic fibrosis (CF) is a genetic disorder caused by mutations in the gene encoding the cystic fibrosis transmembrane conductance regulator (CFTR), a selective anion channel expressed in the epithelium of various organs. The most frequent mutation is F508del. This mutation leads to a misfolded CFTR protein quickly degraded via ubiquitination in the endoplasmic reticulum. Although preventing ubiquitination stabilizes the protein, functionality is not restored due to impaired plasma membrane transport. However, inhibiting the ubiquitination process can improve the effectiveness of correctors which act as chemical chaperones, facilitating F508del CFTR trafficking to the plasma membrane. Previous studies indicate a crosstalk between SUMOylation and ubiquitination in the regulation of CFTR. In this study, we investigated the potential of inhibiting SUMOylation to increase the effects of correctors and enhance the rescue of the F508del mutant across various cell models. In the widely used CFBE41o-cell line expressing F508del-CFTR, inhibiting SUMOylation substantially boosted F508del expression, thereby increasing the efficacy of correctors. Interestingly, this outcome did not result from enhanced stability of the mutant channel, but rather from augmented cytomegalovirus (CMV) promoter-mediated gene expression of F508del-CFTR. Notably, CFTR regulated by endogenous promoters in multiple cell lines or patient cells was not influenced by SUMOylation inhibitors.

Selection of optimised ligands by fluorescence-activated bead sorting

The chemistry of aptamers is largely limited to natural nucleotides, and although modifications of nucleic acids can enhance target aptamer affinity, there has not yet been a technology for selecting the right modifications in the right locations out of the vast number of possibilities, because enzymatic amplification does not transmit sequence-specific modification information. Here we show the first method for the selection of specific nucleoside modifications that increase aptamer binding efficacy, using the oncoprotein EGFR as a model target. Using fluorescence-activated bead sorting (FABS), we have successfully selected optimized aptamers from a library of >65 000 variations. Hits were identified by tandem mass spectrometry and validated by using an EGFR binding assay and computational docking studies. Our results provide proof of concept for this novel strategy for the selection of chemically optimised aptamers and offer a new method for rapidly synthesising and screening large aptamer libraries to accelerate diagnostic and drug discovery.

Clinical Consequences and Functional Impact of the Rare S737F CFTR Variant and Its Responsiveness to CFTR Modulators

S737F is a Cystic Fibrosis (CF) transmembrane conductance regulator (CFTR) missense variant. The aim of our study was to describe the clinical features of a cohort of individuals carrying this variant. In parallel, by exploiting ex vivo functional and molecular analyses on nasal epithelia derived from a subset of S737F carriers, we evaluated its functional impact on CFTR protein as well as its responsiveness to CFTR modulators. We retrospectively collected clinical data of all individuals bearing at least one S737F CFTR variant and followed at the CF Centre of Tuscany region (Italy). Nasal brushing was performed in cooperating individuals. At study end clinical data were available for 10 subjects (mean age: 14 years; range 1–44 years; 3 adult individuals). Five asymptomatic subjects had CF, 2 were CRMS/CFSPID and 3 had an inconclusive diagnosis. Ex vivo analysis on nasal epithelia demonstrated different levels of CF activity. In particular, epithelia derived from asymptomatic CF subjects and from one of the subjects with inconclusive diagnosis showed reduced CFTR activity that could be rescued by treatment with CFTR modulators. On the contrary, in the epithelia derived from the other two individuals with an inconclusive diagnosis, the CFTR-mediated current was similar to that observed in epithelia derived from healthy donors. In vitro functional and biochemical analysis on S737F-CFTR expressed in immortalized bronchial cells highlighted a modest impairment of the channel activity, that was improved by treatment with ivacaftor alone or in combination with tezacaftor/elexacaftor. Our study provide evidence towards the evaluation of CFTR function on ex vivo nasal epithelial cell models as a new assay to help clinicians to classify individuals, in presence of discordance between clinical picture, sweat test and genetic profile.

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

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

Revisiting CFTR Interactions: Old Partners and New Players

Remarkable progress in CFTR research has led to the therapeutic development of modulators that rescue the basic defect in cystic fibrosis. There is continuous interest in studying CFTR molecular disease mechanisms as not all cystic fibrosis patients have a therapeutic option available. Addressing the basis of the problem by comprehensively understanding the critical molecular associations of CFTR interactions remains key. With the availability of CFTR modulators, there is interest in comprehending which interactions are critical to rescue CFTR and which are altered by modulators or CFTR mutations. Here, the current knowledge on interactions that govern CFTR folding, processing, and stability is summarized. Furthermore, we describe protein complexes and signal pathways that modulate the CFTR function. Primary epithelial cells display a spatial control of the CFTR interactions and have become a common system for preclinical and personalized medicine studies. Strikingly, the novel roles of CFTR in development and differentiation have been recently uncovered and it has been revealed that specific CFTR gene interactions also play an important role in transcriptional regulation. For a comprehensive understanding of the molecular environment of CFTR, it is important to consider CFTR mutation-dependent interactions as well as factors affecting the CFTR interactome on the cell type, tissue-specific, and transcriptional levels.

A mutational approach to dissect the functional role of the putative CFTR "PTM-CODE"

Deletion of Phe at position 508 (F508del) in CFTR is the commonest cause of Cystic Fibrosis; this mutation affects the fate of the protein, since most of the F508del-CFTR is retained in the endoplasmic reticulum, ubiquitylated and degraded. CFTR is subjected to different post-translational modifications (PTMs) and the possibility to modulate these PTMs has been suggested as a potential therapeutic strategy for the functional recovery of F508del-CFTR. Recently, it has been suggested the presence of a PTM signature (phosphorylation, methylation and ubiquitylation) in the regulatory insertion element of the CFTR, named PTM-code, which is associated with CFTR maturation and F508del-CFTR recovery. However, the real contribution of these PTMs is still to be deciphered. Here, by using a mutational approach, we show that the PTM-code is dispensable for the functional recovery of F508del-CFTR and therefore its regulation would not be essential in the light of a therapeutical approach.

Polyamines Influence Mouse Sperm Channels Activity

Polyamines are ubiquitous polycationic compounds that are highly charged at physiological pH. While passing through the epididymis, sperm lose their capacity to synthesize the polyamines and, upon ejaculation, again come into contact with the polyamines contained in the seminal fluid, unleashing physiological events that improve sperm motility and capacitation. In the present work, we hypothesize about the influence of polyamines, namely, spermine, spermidine, and putrescine, on the activity of sperm channels, evaluating the intracellular concentrations of chloride [Cl⁻]i, calcium [Ca²⁺]i, sodium [Na⁺]i, potassium [K⁺]i, the membrane V_m, and pHi. The aim of this is to identify the possible regulatory mechanisms mediated by the polyamines on sperm-specific channels under capacitation and non-capacitation conditions. The results showed that the presence of polyamines did not directly influence the activity of calcium and chloride channels. However, the results suggested an interaction of polyamines with sodium and potassium channels, which may contribute to the membrane V_m during capacitation. In addition, alkalization of the pHi revealed the possible activation of sperm-specific Na⁺/H⁺ exchangers (NHEs) by the increased levels of cyclic AMP (cAMP), which were produced by soluble adenylate cyclase (sAC) and interact with the polyamines, evidence that is supported by in silico analysis.

A SYK/SHC1 pathway regulates the amount of CFTR in the plasma membrane

Mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene cause the recessive genetic disease cystic fibrosis, where the chloride transport across the apical membrane of epithelial cells mediated by the CFTR protein is impaired. CFTR protein trafficking to the plasma membrane (PM) is the result of a complex interplay between the secretory and membrane recycling pathways that control the number of channels present at the membrane. In addition, the ion transport activity of CFTR at the PM is modulated through post-translational protein modifications. Previously we described that spleen tyrosine kinase (SYK) phosphorylates a specific tyrosine residue in the nucleotide-binding domain 1 domain and this modification can regulate the PM abundance of CFTR. Here we identified the underlying biochemical mechanism using peptide pull-down assays followed by mass spectrometry. We identified in bronchial epithelial cells that the adaptor protein SHC1 recognizes tyrosine-phosphorylated CFTR through its phosphotyrosine-binding domain and that the formation of a complex between SHC1 and CFTR is induced at the PM in the presence of activated SYK. The depletion of endogenous SHC1 expression was sufficient to promote an increase in CFTR at the PM of these cells. The results identify a SYK/SHC1 pathway that regulates the PM levels of CFTR channels, contributing to a better understanding of how CFTR-mediated chloride secretion is regulated.

Human Sperm Capacitation Involves the Regulation of the Tyr-Phosphorylation Level of the Anion Exchanger 1 (AE1)

Bicarbonate uptake is one of the early steps of capacitation, but the identification of proteins regulating anion fluxes remains elusive. The aim of this study is to investigate the role of sperm solute carrier 4 (SLC4) A1 (spAE1) in the capacitation process. The expression, location, and tyrosine-phosphorylation (Tyr-P) level of spAE1 were assessed. Thereby, it was found that 4,4′-Diisothiocyano-2,2′-stilbenedisulfonic acid (DIDS), an SLC4 family channel blocker, inhibited capacitation in a dose-dependent manner by decreasing acrosome reaction (ARC% 24.5 ± 3.3 vs. 64.9 ± 4.3, p < 0.05) and increasing the percentage of not viable cells (NVC%), comparable to the inhibition by I-172, a cystic fibrosis transmembrane conductance regulator (CFTR) blocker (AR% 30.5 ± 4.4 and NVC% 18.6 ± 2.2). When used in combination, a synergistic inhibitory effect was observed with a remarkable increase of the percentage of NVC (45.3 ± 4.1, p < 0.001). spAE1 was identified in sperm membrane as a substrate for Tyr-protein kinases Lyn and Syk, which were identified as both soluble and membrane-bound pools. spAE1-Tyr-P level increased in the apical region of sperm under capacitating conditions and was negatively affected by I-172 or DIDS, and, to a far greater extent, by a combination of both. In conclusion, we demonstrated that spAE1 is expressed in sperm membranes and it is phosphorylated by Syk, but above all by Lyn on Tyr359, which are involved in sperm viability and capacitation.

PB. Transcriptomic and Proteostasis Networks of CFTR and the Development of Small Molecule Modulators for the Treatment of Cystic Fibrosis Lung Disease

Cystic fibrosis (CF) is a lethal autosomal recessive disease caused by mutations in the CF transmembrane conductance regulator (CFTR) gene. The diversity of mutations and the multiple ways by which the protein is affected present challenges for therapeutic development. The observation that the Phe508del-CFTR mutant protein is temperature sensitive provided proof of principle that mutant CFTR could escape proteosomal degradation and retain partial function. Several specific protein interactors and quality control checkpoints encountered by CFTR during its proteostasis have been investigated for therapeutic purposes, but remain incompletely understood. Furthermore, pharmacological manipulation of many CFTR interactors has not been thoroughly investigated for the rescue of Phe508del-CFTR. However, high-throughput screening technologies helped identify several small molecule modulators that rescue CFTR from proteosomal degradation and restore partial function to the protein. Here, we discuss the current state of CFTR transcriptomic and biogenesis research and small molecule therapy development. We also review recent progress in CFTR proteostasis modulators and discuss how such treatments could complement current FDA-approved small molecules.

Focal adhesion kinase and osmotic responses in ionocytes of Fundulus heteroclitus, a euryhaline teleost fish

Cystic Fibrosis Transmembrane conductance Regulator (CFTR) anion channels are the regulated exit pathway in Cl^- secretion by teleost salt secreting ionocytes of the gill and opercular epithelia of euryhaline teleosts. By confocal light immunocytochemistry using regular and phospho-antibodies directed against conserved sites, we found that killifish CFTR (kfCFTR) and the tyrosine kinase Focal Adhesion Kinase (FAK) phosphorylated at Y407 (FAKpY407) and FAKpY397 are colocalized at the apical membrane and in subjacent membrane vesicles of ionocytes. Hypotonic shock and the α-2 adrenergic agonist clonidine rapidly and reversibly inhibit Cl^- secretion by isolated opercular epithelia, simultaneous with dephosphorylation of FAKpY407 and increased FAKpY397, located in the apical membrane of ionocytes in the opercular epithelium. FAKpY407 is re-phosphorylated at the apical membrane of ionocytes and Cl^- secretion rapidly restored by hypertonic shock, detectable at 2 min., maximum at 5 min and still elevated at 30 min. In isolated opercular epithelia, the FAK phosphorylation inhibitor Y15 and p38MAP kinase inhibitor SB203580 significantly blunted the recovery of short-circuit current (I_sc, equal to Cl^- secretion rate) after hypertonic shock. The cSRC inhibitor saracatinib dephosphorylated FAKpY861 seen near tight junctions of pavement cells, and reduced the increase in epithelial resistance normally seen with clonidine inhibition of ion transport, while FAKpY397 was unaffected. The results show rapid osmosensitive responses in teleost fish ionocytes involve phosphorylation of CFTR by FAKpY407, an opposing role for FAKpY397 and a possible role for FAKpY861 in tight junction dynamics.

Regulation effects of naringin on diesel particulate matter-induced abnormal airway surface liquid secretion

BACKGROUND

PM_2.5 is closely related to the incidence and mortality of respiratory diseases. Diesel particulate matter (DPM) is the main component of particulate air pollution and an important source of PM_2.5.

HYPOTHESIS/PURPOSE

This study mainly explored the effect of DPM on airway surface liquid (ASL) secretion and the regulation of naringin in this process, to evaluate therapeutic potentials of naringin for the treatment of abnormal secretion of the respiratory tract caused by PM_2.5.

METHODS

The concentration of lysozyme was measured by Lysozyme Assay Kit. Total protein content was determined by the BCA Protein Assay Kit. The concentration of cAMP and MUC5AC, expressions of CFTR, AQP1, and AQP5 proteins were measured by ELISA. Expressions of CFTR, AQP1 and AQP5 mRNA were determined by qPCR. Amount of CFTR on the cell membrane was determined by immunofluorescence.

RESULTS

The in vitro and in vivo studies had indicated that DPM could inhibit ASL secretion and increased the viscosity of the liquid. Naringin had the functions to attenuate DPM-induced injury, reduce liquid viscosity by reducing MUC5AC and total protein secretion, increase DPM-induced CFTR, AQP1, and AQP5 mRNA and protein expression, positively regulate apical CFTR insertion and promote CFTR activation by increasing intracellular cAMP.

CONCLUSION

These results demonstrated that naringin had regulating effects on the DPM-induced abnormal secretion of the respiratory tract.

Activation of constitutive androstane receptor inhibits intestinal CFTR-mediated chloride transport

Constitutive androstane receptor (CAR) belonging to the nuclear receptor superfamily plays an important role in the xenobiotic metabolism and disposition. It has been reported that CAR regulates the expression of the ATP-binding cassette (ABC) transporters in the intestine, such as multidrug resistance protein 1 (MDR1) and multidrug resistance-associated protein 2/3 (MRP2 and MRP3). In this study, we investigated the role of CAR in the regulation of cystic fibrosis transmembrane conductance regulator (CFTR)-mediated chloride transport in T84 human colonic epithelial cells and mouse intestinal tissues. Treatments of T84 cell monolayers with specific CAR agonists (CITCO and phenytoin at concentrations of 1 μM and 5 μM, respectively) for 24 h decreased transepithelial Cl^- secretion in response to cAMP-dependent agonist. This inhibition was abolished by coincubation of CITCO with a CAR antagonist, CINPA1. We confirmed that an inhibitory effect of CAR agonists was not due to their cytotoxicity. Basolateral membrane permeabilization experiments also revealed that activation of CAR decreased apical Cl^- current stimulated by both CPT-cAMP and genistein (a direct CFTR activator). Such activation also reduced both mRNA and protein expression of CFTR. Furthermore, CITCO decreased cholera toxin (CT)-induced Cl^- secretion across T84 cell monolayers. In ICR mice, administration of TCPOBOP (3 mg/kgBW), a murine-specific CAR agonist, for 7 days produced significant decreases in CFTR mRNA and protein expressions in intestinal tissues. Interestingly, TCPOBOP also inhibited CT-induced intestinal fluid accumulation in mice. This is the first evidence showing that CFTR was downregulated by CAR activation in the intestine. Our findings suggest that CAR has potential as a new drug target for treatment of condition with hyperactivity/ hyperfunction of CFTR especially secretory diarrheas.

Lignin-carbohydrate complexes: properties, applications, analyses, and methods of extraction: a review

The complexity of lignin and hemicellulose segmentation has been known since the middle of the ninetieth century. Studies confirmed that all lignin units in coniferous species and 47-66% of lignin moieties in deciduous species are bound to hemicelluloses or cellulose molecules in lignin-carbohydrate complexes (LCC). Different types and proportions of lignin and polysaccharides present in biomass lead to the formation of LCC with a great variety of compositions and structures. The nature and amount of LCC linkages and lignin substructures affect the efficiency of pulping, hydrolysis, and digestibility of biomass. This review paper discusses the structures, compositions, and properties of LCC present in biomass and in the products obtained via pretreating biomass. Methods for extracting, fractionating, and analyzing LCC of biomass, pulp, and spent pulping liquors are critically reviewed. The main perspectives and challenges associated with these technologies are extensively discussed. LCC could be extracted from biomass following varied methods, among which dimethyl sulfoxide or dioxane (Björkman's) and acetic acid (LCC-AcOH) processes are the most widely applied. The oxidation and methylation treatments of LCC materials elucidate the locations and frequency of binding sites of hemicelluloses to lignin. The two-dimensional nuclear magnetic resonance analysis allows the identification of the structure and the quantity of lignin-carbohydrate bonds involved in LCC. LCC application seems promising in medicine due to its high anti-HIV, anti-herpes, and anti-microbial activity. In addition, LCC was successfully employed as a precursor for the preparation of spherical biocarriers.

Spleen Tyrosine Kinase as a Target Therapy for Pseudomonas aeruginosa Infection

Spleen tyrosine kinase (SYK) is a nonreceptor tyrosine kinase which associates directly with extracellular receptors, and is critically involved in signal transduction pathways in a variety of cell types for the regulation of cellular responses. SYK is expressed ubiquitously in immune and nonimmune cells, and has a much wider biological role than previously recognized. Several studies have highlighted SYK as a key player in the pathogenesis of a multitude of diseases. Pseudomonas aeruginosa is an opportunistic gram-negative pathogen, which is responsible for systemic infections in immunocompromised individuals, accounting for a major cause of severe chronic lung infection in cystic fibrosis patients and subsequently resulting in a progressive deterioration of lung function. Inhibition of SYK activity has been explored as a therapeutic option in several allergic disorders, autoimmune diseases, and hematological malignancies. This review focuses on SYK as a therapeutic target, and describes the possibility of how current knowledge could be translated for therapeutic purposes to regulate the immune response to the opportunistic pathogen P. aeruginosa.

NM23 proteins: innocent bystanders or local energy boosters for CFTR?

NM23 proteins NDPK-A and -B bind to the cystic fibrosis (CF) protein CFTR in different ways from kinases such as PKA, CK2 and AMPK or linkers to cell calcium such as calmodulin and annexins. NDPK-A (not -B) interacts with CFTR through reciprocal AMPK binding/control, whereas NDPK-B (not -A) binds directly to CFTR. NDPK-B can activate G proteins without ligand-receptor coupling, so perhaps NDPK-B's binding influences energy supply local to a nucleotide-binding site (NBD1) needed for CFTR to function. Curiously, CFTR (ABC-C7) is a member of the ATP-binding cassette (ABC) protein family that does not obey 'clan rules'; CFTR channels anions and is not a pump, regulates disparate processes, is itself regulated by multiple means and is so pleiotropic that it acts as a hub that orchestrates calcium signaling through its consorts such as calmodulin/annexins. Furthermore, its multiple partners make CFTR dance to different tunes in different cellular and subcellular locations as it recycles from the plasma membrane to endosomes. CFTR function in airway apical membranes is inhibited by smoking which has been dubbed 'acquired CF'. CFTR alone among family members possesses a trap for other proteins that it unfurls as a 'fish-net' and which bears consensus phosphorylation sites for many protein kinases, with PKA being the most canonical. Recently, the site of CFTR's commonest mutation has been proposed as a knock-in mutant that alters allosteric control of kinase CK2 by log orders of activity towards calmodulin and other substrates after CFTR fragmentation. This link from CK2 to calmodulin that binds the R region invokes molecular paths that control lumen formation, which is incomplete in the tracheas of some CF-affected babies. Thus, we are poised to understand the many roles of NDPK-A and -B in CFTR function and, especially lumen formation, which is defective in the gut and lungs of many CF babies.

Strategies for the etiological therapy of cystic fibrosis

Etiological therapies aim at repairing the underlying cause of cystic fibrosis (CF), which is the functional defect of the cystic fibrosis transmembrane conductance regulator (CFTR) protein owing to mutations in the CFTR gene. Among these, the F508del CFTR mutation accounts for more than two thirds of CF cases worldwide. Two somehow antinomic schools of thought conceive CFTR repair in a different manner. According to one vision, drugs should directly target the mutated CFTR protein to increase its plasma membrane expression (correctors) or improve its ion transport function (potentiators). An alternative strategy consists in modulating the cellular environment and proteostasis networks in which the mutated CFTR protein is synthesized, traffics to its final destination, the plasma membrane, and is turned over. We will analyze distinctive advantages and drawbacks of these strategies in terms of their scientific and clinical dimensions, and we will propose a global strategy for CF research and development based on a reconciliatory approach. Moreover, we will discuss the utility of preclinical biomarkers that may guide the personalized, patient-specific implementation of CF therapies.

Protein and lipid interactions - Modulating CFTR trafficking and rescue

Different levels of CFTR regulation in the cell contribute to a stringent control of chloride secretion in epithelia. Tuning of chloride transport is achieved by modulating CFTR biogenesis, exit from the endoplasmic reticulum, trafficking, membrane stability and channel activity. In this short review, we summarize recent findings identifying interactions with other proteins - directly or through membrane lipids - and briefly discuss how these observations can provide clues to the design of better therapeutic approaches.

The Mechanistic Links between Insulin and Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) Cl- Channel

The cystic fibrosis transmembrane conductance regulator (CFTR) Cl⁻ channel belongs to the ATP-binding cassette (ABC) transporter superfamily and regulates Cl⁻ secretion in epithelial cells for water secretion. Loss-of-function mutations to the CFTR gene cause dehydrated mucus on the apical side of epithelial cells and increase the susceptibility of bacterial infection, especially in the airway and pulmonary tissues. Therefore, research on the molecular properties of CFTR, such as its gating mechanism and subcellular trafficking, have been intensively pursued. Dysregulated CFTR trafficking is one of the major pathological hallmarks in cystic fibrosis (CF) patients bearing missense mutations in the CFTR gene. Hormones that activate cAMP signaling, such as catecholamine, have been found to regulate the intracellular trafficking of CFTR. Insulin is one of the hormones that regulate cAMP production and promote trafficking of transmembrane proteins to the plasma membrane. The functional interactions between insulin and CFTR have not yet been clearly defined. In this review article, I review the roles of CFTR in epithelial cells, its regulatory role in insulin secretion, and a mechanism of CFTR regulation by insulin.

Hydrogen sulfide stimulates CFTR in Xenopus oocytes by activation of the cAMP/PKA signalling axis

Hydrogen sulfide (H₂S) has been recognized as a signalling molecule which affects the activity of ion channels and transporters in epithelial cells. The cystic fibrosis transmembrane conductance regulator (CFTR) is an epithelial anion channel and a key regulator of electrolyte and fluid homeostasis. In this study, we investigated the regulation of CFTR by H₂S. Human CFTR was heterologously expressed in Xenopus oocytes and its activity was electrophysiologically measured by microelectrode recordings. The H₂S-forming sulphur salt Na₂S as well as the slow-releasing H₂S-liberating compound GYY4137 increased transmembrane currents of CFTR-expressing oocytes. Na₂S had no effect on native, non-injected oocytes. The effect of Na₂S was blocked by the CFTR inhibitor CFTR_inh172, the adenylyl cyclase inhibitor MDL 12330A, and the protein kinase A antagonist cAMPS-Rp. Na₂S potentiated CFTR stimulation by forskolin, but not that by IBMX. Na₂S enhanced CFTR stimulation by membrane-permeable 8Br-cAMP under inhibition of adenylyl cyclase-mediated cAMP production by MDL 12330A. These data indicate that H₂S activates CFTR in Xenopus oocytes by inhibiting phosphodiesterase activity and subsequent stimulation of CFTR by cAMP-dependent protein kinase A. In epithelia, an increased CFTR activity may correspond to a pro-secretory response to H₂S which may be endogenously produced by the epithelium or H₂S-generating microflora.

Rab GTPases regulate the trafficking of channels and transporters - a focus on cystic fibrosis

The amount of ion channels and transporters present at the plasma membrane is a crucial component of the overall regulation of ion transport. The number of channels present result from an intricate network of proteins that controls the late events of channel trafficking, such as endocytosis, recycling and targeting to lysosomal degradation. Small GTPases of the Rab family are key players in these processes thus contributing to regulation of fluid secretion and ion homeostasis. In epithelia, this involves mainly the balance between the chloride channel CFTR and the sodium channel ENaC, whose misfunction is a hallmark of cystic fibrosis - the commonest recessive disorder in Caucasians. Here, we review the role of GTPases in regulating trafficking of ion channels and transporters, comparing what is known for CFTR and ENaC with other types of channels. We also discuss how feasible would be to target the Rab machinery to handle a disorder such as CF.

Identification of the fatty acid activation site on human ClC-2

Fatty acids (including lubiprostone and cobiprostone) are human ClC-2 (hClC-2) Cl^- channel activators. Molecular and cellular mechanisms underlying this activation were examined. Role of a four-amino acid PKA activation site, RGET₆₉₁, of hClC-2 was investigated using wild-type (WT) and mutant (AGET, RGEA, and AGAA) hClC-2 expressed in 293EBNA cells as well as involvement of PKA, intracellular cAMP concentration ([cAMP]_i), EP₂, or EP₄ receptor agonist activity. All fatty acids [lubiprostone, cobiprostone, eicosatetraynoic acid (ETYA), oleic acid, and elaidic acid] caused significant rightward shifts in concentration-dependent Cl^- current activation (increasing EC₅₀s) with mutant compared with WT hClC-2 channels, without changing time and voltage dependence, current-voltage rectification, or methadone inhibition of the channel. As with lubiprostone, cobiprostone activation of hClC-2 occurred with PKA inhibitor (myristoylated protein kinase inhibitor) present or when using double PKA activation site (RRAA₆₅₅/RGEA₆₉₁) mutant. Cobiprostone did not activate human CFTR. Fatty acids did not increase [cAMP]_i in hClC-2/293EBNA or T84 cells. Using T84 CFTR knockdown cells, cobiprostone increased hClC-2 Cl^- currents without increasing [cAMP]_i, while PGE₂ and forskolin-IBMX increased both. Fatty acids were not agonists of EP₂ or EP₄ receptors. L-161,982, a supposed EP₄-selective inhibitor, had no effect on lubiprostone-activated hClC-2 Cl^- currents but significantly decreased T84 cell barrier function measured by transepithelial resistance and fluorescent dextran transepithelial movement. The present findings show that RGET₆₉₁ of hClC-2 (possible binding site) plays an important functional role in fatty acid activation of hClC-2. PKA, [cAMP]_i, and EP₂ or EP₄ receptors are not involved. These studies provide the molecular basis for fatty acid regulation of hClC-2.

Synergy of cAMP and calcium signaling pathways in CFTR regulation

Cystic fibrosis is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene that encodes a chloride channel located in the apical membrane of epithelia cells. The cAMP signaling pathway and protein phosphorylation are known to be primary controlling mechanisms for channel function. In this study, we present an alternative activation pathway that involves calcium-activated calmodulin binding of the intrinsically disordered regulatory (R) region of CFTR. Beyond their potential therapeutic value, these data provide insights into the intersection of calcium signaling with control of ion homeostasis and the ways in which the local CFTR microdomain organizes itself.

Cystic fibrosis results from mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel, leading to defective apical chloride transport. Patients also experience overactivation of inflammatory processes, including increased calcium signaling. Many investigations have described indirect effects of calcium signaling on CFTR or other calcium-activated chloride channels; here, we investigate the direct response of CFTR to calmodulin-mediated calcium signaling. We characterize an interaction between the regulatory region of CFTR and calmodulin, the major calcium signaling molecule, and report protein kinase A (PKA)-independent CFTR activation by calmodulin. We describe the competition between calmodulin binding and PKA phosphorylation and the differential effects of this competition for wild-type CFTR and the major F508del mutant, hinting at potential therapeutic strategies. Evidence of CFTR binding to isolated calmodulin domains/lobes suggests a mechanism for the role of CFTR as a molecular hub. Together, these data provide insights into how loss of active CFTR at the membrane can have additional consequences besides impaired chloride transport.

From the endoplasmic reticulum to the plasma membrane: mechanisms of CFTR folding and trafficking

CFTR biogenesis starts with its co-translational insertion into the membrane of endoplasmic reticulum and folding of the cytosolic domains, towards the acquisition of a fully folded compact native structure. Efficiency of this process is assessed by the ER quality control system that allows the exit of folded proteins but targets unfolded/misfolded CFTR to degradation. If allowed to leave the ER, CFTR is modified at the Golgi and reaches the post-Golgi compartments to be delivered to the plasma membrane where it functions as a cAMP- and phosphorylation-regulated chloride/bicarbonate channel. CFTR residence at the membrane is a balance of membrane delivery, endocytosis, and recycling. Several adaptors, motor, and scaffold proteins contribute to the regulation of CFTR stability and are involved in continuously assessing its structure through peripheral quality control systems. Regulation of CFTR biogenesis and traffic (and its dysregulation by mutations, such as the most common F508del) determine its overall activity and thus contribute to the fine modulation of chloride secretion and hydration of epithelial surfaces. This review covers old and recent knowledge on CFTR folding and trafficking from its synthesis to the regulation of its stability at the plasma membrane and highlights how several of these steps can be modulated to promote the rescue of mutant CFTR.

Cystic fibrosis transmembrane conductance regulator modulators in cystic fibrosis: current perspectives

Mutations of the CFTR gene cause cystic fibrosis (CF), the most common recessive monogenic disease worldwide. These mutations alter the synthesis, processing, function, or half-life of CFTR, the main chloride channel expressed in the apical membrane of epithelial cells in the airway, intestine, pancreas, and reproductive tract. Lung disease is the most critical manifestation of CF. It is characterized by airway obstruction, infection, and inflammation that lead to fatal tissue destruction. In spite of great advances in early and multidisciplinary medical care, and in our understanding of the pathophysiology, CF is still considerably reducing the life expectancy of patients. This review highlights the current development in pharmacological modulators of CFTR, which aim at rescuing the expression and/or function of mutated CFTR. While only Kalydeco® and Orkambi® are currently available to patients, many other families of CFTR modulators are undergoing preclinical and clinical investigations. Drug repositioning and personalized medicine are particularly detailed in this review as they represent the most promising strategies for restoring CFTR function in CF.

Duplicated CFTR isoforms in eels diverged in regulatory structures and osmoregulatory functions

Two cystic fibrosis transmembrane conductance regulator (CFTR) isoforms, CFTRa and CFTRb, were cloned in Japanese eel and their structures and functions were studied in different osmoregulatory tissues in freshwater (FW) and seawater (SW) eels. Molecular phylogenetic results suggested that the CFTR duplication in eels occurred independently of the duplication event in salmonid. CFTRa was expressed in the intestine and kidney and downregulated in both tissues in SW eels, while CFTRb was specifically expressed in the gill and greatly upregulated in SW eels. Structurally, the CFTR isoforms are similar in most functional domains except the regulatory R domain, where the R domain of CFTRa is similar to that of human CFTR but the R domain of CFTRb is unique in having high intrinsic negative charges and fewer phosphorylation sites, suggesting divergence of isoforms in terms of gating properties and hormonal regulation. Immunohistochemical results showed that CFTR was localized on the apical regions of SW ionocytes, suggesting a Cl(-) secretory role as in other teleosts. In intestine and kidney, however, immunoreactive CFTR was mostly found in the cytosolic vesicles in FW eels, indicating that Cl(-) channel activity could be low at basal conditions, but could be rapidly increased by membrane insertion of the stored channels. Guanylin (GN), a known hormone that increases CFTR activity in mammalian intestine, failed to redistribute CFTR and to affect its expression in eel intestine. The results suggested that GN-independent CFTR regulation is present in eel intestine and kidney.