Maria Favia

Na+/H+ Exchanger Regulatory Factor Isoform 1 Overexpression Modulates Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) Expression and Activity in Human Airway 16HBE14o- Cells and Rescues ΔF508 CFTR Functional Expression in Cystic Fibrosis Cells

There is evidence that cystic fibrosis transmembrane conductance regulator (CFTR) interacting proteins play critical roles in the proper expression and function of CFTR. The Na+/H+ exchanger regulatory factor isoform 1 (NHERF1) was the first identified CFTR-binding protein. Here we further clarify the role of NHERF1 in the regulation of CFTR activity in two human bronchial epithelial cell lines: the normal, 16HBE14o-, and the homozygous ΔF508 CFTR, CFBE41o-. Confocal analysis in polarized cell monolayers demonstrated that NHERF1 distribution was associated with the apical membrane in 16HBE14o- cells while being primarily cytoplasmic in CFBE41o- cells. Transfection of 16HBE14o- monolayers with vectors encoding for wild-type (wt) NHERF1 increased both apical CFTR expression and apical protein kinase A (PKA)-dependent CFTR-mediated chloride efflux, whereas transfection with NHERF1 mutated in the binding groove of the PDZ domains or truncated for the ERM domain inhibited both the apical CFTR expression and the CFTR-dependent chloride efflux. These data led us to hypothesize an important role for NHERF1 in regulating CFTR localization and stability on the apical membrane of 16HBE14o- cell monolayers. Importantly, wt NHERF1 overexpression in confluent ΔF508 CFBE41o- and ΔF508 CFT1-C2 cell monolayers induced both a significant redistribution of CFTR from the cytoplasm to the apical membrane and a PKA-dependent activation of CFTR-dependent chloride secretion.

Na/H Exchanger Regulatory Factor 1 Overexpression- dependent Increase of Cytoskeleton Organization Is Fundamental in the Rescue of F508del Cystic Fibrosis Transmembrane Conductance Regulator in Human Airway CFBE41o- Cells

NHERF1 overexpression increases functional apical expression of F508del CFTR in CFBE41o- cells. Here, we show that this occurs via the formation of the multiprotein complex NHERF1-phosphoezrin-actin, which provides a regulated linkage between F508del CFTR and the actin cytoskeleton resulting in an increased F508del CFTR stability in the membrane.

CFTR regulation in human airway epithelial cells requires integrity of the actin cytoskeleton and compartmentalized cAMP and PKA activity

The cystic fibrosis transmembrane conductance regulator (CFTR) mutation ΔF508CFTR still causes regulatory defects when rescued to the apical membrane, suggesting that the intracellular milieu might affect its ability to respond to cAMP regulation. We recently reported that overexpression of the Na+/H+ exchanger regulatory factor NHERF1 in the cystic fibrosis (CF) airway cell line CFBE41o-rescues the functional expression of ΔF508CFTR by promoting F-actin organization and formation of the NHERF1–ezrin–actin complex. Here, using real-time FRET reporters of both PKA activity and cAMP levels, we find that lack of an organized subcortical cytoskeleton in CFBE41o-cells causes both defective accumulation of cAMP in the subcortical compartment and excessive cytosolic accumulation of cAMP. This results in reduced subcortical levels and increased cytosolic levels of PKA activity. NHERF1 overexpression in CFBE41o-cells restores chloride secretion, subcortical cAMP compartmentalization and local PKA activity, indicating that regulation of ΔF508CFTR function requires not only stable expression of the mutant CFTR at the cell surface but also depends on both generation of local cAMP signals of adequate amplitude and activation of PKA in proximity of its target. Moreover, we found that the knockdown of wild-type CFTR in the non-CF 16HBE14o-cells results in both altered cytoskeletal organization and loss of cAMP compartmentalization, whereas stable overexpression of wt CFTR in CF cells restores cytoskeleton organization and re-establishes the compartmentalization of cAMP at the plasma membrane. This suggests that the presence of CFTR on the plasma membrane influences the cytoskeletal organizational state and, consequently, cAMP distribution. Our data show that a sufficiently high concentration of cAMP in the subcortical compartment is required to achieve PKA-mediated regulation of CFTR activity.

NHERF1 and CFTR restore tight junction organisation and function in cystic fibrosis airway epithelial cells: role of ezrin and the RhoA/ROCK pathway

Tight junctions (TJs) restrict the transit of ions and molecules through the paracellular route and act as a barrier to regulate access of inflammatory cells into the airway lumen. The pathophysiology of cystic fibrosis (CF) lung disease is characterised by abnormal ion and fluid transport across the epithelium and polymorphonuclear (PMN) leukocyte-dominated inflammatory response. Na+/H+ exchanger regulatory factor 1 (NHERF1) is a protein involved in PKA-dependent activation of CFTR by interacting with CFTR via its PDZ domains and with ezrin via its C-terminal domain. We have previously found that the NHERF1-overexpression dependent rescue CFTR-dependent chloride secretion is due to the re-organisation of the actin cytoskeleton network induced by the formation of the multiprotein complex NHERF1–RhoA–ezrin–actin. In this context, we here studied whether NHERF1 and CFTR are involved in the organisation and function of TJs. F508del CFBE41o− monolayers presented nuclear localisation of zonula occludens (ZO-1) and occludin as well as disorganisation of claudin 1 and junction-associated adhesion molecule 1 as compared with wild-type 16HBE14o− monolayers, paralleled by increased permeability to dextrans and PMN transmigration. Overexpression of either NHERF1 or CFTR in CFBE41o− cells rescued TJ proteins to their proper intercellular location and decreased permeability and PMN transmigration, while this effect was not achieved by overexpressing either NHERF1 deprived of ezrin-binding domain. Further, expression of a phospho-dead ezrin mutant, T567A, increased permeability in both 16HBE14o− cells and in a CFBE clone stably overexpressing NHERF1 (CFBE/sNHERF1), whereas a constitutively active form of ezrin, T567D, achieved the opposite effect in CFBE41o− cells. A dominant-negative form of RhoA (RhoA-N19) also disrupted ZO-1 localisation at the intercellular contacts dislodging it to the nucleus and increased permeability in CFBE/sNHERF1. The inhibitor Y27632 of Rho kinase (ROCK) increased permeability as well. Overall, these data suggest a significant role for the multiprotein complex CFTR–NHERF1–ezrin–actin in maintaining TJ organisation and barrier function, and suggest that the RhoA/ROCK pathway is involved.

β-Oestradiol rescues ΔF508CFTR functional expression in human cystic fibrosis airway CFBE41o - cells through the up-regulation of NHERF1

Background information. CF (cystic fibrosis) is a disease caused by mutations within the CFTR (CF transmembrane conductance regulator) gene. The most common mutation, ΔF508 (deletion of Phe‐508), results in a protein that is defective in folding and trafficking to the cell surface but is functional if properly localized in the plasma membrane. We have recently demonstrated that overexpression of the PDZ protein NHERF1 (Na+/H+‐exchanger regulatory factor 1) in CF airway cells induced both a redistribution of ΔF508CFTR from the cytoplasm to the apical membrane and the PKA (protein kinase A)‐dependent activation of ΔF508CFTR‐dependent chloride secretion. In view of the potential importance of the targeted up‐regulation of NHERF1 in a therapeutic context, and since it has been demonstrated that oestrogen treatment increases endogenous NHERF1 expression, we tested the hypothesis that oestrogen treatment can increase NHERF1 expression in a human bronchiolar epithelial CF cell line, CFBE41o−, with subsequent rescue of apical ΔF508CFTR chloride transport activity.

Stimulation of Xenopus P2Y1 receptor activates CFTR in A6 cells

AbstractNucleotide binding to purinergic P2Y receptors contributes to the regulation of a variety of physiological functions in renal epithelial cells. Here, we investigate the regulatory mechanism of the P2Y1 receptor agonist 2-methylthioadenosine diphosphate (2-MeSADP) on Cl− transport in A6 cells, a commonly used model of the distal section of the Xenopus laevis nephron. Protein and mRNA expression analysis together with functional measurements demonstrated the basolateral location of the Xenopus P2Y1 receptor. 2-MeSADP increased intracellular [Ca2+] and cAMP and Cl− efflux, responses that were all inhibited by the specific P2Y1 receptor antagonist MRS 2179. Cl− efflux was also inhibited by the cystic fibrosis transmembrane conductance regulator (CFTR) blocker glibenclamide. Inhibition of either protein kinase A (PKA) or the binding between A-kinase-anchoring proteins (AKAPs) and the regulatory PKA RII subunit blocked the 2-MeSADP-induced activation of CFTR, suggesting that PKA mediates P2Y1 receptor regulation of CFTR through one or more AKAPs. Further, the truncation of the PDZ1 domain of the scaffolding protein Na+/H+ exchanger regulatory factor-2 (NHERF-2) inhibited 2-MeSADP-dependent stimulation of Cl− efflux, suggesting the involvement of this scaffolding protein. Activation or inhibition of PKC had no effect per se on basal Cl− efflux but potentiated or reduced the 2-MeSADP-dependent stimulation of Cl− efflux, respectively. These data suggest that the X. laevis P2Y1 receptor in A6 cells can increase both cAMP/PKA and Ca2+/PKC intracellular levels and that the PKC pathway is involved in CFTR activation via potentiation of the PKA pathway.

Goblet Cell Hyperplasia Requires High Bicarbonate Transport To Support Mucin Release.

Goblet cell hyperplasia, a feature of asthma and other respiratory diseases, is driven by the Th-2 cytokines IL-4 and IL-13. In human bronchial epithelial cells, we find that IL-4 induces the expression of many genes coding for ion channels and transporters, including TMEM16A, SLC26A4, SLC12A2, and ATP12A. At the functional level, we find that IL-4 enhances calcium- and cAMP-activated chloride/bicarbonate secretion, resulting in high bicarbonate concentration and alkaline pH in the fluid covering the apical surface of epithelia. Importantly, mucin release, elicited by purinergic stimulation, requires the presence of bicarbonate in the basolateral solution and is defective in cells derived from cystic fibrosis patients. In conclusion, our results suggest that Th-2 cytokines induce a profound change in expression and function in multiple ion channels and transporters that results in enhanced bicarbonate transport ability. This change is required as an important mechanism to favor release and clearance of mucus.

Extracellular Adenine Nucleotides Regulate Na+/H+ Exchanger NHE3 Activity in A6-NHE3 Transfectants by a cAMP/PKA-dependent Mechanism

As potential autocrine or paracrine factors, extracellular nucleotides are known to be important regulators of renal ion transporters by activating cell surface receptors and intracellular signaling pathways. We investigated the influence of extracellular adenine nucleotides on Na+/H+ exchanger isoform 3 (NHE3) activity in A6-NHE3 cells. This is a polarized cell line obtained by stable transfection of A6 cells with the cDNA encoding the rat isoform of NHE3, which is expressed on the apical membrane. Basolateral addition of the P2Y1 agonist, 2-MeSADP, induced an inhibition of NHE3 activity, which was prevented by preincubation with selective P2Y1 antagonists, MRS 2179 (N6-methyl-2?-deoxyadenosine-3?,5?-bisphosphate) and MRS 2286 (2-[2-(2-chloro-6-methylamino-purin-9-yl)-ethyl]-propane-1,3-bisoxy(diammoniumphosphate)). NHE3 activity was also significantly inhibited by ATP and ATP-g-S but not by UTP. 2-MeSADP induced a P2Y1 antagonist-sensitive increase in both [Ca2+]i and cAMP production. Pre-incubation with a PKC inhibitor, Calphostin C, or the calcium chelator BAPTA-AM, had no effect on the 2-MeSADP-dependent inhibition of NHE3 activity, whereas this inhibition was reversed by either incubation with the PKA inhibitor H89 or by mutation of two PKA target serines (S552 and S605) on NHE3. Pre-incubation of the A6-NHE3 cells with the synthetic peptide, Ht31, which prevents the binding between AKAPs and the regulatory PKA subunits RII, also prevented the 2-MeSADP-induced inhibition of NHE3. We conclude that only the cAMP/PKA pathway is involved in the inhibition of NHE3 activity.

Hematopoietic Stem/Progenitor Cells Express Functional Mitochondrial Energy-Dependent Cystic Fibrosis Transmembrane Conductance Regulator

Bone marrow-derived hematopoietic stem/progenitor cells (HSPCs) encompass a wide array of cell subsets with different capacities of engraftment and injured tissue-regenerating potential. The characterization/isolation of the stem cell subpopulations represents a major challenge to improve the efficacy of transplantation protocols used in regenerative medicine. Cystic fibrosis (CF) is one of the diseases whose hope of cure relies on the successful application of cell-based gene therapy. This study was aimed at characterizing murine HSPCs on the basis of their bioenergetic competence and CF transmembrane conductance regulator (CFTR) expression. Positively immunoselected Sca-1(+) HSPCs encompassed 2 populations distinguished by their different size, Sca-1 expression and mitochondrial content. The smaller were the cells, the higher was Sca-1 expression and the lower was the intracellular density of functional mitochondria. Reverse transcription-polymerase chain reaction and western blotting revealed that HSPCs expressed CFTR mRNA and protein, which was also functional, as assessed by spectrofluorimetric and patch-clamp techniques. Inhibition of mitochondrial oxidative phosphorylation by oligomycin resulted in a 70% decrease of both the intracelluar adenosine triphosphate content and CFTR-mediated channel activity. Finally, HSPCs with lower Sca-1 expression and higher mitochondrial content displayed higher CFTR levels. Our findings identify 2 subpopulations in HSPCs and unveil a so-far unappreciated relationship between bioenergetic metabolism and CFTR in HSPC biology.

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

ABSTRACT 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. Highlighted Article: Correctors of F508del CFTR, by activating ezrin and actin cytoskeleton re-organization, generate a sub-cortical pool of cAMP of adequate amplitude to activate chloride secretion.