Search and rescue: finding ways to correct deltaF508 CFTR

Augmentation of CFTR maturation by S-nitrosoglutathione reductase

S-nitrosoglutathione (GSNO) reductase regulates novel endogenous S-nitrosothiol signaling pathways, and mice deficient in GSNO reductase are protected from airways hyperreactivity. S-nitrosothiols are present in the airway, and patients with cystic fibrosis (CF) tend to have low S-nitrosothiol levels that may be attributed to upregulation of GSNO reductase activity. The present study demonstrates that 1) GSNO reductase activity is increased in the cystic fibrosis bronchial epithelial (CFBE41o(-)) cells expressing mutant F508del-cystic fibrosis transmembrane regulator (CFTR) compared with the wild-type CFBE41o(-) cells, 2) GSNO reductase expression level is increased in the primary human bronchial epithelial cells expressing mutant F508del-CFTR compared with the wild-type cells, 3) GSNO reductase colocalizes with cochaperone Hsp70/Hsp90 organizing protein (Hop; Stip1) in human airway epithelial cells, 4) GSNO reductase knockdown with siRNA increases the expression and maturation of CFTR and decreases Stip1 expression in human airway epithelial cells, 5) increased levels of GSNO reductase cause a decrease in maturation of CFTR, and 6) a GSNO reductase inhibitor effectively reverses the effects of GSNO reductase on CFTR maturation. These studies provide a novel approach to define the subcellular location of the interactions between Stip1 and GSNO reductase and the role of S-nitrosothiols in these interactions.

ΔF508 CFTR surface stability is regulated by DAB2 and CHIP-mediated ubiquitination in post-endocytic compartments

The ΔF508 mutant form of the cystic fibrosis transmembrane conductance regulator (ΔF508 CFTR) that is normally degraded by the ER-associated degradative pathway can be rescued to the cell surface through low-temperature (27°C) culture or small molecular corrector treatment. However, it is unstable on the cell surface, and rapidly internalized and targeted to the lysosomal compartment for degradation. To understand the mechanism of this rapid turnover, we examined the role of two adaptor complexes (AP-2 and Dab2) and three E3 ubiquitin ligases (c-Cbl, CHIP, and Nedd4-2) on low-temperature rescued ΔF508 CFTR endocytosis and degradation in human airway epithelial cells. Our results demonstrate that siRNA depletion of either AP-2 or Dab2 inhibits ΔF508 CFTR endocytosis by 69% and 83%, respectively. AP-2 or Dab2 depletion also increases the rescued protein half-life of ΔF508 CFTR by ~18% and ~91%, respectively. In contrast, the depletion of each of the E3 ligases had no effect on ΔF508 CFTR endocytosis, whereas CHIP depletion significantly increased the surface half-life of ΔF508 CFTR. To determine where and when the ubiquitination occurs during ΔF508 CFTR turnover, we monitored the ubiquitination of rescued ΔF508 CFTR during the time course of CFTR endocytosis. Our results indicate that ubiquitination of the surface pool of ΔF508 CFTR begins to increase 15 min after internalization, suggesting that CFTR is ubiquitinated in a post-endocytic compartment. This post-endocytic ubiquination of ΔF508 CFTR could be blocked by either inhibiting endocytosis, by siRNA knockdown of CHIP, or by treating cells with the CFTR corrector, VX-809. Our results indicate that the post-endocytic ubiquitination of CFTR by CHIP is a critical step in the peripheral quality control of cell surface ΔF508 CFTR.

Multicenter intestinal current measurements in rectal biopsies from CF and non-CF subjects to monitor CFTR function

Intestinal current measurements (ICM) from rectal biopsies are a sensitive means to detect cystic fibrosis transmembrane conductance regulator (CFTR) function, but have not been optimized for multicenter use. We piloted multicenter standard operating procedures (SOPs) to detect CFTR activity by ICM and examined key questions for use in clinical trials. SOPs for ICM using human rectal biopsies were developed across three centers and used to characterize ion transport from non-CF and CF subjects (two severe CFTR mutations). All data were centrally evaluated by a blinded interpreter. SOPs were then used across four centers to examine the effect of cold storage on CFTR currents and compare CFTR currents in biopsies from one subject studied simultaneously either at two sites (24 hours post-biopsy) or when biopsies were obtained by either forceps or suction. Rectal biopsies from 44 non-CF and 17 CF subjects were analyzed. Mean differences (µA/cm²; 95% confidence intervals) between CF and non-CF were forskolin/IBMX=102.6(128.0 to 81.1), carbachol=96.3(118.7 to 73.9), forskolin/IBMX+carbachol=200.9(243.1 to 158.6), and bumetanide=-44.6 (-33.7 to -55.6) (P<0.005, CF vs non-CF for all parameters). Receiver Operating Characteristic curves indicated that each parameter discriminated CF from non-CF subjects (area under the curve of 0.94-0.98). CFTR dependent currents following 18-24 hours of cold storage for forskolin/IBMX, carbachol, and forskolin/IBMX+carbachol stimulation (n=17 non-CF subjects) were 44%, 47.5%, and 47.3%, respectively of those in fresh biopsies. CFTR-dependent currents from biopsies studied after cold storage at two sites simultaneously demonstrated moderate correlation (n=14 non-CF subjects, Pearson correlation coefficients 0.389, 0.484, and 0.533). Similar CFTR dependent currents were detected from fresh biopsies obtained by either forceps or suction (within-subject comparisons, n=22 biopsies from three non-CF subjects). Multicenter ICM is a feasible CFTR outcome measure that discriminates CF from non-CF ion transport, offers unique advantages over other CFTR bioassays, and warrants further development as a potential CFTR biomarker.

Polymorphisms in ADRB2 gene can modulate the response to bronchodilators and the severity of cystic fibrosis

Background

The most common cystic fibrosis (CF) manifestation is the progressive chronic obstructive pulmonary disease caused by deficiency, dysfunction, or absence of the CFTR (Cystic Fibrosis Transmembrane Regulator) protein on the apical surface of the cells in the respiratory tract. The use of bronchodilators (BD), and inhaled corticosteroids (IC) have been suggested for the management of airway inflammation in CF. The effectiveness of BD and IC have been verified, proven in laboratory and in the clinical treatment for asthma patients. However, in CF, the effectiveness of these drugs is controversial. The extent of asthma’s response to BD depends on the presence of polymorphisms in the ADRB2 gene. In contrast, in CF, little is known about the response to the BD and the association of CF´s severity with the different polymorphisms in ADRB2 gene. In this context, our objective was to verify whether the Arg16Gly and Glu27Gln polymorphisms in ADRB2 gene are associated with severity and with the bronchodilator response in CF patients.

Method

Cross-sectional study of 122 CF patients subjected to analysis of mutations in the CFTR gene, polymorphisms in ADRB2 gene, along with clinical and laboratorial characteristics of severity.

Result

The Arg16Gly polymorphism in ADRB2 gene was associated with pancreatic insufficiency(p:0.009), Bhalla score(p:0.039), forced expiratory volume in the first second[FEV₁(%)](p:0.003), forced expiratory flow between 25 and 75% of the forced vital capacity-FVC[FEF25-75(%)](p:0.008) and lower age at the first isolation of the Pseudomonas aeruginosa(p:0.012). The response to the BD spirometry was associated with clinical severity markers, FEV₁(%)(p:0.011) and FEF25-75(%)(p:0.019), for the Arg16Gly polymorphism in the ADRB2 gene. The haplotype analysis showed association with the FEV₁/FVC marker from the spirometry test, before and after using the BD, with higher values in the group with Gly/Gly and Glu/Glu, respectively, for the Arg16Gly and Gln27Glu polymorphisms. The analysis by MDR2.0 software, showed association with FEF25-75%; the response to Arg16Gly was respondent by 17.35% and Gln27Glu by 6.8% in variation found.

Conclusion

There was an association between the Arg16Gly and Gln27Glu polymorphisms in ADRB2 gene with CF´s severity and bronchodilator response.

S-Nitrosylation signaling regulates cellular protein interactions

BACKGROUND

S-Nitrosothiols are made by nitric oxide synthases and other metalloproteins. Unlike nitric oxide, S-nitrosothiols are involved in localized, covalent signaling reactions in specific cellular compartments. These reactions are enzymatically regulated.

SCOPE

S-Nitrosylation affects interactions involved in virtually every aspect of normal cell biology. This article is part of a Special Issue entitled Regulation of Cellular Processes by S-nitrosylation.

MAJOR CONCLUSIONS AND SIGNIFICANCE

S-Nitrosylation is a regulated signaling reaction.

Reciprocal control of hERG stability by Hsp70 and Hsc70 with implication for restoration of LQT2 mutant stability

RATIONALE

The human ether-a-go-go-related gene (hERG) encodes the α subunit of the potassium current I(Kr). It is highly expressed in cardiomyocytes and its mutations cause long QT syndrome type 2. Heat shock protein (Hsp)70 is known to promote maturation of hERG. Hsp70 and heat shock cognate (Hsc70) 70 has been suggested to play a similar function. However, Hsc70 has recently been reported to counteract Hsp70.

OBJECTIVE

We investigated whether Hsc70 counteracts Hsp70 in the control of wild-type and mutant hERG stability.

METHODS AND RESULTS

Coexpression of Hsp70 with hERG in HEK293 cells suppressed hERG ubiquitination and increased the levels of both immature and mature forms of hERG. Immunocytochemistry revealed increased levels of hERG in the endoplasmic reticulum and on the cell surface. Electrophysiological studies showed increased I(Kr). All these effects of Hsp70 were abolished by Hsc70 coexpression. Heat shock treatment of HL-1 mouse cardiomyocytes induced endogenous Hsp70, switched mouse ERG associated with Hsc70 to Hsp70, increased I(Kr), and shortened action potential duration. Channels with disease-causing missense mutations in intracellular domains had a higher binding capacity to Hsc70 than wild-type channels and channels with mutations in the pore region. Knockdown of Hsc70 by small interfering RNA or heat shock prevented degradation of mutant hERG proteins with mutations in intracellular domains.

CONCLUSIONS

These results indicate reciprocal control of hERG stability by Hsp70 and Hsc70. Hsc70 is a potential target in the treatment of LQT2 resulting from missense hERG mutations.

Hsp 70/Hsp 90 organizing protein as a nitrosylation target in cystic fibrosis therapy

The endogenous signaling molecule S-nitrosoglutathione (GSNO) and other S-nitrosylating agents can cause full maturation of the abnormal gene product DeltaF508 cystic fibrosis (CF) transmembrane conductance regulator (CFTR). However, the molecular mechanism of action is not known. Here we show that Hsp70/Hsp90 organizing protein (Hop) is a critical target of GSNO, and its S-nitrosylation results in DeltaF508 CFTR maturation and cell surface expression. S-nitrosylation by GSNO inhibited the association of Hop with CFTR in the endoplasmic reticulum. This effect was necessary and sufficient to mediate GSNO-induced cell-surface expression of DeltaF508 CFTR. Hop knockdown using siRNA recapitulated the effect of GSNO on DeltaF508 CFTR maturation and expression. Moreover, GSNO acted additively with decreased temperature, which promoted mutant CFTR maturation through a Hop-independent mechanism. We conclude that GSNO corrects DeltaF508 CFTR trafficking by inhibiting Hop expression, and that combination therapies--using differing mechanisms of action--may have additive benefits in treating CF.