Novel pharmacologic therapies for cystic fibrosis

Osmolytes: Wonder molecules to combat protein misfolding against stress conditions

The proper functioning of any protein depends on its three dimensional conformation which is achieved by the accurate folding mechanism. Keeping away from the exposed stress conditions leads to cooperative unfolding and sometimes partial folding, forming the structures like protofibrils, fibrils, aggregates, oligomers, etc. leading to several neurodegenerative diseases like Parkinson's disease, Alzheimer's, Cystic fibrosis, Huntington, Marfan syndrome, and also cancers in some cases, too. Hydration of proteins is necessary, which may be achieved by the presence of organic solutes called osmolytes within the cell. Osmolytes belong to different classes in different organisms and play their role by preferential exclusion of osmolytes and preferential hydration of water molecules and achieves the osmotic balance in the cell otherwise it may cause problems like cellular infection, cell shrinkage leading to apoptosis and cell swelling which is also the major injury to the cell. Osmolyte interacts with protein, nucleic acids, intrinsically disordered proteins by non-covalent forces. Stabilizing osmolytes increases the Gibbs free energy of the unfolded protein and decreases that of folded protein and vice versa with denaturants (urea and guanidinium hydrochloride). The efficacy of each osmolyte with the protein is determined by the calculation of m value which reflects its efficiency with protein. Hence osmolytes can be therapeutically considered and used in drugs.

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.

Bioactive Peptide Natural Products as Lead Structures for Medicinal Use

The need for new drugs for the treatment of various diseases is enormous. From the previous century until the present, numerous peptide and peptide-derived natural products have been isolated from bacteria and fungi. Hence, microorganisms play a pivotal role as sources for novel drugs with an emphasis on anti-infective agents. Various disciplines from biology, chemistry, and medicine are involved in early stages of the search for peptide natural products including taxonomy, microbiology, bioanalytics, bioinformatics, and medicinal chemistry. Under biochemical aspects, small peptide drugs are basically either ribosomally synthesized and post-translationally modified (RiPPs) or synthesized by multimodular nonribosomal peptide synthetases (NRPSs). Within the context of current developments on bioactive peptide natural products, this Account predominantly highlights recent discoveries, approaches, and research from our laboratory on RiPPs and NRPSs from bacteria and fungi. In our search for peptides showing bioactivities of interest, different approaches were applied: classical screening, in silico prediction, in vitro reconstitution, site-directed mutagenesis, chemoenzymatics, heterologous expression, and total synthesis including structure-activity relationship (SAR) studies in the research on the labyrinthopeptins, albicidin, and the cyclodepsipeptides (CDPs). The ribosomally synthesized labyrinthopeptins, class III lanthipeptides, which have been discovered in a classical screening campaign, display highly attractive antiallodynic (against neuropathic pain caused by dysfunction of the nervous system) and antiviral activities. Therefore, the biosynthetic assembly was investigated by extensive enzymatic studies of the modifying enzymes, and site-directed mutagenesis was performed for the generation of analogs. By genome mining, other class III lanthipeptides have been uncovered, while synthetic access proved to be an unmet challenge for the labyrinthopeptins. In contrast, for the gyrase inhibitor albicidin, the establishment of a chemical synthesis followed by medicinal chemistry studies was the only viable option to gain access to derivatives. Albicidin, which has been discovered investigating plant host-pathogen interactions, has a strong activity against Gram-negative bacteria, for example, Escherichia coli and Pseudomonas aeruginosa, and a future synthetic derivative may become a lead structure for development of an anti-Gram-negative drug. The compound class of the cyclodepsipeptides contributes already two marketed drugs, enniatin (fusafungine) and emodepside. Cyclodepsipeptides show general antibacterial and antifungal effects, whereas specific insecticidal and anthelmintic activities provide lead structures for drug development. Hence, exploiting the chances of reprogramming NRPSs, the generation of chimeric or otherwise designed synthetases could render a new untapped structural space and thus novel bioactivities. While current developments in the fields of genomics, bioinformatics, and molecular biology facilitate the search for new natural products and the design of new peptide structures, the next decade will show which compounds have been carried on further applications and whether current developments have led to an increase in drug candidates.

Interference with ubiquitination in CFTR modifies stability of core glycosylated and cell surface pools

It is recognized that both wild-type and mutant CFTR proteins undergo ubiquitination at multiple lysines in the proteins and in one or more subcellular locations. We hypothesized that ubiquitin is added to specific sites in wild-type CFTR to stabilize it and at other sites to signal for proteolysis. Mass spectrometric analysis of wild-type CFTR identified ubiquitinated lysines 68, 710, 716, 1041, and 1080. We demonstrate that the ubiquitinated K710, K716, and K1041 residues stabilize wild-type CFTR, protecting it from proteolysis. The polyubiquitin linkage is predominantly K63. N-tail mutants, K14R and K68R, lead to increased mature band CCFTR, which can be augmented by proteasomal (but not lysosomal) inhibition, allowing trafficking to the surface. The amount of CFTR in the K1041R mutant was drastically reduced and consisted of bands A/B, suggesting that the site in transmembrane 10 (TM10) was critical to further processing beyond the proteasome. The K1218R mutant increases total and cell surface CFTR, which is further accumulated by proteasomal and lysosomal inhibition. Thus, ubiquitination at residue 1218 may destabilize wild-type CFTR in both the endoplasmic reticulum (ER) and recycling pools. Small molecules targeting the region of residue 1218 to block ubiquitination or to preserving structure at residues 710 to 716 might be protein sparing for some forms of cystic fibrosis.

The iodide-transport-defect-causing mutation R124H: a δ-amino group at position 124 is critical for maturation and trafficking of the Na+/I- symporter

Na(+)/I(-) symporter (NIS)-mediated active accumulation of I(-) in thyrocytes is a key step in the biosynthesis of the iodine-containing thyroid hormones T3 and T4. Several NIS mutants have been identified as a cause of congenital I(-) transport defect (ITD), and their investigation has yielded valuable mechanistic information on NIS. Here we report novel findings derived from the thorough characterization of the ITD-causing mutation R124H, located in the second intracellular loop (IL-2). R124H NIS is incompletely glycosylated and colocalizes with endoplasmic reticulum (ER)-resident protein markers. As a result, R124H NIS is not targeted to the plasma membrane and therefore does not mediate any I(-) transport in transfected COS-7 cells. Strikingly, however, the mutant is intrinsically active, as revealed by its ability to mediate I(-) transport in membrane vesicles. Of all the amino acid substitutions we carried out at position 124 (K, D, E, A, W, N and Q), only Gln restored targeting of NIS to the plasma membrane and NIS activity, suggesting a key structural role for the δ-amino group of R124 in the transporter's maturation and cell surface targeting. Using our NIS homology model based on the structure of the Vibrio parahaemolyticus Na(+)/galactose symporter, we propose an interaction between the δ-amino group of either R or Q124 and the thiol group of C440, located in IL-6. We conclude that the interaction between IL-2 and IL-6 is critical for the local folding required for NIS maturation and plasma membrane trafficking.

Applications of proteomic technologies for understanding the premature proteolysis of CFTR

Cystic fibrosis (CF) is caused by mutations in the CF transmembrane conductance regulator (CFTR) gene, which encodes an ATP-dependent anion channel. Disease-causing mutations can affect channel biogenesis, trafficking or function, and result in reduced ion transport at the apical surface of many tissues. The most common CFTR mutation is a deletion of phenylalanine at position 508 (DeltaF508), which results in a misfolded protein that is prematurely targeted for degradation. This article focuses on how proteomic approaches have been utilized to explore the mechanisms of premature proteolysis in CF. Additionally, we emphasize the potential for proteomic-based technologies in expanding our understanding of CF pathophysiology and therapeutic approaches.

Heterozygous variant at nucleotide position 875+11A>T in exon 6A cystic fibrosis transmembrane conductance regulator gene induces 852del22 mutation false-positivity by line probe assay

OBJECTIVE

To explain the lack of genotype-phenotype correlation observed in a patient double heterozygous for the 852del22 and F508del mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene.

DESIGN

Case report.

SETTING

Medical laboratory department.

PATIENT(S)

A 42-year-old asymptomatic patient underwent genetic screening for in vitro fertilization (IVF).

INTERVENTION(S)

CFTR genetic screening (commercial kit aimed at detecting 57 mutations), segregation analysis, evaluation of the polymerase chain reaction (PCR) products using a denaturing high performance liquid chromatography (DHPLC), and sequence analysis.

MAIN OUTCOME MEASURE(S)

To avoid diagnostic errors and improve genetic counseling.

RESULT(S)

Segregation analysis allowed us to establish that the mutations were in trans. Analysis of the PCR products using a DHPLC apparatus showed a heteroduplex formation indicative of a heterozygous variant in exon 6A. Direct sequencing characterized the heterozygous variant as an A to T transversion at nucleotide position 875+11. Therefore, the change of one single nucleotide in a portion surrounding the 852del22 mutation facilitated the aspecific interaction between the commercial oligonucleotide probe and the amplified genomic DNA, which explains the 852del22 mutation false molecular positivity that was detected by the line probe assay.

CONCLUSION(S)

The individualization of 852del22 mutation by a standard genetic panel should be confirmed by more extensive analyses.

Diagnosis of cystic fibrosis

Cystic fibrosis (CF) is an autosomal recessive disease caused by mutations in the CF transmembrane conductance regulator (CFTR) gene that results in abnormal viscous mucoid secretions in multiple organs and whose main clinical features are pancreatic insufficiency and chronic endobronchial infection. Although it was initially defined and diagnosed based on clinical features and sweat chloride measurement, an in vivo method of assessing CFTR function, the discovery of the CFTR gene in 1989 revealed a broad spectrum of CF phenotypes associated with specific CFTR gene mutations. In this article, we will review the indications for sweat testing, alternative techniques to diagnose CF, and the approach to patients with an ambiguous or indeterminate diagnosis of CF.

Animal models of chronic lung infection with Pseudomonas aeruginosa: useful tools for cystic fibrosis studies

Cystic fibrosis (CF) is caused by a defect in the transmembrane conductance regulator (CFTR) protein that functions as a chloride channel. Dysfunction of the CFTR protein results in salty sweat, pancreatic insufficiency, intestinal obstruction, male infertility and severe pulmonary disease. In most patients with CF life expectancy is limited due to a progressive loss of functional lung tissue. Early in life a persistent neutrophylic inflammation can be demonstrated in the airways. The cause of this inflammation, the role of CFTR and the cause of lung morbidity by different CF-specific bacteria, mostly Pseudomonas aeruginosa, are not well understood. The lack of an appropriate animal model with multi-organ pathology having the characteristics of the human form of CF has hampered our understanding of the pathobiology and chronic lung infections of the disease for many years. This review summarizes the main characteristics of CF and focuses on several available animal models that have been frequently used in CF research. A better understanding of the chronic lung infection caused particularly by P. aeruginosa, the pathophysiology of lung inflammation and the pathogenesis of lung disease necessitates animal models to understand CF, and to develop and improve treatment.

Transcription factor oligodeoxynucleotides to NF-kappaB inhibit transcription of IL-8 in bronchial cells

Chronic pulmonary inflammation in patients affected by cystic fibrosis (CF) is characterized by massive bronchial infiltrates of neutrophils, which is sustained by the interaction of pathogens (e.g., Pseudomonas aeruginosa) with surface bronchial cells. To explore new treatment options focused on the reduction of neutrophil chemotaxis, we applied the transcription factor (TF) decoy approach, based on the intracellular delivery of double-stranded oligodeoxynucleotides (ODNs) causing inhibition of the binding of TF-related proteins to the different consensus sequences in the promoter of specific genes. In CF bronchial IB3-1 cells, P. aeruginosa induced transcription of the neutrophil chemokines IL-8 and GRO-gamma, of the adhesion molecule intercellular adhesion molecule (ICAM)-1, and of the cytokines IL-1beta and IL-6. Since consensus sequences for the TF, NF-kappaB, are contained in the promoters of all these genes, IB3-1, CuFi-1, Beas-2B, and CaLu-3 cells were transfected with double-stranded TF "decoy" ODNs mimicking different NF-kappaB consensus sequences. IL-8 NF-kappaB decoy ODN partially inhibited the P. aeruginosa-dependent transcription of IL-8, GRO-gamma, and IL-6, whereas decoy ODNs to both HIV-1 long terminal repeat and Igk produced a strong, 80 to 85% inhibition of transcription of IL-8, without reducing that of GRO-gamma, ICAM-1, IL-1beta, and IL-6. In conclusion, intracellular delivery of "decoy" molecules aimed to compete with the TF, NF-kappaB, is a promising strategy to obtain inhibition of IL-8 gene transcription.

Targeted therapy for cystic fibrosis: cystic fibrosis transmembrane conductance regulator mutation-specific pharmacologic strategies

Cystic fibrosis (CF) results from the absence or dysfunction of a single protein, the CF transmembrane conductance regulator (CFTR). CFTR plays a critical role in the regulation of ion transport in a number of exocrine epithelia. Improvement or restoration of CFTR function, where it is deficient, should improve the CF phenotype. There are >1000 reported disease-causing mutations of the CFTR gene. Recent investigations have afforded a better understanding of the mechanism of dysfunction of many of these mutant CFTRs, and have allowed them to be classified according to their mechanism of dysfunction. These data, as well as an enhanced understanding of the role of CFTR in regulating epithelial ion transport, have led to the development of therapeutic strategies based on pharmacologic enhancement or repair of mutant CFTR dysfunction. The strategy, termed 'protein repair therapy', is aimed at improving the regulation of epithelial ion transport by mutant CFTRs in a mutation-specific fashion. The grouping of CFTR gene mutations, according to mechanism of dysfunction, yields some guidance as to which pharmacologic repair agents may be useful for specific CFTR mutations. Recent data has suggested that combinations of pharmacologic repair agents may be necessary to obtain clinically meaningful CFTR repair. Nevertheless, such strategies to improve mutant CFTR function hold great promise for the development of novel therapies aimed at correcting the underlying pathophysiology of CF.

Characterization of vasopressin V2 receptor mutants in nephrogenic diabetes insipidus in a polarized cell model

X-linked nephrogenic diabetes insipidus (NDI) is caused by mutations in the gene encoding the vasopressin V2 receptor (V2R). For the development of a tailored therapy for NDI, knowledge of the cellular fate of V2R mutants is needed. It would be useful when this fate could be predicted from the location and type of mutation. To identify similarities and differences in localization, maturation, stability, and degradation of COOH-terminal GFP-tagged V2R mutants, we stably expressed nine mutants in polarized Madin-Darby canine kidney cells. The mutants V2R-L44P, -Delta62-64, -I130F, -S167T, -S167L, and -V206D were mainly expressed in the endoplasmic reticulum (ER) as immature proteins. These mutants had relatively short half-lives due to proteasomal degradation, except for V2R-Delta62-64. In contrast, V2R-R113W, -G201D, and -T204N were expressed in the ER and in the basolateral membrane as immature, high-mannose glycosylated, and mature complex-glycosylated proteins. The immature forms of V2R-R113W and -T204N, but not V2R-G201D, were rapidly degraded. The mature forms varied extensively in their stability and were degraded by only lysosomes (V2R-T204N and wild-type V2R) or lysosomes and proteasomes (V2R-G201D, -R113W). These data reveal that most missense V2R mutations lead to retention in the ER and suggest that mutations that likely distort a transmembrane domain or introduce a charged amino acid close to it make a V2R mutant more prone to ER retention. Because six of the mutants tested showed significant increases in intracellular cAMP levels on transient expression in COS cells, activation of these six receptors following rescue of cell-surface expression might provide a cure for NDI patients.

Molecular analysis of a congenital iodide transport defect: G543E impairs maturation and trafficking of the Na+/I- symporter

The Na+/I- symporter (NIS) is a key membrane glycoprotein that mediates active I- transport in the thyroid and other tissues. Upon isolation of the cDNA encoding NIS, 10 NIS mutations that cause congenital iodide transport defect have been identified. Three of these mutations (T354P, G395R, and Q267E) have been thoroughly characterized at the molecular level. All three NIS mutant proteins are correctly targeted to the plasma membrane; however, whereas Q267E displays minimal activity, T354P and G395R are inactive. Here, we show that in contrast to these mutants, G543E NIS matures only partially and is retained intracellularly; thus, it is not targeted properly to the cell surface, apparently because of faulty folding. These findings indicate that the G543 residue plays significant roles in NIS maturation and trafficking. Remarkably, NIS activity was rescued by small neutral amino acid substitutions (volume < 129 A3) at this position, suggesting that G543 is in a tightly packed region of NIS.

Novel, mechanism-based therapies for cystic fibrosis

PURPOSE OF REVIEW

Cystic fibrosis results from disruption of the biosynthesis or function of the cystic fibrosis transmembrane conductance regulator. Cystic fibrosis transmembrane conductance regulator plays a critical role in the regulation of epithelial ion transport. Restoration of cystic fibrosis transmembrane conductance regulator function should improve the cystic fibrosis phenotype.

RECENT FINDINGS

Recent investigations affording a better understanding of the mechanism of dysfunction of mutant cystic fibrosis transmembrane conductance regulators, as well as the roles of cystic fibrosis transmembrane conductance regulator in regulating epithelial ion transport, have led to development of therapeutic strategies based on repair or bypass of mutant cystic fibrosis transmembrane conductance regulator dysfunction. The former strategy, coined 'protein repair therapy,' is aimed at improving or restoring the function of mutant cystic fibrosis transmembrane conductance regulators, whereas the latter approach aims to augment epithelial ion transport to compensate for the absent function mutant cystic fibrosis transmembrane conductance regulator.

SUMMARY

Strategies to improve mutant cystic fibrosis transmembrane conductance regulator function or to bypass mutant cystic fibrosis transmembrane conductance regulator function hold great promise for development of novel therapies aimed at correcting the underlying pathophysiology of cystic fibrosis.

Beyond the signal sequence: protein routing in health and disease

Receptors, hormones, enzymes, ion channels, and structural components of the cell are created by the act of protein synthesis. Synthesis alone is insufficient for proper function, of course; for a cell to operate effectively, its components must be correctly compartmentalized. The mechanism by which proteins maintain the fidelity of localization warrants attention in light of the large number of different molecules that must be routed to distinct subcellular loci, the potential for error, and resultant disease. This review summarizes diseases known to have etiologies based on defective protein folding or failure of the cell's quality control apparatus and presents approaches for therapeutic intervention.

Phenotypic expression of genotype-phenotype correlation in cystic fibrosis patients carrying the 852del22 mutation

Currently, more than 1,000 mutations have been identified in the cystic fibrosis transmembrane regulator (CFTR) gene. While some mutations are common worldwide, the majority are restricted in certain ethnic groups. We have found that in Southern Italy, the 852del22 mutation is well represented with a frequency of 3.5%. We have screened, by reverse dot blot, denaturing gradient gel electrophoresis (DGGE), and gene sequencing, the entire coding regions of CFTR gene in 371 consecutive cystic fibrosis (CF) patients from Southern Italy and have identified 17 patients carrying rare genotypes, among which 13 [6 M; median age 21.7 years (range: 4.5-47.7 years)] carry the 852del22 mutation. To assess the phenotypic expression of CF in patients with the 852del22 mutations we have compared these patients with a group of age and gender matched patients homozygous for the DeltaF508 mutation [n = 34; 19 M; median age 19.9 years (range: 3.8-34.6 years)]. Overall, we found no difference in terms of complications, patient survival (17.6% vs. 30.7%; P = NS), estimated time needed to develop a severe lung disease (22.1 vs. 24.5 years; P = NS), nutritional status, and rate of infection or colonization by most common pathogens between patients in the two groups. Finally, we have found that a late diagnosis was associated with a poor outcome (severe lung disease) regardless of genotype. Our data show that 852del22 mutation results in a phenotypic expression of disease as severe as that determined by the more typical DeltaF508 and, as in the latter case, there is no strict genotype/phenotype correlation.

Sildenafil (Viagra) corrects DeltaF508-CFTR location in nasal epithelial cells from patients with cystic fibrosis

BACKGROUND

Most patients with cystic fibrosis (CF) have a DeltaF508 mutation resulting in abnormal retention of mutant gene protein (DeltaF508-CFTR) within the cell. This study was undertaken to investigate DeltaF508-CFTR trafficking in native cells from patients with CF with the aim of discovering pharmacological agents that can move DeltaF508-CFTR to its correct location in the apical cell membrane.

METHOD

Nasal epithelial cells were obtained by brushing from individuals with CF. CFTR location was determined using immunofluorescence and confocal imaging in untreated cells and cells treated with sildenafil. The effect of sildenafil treatment on CFTR chloride transport function was measured in CF15 cells using an iodide efflux assay.

RESULTS

In most untreated CF cells DeltaF508-CFTR was mislocalised within the cell at a site close to the nucleus. Exposure of cells to sildenafil (2 hours at 37 degrees C) resulted in recruitment of DeltaF508-CFTR to the apical membrane and the appearance of chloride transport activity. Sildenafil also increased DeltaF508-CFTR trafficking in cells from individuals with CF with a single copy DeltaF508 (DeltaF508/4016ins) or with a newly described CF trafficking mutation (R1283M).

CONCLUSIONS

The findings provide proof of principle for sildenafil as a DeltaF508-CFTR trafficking drug and give encouragement for future testing of sildenafil and related PDE5 inhibitors in patients with CF.

A comparison of 14 antibodies for the biochemical detection of the cystic fibrosis transmembrane conductance regulator protein

Interest in the biochemical detection of the cystic fibrosis (CF) transmembrane conductance regulator (CFTR) protein followed soon after cloning of the gene and prediction of the protein structure. Ever since, antibodies (Abs) have been produced and used to detect CFTR in both heterologously and endogenously expressing cells and tissues. Although designed to be sensitive and specific, these Abs produce, in most cases, unsatisfactory results when used for the biochemical detection of CFTR either by Western blot or by immunoprecipitation. The lack of Abs that can reliably detect the CFTR protein is a major constraint to studies of CF. We compared 14 different Abs for their ability to detect CFTR in both stably transfected and endogenously expressing cell lines.

Extracellular Zinc and ATP Restore Chloride Secretion across Cystic Fibrosis Airway Epithelia by Triggering Calcium Entry

Cystic fibrosis (CF) is caused by defective cyclic AMP-dependent cystic fibrosis transmembrane conductance regulator Cl(-) channels. Thus, CF epithelia fail to transport Cl(-) and water. A postulated therapeutic avenue in CF is activation of alternative Ca(2+)-dependent Cl(-) channels. We hypothesized that stimulation of Ca(2+) entry from the extracellular space could trigger a sustained Ca(2+) signal to activate Ca(2+)-dependent Cl(-) channels. Cytosolic [Ca(2+)](i) was measured in non-polarized human CF (IB3-1) and non-CF (16HBE14o(-)) airway epithelial cells. Primary human CF and non-CF airway epithelial monolayers as well as Calu-3 monolayers were used to assess anion secretion. In vivo nasal potential difference measurements were performed in non-CF and two different CF mouse (DeltaF508 homozygous and bitransgenic gut-corrected but lung-null) models. Zinc and ATP induced a sustained, reversible, and reproducible increase in cytosolic Ca(2+) in CF and non-CF cells with chemistry and pharmacology most consistent with activation of P2X purinergic receptor channels. P2X purinergic receptor channel-mediated Ca(2+) entry stimulated sustained Cl(-) and HCO(3)(-) secretion in CF and non-CF epithelial monolayers. In non-CF mice, zinc and ATP induced a significant Cl(-) secretory response similar to the effects of agonists that increase intracellular cAMP levels. More importantly, in both CF mouse models, Cl(-) permeability of nasal epithelia was restored in a sustained manner by zinc and ATP. These effects were reversible and reacquirable upon removal and readdition of agonists. Our data suggest that activation of P2X calcium entry channels may have profound therapeutic benefit for CF that is independent of cystic fibrosis transmembrane conductance regulator genotype.

Bringing new treatments to the bedside in cystic fibrosis

The discovery of the cystic fibrosis transmembrane conductance regulator gene in 1989 led to a dramatic increase in the understanding of the molecular basis of CF. Increased knowledge has provided the opportunity to target drug development at correcting the basic defect either by gene therapy or pharmacological modulation of the abnormal physiological processes. Development of new medications for the CF population poses many challenges. The discovery and development of new medications is always time consuming and expensive. Since CF affects a small population worldwide, the potential for a drug company to profit from a new treatment is limited. In addition, each new therapy must have an additional and proven benefit to be attractive to clinicians and consumers, otherwise it will not be commercially viable. Demonstrating clinical benefit is problematic as a limited number of patients are available to participate in clinical trails and outcome measures, such as length of life, are hard to measure. In this review we will illustrate these challenges by discussing the development of treatments which have successfully reached the bedside and those that were unsuccessful.

Benzoflavone activators of the cystic fibrosis transmembrane conductance regulator: towards a pharmacophore model for the nucleotide-binding domain

Our previous screen of flavones and related heterocycles for the ability to activate the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel indicated that UCCF-029, a 7,8-benzoflavone, was a potent activator. In the present study, we describe the synthesis and evaluation, using cell-based assays, of a series of benzoflavone analogues to examine structure-activity relationships and to identify compounds having greater potency for activation of both wild type CFTR and a mutant CFTR (G551D-CFTR) that causes cystic fibrosis in some human subjects. Using UCCF-029 as a structural guide, a panel of 77 flavonoid analogues was prepared. Analysis of the panel in FRT cells indicated that benzannulation of the flavone A-ring at the 7,8-position greatly improved compound activity and potency for several flavonoids. Incorporation of a B-ring pyridyl nitrogen either at the 3- or 4-position also elevated CFTR activity, but the influence of this structural modification was not as uniform as the influence of benzannulation. The most potent new analogue, UCCF-339, activated wild-type CFTR with a K(d) of 1.7 microM, which is more active than the previous most potent flavonoid activator of CFTR, apigenin. Several compounds in the benzoflavone panel also activated G551D-CFTR, but none were as active as apigenin. Pharmacophore modeling suggests a common binding mode for the flavones and other known CFTR activators at one of the nucleotide-binding sites, allowing for the rational development of more potent flavone analogues.

Classification and rescue of ROMK mutations underlying hyperprostaglandin E syndrome/antenatal Bartter syndrome

BACKGROUND

Mutations in the renal K+ channel ROMK (Kir 1.1) cause hyperprostaglandin E syndrome/antenatal Bartter syndrome (HPS/aBS), a severe tubular disorder leading to renal salt and water wasting. Several studies confirmed the predominance of alterations of current properties in ROMK mutants. However, in most of these studies, analysis was restricted to nonmammalian cells and electrophysiologic methods. Therefore, for the majority of ROMK mutations, disturbances in protein trafficking remained unclear. The aim of the present study was the evaluation of different pathogenic mechanisms of 20 naturally occurring ROMK mutations with consecutive classification into mutational classes and identification of distinct rescue mechanisms according to the underlying defect.

METHODS

Mutated ROMK potassium channels were expressed in Xenopus oocytes and a human kidney cell line and analyzed by two electrode voltage clamp analysis, immunofluorescence, and Western blot analysis.

RESULTS

We identified 14 out of 20 ROMK mutations that did not reach the cell surface, indicating defective membrane trafficking. High expression levels rescued six out of 14 ROMK mutants, leading to significant K+ currents. In addition, two early inframe stop mutations could be rescued by aminoglycosides, resulting in full-length ROMK and correct trafficking to the plasma membrane in a subset of transfected cells.

CONCLUSION

In contrast to previous reports, most of the investigated ROMK mutations displayed a trafficking defect that might be rescued by pharmacologic agents acting as molecular chaperones. The evaluation of different disease-causing mechanisms will be essential for establishing new and more specific therapeutic strategies for HPS/aBS patients.

CFTR et les anomalies des transports ioniques dans la mucoviscidose

The genetic disease cystic fibrosis (CF) is caused by mutations of the CF gene and generates defective Cl- transport across the affected epithelium. Recent progress have been made to understand CFTR activity and regulation in epithelia and its role in the muco-ciliary clearance of airway. This revue-overviews the mechanisms of transepithelial ion transport, the role of CFTR in that process and the consequences for CF of CFTR mutations.

Distinct cytokine production by lung and blood neutrophils from children with cystic fibrosis

Inflammation plays a critical role in lung disease progression in cystic fibrosis (CF). This inflammatory process is dominated by a neutrophil influx in the airways. To determine whether the accumulation of neutrophils in the airways of CF patients is associated with an altered function, we analyzed the capacity of neutrophils isolated from the lung compartment and the blood to release the major neutrophil pro- and anti-inflammatory cytokines IL-8 and IL-1-receptor antagonist (ra) spontaneously and in the presence of LPS. Comparison of cytokine production by blood neutrophils from CF patients and from control subjects showed significantly increased IL-8 and decreased IL-1ra release by CF neutrophils. Comparison of cytokine production by airway and blood neutrophils from CF patients also documented distinct profiles: the spontaneous release of IL-8 and IL-1ra by airway neutrophils was significantly higher than that from blood neutrophils. Culture in the presence of LPS failed to further enhance cytokine production. Analysis of the effect of dexamethasone confirmed the difference in the responsiveness of lung and blood neutrophils in CF. Used at a concentration effective in reducing IL-8 production by blood neutrophils, dexamethasone (10(-6) M) was unable to repress secretion of IL-8 by airway neutrophils. In addition, comparison of cytokine production by airway neutrophils from children with CF and children with dyskinetic cilia syndrome also documented distinct profiles of secretion. These results are consistent with a dysregulated cytokine production by lung and blood neutrophils in CF. They provide support to the hypothesis that not only the CF genotype but also the local environment may modify the functional properties of the neutrophils.

Transition of care between paediatric and adult gastroenterology. Cystic fibrosis

Advancement in treatment has resulted in a dramatic increase in life expectancy of patients with cystic fibrosis (CF) to well beyond 30 years of age in most centres. What was once a fatal genetic disease in childhood now sees over a third of its CF populations in adult clinics. The improved survival is multifactorial, but most attribute the reasons to more aggressive nutritional care along with better management of the gastrointestinal and pulmonary systems. Many of the nutritional and GI issues of paediatric patients remain similar when they become adults, except that there is the added psychosocial stress associated with this transition. This chapter aims to highlight the gastrointestinal and nutritional issues manifesting at the different stages of life from infancy to adulthood, and the recommended management.

Disease genes: flattery and deception

Completion of the human genome project raises the possibility of genetically based treatments for a multitude of human diseases. As yet only a handful of patients have benefited clinically from this approach. Why gene transfer is such a complex issue is discussed in this article. Theoretically, the easiest diseases to treat are single gene recessive diseases, where, presumably, gene delivery to somatic cells is all that is required. Two prime candidates for gene therapy are severe combined immunodeficiency disease (SCID) and cystic fibrosis (CF). Attempts to treat both of these diseases by gene therapy commenced in the late 1980s. Some clinical benefit has been recorded with SCID, but none, as yet, has been recorded with CF.

The cystic fibrosis mutation G551D alters the non-Michaelis-Menten behavior of the cystic fibrosis transmembrane conductance regulator (CFTR) channel and abolishes the inhibitory Genistein binding site

Loss of cystic fibrosis transmembrane conductance regulator (CFTR) channel activity explains most of the manifestations of the cystic fibrosis (CF) disease. To understand the consequences of CF mutations on CFTR channel activity, we compared the pharmacological properties of wild-type (wt) and G551D-CFTR. Dose-dependent relationships of wt-CFTR activated by genistein follows a non-Michaelis-Menten behavior consistent with the presence of two binding sites. With phosphorylated CFTR, a high affinity site for genistein is the activator (K(s) approximately 3 microm), whereas a second site of low affinity (K(i) approximately 75 microm) is the inhibitor. With non-phosphorylated CFTR, K(s) was increased (K(s) approximately 12 microm), but K(i) was not affected (K(i) approximately 70 microm). In G551D-CFTR cells, channel activity was recovered by co-application of forskolin and genistein in a dose-dependent manner. A further stimulation of G551D-CFTR channel activity was measured at concentrations from 30 microm to 1 mm. The dose response is described by a classical Michaelis-Menten kinetics with only a single apparent site (K(m) approximately 11 microm). Our results suggest glycine 551 in NBD1 as an important location within the low affinity inhibitory site for genistein and offers new evidence for pharmacological alteration caused by an NBD1 mutation of CFTR. This study also reveals how a mutation of an ion channel converts a non-Michaelis-Menten behavior (two binding sites) into a classical Michaelis-Menten model (one binding site).

Cell-biologic and functional analyses of five new Aquaporin-2 missense mutations that cause recessive nephrogenic diabetes insipidus

Mutations in the Aquaporin-2 gene, which encodes a renal water channel, have been shown to cause autosomal nephrogenic diabetes insipidus (NDI), a disease in which the kidney is unable to concentrate urine in response to vasopressin. Most AQP2 missense mutants in recessive NDI are retained in the endoplasmic reticulum (ER), but AQP2-T125M and AQP2-G175R were reported to be nonfunctional channels unimpaired in their routing to the plasma membrane. In five families, seven novel AQP2 gene mutations were identified and their cell-biologic basis for causing recessive NDI was analyzed. The patients in four families were homozygous for mutations, encoding AQP2-L28P, AQP2-A47V, AQP2-V71M, or AQP2-P185A. Expression in oocytes revealed that all these mutants, and also AQP2-T125M and AQP2-G175R, conferred a reduced water permeability compared with wt-AQP2, which was due to ER retardation. The patient in the fifth family had a G>A nucleotide substitution in the splice donor site of one allele that results in an out-of-frame protein. The other allele has a nucleotide deletion (c652delC) and a missense mutation (V194I). The routing and function of AQP2-V194I in oocytes was not different from wt-AQP2; it was therefore concluded that c652delC, which leads to an out-of-frame protein, is the NDI-causing mutation of the second allele. This study indicates that misfolding and ER retention is the main, and possibly only, cell-biologic basis for recessive NDI caused by missense AQP2 proteins. In addition, the reduced single channel water permeability of AQP2-A47V (40%) and AQP2-T125M (25%) might become of therapeutic value when chemical chaperones can be found that restore their routing to the plasma membrane.

Methoxsalen stimulates electrogenic Cl- secretion in the mouse jejunum

We used the short-circuit current (I(sc)) and patch-clamp techniques to investigate the effects of methoxsalen (MTX) on the electrogenic Cl- secretion of the mouse jejunum. MTX stimulated a sustained increase in Isc that was dose dependent. Bumetanide inhibited MTX-stimulated Isc in a dose-dependent manner consistent with activation of Cl- secretion. MTX failed to stimulate I(sc) following maximal activation of the cAMP pathway by forskolin, but did increase Isc after a submaximal dose of forskolin. Glibenclamide, a blocker of the cystic fibrosis transmembrane conductance regulator (CFTR), reduced the MTX-stimulated increase of Isc by 59 +/- 6%. The cAMP-dependent K+ channel blocker 293B did not alter the MTX-activated I(sc); however, clotrimazole, an intermediate Ca2(+)-activated K+ channel (IK(Ca)) blocker, reduced the MTX-stimulated I(sc). MTX did not alter Na(+)-glucose cotransport across the mouse jejunum. In cell-attached membrane patches, MTX increased the open probability of the basolateral IK(Ca) channel of isolated crypts. These data suggest that the CFTR and IK(Ca) channels participate in the MTX-activated, sustained Cl- secretory response of the mouse jejunum.

Synthesis of a molecular mimic of the Glc1Man9 oligoside as potential inhibitor of calnexin binding to DeltaF508 CFTR protein

Deletion of phenylalanine at position 508 of the CFTR protein is associated with a severe form of cystic fibrosis. Biosynthetic arrest of the misfolded DeltaF508 CFTR protein in the endoplasmic reticulum is due to prolonged interaction with protein chaperones. In order to overcome this retention and thereby restore the delivery of the protein to the plasma membrane, a molecular mimic of the glycoprotein oligoside moiety has been designed and synthesized. Ability of this mimic to inhibit the binding of the natural Glc1Man9GlcNAc oligoside to calnexin has been measured.

Calcium-pump inhibitors induce functional surface expression of Delta F508-CFTR protein in cystic fibrosis epithelial cells

The most common mutation in cystic fibrosis, Delta F508, results in a cystic fibrosis transmembrane conductance regulator (CFTR) protein that is retained in the endoplasmic reticulum (ER). Retention is dependent upon chaperone proteins, many of which require Ca(++) for optimal activity. Interfering with chaperone activity by depleting ER Ca(++) stores might allow functional Delta F508-CFTR to reach the cell surface. We exposed several cystic fibrosis cell lines to the ER Ca(++) pump inhibitor thapsigargin and evaluated surface expression of Delta F508-CFTR. Treatment released ER-retained Delta F508-CFTR to the plasma membrane, where it functioned effectively as a Cl(-) channel. Treatment with aerosolized calcium-pump inhibitors reversed the nasal epithelial potential defect observed in a mouse model of Delta F508-CFTR expression. Thus, ER calcium-pump inhibitors represent a potential target for correcting the cystic fibrosis defect.

Hyperacidification of cellubrevin endocytic compartments and defective endosomal recycling in cystic fibrosis respiratory epithelial cells

The cystic fibrosis transmembrane conductance regulator (CFTR), which is aberrant in patients with cystic fibrosis, normally functions both as a chloride channel and as a pleiotropic regulator of other ion transporters. Here we show, by ratiometric imaging with luminally exposed pH-sensitive green fluorescent protein, that CFTR affects the pH of cellubrevin-labeled endosomal organelles resulting in hyperacidification of these compartments in cystic fibrosis lung epithelial cells. The excessive acidification of intracellular organelles was corrected with low concentrations of weak base. Studies with proton ATPase and sodium channel inhibitors showed that the increased acidification was dependent on proton pump activity and sodium transport. These observations implicate sodium efflux in the pH homeostasis of a subset of endocytic organelles and indicate that a dysfunctional CFTR in cystic fibrosis leads to organellar hyperacidification in lung epithelial cells because of a loss of CFTR inhibitory effects on sodium transport. Furthermore, recycling of transferrin receptor was altered in CFTR mutant cells, suggesting a previously unrecognized cellular defect in cystic fibrosis, which may have functional consequences for the receptors on the plasma membrane or within endosomal compartments.

Cystic fibrosis

Cystic fibrosis is the most common lethal inherited disorder with autosomal recessive inheritance. Major progress has been made in understanding the molecular mechanisms leading to increased susceptibility to Pseudomonas aeruginosa colonization. Persistent respiratory infection with P. aeruginosa leads to progressive pulmonary inflammation and is the major cause of morbidity and mortality. Treatment and prophylaxis of respiratory infection has improved the median survival and quality of life of cystic fibrosis patients. In the future, treatment of the underlying genetic defect may be possible.

Correction of CFTR malfunction and stimulation of Ca-activated Cl channels restore HCO3- secretion in cystic fibrosis bile ductular cells

In view of the occurrence of hepatobiliary disorders in cystic fibrosis (CF) this study addresses the role of the cystic fibrosis transmembrane conductance regulator (CFTR) and of Ca(2+)-activated Cl(-) channels in promoting HCO3- secretion in bile ductular cells. Human cholangiocytes were isolated from control livers and from 1 patient with CF (DeltaF508/G542X mutations). Single channel and whole cell currents were analyzed by patch clamp techniques, and HCO3- secretion was determined by fluorometric analysis of the rate of recovery of intracellular pH following alkaline loading. In control cholangiocytes, both cyclic adenosine monophosphate (cAMP) and protein kinase A (PKA) catalytic subunit, activated CFTR Cl(-) channels that exhibited a nonrectifying conductance of 8 pS and appeared in clusters. Activation of Cl(-) current by cAMP was associated with an increase in the rate of HCO3- secretion. The basal rate of HCO3- secretion was lower in CF than in control cholangiocytes. In both control and CF cholangiocytes, raising intracellular Ca(2+) concentrations with ionomycin led to a parallel activation of Cl(-) current and HCO3- secretion. Consistent with reports that premature stop codon mutations (class I; e.g., G542X) can be read over by treatment with aminoglycoside antibiotics, exposure of CF cholangiocytes to gentamicin restored activation by cAMP of Cl(-) current and HCO3- secretion. The observation that activation of Ca(2+)-dependent Cl(-) channels can substitute for cystic fibrosis transmembrane conductance regulator (CFTR) in supporting HCO3- secretion and the efficacy of gentamicin in restoring CFTR function and HCO3- secretion in class I mutations are of potential clinical interest.

Trafficking of GFP-tagged DeltaF508-CFTR to the plasma membrane in a polarized epithelial cell line

The DeltaF508 mutation reduces the amount of cystic fibrosis transmembrane conductance regulator (CFTR) expressed in the plasma membrane of epithelial cells. However, a reduced temperature, butyrate compounds, and "chemical chaperones" allow DeltaF508-CFTR to traffic to the plasma membrane and increase Cl(-) permeability in heterologous and nonpolarized cells. Because trafficking is affected by the polarized state of epithelial cells and is cell-type dependent, our goal was to determine whether these maneuvers induce DeltaF508-CFTR trafficking to the apical plasma membrane in polarized epithelial cells. To this end, we generated and characterized a line of polarized Madin-Darby canine kidney (MDCK) cells stably expressing DeltaF508-CFTR tagged with green fluorescent protein (GFP). A reduced temperature, glycerol, butyrate, or DMSO had no effect on 8-(4-chlorophenylthio)-cAMP (CPT-cAMP)-stimulated transepithelial Cl(-) secretion across polarized monolayers. However, when the basolateral membrane was permeabilized, butyrate, but not the other experimental maneuvers, increased the CPT-cAMP-stimulated Cl(-) current across the apical plasma membrane. Thus butyrate increased the amount of functional DeltaF508-CFTR in the apical plasma membrane. Butyrate failed to stimulate transepithelial Cl(-) secretion because of inhibitory effects on Cl(-) uptake across the basolateral membrane. These observations suggest that studies on heterologous and nonpolarized cells should be interpreted cautiously. The GFP tag on DeltaF508-CFTR will allow investigation of DeltaF508-CFTR trafficking in living, polarized MDCK epithelial cells in real time.

Recent advances in cystic fibrosis

The median life expectancy for cystic fibrosis is now over 30 years, and it is projected that in newborn infants it will become more than 40 years. The identification of the cystic fibrosis gene and its product, cystic fibrosis transmembrane conductance regulator (CFTR), has widened the spectrum of the disease from the classical case of the infant with cystic fibrosis to the elderly childless man with unexplained bronchiectasis. There is increasing evidence of the advantages of newborn screening for cystic fibrosis and subsequent specialist care. Management concentrates on optimising nutritional status and preventing lung infection and inflammation.

Glycerol increases the yield and activity of human phenylalanine hydroxylase mutant enzymes produced in a prokaryotic expression system

Chemical chaperones are low molecular weight compounds known to stabilize proteins in vitro. Recently it was shown that, in transfected cells, these molecules can also correct the defective folding of some mutant proteins. Hyperphenylalaninemia (HPA) has been proposed to be classified as a "conformational disease," since it has been shown that the majority of the PAH mutations affect protein folding, thereby causing an increasing tendency toward aggregation and proteolytic degradation. Based on these observations, the effect of glycerol as a stabilizer agent of recombinant mutant forms of human phenylalanine hydroxylase enzymes (hPAH) produced in a prokaryotic expression system was investigated. The wild-type and two mutant forms of the hPAH protein (R270K and V388M) were expressed in the presence of glycerol in the culture medium. The yield, specific enzymatic activities, and kinetic properties of the recombinant proteins were determined and compared with the data obtained under normal growth conditions. The results obtained demonstrate that glycerol not only improved the yield of the soluble hPAH proteins (2- to 3-fold depending on the mutant enzyme) produced but also increased the specific activity of the purified recombinant enzymes. We speculate that correction of protein folding abnormalities by chemical chaperones may be a possible therapeutic approach to correct conformational diseases.