Tissue and cellular expression patterns of porcine CFTR: similarities to and differences from human CFTR

Genomic, biochemical and expressional properties reveal strong conservation of the CLCA2 gene in birds and mammals

Recent studies have revealed the dynamic and complex evolution of CLCA1 gene homologues in and between mammals and birds with a particularly high diversity in mammals. In contrast, CLCA2 has only been found as a single copy gene in mammals, to date. Furthermore, CLCA2 has only been investigated in few mammalian species but not in birds. Here, we established core genomic, protein biochemical and expressional properties of CLCA2 in several bird species and compared them with mammalian CLCA2. Chicken, turkey, quail and ostrich CLCA2 were compared to their mammalian orthologues using in silico, biochemical and expressional analyses. CLCA2 was found highly conserved not only at the level of genomic and exon architecture but also in terms of the canonical CLCA2 protein domain organization. The putatively prototypical galline CLCA2 (gCLCA2) was cloned and immunoblotting as well as immunofluorescence analyses of heterologously expressed gCLCA2 revealed protein cleavage, glycosylation patterns and anchoring in the plasma membrane similar to those of most mammalian CLCA2 orthologues. Immunohistochemistry found highly conserved CLCA2 expression in epidermal keratinocytes in all birds and mammals investigated. Our results suggest a highly conserved and likely evolutionarily indispensable role of CLCA2 in keratinocyte function. Its high degree of conservation on the genomic, biochemical and expressional levels stands in contrast to the dynamic structural complexities and proposed functional diversifications between mammalian and avian CLCA1 homologues, insinuating a significant degree of negative selection of CLCA2 orthologues among birds and mammals. Finally, and again in contrast to CLCA1, the high conservation of CLCA2 makes it a strong candidate for studying basic properties of the functionally still widely unresolved CLCA gene family.

Evolutionarily conserved properties of CLCA proteins 1, 3 and 4, as revealed by phylogenetic and biochemical studies in avian homologues

Species-specific diversities are particular features of mammalian chloride channel regulator, calcium activated (CLCA) genes. In contrast to four complex gene clusters in mammals, only two CLCA genes appear to exist in chickens. CLCA2 is conserved in both, while only the galline CLCA1 (gCLCA1) displays close genetic distance to mammalian clusters 1, 3 and 4. In this study, sequence analyses and biochemical characterizations revealed that gCLCA1 as a putative avian prototype shares common protein domains and processing features with all mammalian CLCA homologues. It has a transmembrane (TM) domain in the carboxy terminal region and its mRNA and protein were detected in the alimentary canal, where the protein was localized in the apical membrane of enterocytes, similar to CLCA4. Both mammals and birds seem to have at least one TM domain containing CLCA protein with complex glycosylation in the apical membrane of enterocytes. However, some characteristic features of mammalian CLCA1 and 3 including entire protein secretion and expression in cell types other than enterocytes seem to be dispensable for chicken. Phylogenetic analyses including twelve bird species revealed that avian CLCA1 and mammalian CLCA3 form clades separate from a major branch containing mammalian CLCA1 and 4. Overall, our data suggest that gCLCA1 and mammalian CLCA clusters 1, 3 and 4 stem from a common ancestor which underwent complex gene diversification in mammals but not in birds.

The molecular evolution of function in the CFTR chloride channel

The ATP-binding cassette (ABC) transporter superfamily includes many proteins of clinical relevance, with genes expressed in all domains of life. Although most members use the energy of ATP binding and hydrolysis to accomplish the active import or export of various substrates across membranes, the cystic fibrosis transmembrane conductance regulator (CFTR) is the only known animal ABC transporter that functions primarily as an ion channel. Defects in CFTR, which is closely related to ABCC subfamily members that bear function as bona fide transporters, underlie the lethal genetic disease cystic fibrosis. This article seeks to integrate structural, functional, and genomic data to begin to answer the critical question of how the function of CFTR evolved to exhibit regulated channel activity. We highlight several examples wherein preexisting features in ABCC transporters were functionally leveraged as is, or altered by molecular evolution, to ultimately support channel function. This includes features that may underlie (1) construction of an anionic channel pore from an anionic substrate transport pathway, (2) establishment and tuning of phosphoregulation, and (3) optimization of channel function by specialized ligand–channel interactions. We also discuss how divergence and conservation may help elucidate the pharmacology of important CFTR modulators.

The Family of Chloride Channel Regulator, Calcium-activated Proteins in the Feline Respiratory Tract: A Comparative Perspective on Airway Diseases in Man and Animal Models

Members of the chloride channel regulator, calcium-activated (CLCA) family are considered to be modifiers in inflammatory, mucus-based respiratory conditions such as asthma and cystic fibrosis. Previous work has shown substantial differences between human and murine CLCA orthologues that limit the value of mouse models. As an alternative, the cat is an unfamiliar but powerful model of human asthma. We therefore characterized the expression profiles of CLCA proteins in the feline respiratory tract. Identical to other species, the feline CLCA1 protein was immunohistochemically localized to virtually all goblet cells and found to be secreted into the mucus. However, it was not detected in submucosal glands where it is expressed in other species. In contrast to all other species studied to date, feline CLCA2 was not found in submucosal glands or any other airway cells. Similar to mice, but in contrast to man and pigs, the feline respiratory tract was devoid of CLCA4 expression. In the airways of asthmatic cats, CLCA1 was strongly overexpressed, similar to human patients. Therefore, despite some similarities in CLCA1 protein expression and secretion, substantial differences were identified between several feline CLCA family members and their respective orthologues in man, mice and pigs, which must be considered in comparative medicine.

Cystic fibrosis transmembrane conductance regulator-emerging regulator of cancer

Mutations of cystic fibrosis transmembrane conductance regulator (CFTR) cause cystic fibrosis, the most common life-limiting recessive genetic disease among Caucasians. CFTR mutations have also been linked to increased risk of various cancers but remained controversial for a long time. Recent studies have begun to reveal that CFTR is not merely an ion channel but also an important regulator of cancer development and progression with multiple signaling pathways identified. In this review, we will first present clinical findings showing the correlation of genetic mutations or aberrant expression of CFTR with cancer incidence in multiple cancers. We will then focus on the roles of CFTR in fundamental cellular processes including transformation, survival, proliferation, migration, invasion and epithelial-mesenchymal transition in cancer cells, highlighting the signaling pathways involved. Finally, the association of CFTR expression levels with patient prognosis, and the potential of CFTR as a cancer prognosis indicator in human malignancies will be discussed.

A sequence upstream of canonical PDZ-binding motif within CFTR COOH-terminus enhances NHERF1 interaction

The development of cystic fibrosis transmembrane conductance regulator (CFTR) targeted therapy for cystic fibrosis has generated interest in maximizing membrane residence of mutant forms of CFTR by manipulating interactions with scaffold proteins, such as sodium/hydrogen exchange regulatory factor-1 (NHERF1). In this study, we explored whether COOH-terminal sequences in CFTR beyond the PDZ-binding motif influence its interaction with NHERF1. NHERF1 displayed minimal self-association in blot overlays (NHERF1, Kd = 1,382 ± 61.1 nM) at concentrations well above physiological levels, estimated at 240 nM from RNA-sequencing and 260 nM by liquid chromatography tandem mass spectrometry in sweat gland, a key site of CFTR function in vivo. However, NHERF1 oligomerized at considerably lower concentrations (10 nM) in the presence of the last 111 amino acids of CFTR (20 nM) in blot overlays and cross-linking assays and in coimmunoprecipitations using differently tagged versions of NHERF1. Deletion and alanine mutagenesis revealed that a six-amino acid sequence 1417EENKVR1422 and the terminal 1478TRL1480 (PDZ-binding motif) in the COOH-terminus were essential for the enhanced oligomerization of NHERF1. Full-length CFTR stably expressed in Madin-Darby canine kidney epithelial cells fostered NHERF1 oligomerization that was substantially reduced (∼5-fold) on alanine substitution of EEN, KVR, or EENKVR residues or deletion of the TRL motif. Confocal fluorescent microscopy revealed that the EENKVR and TRL sequences contribute to preferential localization of CFTR to the apical membrane. Together, these results indicate that COOH-terminal sequences mediate enhanced NHERF1 interaction and facilitate the localization of CFTR, a property that could be manipulated to stabilize mutant forms of CFTR at the apical surface to maximize the effect of CFTR-targeted therapeutics.

Ubiquitous LEA29Y Expression Blocks T Cell Co-Stimulation but Permits Sexual Reproduction in Genetically Modified Pigs

We have successfully established and characterized a genetically modified pig line with ubiquitous expression of LEA29Y, a human CTLA4-Ig derivate. LEA29Y binds human B7.1/CD80 and B7.2/CD86 with high affinity and is thus a potent inhibitor of T cell co-stimulation via this pathway. We have characterized the expression pattern and the biological function of the transgene as well as its impact on the porcine immune system and have evaluated the potential of these transgenic pigs to propagate via assisted breeding methods. The analysis of LEA29Y expression in serum and multiple organs of CAG-LEA transgenic pigs revealed that these animals produce a biologically active transgenic product at a considerable level. They present with an immune system affected by transgene expression, but can be maintained until sexual maturity and propagated by assisted reproduction techniques. Based on previous experience with pancreatic islets expressing LEA29Y, tissues from CAG-LEA29Y transgenic pigs should be protected against rejection by human T cells. Furthermore, their immune-compromised phenotype makes CAG-LEA29Y transgenic pigs an interesting large animal model for testing human cell therapies and will provide an important tool for further clarifying the LEA29Y mode of action.

Naturally Occurring Deletion Mutants of the Pig-Specific, Intestinal Crypt Epithelial Cell Protein CLCA4b without Apparent Phenotype

The human CLCA4 (chloride channel regulator, calcium-activated) modulates the intestinal phenotype of cystic fibrosis (CF) patients via an as yet unknown pathway. With the generation of new porcine CF models, species-specific differences between human modifiers of CF and their porcine orthologs are considered critical for the translation of experimental data. Specifically, the porcine ortholog to the human CF modulator gene CLCA4 has recently been shown to be duplicated into two separate genes, CLCA4a and CLCA4b. Here, we characterize the duplication product, CLCA4b, in terms of its genomic structure, tissue and cellular expression patterns as well as its in vitro electrophysiological properties. The CLCA4b gene is a pig-specific duplication product of the CLCA4 ancestor and its protein is exclusively expressed in small and large intestinal crypt epithelial cells, a niche specifically occupied by no other porcine CLCA family member. Surprisingly, a unique deleterious mutation of the CLCA4b gene is spread among modern and ancient breeds in the pig population, but this mutation did not result in an apparent phenotype in homozygously affected animals. Electrophysiologically, neither the products of the wild type nor of the mutated CLCA4b genes were able to evoke a calcium-activated anion conductance, a consensus feature of other CLCA proteins. The apparently pig-specific duplication of the CLCA4 gene with unique expression of the CLCA4b protein variant in intestinal crypt epithelial cells where the porcine CFTR is also present raises the question of whether it may modulate the porcine CF phenotype. Moreover, the naturally occurring null variant of CLCA4b will be valuable for the understanding of CLCA protein function and their relevance in modulating the CF phenotype.

Characterizing diverse orthologues of the cystic fibrosis transmembrane conductance regulator protein for structural studies

As an ion channel, the cystic fibrosis transmembrane conductance regulator (CFTR) protein occupies a unique niche within the ABC family. Orthologues of CFTR are extant throughout the animal kingdom from sharks to platypods to sheep, where the osmoregulatory function of the protein has been applied to differing lifestyles and diverse organ systems. In humans, loss-of-function mutations to CFTR cause the disease cystic fibrosis, which is a significant health burden in populations of white European descent. Orthologue screening has proved fruitful in the pursuit of high-resolution structural data for several membrane proteins, and we have applied some of the princples developed in previous studies to the expression and purification of CFTR. We have overexpressed this protein, along with evolutionarily diverse orthologues, in Saccharomyces cerevisiae and developed a purification to isolate it in quantities sufficient for structural and functional studies.

Pyocyanin inhibits both nitric oxide-dependent and -independent relaxation in porcine coronary arteries

The effects of the Pseudomonas aeruginosa virulence factor pyocyanin (PCN) on the contractile function of porcine coronary arteries was investigated in vitro. Artery rings (5 mm) were suspended in organ baths containing Krebs' solution for the measurement of isometric tension. The effect of PCN on resting and precontracted coronary arteries was initially investigated with various agents. Arteries were precontracted with prostaglandin (PG) F2α or potassium chloride and endothelium-dependent relaxations were induced by various agents in the presence of PCN. Pyocyanin (0.1-10 μmol/L) evoked small-amplitude, dose-dependent contractions in resting porcine coronary arteries. In addition, PCN amplified the contractile response to PGF2α , but did not alter responses to carbachol. Pyocyanin (0.1-10 μmol/L) significantly inhibited endothelium-dependent relaxations evoked by neurokinin A. Pyocyanin also inhibited relaxations evoked by diethylamine nitric oxide (a nitric oxide donor), forskolin (an adenylate cyclase activator), dibuytyryl-cAMP (a cAMP analogue), 8-bromo-cGMP (a cGMP analogue) and P1075 (a KATP channel activator), but not isoprenaline (β-adrenoceceptor agonist). These results indicate that physiological concentrations of PCN interfere with multiple intracellular processes involved in vascular smooth muscle relaxation, in particular pathways downstream of nitric oxide release. Thus, PCN may alter normal vascular function in patients infected with P. aeruginosa.

Differentiation between human ClC-2 and CFTR Cl− channels with pharmacological agents

It has been difficult to separate/identify the roles of ClC-2 and CFTR in Cl− transport studies. Using pharmacological agents, we aimed to differentiate functionally between ClC-2 and CFTR Cl− channel currents. Effects of CFTR inhibitor 172 (CFTRinh172), N-(4-methylphenylsulfonyl)- N′-(4-trifluoromethylphenyl)urea (DASU-02), and methadone were examined by whole cell patch clamp on Cl− currents in recombinant human ClC-2/human embryonic kidney 293 (ClC-2/HEK293) cells stably transformed with Epstein-Barr nuclear antigen 1 (hClC-2/293EBNA) and human CFTR/HEK293 (hCFTR/HEK293) cells and by short-circuit current ( Isc) measurements in T84 cells. Lubiprostone and forskolin-IBMX were used as activators. CFTRinh172 inhibited forskolin-IBMX-stimulated recombinant human CFTR (hCFTR) and lubiprostone-stimulated recombinant human ClC-2 (hClC-2) Cl− currents in a concentration-dependent manner equipotently. DASU-02 inhibited forskolin-IBMX-stimulated Cl− currents in hCFTR/HEK293 cells, but not lubiprostone-stimulated Cl− currents in hClC-2/293EBNA cells. In T84 cells with basolateral nystatin or 1-ethyl-2-benzimidazolinone (1-EBIO), lubiprostone-stimulated and forskolin-IBMX-cyclosporin A (FICA)-stimulated Isc components were observed. CFTRinh172 inhibited major portions of both components. DASU-02 had no effect on lubiprostone-stimulated Isc but partially inhibited FICA-stimulated Isc. T84 cells in which ClC-2 or CFTR was knocked down using siRNAs were constructed. T84 ClC-2 knockdown cells did not respond to lubiprostone but did respond to forskolin-IBMX in a methadone-insensitive, DASU-02-sensitive manner, indicating CFTR function. T84 CFTR knockdown cells responded separately to lubiprostone and forskolin-IBMX in a methadone-sensitive and DASU-02-insensitive manner, indicating ClC-2 function. Low lubiprostone concentrations activated ClC-2, but not CFTR, and both channels were activated by forskolin-IBMX but have different inhibitor sensitivities. Methadone, but not DASU-02, inhibited ClC-2. DASU-02, but not methadone, inhibited CFTR. In T84 cells, both ClC-2 and CFTR are present and likely play roles in Cl− secretion.

Regulation of ACh release from guinea pig bladder urothelial cells: potential role in bladder filling sensations

BACKGROUND AND PURPOSE

The aim of this study was to quantify and characterize the mechanism of non-neuronal ACh release from bladder urothelial cells and to determine if urothelial cells could be a site of action of anti-muscarinic drugs.

EXPERIMENTAL APPROACH

A novel technique was developed whereby ACh could be measured from freshly isolated guinea pig urothelial cells in suspension following mechanical stimulation. Various agents were used to manipulate possible ACh release pathways in turn and to study the effects of muscarinic receptor activation and inhibition on urothelial ATP release.

KEY RESULTS

Minimal mechanical stimulus achieved full ACh release, indicating a small dynamic range and possible all-or-none signal. ACh release involved a mechanism dependent on the anion channel CFTR and intracellular calcium concentration, but was independent of extracellular calcium, vesicular trafficking, connexins or pannexins, organic cation transporters and was not affected by botulinum-A toxin. Stimulating ACh receptors increased ATP production and antagonizing them reduced ATP release, suggesting a link between ACh and ATP release.

CONCLUSIONS AND IMPLICATIONS

These results suggest that release of non-neuronal ACh from the urothelium is large enough and well located to act as a modulator of ATP release. It is hypothesized that this pathway may contribute to the actions of anti-muscarinic drugs in reducing the symptoms of lower urinary tract syndromes. Additionally the involvement of CFTR in ACh release suggests an exciting new direction for the treatment of these conditions.

Airway deposition of nebulized gene delivery nanocomplexes monitored by radioimaging agents

Receptor-targeted nanocomplexes are nonviral vectors developed for gene delivery to the airway epithelium for the treatment of pulmonary disease associated with cystic fibrosis. The present study aimed to optimize the delivery of the nanocomplex by nebulization, and to monitor the in vivo deposition of radiolabeled vector in the airways of a large animal model by γ-camera scintigraphy. Large White weaner pigs were nebulized with nanocomplexes mixed with technetium-99m radiopharmaceuticals. The aerosol deposition scans suggested that the nebulized radiovectors were deposited mainly in the trachea-main bronchi and in the midregion of the lungs. The plasmid biodistribution, assessed by real-time PCR, correlated with the scintigraphy images. The highest plasmid copy numbers were found in the bronchial areas and in the tissues proximal to the main bronchi bifurcation. Immunohistochemistry detected transgene expression in the tracheal and bronchial ciliated epithelium. Histological analysis of lung tissue showed no evidence of inflammation, and no increase in inflammatory cytokines or inflammatory cells was detected in the bronchoalveolar lavage. The deposition of nebulized nanocomplexes coassociated with technetium-99m can be monitored by nuclear medicine techniques. The use of a noninvasive strategy to follow the delivery of the vector could improve the clinical management of patients undergoing cystic fibrosis gene therapy.

Efficient gene delivery to pig airway epithelia and submucosal glands using helper-dependent adenoviral vectors

Airway gene delivery is a promising strategy to treat patients with life-threatening lung diseases such as cystic fibrosis (CF). However, this strategy has to be evaluated in large animal preclinical studies in order to translate it to human applications. Because of anatomic and physiological similarities between the human and pig lungs, we utilized pig as a large animal model to examine the safety and efficiency of airway gene delivery with helper-dependent adenoviral vectors. Helper-dependent vectors carrying human CFTR or reporter gene LacZ were aerosolized intratracheally into pigs under bronchoscopic guidance. We found that the LacZ reporter and hCFTR transgene products were efficiently expressed in lung airway epithelial cells. The transgene vectors with this delivery can also reach to submucosal glands. Moreover, the hCFTR transgene protein localized to the apical membrane of both ciliated and nonciliated epithelial cells, mirroring the location of wild-type CF transmembrane conductance regulator (CFTR). Aerosol delivery procedure was well tolerated by pigs without showing systemic toxicity based on the limited number of pigs tested. These results provide important insights into developing clinical strategies for human CF lung gene therapy.Molecular Therapy-Nucleic Acids (2013) 2, e127; doi:10.1038/mtna.2013.55; published online 8 October 2013.

Synthesis of porcine pCLCA2 protein during late differentiation of keratinocytes of epidermis and hair follicle inner root sheath

Despite the discovery of the widely expressed CLCA (chloride channel regulators, calcium-activated) proteins more than 15 years ago, their seemingly diverse functions are still poorly understood. With the recent generation of porcine animal models for cystic fibrosis (CF), members of the porcine CLCA family are becoming of interest as possible modulators of the disease in the pig. Here, we characterize pCLCA2, the porcine ortholog of the human hCLCA2 and the murine mCLCA5, which are the only CLCA members expressed in the skin. Immunohistochemical studies with a specific antibody against pCLCA2 have revealed a highly restricted pCLCA2 protein expression in the skin. The protein is strictly co-localized with filaggrin and trichohyalin in the granular layer of the epidermis and the inner root sheath of the hair follicles, respectively. No differences have been observed between the expression patterns of wild-type pigs and CF transmembrane conductance regulator(-/-) pigs. We speculate that pCLCA2 plays an as yet undefined role in the structural integrity of the skin or, possibly, in specialized functions of the epidermis, including barrier or defense mechanisms.

The porcine chloride channel calcium-activated family member pCLCA4a mirrors lung expression of the human hCLCA4

Pig models of cystic fibrosis (CF) have recently been established that are expected to mimic the human disease closer than mouse models do. The human CLCA (originally named chloride channels, calcium-activated) member hCLCA4 is considered a potential modifier of disease severity in CF, but its murine ortholog, mCLCA6, is not expressed in the mouse lung. Here, we have characterized the genomic structure, protein processing, and tissue expression patterns of the porcine ortholog to hCLCA4, pCLCA4a. The genomic structure and cellular protein processing of pCLCA4a were found to closely mirror those of hCLCA4 and mCLCA6. Similar to human lung, pCLCA4a mRNA was strongly expressed in porcine lungs, and the pCLCA4a protein was immunohistochemically detected on the apical membranes of tracheal and bronchial epithelial cells. This stands in sharp contrast to mouse mCLCA6, which has been detected exclusively in intestinal epithelia but not the murine lung. The results may add to the understanding of species-specific differences in the CF phenotype and support the notion that the CF pig model may be more suitable than murine models to study the role of hCLCA4.

Sequential targeting of CFTR by BAC vectors generates a novel pig model of cystic fibrosis

Cystic fibrosis (CF) is the most common lethal inherited disease in Caucasians and is caused by mutations in the CFTR gene. The disease is incurable and medical treatment is limited to the amelioration of symptoms or secondary complications. A comprehensive understanding of the disease mechanisms and the development of novel treatment options require appropriate animal models. Existing CF mouse models fail to reflect important aspects of human CF. We thus generated a CF pig model by inactivating the CFTR gene in primary porcine cells by sequential targeting using modified bacterial artificial chromosome vectors. These cells were then used to generate homozygous CFTR mutant piglets by somatic cell nuclear transfer. The homozygous CFTR mutants lack CFTR protein expression and display severe malformations in the intestine, respiratory tract, pancreas, liver, gallbladder, and male reproductive tract. These phenotypic abnormalities closely resemble both the human CF pathology as well as alterations observed in a recently published CF pig model which was generated by a different gene targeting strategy. Our new CF pig model underlines the value of the CFTR-deficient pig for gaining new insight into the disease mechanisms of CF and for the development and evaluation of new therapeutic strategies. This model will furthermore increase the availability of CF pigs to the scientific community.

The murine goblet cell protein mCLCA3 is a zinc-dependent metalloprotease with autoproteolytic activity

Several members of the CLCA family of proteins, originally named chloride channels, calcium-activated, have been shown to modulate chloride conductance in various cell types via an unknown mechanism. Moreover, the human (h) hCLCA1 is thought to modulate the severity of disease in asthma and cystic fibrosis (CF) patients. All CLCA proteins are post-translationally cleaved into two subunits, and recently, a conserved HEXXH zinc-binding amino acid motif has been identified, suggesting a role for CLCA proteins as metalloproteases. Here, we have characterized the cleavage and autoproteolytic activity of the murine model protein mCLCA3, which represents the murine orthologue of human hCLCA1. Using crude membrane fractions from transfected HEK293 cells, we demonstrate that mCLCA3 cleavage is zinc-dependent and exclusively inhibited by cation-chelating metalloprotease inhibitors. Cellular transport and secretion were not affected in response to a cleavage defect that was introduced by the insertion of an E157Q mutation within the HEXXH motif of mCLCA3. Interspecies conservation of these key results was further confirmed with the porcine (p) orthologue of hCLCA1 and mCLCA3, pCLCA1. Importantly, the mCLCA3E157Q mutant was cleaved after co-transfection with the wild-type mCLCA3 in HEK293 cells, suggesting that an intermolecular autoproteolytic event takes place. Edman degradation and MALDI-TOF-MS of the protein fragments identified a single cleavage site in mCLCA3 between amino acids 695 and 696. The data strongly suggest that secreted CLCA proteins have zinc-dependent autoproteolytic activity and that they may cleave additional proteins.