Chloride channels and cystic fibrosis of the pancreas

Ion Channel Signature in Healthy Pancreas and Pancreatic Ductal Adenocarcinoma

Pancreatic ductal adenocarcinoma (PDAC) is the fourth most common cause of cancer-related deaths in United States and Europe. It is predicted that PDAC will become the second leading cause of cancer-related deaths during the next decades. The development of PDAC is not well understood, however, studies have shown that dysregulated exocrine pancreatic fluid secretion can contribute to pathologies of exocrine pancreas, including PDAC. The major roles of healthy exocrine pancreatic tissue are secretion of enzymes and bicarbonate rich fluid, where ion channels participate to fine-tune these biological processes. It is well known that ion channels located in the plasma membrane regulate multiple cellular functions and are involved in the communication between extracellular events and intracellular signaling pathways and can function as signal transducers themselves. Hereby, they contribute to maintain resting membrane potential, electrical signaling in excitable cells, and ion homeostasis. Despite their contribution to basic cellular processes, ion channels are also involved in the malignant transformation from a normal to a malignant phenotype. Aberrant expression and activity of ion channels have an impact on essentially all hallmarks of cancer defined as; uncontrolled proliferation, evasion of apoptosis, sustained angiogenesis and promotion of invasion and migration. Research indicates that certain ion channels are involved in the aberrant tumor growth and metastatic processes of PDAC. The purpose of this review is to summarize the important expression, localization, and function of ion channels in normal exocrine pancreatic tissue and how they are involved in PDAC progression and development. As ion channels are suggested to be potential targets of treatment they are furthermore suggested to be biomarkers of different cancers. Therefore, we describe the importance of ion channels in PDAC as markers of diagnosis and clinical factors.

The road to aryl CHanion binding was paved with good intentions: fundamental studies, host design, and historical perspectives in CH hydrogen bonding

Throughout the design and development of supramolecular receptors for anion binding, many different non-covalent anion-binding motifs have been employed. One motif seen in many host-guest systems is the sometimes weaker, 'non-traditional' aryl CH hydrogen bond. From June Sutor's discovery of the interaction and its subsequent dismissal by the field in the 1960s to today's use of the aryl CH hydrogen bond in synthetic anion receptors, the path our lab took to begin studying this interaction has been influenced by many other researchers in the field. This feature article highlights the history and properties of the CH hydrogen bond, with a particular focus on aryl CH hydrogen bonds in anion recognition. We highlight select recent developments in the field of anion receptors utilizing aryl CH hydrogen bonds, with an emphasis on how this has influenced the evolution of our approach in designing fundamental studies on CH hydrogen bonding and exploiting this interaction in efforts aimed toward preferential anion binding.

Animal Models in the Pathophysiology of Cystic Fibrosis

Our understanding of the multiorgan pathology of cystic fibrosis (CF) has improved impressively during the last decades, but we still lack a full comprehension of the disease progression. Animal models have greatly contributed to the elucidation of specific mechanisms involved in CF pathophysiology and the development of new therapies. Soon after the cloning of the CF transmembrane conductance regulator (CFTR) gene in 1989, the first mouse model was generated and this model has dominated in vivo CF research ever since. Nonetheless, the failure of murine models to mirror human disease severity in the pancreas and lung has led to the generation of larger animal models such as pigs and ferrets. The following review presents and discusses data from the current animal models used in CF research.

Personalized medicine for cystic fibrosis: establishing human model systems

With over 1,500 identifiable mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene that result in distinct functional and phenotypical abnormalities, it is virtually impossible to perform randomized clinical trials to identify the best therapeutics for all patients. Therefore, a personalized medicine approach is essential. The only way to realistically accomplish this is through the development of improved in vitro human model systems. The lack of a readily available and infinite supply of human CFTR-expressing airway epithelial cells is a key bottleneck. We propose that a concerted two-pronged approach is necessary for patient-specific cystic fibrosis research to continue to prosper and realize its potential: (1) more effective culture and differentiation conditions for growing primary human airway and nasal epithelial cells and (2) the development of collective protocols for efficiently differentiating disease- and patient-specific induced pluripotent stem cells (iPSC) into pure populations of adult epithelial cells. Ultimately, we need a personalized human model system for cystic fibrosis with the capacity for uncomplicated bankability, widespread availability, and universal applicability for patient-specific disease modeling, novel pharmacotherapy investigation and screening, and readily executable genetic modification.

Ferret and pig models of cystic fibrosis: prospects and promise for gene therapy

Large animal models of genetic diseases are rapidly becoming integral to biomedical research as technologies to manipulate the mammalian genome improve. The creation of cystic fibrosis (CF) ferrets and pigs is an example of such progress in animal modeling, with the disease phenotypes in the ferret and pig models more reflective of human CF disease than mouse models. The ferret and pig CF models also provide unique opportunities to develop and assess the effectiveness of gene and cell therapies to treat affected organs. In this review, we examine the organ disease phenotypes in these new CF models and the opportunities to test gene therapies at various stages of disease progression in affected organs. We then discuss the progress in developing recombinant replication-defective adenoviral, adeno-associated viral, and lentiviral vectors to target genes to the lung and pancreas in ferrets and pigs, the two most affected organs in CF. Through this review, we hope to convey the potential of these new animal models for developing CF gene and cell therapies.

Porcine nasal epithelial cultures for studies of cystic fibrosis sinusitis

BACKGROUND

Transgenic cystic fibrosis (CF) murine models do not develop spontaneous lung or sinus disease, 2 major causes of morbidity in human CF patients. Because of these limitations, transgenic cystic fibrosis transmembrane conductance regulator (CFTR)(-/-) pigs have been developed and are currently being characterized. These CF animal models have phenotypes closely resembling that of human CF subjects. The objectives of the current study were to develop primary porcine nasal epithelial (PNE) cultures and evaluate their usefulness as a means to investigate sinonasal transepithelial transport and CFTR function.

METHODS

PNE derived from the septum or turbinates of CFTR(+/+) and CFTR(-/-) pigs were cultured at an air-liquid interface to confluence and full differentiation. Epithelial monolayers were mounted in Ussing chambers to investigate pharmacologic manipulation of ion transport. Ciliary beat frequency (CBF) and scanning electron microscopy of monolayers were used to indicate degree of ciliation and cell differentiation.

RESULTS

Stimulation of CFTR-mediated anion transport (ΔIsc in μA/cm(2) ) was significantly greater in epithelia derived from the septum when compared to turbinates (33.04 ± 1.17 vs 18.9 ± 0.73; p < 0.05). Cyclic adenosine monophosphate (cAMP)-activated Cl(-) secretion was absent in CFTR(-/-) and present in CFTR(+/+) epithelia. Calcium-activated Cl(-) (CaCC) secretion was increased in CF; however, overall Cl(-) transport through CaCCs was very low. Degree of ciliation (90%) and CBF were similar between groups.

CONCLUSION

Septal PNE exhibit a robust ion transport phenotype and indicate CFTR(-/-) sinus disease could be attributable to diminished alternative pathways for Cl(-) transport. Overall, PNE have similarities to human respiratory epithelia not demonstrated in murine cells and represent useful in vitro models for studying CF sinus disease.

CFTR structure and cystic fibrosis

CFTR (cystic fibrosis transmembrane conductance regulator) is a member of the ATP-binding cassette family of membrane proteins. Although almost all members of this family are transporters, CFTR functions as a channel with specificity for anions, in particular chloride and bicarbonate. In this review we look at what is known about CFTR structure and function within the context of the ATP-binding cassette family. We also review current strategies aimed at obtaining the high resolution structure of the protein.

New animal models of cystic fibrosis: what are they teaching us?

PURPOSE OF REVIEW

Cystic fibrosis is the first human genetic disease to benefit from the directed engineering of three different species of animal models (mice, pigs, and ferrets). Recent studies on the cystic fibrosis pig and ferret models are providing new information about the pathophysiology of cystic fibrosis in various organ systems. Additionally, new conditional cystic fibrosis transmembrane conductance regulator (CFTR) knockout mice are teaching unexpected lessons about CFTR function in surprising cellular locations. Comparisons between these animal models and the human condition are key to dissecting the complexities of disease pathophysiology in cystic fibrosis.

RECENT FINDINGS

Cystic fibrosis pigs and ferrets have provided new models to study the spontaneous development of disease in the lung and pancreas, two organs that are largely spared overt spontaneous disease in cystic fibrosis mice. New cystic fibrosis mouse models are now interrogating CFTR functions involved in growth and inflammation at an organ-based level using conditional knockout technology. Together, these models are providing new insights on the human condition.

SUMMARY

Basic and clinical cystic fibrosis research will benefit greatly from the comparative pathophysiology of cystic fibrosis mice, pigs, and ferrets. Both similarities and differences between these three cystic fibrosis models will inform pathophysiologically important mechanisms of CFTR function in humans and aid in the development of both organ-specific and general therapies for cystic fibrosis.

The cystic fibrosis transmembrane conductance regulator: an intriguing protein with pleiotropic functions

Cystic fibrosis is a frequent autosomal recessive disorder that is caused by the malfunctioning of a small chloride channel, the cystic fibrosis transmembrane conductance regulator. The protein is found in the apical membrane of epithelial cells lining exocrine glands. Absence of this channel results in imbalance of ion concentrations across the cell membrane. As a result, fluids secreted through these glands become more viscous and, in the end, ducts become plugged and atrophic. Little is known about the pathways that link the malfunctioning of the CFTR protein with the observed clinical phenotype. Moreover, there is no strict correlation between specific CFTR mutations and the CF phenotype. This might be explained by the fact that environmental and additional genetic factors may influence the phenotype. The CFTR protein itself is regulated at the maturational level by chaperones and SNARE proteins and at the functional level by several protein kinases. Moreover, CFTR functions also as a regulator of other ion channels and of intracellular membrane transport processes. In order to be able to function as a protein with pleiotropic actions, CFTR seems to be linked with other proteins and with the cytoskeleton through interaction with PDZ-domain-containing proteins at the apical pole of the cell. Progress in cystic fibrosis research is substantial, but still leaves many questions unanswered.

Animal models of cystic fibrosis

Animal models of cystic fibrosis, in particular several different mutant mouse strains obtained by homologous recombination, have contributed considerably to our understanding of CF pathology. In this review, we describe and compare the main phenotypic features of these models. Recent and possible future developments in this field are discussed.

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.

Mouse models of cystic fibrosis

The development of mouse models for cystic fibrosis has provided the opportunity to dissect disease pathogenesis, correlate genotype and phenotype, study disease-modifying genes and develop novel therapeutics. This review discusses the successes and the challenges encountered in characterizing and optimizing these models.

Electrophysiological characteristics of the Ca2+-activated Cl- channel family of anion transport proteins

1. A protein isolated from the bovine tracheal epithelium behaves as a Ca2+-activated Cl- channel (CaCC) when incorporated into planar lipid bilayers. 2. An antibody raised against this protein was used to screen a cDNA expression library and resulted in the isolation of a cDNA clone that exhibited nearly identical electrophysiological characteristics to the isolated endogenous protein when expressed. 3. Recent cloning of several related proteins has revealed that the cloned bovine CaCC is one of a large and growing family. All new family members so far examined are associated with the appearance of a novel Ca2+-mediated Cl- conductance when heterologously expressed. 4. This new group of proteins may underlie the Ca2+-mediated Cl- conductance upregulated in the cystic fibrosis (CF) knockout mouse and thought to be responsible for the escape from the significant airway pathology associated with CF.

Modulation of the Ca(2+)-activated Cl(-) channel by 14-3-3epsilon

We have previously reported an association of 14-3-3epsilon isoform with calmodulin. Using the voltage-clamp technique, the present study investigated the potential role of 14-3-3 in modulating the Ca(2+)-activated Cl(-) channel (CaCC) endogenously expressed in Xenopus oocytes. Injection of 14-3-3epsilon antisense oligodeoxynucleotides resulted in potentiation of the ionomycin-induced Cl(-) current, while 14-3-3 peptide and calmodulin inhibitor, W13, suppressed the antisense-potentiated current. The data suggest that 14-3-3epsilon plays an inhibitory role in modulating the CaCC by interacting with the calmodulin-dependent pathway. The potential role of 14-3-3epsilon in other tissues and its therapeutic potential for cystic fibrosis are discussed.

Bimodal control of a Ca(2+)-activated Cl(-) channel by different Ca(2+) signals

Ca(2+)-activated Cl(-) channels play important roles in a variety of physiological processes, including epithelial secretion, maintenance of smooth muscle tone, and repolarization of the cardiac action potential. It remains unclear, however, exactly how these channels are controlled by Ca(2+) and voltage. Excised inside-out patches containing many Ca(2+)-activated Cl(-) channels from Xenopus oocytes were used to study channel regulation. The currents were mediated by a single type of Cl(-) channel that exhibited an anionic selectivity of I(-) > Br(-) > Cl(-) (3.6:1.9:1.0), irrespective of the direction of the current flow or [Ca(2+)]. However, depending on the amplitude of the Ca(2+) signal, this channel exhibited qualitatively different behaviors. At [Ca(2+)] < 1 microM, the currents activated slowly upon depolarization and deactivated upon hyperpolarization and the steady state current-voltage relationship was strongly outwardly rectifying. At higher [Ca(2+)], the currents did not rectify and were time independent. This difference in behavior at different [Ca(2+)] was explained by an apparent voltage-dependent Ca(2+) sensitivity of the channel. At +120 mV, the EC(50) for channel activation by Ca(2+) was approximately fourfold less than at -120 mV (0.9 vs. 4 microM). Thus, at [Ca(2+)] < 1 microM, inward current was smaller than outward current and the currents were time dependent as a consequence of voltage-dependent changes in Ca(2+) binding. The voltage-dependent Ca(2+) sensitivity was explained by a kinetic gating scheme in which channel activation was Ca(2+) dependent and channel closing was voltage sensitive. This scheme was supported by the observation that deactivation time constants of currents produced by rapid Ca(2+) concentration jumps were voltage sensitive, but that the activation time constants were Ca(2+) sensitive. The deactivation time constants increased linearly with the log of membrane potential. The qualitatively different behaviors of this channel in response to different Ca(2+) concentrations adds a new dimension to Ca(2+) signaling: the same channel can mediate either excitatory or inhibitory responses, depending on the amplitude of the cellular Ca(2+) signal.

Cystic fibrosis transmembrane conductance regulator regulates luminal Cl-/HCO3- exchange in mouse submandibular and pancreatic ducts

We have demonstrated previously the regulation of Cl-/HCO3- exchange activity by the cystic fibrosis transmembrane conductance regulator (CFTR) in model systems of cells stably or transiently transfected with CFTR (Lee, M. G., Wigley, W. C., Zeng, W., Noel, L. E., Marino, C. R., Thomas, P. J., and Muallem, S. (1999) J. Biol. Chem. 274, 3414-3421). In the present work we examine the significance of this regulation in cells naturally expressing CFTR. These include the human colonic T84 cell line and the mouse submandibular gland and pancreatic ducts, tissues that express high levels of CFTR in the luminal membrane. As in heterologous expression systems, stimulation of T84 cells with forskolin increased the Cl-/HCO3- exchange activity independently of CFTR Cl- channel activity. Freshly isolated submandibular gland ducts from wild type mice showed variable Cl-/HCO3- exchange activity. Measurement of [Cl-]i revealed that this was largely the result of variable steady-state [Cl-]i. Membrane depolarization with 5 mM Ba2+ or 100 mM K+ increased and stabilized [Cl-]i. Under depolarized conditions wild type and DeltaF/DeltaF mice had comparable basal Cl-/HCO3- exchange activity. Notably, stimulation with forskolin increased Cl-/HCO3- exchange activity in submandibular gland ducts from wild type but not DeltaF/DeltaF mice. Microperfusion of the main pancreatic duct showed Cl-/HCO3- exchange activity in both the basolateral and luminal membranes. Stimulation of ducts from wild type animals with forskolin had no effect on basolateral but markedly stimulated luminal Cl-/HCO3- exchange activity. By contrast, forskolin had no effect on either basolateral or luminal Cl-/HCO3- exchange activity of ducts from DeltaF/DeltaF animals. We conclude that CFTR regulates luminal Cl-/HCO3- exchange activity in CFTR-expressing cells, and we discuss the possible physiological significance of these findings regarding cystic fibrosis.

Regulation of Cl-/ HCO3- exchange by cystic fibrosis transmembrane conductance regulator expressed in NIH 3T3 and HEK 293 cells

A central function of cystic fibrosis transmembrane conductance regulator (CFTR)-expressing tissues is the secretion of fluid containing 100-140 mM HCO3-. High levels of HCO3- maintain secreted proteins such as mucins (all tissues) and digestive enzymes (pancreas) in a soluble and/or inactive state. HCO3- secretion is impaired in CF in all CFTR-expressing, HCO3--secreting tissues examined. The mechanism responsible for this critical problem in CF is unknown. Since a major component of HCO3- secretion in CFTR-expressing cells is mediated by the action of a Cl-/HCO3- exchanger (AE), in the present work we examined the regulation of AE activity by CFTR. In NIH 3T3 cells stably transfected with wild type CFTR and in HEK 293 cells expressing WT and several mutant CFTR, activation of CFTR by cAMP stimulated AE activity. Pharmacological and mutagenesis studies indicated that expression of CFTR in the plasma membrane, but not the Cl- conductive function of CFTR was required for activation of AE. Furthermore, mutations in NBD2 altered regulation of AE activity by CFTR independent of their effect on Cl- channel activity. At very high expression levels CFTR modified the sensitivity of AE to 4,4'-diisothiocyanatostilbene-2, 2'-disulfonate. The novel finding of regulation of Cl-/HCO3- exchange by CFTR reported here may have important physiological implications and explain, at least in part, the impaired HCO3- secretion in CF.