Mechanism and synergism in epithelial fluid and electrolyte secretion

Heavy metals in cigarette smoke strongly inhibit pancreatic ductal function and promote development of chronic pancreatitis

BACKGROUND AND AIMS

Smoking is recognised as an independent risk factor in the development of chronic pancreatitis (CP). Cystic fibrosis transmembrane conductance regulator (CFTR) function and ductal fluid and bicarbonate secretion are also known to be impaired in CP, so it is crucial to understand the relationships between smoking, pancreatic ductal function and the development of CP.

METHODS

We measured sweat chloride (Cl-) concentrations in patients with and without CP, both smokers and non-smokers, to assess CFTR activity. Serum heavy metal levels and tissue cadmium concentrations were determined by mass spectrometry in smoking and non-smoking patients. Guinea pigs were exposed to cigarette smoke, and cigarette smoke extract (CSE) was prepared to characterise its effects on pancreatic HCO3 - and fluid secretion and CFTR function. We administered cerulein to both the smoking and non-smoking groups of mice to induce pancreatitis.

RESULTS

Sweat samples from smokers, both with and without CP, exhibited elevated Cl- concentrations compared to those from non-smokers, indicating a decrease in CFTR activity due to smoking. Pancreatic tissues from smokers, regardless of CP status, displayed lower CFTR expression than those from non-smokers. Serum levels of cadmium and mercury, as well as pancreatic tissue cadmium, were increased in smokers. Smoking, CSE, cadmium, mercury and nicotine all hindered fluid and HCO3 - secretion and CFTR activity in pancreatic ductal cells. These effects were mediated by sustained increases in intracellular calcium ([Ca2+]i), depletion of intracellular ATP (ATPi) and mitochondrial membrane depolarisation.

CONCLUSION

Smoking impairs pancreatic ductal function and contributes to the development of CP. Heavy metals, notably cadmium, play a significant role in the harmful effects of smoking.

KEY POINTS

Smoking and cigarette smoke extract diminish pancreatic ductal fluid and HCO3 - secretion as well as the expression and function of CFTR Cd and Hg concentrations are significantly higher in the serum samples of smokers Cd accumulates in the pancreatic tissue of smokers.

Deficiency of the BKCa potassium channel displayed significant implications for the physiology of the human bronchial epithelium

Plasma membrane large-conductance calcium-activated potassium (BKCa) channels are important players in various physiological processes, including those mediated by epithelia. Like other cell types, human bronchial epithelial (HBE) cells also express BKCa in the inner mitochondrial membrane (mitoBKCa). The genetic relationships between these mitochondrial and plasma membrane channels and the precise role of mitoBKCa in epithelium physiology are still unclear. Here, we tested the hypothesis that the mitoBKCa channel is encoded by the same gene as the plasma membrane BKCa channel in HBE cells. We also examined the impact of channel loss on the basic function of HBE cells, which is to create a tight barrier. For this purpose, we used CRISPR/Cas9 technology in 16HBE14o- cells to disrupt the KCNMA1 gene, which encodes the α-subunit responsible for forming the pore of the plasma membrane BKCa channel. Electrophysiological experiments demonstrated that the disruption of the KCNMA1 gene resulted in the loss of BKCa-type channels in the plasma membrane and mitochondria. We have also shown that HBE ΔαBKCa cells exhibited a significant decrease in transepithelial electrical resistance which indicates a loss of tightness of the barrier created by these cells. We have also observed a decrease in mitochondrial respiration, which indicates a significant impairment of these organelles. In conclusion, our findings indicate that a single gene encodes both populations of the channel in HBE cells. Furthermore, this channel is critical for maintaining the proper function of epithelial cells as a cellular barrier.

SLC26A9 in airways and intestine: secretion or absorption?

SLC26A9 is one out of 11 proteins that belong to the SLC26A family of anion transporters. Apart from expression in the gastrointestinal tract, SLC26A9 is also found in the respiratory system, in male tissues and in the skin. SLC26A9 has gained attention because of its modifier role in the gastrointestinal manifestation of cystic fibrosis (CF). SLC26A9 appears to have an impact on the extent of intestinal obstruction caused by meconium ileus. SLC26A9 supports duodenal bicarbonate secretion, but was assumed to provide a basal Cl- secretory pathway in airways. However, recent results show that basal airway Cl- secretion is due to cystic fibrosis conductance regulator (CFTR), while SLC26A9 may rather secrete HCO₃-, thereby maintaining proper airway surface liquid (ASL) pH. Moreover, SLC26A9 does not secrete but probably supports reabsorption of fluid particularly in the alveolar space, which explains early death by neonatal distress in Slc26a9-knockout animals. While the novel SLC26A9 inhibitor S9-A13 helped to unmask the role of SLC26A9 in the airways, it also provided evidence for an additional role in acid secretion by gastric parietal cells. Here we discuss recent data on the function of SLC26A9 in airways and gut, and how S9-A13 may be useful in unraveling the physiological role of SLC26A9.

Normal and Sjogren's syndrome models of the murine lacrimal gland studied at single-cell resolution

The lacrimal gland is of central interest in ophthalmology both as the source of the aqueous component of tear fluid and as the site of autoimmune pathology in the context of Sjogren's syndrome (SjS). To provide a foundational description of mouse lacrimal gland cell types and their patterns of gene expression, we have analyzed single-cell transcriptomes from wild-type (Balb/c) mice and from two genetically based SjS models, MRL/lpr and NOD (nonobese diabetic).H2b, and defined the localization of multiple cell-type-specific protein and mRNA markers. This analysis has uncovered a previously undescribed cell type, Car6+ cells, which are located at the junction of the acini and the connecting ducts. More than a dozen secreted polypeptides that are likely to be components of tear fluid are expressed by acinar cells and show pronounced sex differences in expression. Additional examples of gene expression heterogeneity within a single cell type were identified, including a gradient of Claudin4 along the length of the ductal system and cell-to-cell heterogeneity in transcription factor expression within acinar and myoepithelial cells. The patterns of expression of channels, transporters, and pumps in acinar, Car6+, and ductal cells make strong predictions regarding the mechanisms of water and electrolyte secretion. In MRL/lpr and NOD.H2b lacrimal glands, distinctive changes in parenchymal gene expression and in immune cell subsets reveal widespread interferon responses, a T cell-dominated infiltrate in the MRL/lpr model, and a mixed B cell and T cell infiltrate in the NOD.H2b model.

Simulation of Calcium Dynamics in Realistic Three-Dimensional Domains

The cytosolic concentration of free calcium ions ([Ca2+]) is an important intracellular messenger in most cell types, and the spatial distribution of [Ca2+] is often critical. In a salivary gland acinar cell, a polarised epithelial cell, whose principal function is to transport water and thus secrete saliva, [Ca2+] controls the secretion of primary saliva, but increases in [Ca2+] are localised to the apical regions of the cell. Hence, any quantitative explanation of how [Ca2+] controls saliva secretion must take into careful account the spatial distribution of the various Ca2+ sources, Ca2+ sinks, and Ca2+-sensitive ion channels. Based on optical slices, we have previously constructed anatomically accurate three-dimensional models of seven salivary gland acinar cells, and thus shown that a model in which Ca2+ responses are confined to the apical regions of the cell is sufficient to provide a quantitative and predictive explanation of primary saliva secretion. However, reconstruction of such anatomically accurate cells is extremely time consuming and inefficient. Here, we present an alternative, mostly automated method of constructing three-dimensional cells that are approximately anatomically accurate and show that the new construction preserves the quantitative accuracy of the model.

Pathophysiological aspects of the thick ascending limb and novel genetic defects: HELIX syndrome and transient antenatal Bartter syndrome

The thick ascending limb plays a central role in human kidney physiology, participating in sodium reabsorption, urine concentrating mechanisms, calcium and magnesium homeostasis, bicarbonate and ammonium homeostasis, and uromodulin synthesis. This review aims to illustrate the importance of these roles from a pathophysiological point of view by describing the interactions of the key proteins of this segment and by discussing how recently identified and long-known hereditary diseases affect this segment. The descriptions of two recently described salt-losing tubulopathies, transient antenatal Bartter syndrome and HELIX syndrome, which are caused by mutations in MAGED2 and CLDN10 genes, respectively, highlight the role of new players in the modulation of sodium reabsorption the thick ascending limb.

Bicarbonate Transport in Cystic Fibrosis and Pancreatitis

CFTR, the cystic fibrosis (CF) gene-encoded epithelial anion channel, has a prominent role in driving chloride, bicarbonate and fluid secretion in the ductal cells of the exocrine pancreas. Whereas severe mutations in CFTR cause fibrosis of the pancreas in utero, CFTR mutants with residual function, or CFTR variants with a normal chloride but defective bicarbonate permeability (CFTR^BD), are associated with an enhanced risk of pancreatitis. Recent studies indicate that CFTR function is not only compromised in genetic but also in selected patients with an acquired form of pancreatitis induced by alcohol, bile salts or smoking. In this review, we summarize recent insights into the mechanism and regulation of CFTR-mediated and modulated bicarbonate secretion in the pancreatic duct, including the role of the osmotic stress/chloride sensor WNK1 and the scaffolding protein IRBIT, and current knowledge about the role of CFTR in genetic and acquired forms of pancreatitis. Furthermore, we discuss the perspectives for CFTR modulator therapy in the treatment of exocrine pancreatic insufficiency and pancreatitis and introduce pancreatic organoids as a promising model system to study CFTR function in the human pancreas, its role in the pathology of pancreatitis and its sensitivity to CFTR modulators on a personalized basis.

Calcium Dynamics and Water Transport in Salivary Acinar Cells

Saliva is secreted from the acinar cells of the salivary glands, using mechanisms that are similar to other types of water-transporting epithelial cells. Using a combination of theoretical and experimental techniques, over the past 20 years we have continually developed and modified a quantitative model of saliva secretion, and how it is controlled by the dynamics of intracellular calcium. However, over approximately the past 5 years there have been significant developments both in our understanding of the underlying mechanisms and in the way these mechanisms should best be modelled. Here, we review the traditional understanding of how saliva is secreted, and describe how our work has suggested important modifications to this traditional view. We end with a brief description of the most recent data from living animals and discuss how this is now contributing to yet another iteration of model construction and experimental investigation.

The regulatory role of vasoactive intestinal peptide in lacrimal gland ductal fluid secretion: A new piece of the puzzle in tear production

Purpose

Vasoactive intestinal peptide (VIP) is an important regulator of lacrimal gland (LG) function although the effect of VIP on ductal fluid secretion is unknown. Therefore, the aim of the present study was to investigate the role of VIP in the regulation of fluid secretion of isolated LG ducts and to analyze the underlying intracellular mechanisms.

Methods

LGs from wild-type (WT) and cystic fibrosis transmembrane conductance regulator (CFTR) knockout (KO) mice were used. Immunofluorescence was applied to confirm the presence of VIP receptors termed VPAC1 and VPAC2 in LG duct cells. Ductal fluid secretion evoked by VIP (100 nM) was measured in isolated ducts using videomicroscopy. Intracellular Ca²⁺ signaling underlying VIP stimulation was investigated with microfluorometry.

Results

VIP stimulation resulted in a robust and continuous fluid secretory response in isolated duct segments originated from WT mice. In contrast, CFTR KO ducts exhibited only a weak pulse-like secretion. A small but statistically significant increase was detected in the intracellular Ca²⁺ level [Ca²⁺]_i during VIP stimulation in the WT and in CFTR KO ducts. VIP-evoked changes in [Ca²⁺]_i did not differ considerably between the WT and CFTR KO ducts.

Conclusions

These results suggest the importance of VIP in the regulation of ductal fluid secretion and the determining role of the adenylyl cyclase-cAMP-CFTR route in this process.

Cellular channelopathy mediated by hypergravity: IL-6-mediated Nkcc1 activation and enhanced Trpm2 expression in rat atrium

Although cardiac tissue is considered a target of gravitational force (g-force), the mechanism of hypergravity on the ion modulation or identification of ion transporters is still unknown. Thus, we determine the effect of hypergravity on a physical force-sensitive cytokine, IL-6 and its related channel activity to investigate rat cardiac function changes in response to accelerated g-force. Serum IL-6 levels and intracellular calcium levels of the right atrium were moderately increased under hypergravity stimulation (4g). IL-6 was involved in the modulation of sodium-potassium-chloride cotransporter (Nkcc) activity. Surprisingly, the right atrium under 4g revealed significantly enhanced Nkcc1 activity. The use of IL-6 on the NKCC1-overexpressed or native NKCC-expressing cells also showed enhanced NKCC1 activity. Hypergravity conditions were also involved in the oxidative stress activated Trpm2 channel and revealed an enhanced expression of the Trpm2 channel under 4g in the rat right atrium. In conclusion, hypergravity revealed that moderate increases in serum IL-6 and enhanced Nkcc1 activity was modulated by IL-6. In addition, enhanced Trpm2 channel expression could be involved in the increased intracellular calcium levels of the right atrium under hypergravitational force. We therefore address that enhanced physical force-sensitive cytokine and oxidative stress by the gravitational force mediate activation of the cotransporter involved in possibilities of edema and calcium loading in cardiac tissue.

Enhancement of Antitumor Efficacy of Paclitaxel-Loaded PEGylated Liposomes by N,N-Dimethyl Tertiary Amino Moiety in Pancreatic Cancer

Introduction

Pancreatic cancer, or pancreatic duct adenocarcinoma (PDAC), remains one of the most lethal cancers and features insidious onset, highly aggressive behavior and early distant metastasis. The dense fibrotic stroma surrounding tumor cells is thought to be a shield to resist the permeation of chemotherapy drugs in the treatment of PDAC. Thus, we synthesized a pancreas-targeting paclitaxel-loaded PEGylated liposome and investigated its antitumor efficacy in the patient-derived orthotopic xenograft (PDOX) nude mouse models of PDAC.

Methods

The PTX-loaded PEGylated liposomes were prepared by film dispersion-ultrasonic method and modified by an N,N-dimethyl tertiary amino residue. Morphology characteristics of the PTX-loaded liposomes were observed by transmission electron microscope (TEM). The PDOX models of PDAC were established by orthotopic implantation and imaged by a micro positron emission tomography/computed tomography (PET/CT) imaging system. The in vivo distribution and antitumor study were then carried out to observe the pancreas-targeting accumulation and the antitumor efficacy of the proposed PTX liposomes.

Results

PTX loaded well into both modified (PTX-Lip2N) and unmodified (PTX-Lip) PEGylated liposomes with spherical shapes and suitable parameters for the endocytosis process. The PDOX nude mouse models were successfully created in which high ¹⁸F-FDG intaking regions were observed by micro-PET/CT. In addition to higher cellular uptakes of PTX-Lip2N by the BxPC-3 cells, the proposed nanoparticle had a notable penetrating ability towards PDAC tumor tissues, and consequently, the antitumor ability of PTX-Lip2N was significantly superior to the unmodified PTX-Lip in vivo PDOX models and even more effective than nab-PTX in restraining tumor growth.

Conclusion

The modified pancreas-targeting PTX-loaded PEGylated liposomes provide a promising platform for the treatment of pancreatic cancer.

Regulation of Cl- Electrolyte Permeability in Epithelia by Active Traditional Chinese Medicine Monomers for Diarrhea

The epithelial layer, lining the inner surface of the mammalian alveolar, kidney, brain and colon, is a typical electrolyte transporting tissue. Large quantities of salt and fluid are actively moved from the mucosal side toward the blood vessel. Transepithelial salt re-absorption in epithelial tissues plays an important role in maintaining fluid homeostasis. In absorptive epithelium, fluid and salt flux is controlled by the machinery mainly composed of epithelial sodium channel, cystic fibrosis transmembrane conductance regulator, Na+-K+-2Cl- cotransporter, Na+/H+ exchanger, and Na+/K+-ATPase. Dysregulation of salt permeability across epithelium contributes to the pathogenesis of organ edema. In numerous ion transporters, epithelial Cl- transportation plays an important role in water secretion across epithelial tissues and regulation of body fluid content. Many traditional Chinese medicines treat diarrhea by regulating the Cl- electrolyte transport. We systematically summarized the recent progress regarding the traditional Chinese medicine on Cl- electrolyte transport in the intestinal epithelial tissues. The pharmaceutical relevance of developing advanced strategies to mitigate edematous disorders is also implicated. In conclusion, the crosstalk between Cl- electrolyte transport and active traditional Chinese medicine monomers may lead to the development of new strategies for diarrhea by manipulating the function and expression of ion channels.

Mouse pancreatic ductal organoid culture as a relevant model to study exocrine pancreatic ion secretion

Pancreatic exocrine secretory processes are challenging to investigate on primary epithelial cells. Pancreatic organoid cultures may help to overcome shortcomings of the current models, however the ion secretory processes in pancreatic organoids-and therefore their physiological relevance or their utility in disease modeling-are not known. To answer these questions, we provide side-by-side comparison of gene expression, morphology, and function of epithelial cells in primary isolated pancreatic ducts and organoids. We used mouse pancreatic ductal fragments for experiments or were grown in Matrigel to obtain organoid cultures. Using PCR analysis we showed that gene expression of ion channels and transporters remarkably overlap in primary ductal cells and organoids. Morphological analysis with scanning electron microscopy revealed that pancreatic organoids form polarized monolayers with brush border on the apical membrane. Whereas the expression and localization of key proteins involved in ductal secretion (cystic fibrosis transmembrane conductance regulator, Na⁺/H⁺ exchanger 1 and electrogenic Na⁺/HCO₃^- cotransporter 1) are equivalent to the primary ductal fragments. Measurements of intracellular pH and Cl^- levels revealed no significant difference in the activities of the apical Cl^-/HCO₃^- exchange, or in the basolateral Na⁺ dependent HCO₃^- uptake. In summary we found that ion transport activities in the mouse pancreatic organoids are remarkably similar to those observed in freshly isolated primary ductal fragments. These results suggest that organoids can be suitable and robust model to study pancreatic ductal epithelial ion transport in health and diseases and facilitate drug development for secretory pancreatic disorders like cystic fibrosis, or chronic pancreatitis.

Cl- as a bona fide signaling ion

Cl^- is the major extracellular (Cl^-_out) and intracellular (Cl^-_in) anion whose concentration is actively regulated by multiple transporters. These transporters generate Cl^- gradients across the plasma membrane and between the cytoplasm and intracellular organelles. [Cl^-]_in changes rapidly in response to cell stimulation and influences many physiological functions, as well as cellular and systemic homeostasis. However, less appreciated is the signaling function of Cl^-. Cl^- interacts with multiple proteins to directly modify their activity. This review highlights the signaling function of Cl^- and argues that Cl^- is a bona fide signaling ion, a function deserving extensive exploration.

Defense Mechanisms Against Acid Exposure by Dental Enamel Formation, Saliva and Pancreatic Juice Production

The pancreas, the salivary glands and the dental enamel producing ameloblasts have marked developmental, structural and functional similarities. One of the most striking similarities is their bicarbonate-rich secretory product, serving acid neutralization. An important difference between them is that while pancreatic juice and saliva are delivered into a lumen where they can be collected and analyzed, ameloblasts produce locally precipitating hydroxyapatite which cannot be easily studied. Interestingly, the ion and protein secretion by the pancreas, the salivary glands, and maturation ameloblasts are all two-step processes, of course with significant differences too. As they all have to defend against acid exposure by producing extremely large quantities of bicarbonate, the failure of this function leads to deteriorating consequences. The aim of the present review is to describe and characterize the defense mechanisms of the pancreas, the salivary glands and enamel-producing ameloblasts against acid exposure and to compare their functional capabilities to do this by producing bicarbonate.

The apical Na+ -HCO3 - cotransporter Slc4a7 (NBCn1) does not contribute to bicarbonate transport by mouse salivary gland ducts

The HCO₃ ^- secretion mechanism in salivary glands is unclear but is thought to rely on the co-ordinated activity of multiple ion transport proteins including members of the Slc4 family of bicarbonate transporters. Slc4a7 was immunolocalized to the apical membrane of mouse submandibular duct cells. In contrast, Slc4a7 was not detected in acinar cells, and correspondingly, Slc4a7 disruption did not affect fluid secretion in response to cholinergic or β-adrenergic stimulation in the submandibular gland (SMG). Much of the Na ⁺ -dependent intracellular pH (pH _i ) regulation in SMG duct cells was insensitive to 4,4'-diisothiocyano-2,2'-stilbenedisulfonic acid, S0859, and to the removal of extracellular HCO ₃ ^- . Consistent with these latter observations, the Slc4a7 null mutation had no impact on HCO ₃ ^- secretion nor on pH _i regulation in duct cells. Taken together, our results revealed that Slc4a7 targets to the apical membrane of mouse SMG duct cells where it contributes little if any to pH _i regulation or stimulated HCO ₃ ^- secretion.

Cholecystokinin (CCK) Regulation of Pancreatic Acinar Cells: Physiological Actions and Signal Transduction Mechanisms

Pancreatic acinar cells synthesize and secrete about 20 digestive enzymes and ancillary proteins with the processes that match the supply of these enzymes to their need in digestion being regulated by a number of hormones (CCK, secretin and insulin), neurotransmitters (acetylcholine and VIP) and growth factors (EGF and IGF). Of these regulators, one of the most important and best studied is the gastrointestinal hormone, cholecystokinin (CCK). Furthermore, the acinar cell has become a model for seven transmembrane, heterotrimeric G protein coupled receptors to regulate multiple processes by distinct signal transduction cascades. In this review, we briefly describe the chemistry and physiology of CCK and then consider the major physiological effects of CCK on pancreatic acinar cells. The majority of the review is devoted to the physiologic signaling pathways activated by CCK receptors and heterotrimeric G proteins and the functions they affect. The pathways covered include the traditional second messenger pathways PLC-IP3-Ca²⁺ , DAG-PKC, and AC-cAMP-PKA/EPAC that primarily relate to secretion. Then there are the protein-protein interaction pathways Akt-mTOR-S6K, the three major MAPK pathways (ERK, JNK, and p38 MAPK), and Ca²⁺ -calcineurin-NFAT pathways that primarily regulate non-secretory processes including biosynthesis and growth, and several miscellaneous pathways that include the Rho family small G proteins, PKD, FAK, and Src that may regulate both secretory and nonsecretory processes but are not as well understood. © 2019 American Physiological Society. Compr Physiol 9:535-564, 2019.

Cystic Fibrosis of the Pancreas: The Role of CFTR Channel in the Regulation of Intracellular Ca2+ Signaling and Mitochondrial Function in the Exocrine Pancreas

Cystic fibrosis (CF) is the most common genetic disorder that causes a significant damage in secretory epithelial cells due to the defective ion flux across the cystic fibrosis transmembrane conductance regulator (CFTR) Cl^- channel. Pancreas is one of the organs most frequently damaged by the disease leading to pancreatic insufficiency, abdominal pain and an increased risk of acute pancreatitis in CF patients causing a significant decrease in the quality of life. CFTR plays a central role in the pancreatic ductal secretory functions by carrying Cl^- and HCO₃ ^- ions across the apical membrane. Therefore pathophysiological studies in CF mostly focused on the effects of impaired ion secretion by pancreatic ductal epithelial cells leading to exocrine pancreatic damage. However, several studies indicated that CFTR has a central role in the regulation of intracellular signaling processes and is now more widely considered as a signaling hub in epithelial cells. In contrast, elevated intracellular Ca²⁺ level was observed in the lack of functional CFTR in different cell types including airway epithelial cells. In addition, impaired CFTR expression has been correlated with damaged mitochondrial function in epithelial cells. These alterations of intracellular signaling in CF are not well characterized in the exocrine pancreas yet. Therefore in this review we would like to summarize the complex role of CFTR in the exocrine pancreas with a special focus on the intracellular signaling and mitochondrial function.

Expression of the B splice variant of NBCe1 (SLC4A4) in the mouse kidney

Sodium-coupled bicarbonate transporters are critical for renal electrolyte transport. The electrogenic, sodium-coupled bicarbonate cotransporter, isoform 1 (NBCe1), encoded by the SLC4A4 geneencoded by the SLC4A4 gene has five multiple splice variants; the A splice variant, NBCe1-A, is the primary basolateral bicarbonate transporter in the proximal convoluted tubule. This study's purpose was to determine if there is expression of additional NBCe1 splice variants in the mouse kidney, their cellular distribution, and their regulation by metabolic acidosis. In wild-type mice, an antibody reactive only to NBCe1-A showed basolateral immunolabel only in cortical proximal tubule (PT) segments, whereas an antibody reactive to all NBCe1 splice variants (pan-NBCe1) showed basolateral immunolabel in PT segments in both the cortex and outer medulla. In mice with NBCe1-A deletion, the pan-NBCe1 antibody showed basolateral PT immunolabel in both the renal cortex and outer stripe of the outer medulla, and immunoblot analysis showed expression of a ~121-kDa protein. RT-PCR of mRNA from NBCe1-A knockout mice directed at splice variant-specific regions showed expression of only NBCe1-B mRNA. In wild-type kidney, RT-PCR confirmed expression of mRNA for the NBCe1-B splice variant and absence of mRNA for the C, D, and E splice variants. Finally, exogenous acid loading increased expression in the proximal straight tubule in the outer stripe of the outer medulla. These studies demonstrate that the NBCe1-B splice variant is present in the PT, and its expression increases in response to exogenous acid loading, suggesting it participates in the PT contribution to acid-base homeostasis.

Novel Insight Into the Role of CFTR in Lacrimal Gland Duct Function in Mice

Purpose

The role of cystic fibrosis transmembrane conductance regulator (CFTR) in lacrimal gland (LG) function has only recently received some attention, mainly from our group. In the present study, we investigated the potential changes of LG pathology, tear secretion, ocular surface integrity, and fluid secretion in isolated LG ducts from CFTR knockout (KO) mice.

Methods

Tear production and ocular surface integrity were investigated in anesthetized wild-type (WT) and KO mice using cotton threads and fluorescein staining, respectively. Immunofluorescence was used to localize CFTR protein in the LGs. Ductal fluid secretions evoked by forskolin (10 μM); cell-permeable cAMP analogue (8-bromo cAMP, 100 μM); or carbachol (100 μM) were measured in isolated LG ducts using video-microscopy. Intracellular Ca2+ homeostasis underlying carbachol stimulation was investigated with microfluorometry.

Results

Significant decrease in tear secretion and impaired ocular surface integrity were observed in KO mice. Immunofluorescence demonstrated the predominant presence of CFTR protein in the apical membranes of the duct cells from WT mice. Continuous fluid secretion was evoked by forskolin and 8-bromo cAMP in LG ducts from WT mice, while no secretory response was observed in ducts from KO mice. Carbachol caused similar secretory responses in ducts from WT and KO animals without significant differences in cytosolic Ca²⁺ signaling.

Conclusions

Our results suggest the important role of CFTR in LG ductal secretion and in the maintenance of ocular surface integrity, suggesting that CFTR may be a promising target of novel therapeutic approaches in the treatment of dry eye.

Deconstructing the principles of ductal network formation in the pancreas

The mammalian pancreas is a branched organ that does not exhibit stereotypic branching patterns, similarly to most other glands. Inside branches, it contains a network of ducts that undergo a transition from unconnected microlumen to a mesh of interconnected ducts and finally to a treelike structure. This ductal remodeling is poorly understood, both on a microscopic and macroscopic level. In this article, we quantify the network properties at different developmental stages. We find that the pancreatic network exhibits stereotypic traits at each stage and that the network properties change with time toward the most economical and optimized delivery of exocrine products into the duodenum. Using in silico modeling, we show how steps of pancreatic network development can be deconstructed into two simple rules likely to be conserved for many other glands. The early stage of the network is explained by noisy, redundant duct connection as new microlumens form. The later transition is attributed to pruning of the network based on the flux of fluid running through the pancreatic network into the duodenum.

In the pancreas of mammals, digestive enzymes are transported from their production site in acini (clusters of cells that secrete the enzymes) to the intestine via a network of ducts. During organ development in fetuses, the ducts initially form by the coordinated polarization of cells to form small holes, which will connect and fuse, to constitute a meshwork. This hyperconnected network further develops into a treelike structure by the time of birth. In this article, we use methods originally developed to analyze road, rail, web, or river networks to quantify the network properties at different developmental stages. We find that the pancreatic network properties are similar between individuals at specific time points but eventually change to achieve the most economical and optimized structure to deliver pancreatic juice into the duodenum. Using in silico modeling, we show how the stages of pancreatic network development follow two simple rules, which are likely to be conserved for the development of other glands. The early stage of the network is explained by noisy, redundant duct connection as new small ductal holes form. Later on, the secretion of fluid that runs through the pancreatic network into the duodenum leads to the widening of ducts with the greatest flow, while nonnecessary ducts are eliminated, akin to how river beds are formed.

The apical anion exchanger Slc26a6 promotes oxalate secretion by murine submandibular gland acinar cells

The solute carrier family 26 (SLC26) gene family encodes at least 10 different anion exchangers. SLC26 member 6 (SLC26A6 or CFEX/PAT-1) and the cystic fibrosis transmembrane conductance regulator (CFTR) co-localize to the apical membrane of pancreatic duct cells, where they act in concert to drive HCO3- and fluid secretion. In contrast, in the small intestine, SLC26A6 serves as the major pathway for oxalate secretion. However, little is known about the function of Slc26a6 in murine salivary glands. Here, RNA sequencing-based transcriptional profiling and Western blots revealed that Slc26a6 is highly expressed in mouse submandibular and sublingual salivary glands. Slc26a6 localized to the apical membrane of salivary gland acinar cells with no detectable immunostaining in the ducts. CHO-K1 cells transfected with mouse Slc26a6 exchanged Cl- for oxalate and HCO3-, whereas two other anion exchangers known to be expressed in salivary gland acinar cells, Slc4a4 and Slc4a9, mediated little, if any, Cl-/oxalate exchange. Of note, both Cl-/oxalate exchange and Cl-/HCO3- exchange were significantly reduced in acinar cells isolated from the submandibular glands of Slc26a6-/- mice. Oxalate secretion in submandibular saliva also decreased significantly in Slc26a6-/- mice, but HCO3- secretion was unaffected. Taken together, our findings indicate that Slc26a6 is located at the apical membrane of salivary gland acinar cells, where it mediates Cl-/oxalate exchange and plays a critical role in the secretion of oxalate into saliva.

Renoguanylin stimulates apical CFTR translocation and decreases HCO3- secretion through PKA activity in the Gulf toadfish (Opsanus beta)

The guanylin peptides - guanylin, uroguanylin and renoguanylin (RGN) - are endogenously produced hormones in teleost fish enterocytes that are activators of guanylyl cyclase-C (GC-C) and are potent modulators of intestinal physiology, particularly in seawater teleosts. Most notably, they reverse normal net ion-absorbing mechanisms that are vital to water absorption, an important process for seawater teleost survival. The role of guanylin-peptide stimulation of the intestine remains unclear, but it is hypothesized to facilitate the removal of solids from the intestine by providing fluid to enable their removal by peristalsis. The present study used one member of this group of peptides - RGN - to provide evidence for the prominent role that protein kinase A (PKA) plays in mediating the effects of guanylin-peptide stimulation in the posterior intestine of the Gulf toadfish (Opsanus beta). Protein kinase G was found to not mediate the intracellular effects of RGN, despite previous evidence showing that GC-C activation leads to higher cyclic guanosine monophosphate formation. RGN reversed the absorptive short-circuit current and increased conductance in the Gulf toadfish intestine. These effects are correlated to increased trafficking of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl^- channel to the apical membrane, which is negated by PKA inhibition. Moreover, RGN decreased HCO₃^- secretion, likely by limiting apical HCO₃^-/Cl^- exchange (possibly by reducing SLC26a6 activity), a reduction that was enhanced by PKA inhibition. RGN seems to alter PKA activity in the posterior intestine to recruit CFTR to the apical membrane and reduce HCO₃^- secretion.

Transcriptional profiling reveals gland-specific differential expression in the three major salivary glands of the adult mouse

RNA-Seq was used to better understand the molecular nature of the biological differences among the three major exocrine salivary glands in mammals. Transcriptional profiling found that the adult murine parotid, submandibular, and sublingual salivary glands express greater than 14,300 protein-coding genes, and nearly 2,000 of these genes were differentially expressed. Principle component analysis of the differentially expressed genes revealed three distinct clusters according to gland type. The three salivary gland transcriptomes were dominated by a relatively few number of highly expressed genes (6.3%) that accounted for more than 90% of transcriptional output. Of the 912 transcription factors expressed in the major salivary glands, greater than 90% of them were detected in all three glands, while expression for ~2% of them was enriched in an individual gland. Expression of these unique transcription factors correlated with sublingual and parotid specific subsets of both highly expressed and differentially expressed genes. Gene ontology analyses revealed that the highly expressed genes common to all glands were associated with global functions, while many of the genes expressed in a single gland play a major role in the function of that gland. In summary, transcriptional profiling of the three murine major salivary glands identified a limited number of highly expressed genes, differentially expressed genes, and unique transcription factors that represent the transcriptional signatures underlying gland-specific biological properties.

Salivary gland hypofunction in KK-Ay type 2 diabetic mice

BACKGROUND

Hypofunction of different organs in the body is associated with diabetes, including in the oral cavity. Diabetes is often associated with xerostomia, but the underlying mechanism is not well characterized. Thus, the mechanisms underlying diabetes-induced xerostomia were investigated in this study in KK-A ^y mice as an experimental model of type 2 diabetes.

METHODS

The mechanisms involved in diabetes-induced xerostomia were investigated using the ex vivo glandular perfusion technique, histological analysis, and immunohistochemical and intracellular signaling analyses.

RESULTS

Ex vivo submandibular gland secretions from KK-A^y mice decreased by 30% following stimulation with 0.3 μmol/L carbachol (CCh), a cholinergic agonist. Acinar cell weight was comparable between KK-A^y and control mice, whereas duct cell weight was significantly greater in KK-A^y mice. Concentrations of Na⁺ and Cl^- in the secreted saliva decreased significantly in KK-A^y mice, supporting the finding of increased ductal tissue in KK-A^y mice. Immunohistochemistry revealed no significant differences between KK-A^y and control mice in terms of the expression of Cl^- and water channels, Na⁺ -K⁺ -2Cl^- cotransporters, and membrane proteins critical for fluid secretion. Cellular signaling analysis revealed that the increase in [Ca²⁺ ]_i in response to 0.3 μmol/L CCh was reduced by 30% in KK-A^y mice, although there was no significant difference in the thapsigargin (1.0 μmol/L)-induced increase in store-depleted calcium between KK-A^y and control mice.

CONCLUSIONS

These results demonstrate that submandibular fluid secretion is diminished in KK-A^y mice because of a diminished increase in [Ca²⁺ ]_i . Duct cell weight increased in KK-A^y mice, possibly leading to increased ion reabsorption and thus decreased Na⁺ and Cl^- concentrations in the secreted saliva.

Restoration of CFTR Activity in Ducts Rescues Acinar Cell Function and Reduces Inflammation in Pancreatic and Salivary Glands of Mice

BACKGROUND & AIMS

Sjögren's syndrome and autoimmune pancreatitis are disorders with decreased function of salivary, lacrimal glands, and the exocrine pancreas. Nonobese diabetic/ShiLTJ mice and mice transduced with the cytokine BMP6 develop Sjögren's syndrome and chronic pancreatitis and MRL/Mp mice are models of autoimmune pancreatitis. Cystic fibrosis transmembrane conductance regulator (CFTR) is a ductal Cl^- channel essential for ductal fluid and HCO₃^- secretion. We used these models to ask the following questions: is CFTR expression altered in these diseases, does correction of CFTR correct gland function, and most notably, does correcting ductal function correct acinar function?

METHODS

We treated the mice models with the CFTR corrector C18 and the potentiator VX770. Glandular, ductal, and acinar cells damage, infiltration, immune cells and function were measured in vivo and in isolated duct/acini.

RESULTS

In the disease models, CFTR expression is markedly reduced. The salivary glands and pancreas are inflamed with increased fibrosis and tissue damage. Treatment with VX770 and, in particular, C18 restored salivation, rescued CFTR expression and localization, and nearly eliminated the inflammation and tissue damage. Transgenic overexpression of CFTR exclusively in the duct had similar effects. Most notably, the markedly reduced acinar cell Ca²⁺ signaling, Orai1, inositol triphosphate receptors, Aquaporin 5 expression, and fluid secretion were restored by rescuing ductal CFTR.

CONCLUSIONS

Our findings reveal that correcting ductal function is sufficient to rescue acinar cell function and suggests that CFTR correctors are strong candidates for the treatment of Sjögren's syndrome and pancreatitis.

Progress in understanding mucus abnormalities in cystic fibrosis airways

Normal airways below the carina maintain an essentially sterile environment via a multi-pronged innate defence system that includes mucus clearance via mucociliary clearance and cough, multiple antimicrobials and cellular components including macrophages and neutrophils. In cystic fibrosis (CF), loss of CFTR function compromises these defences, and with present standard of care virtually all people with CF eventually develop mucus accumulation, plugging and chronic infections. This review focuses on how mucus is affected by CFTR loss.

Pro-Secretory Activity and Pharmacology in Rabbits of an Aminophenyl-1,3,5-Triazine CFTR Activator for Dry Eye Disorders

Purpose

Pharmacological activation of ocular surface cystic fibrosis transmembrane conductance regulator (CFTR) chloride channels is a potential pro-secretory approach to treat dry eye disorders. We previously reported the discovery of aminophenyl-1,3,5-triazines, one of which, N-methyl-N-phenyl-6-(2,2,3,3-tetrafluoropropoxy)-1,3,5-triazine-2,4-diamine (herein called CFTRact-K267), fully activated human wildtype CFTR with EC50 ∼ 30 nM and increased tear volume for 8 hours in mice. Here, functional and pharmacological studies of CFTRact-K267 were done in adult New Zealand white rabbits.

Methods

CFTR chloride conductance was measured in vivo by ocular surface potential differences and in ex vivo conjunctiva by short-circuit current. Tear volume was measured by the Schirmer tear test II and CFTRact-K267 pharmacokinetics and tissue distribution by liquid chromatography/mass spectrometry. Toxicity profile was studied for 28 days with twice-daily topical administration.

Results

Electrophysiological measurements in vivo and in ex vivo conjunctiva demonstrated CFTR activation by CFTRact-K267. A single topical dose of 3 nmol CFTRact-K267 increased tear production by >5 mm for 9 hours by the Schirmer tear test, with predicted therapeutic concentrations maintained in tear fluid. No tachyphylaxis was seen following 28-day twice-daily administration, and changes were not observed in corneal surface integrity or thickness, intraocular pressure, or ocular histology. At day 28, CFTRact-K267 was concentrated in the cornea and conjunctiva and was not detectable in blood or peripheral organs.

Conclusions

These studies support the development of CFTRact-K267 as a pro-secretory therapy for dry eye disorders.

The Mechanistic Links between Insulin and Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) Cl- Channel

The cystic fibrosis transmembrane conductance regulator (CFTR) Cl⁻ channel belongs to the ATP-binding cassette (ABC) transporter superfamily and regulates Cl⁻ secretion in epithelial cells for water secretion. Loss-of-function mutations to the CFTR gene cause dehydrated mucus on the apical side of epithelial cells and increase the susceptibility of bacterial infection, especially in the airway and pulmonary tissues. Therefore, research on the molecular properties of CFTR, such as its gating mechanism and subcellular trafficking, have been intensively pursued. Dysregulated CFTR trafficking is one of the major pathological hallmarks in cystic fibrosis (CF) patients bearing missense mutations in the CFTR gene. Hormones that activate cAMP signaling, such as catecholamine, have been found to regulate the intracellular trafficking of CFTR. Insulin is one of the hormones that regulate cAMP production and promote trafficking of transmembrane proteins to the plasma membrane. The functional interactions between insulin and CFTR have not yet been clearly defined. In this review article, I review the roles of CFTR in epithelial cells, its regulatory role in insulin secretion, and a mechanism of CFTR regulation by insulin.

Pharmacological targeting of SPAK kinase in disorders of impaired epithelial transport

The mammalian SPS1-related proline/alanine-rich serine-threonine kinase SPAK (STK39) modulates ion transport across and between epithelial cells in response to environmental stimuli such osmotic stress and inflammation. Research over the last decade has established a central role for SPAK in the regulation of ion and water transport in the distal nephron, colonic crypts, and pancreatic ducts, and has implicated deregulated SPAK signaling in NaCl-sensitive hypertension, ulcerative colitis and Crohn's disease, and cystic fibrosis. Areas covered: We review recent advances in our understanding of the role of SPAK kinase in the regulation of epithelial transport. We highlight how SPAK signaling - including its upstream Cl^- sensitive activators, the WNK kinases, and its downstream ion transport targets, the cation- Cl^- cotransporters contribute to human disease. We discuss prospects for the pharmacotherapeutic targeting of SPAK kinase in specific human disorders that feature impaired epithelial homeostasis. Expert opinion: The development of novel drugs that antagonize the SPAK-WNK interaction, inhibit SPAK kinase activity, or disrupt SPAK kinase activation by interfering with its binding to MO25α/β could be useful adjuncts in essential hypertension, inflammatory colitis, and cystic fibrosis.

Two Phase Modulation of [Formula: see text] Entry and Cl-/[Formula: see text] Exchanger in Submandibular Glands Cells by Dexmedetomidine

Dexmedetomidine (Dex), a highly selective α2-adrenoceptor agonist, attenuates inflammatory responses induced by lipopolysaccharide (LPS) and induces sedative and analgesic effects. Administration of Dex also reduces salivary secretion in human subjects and inhibits osmotic water permeability in rat cortical collecting ducts. However, little is known about the mechanisms underlying the effects of Dex on salivary glands fluid secretion. We demonstrated the α2-adrenoceptor expression in the basolateral membrane of mouse submandibular glands (SMG). To investigate fluid secretion upon treatment with Dex, we studied the effects of Dex on the activity of Na+-K+-2Cl- cotransporter1 (NKCC1) and Cl-/[Formula: see text] exchange (CBE), and on downstream pro-inflammatory cytokine expression in isolated primary mouse SMG cells. Dex acutely increased CBE activity and NKCC1-mediated and independent [Formula: see text] entry in SMG duct cells, and enhanced ductal fluid secretion in a sealed duct system. Dex showed differential effects on cholinergic/adrenergic stimulations and inflammatory mediators, histamine, and LPS, stimulations-induced Ca2+ in mouse SMG cells. Both, histamine- and LPS-induced intracellular Ca2+ increases were inhibited by Dex, whereas carbachol-stimulated Ca2+ signals were not. Long-lasting (2 h) treatment with Dex reduced CBE activity in SMG and in human submandibular glands (HSG) cells. Moreover, when isolated SMG cells were stimulated with Dex for 2 h, phosphodiesterase 4D (PDE4D) expression was enhanced. These results confirm the anti-inflammatory properties of Dex on LPS-mediated signaling. Further, Dex also inhibited mRNA expression of interleukin-6 and NADPH oxidase 4. The present study also showed that α2-adrenoceptor activation by Dex reduces salivary glands fluid secretion by increasing PDE4D expression, and subsequently reducing the concentration of cAMP. These findings reveal an interaction between the α2-adrenoceptor and PDE4D, which should be considered when using α2-adrenoceptor agonists as sedative or analgesics.

WNK Kinase Signaling in Ion Homeostasis and Human Disease

WNK kinases, along with their upstream regulators (CUL3/KLHL3) and downstream targets (the SPAK/OSR1 kinases and the cation-Cl^- cotransporters [CCCs]), comprise a signaling cascade essential for ion homeostasis in the kidney and nervous system. Recent work has furthered our understanding of the WNKs in epithelial transport, cell volume homeostasis, and GABA signaling, and uncovered novel roles for this pathway in immune cell function and cell proliferation.

MDCK cells are capable of water secretion and reabsorption in response to changes in the ionic environment

A prerequisite for tissue electrolyte homeostasis is highly regulated ion and water transport through kidney or intestinal epithelia. In the present work, we monitored changes in the cell and luminal volumes of type II Madin-Darby canine kidney (MDCK) cells grown in a 3D environment in response to drugs, or to changes in the composition of the basal extracellular fluid. Using fluorescent markers and high-resolution spinning disc confocal microscopy, we could show that lack of sodium and potassium ions in the basal fluid (tetramethylammonium chloride (TMACl) buffer) induces a rapid increase in the cell and luminal volumes. This transepithelial water flow could be regulated by inhibitors and agonists of chloride channels. Hence, the driving force for the transepithelial water flow is chloride secretion, stimulated by hyperpolarization. Chloride ion depletion of the basal fluid (using sodium gluconate buffer) induces a strong reduction in the lumen size, indicating reabsorption of water from the lumen to the basal side. Lumen size also decreased following depolarization of the cell interior by rendering the membrane permeable to potassium. Hence, MDCK cells are capable of both absorption and secretion of chloride ions and water; negative potential within the lumen supports secretion, while depolarizing conditions promote reabsorption.

Regulation of epithelial ion transport in exocrine glands by store-operated Ca2+ entry

Store-operated Ca²⁺ entry (SOCE) is a conserved mechanism of Ca²⁺ influx that regulates Ca²⁺ signaling in many cell types. SOCE is activated by depletion of endoplasmic reticulum (ER) Ca²⁺ stores in response to physiological agonist stimulation. After it was first postulated by J.W. Putney Jr. in 1986, SOCE has been described in a large number of non-excitable cell types including secretory cells of different exocrine glands. Here we discuss the mechanisms by which SOCE controls salt and fluid secretion in exocrine glands, with a special focus on eccrine sweat glands. In sweat glands, SOCE plays an important, non-redundant role in regulating the function of Ca²⁺-activated Cl^- channels (CaCC), Cl^- secretion and sweat production. In the absence of key regulators of SOCE such as the CRAC channel pore subunit ORAI1 and its activator STIM1, the Ca²⁺-activated chloride channel TMEM16A is inactive and fails to secrete Cl^-, resulting in anhidrosis in mice and human patients.

Ca2+-activated Cl− channel currents in mammary secretory cells from lactating mouse

The Cl− secretion via Ca2+-activated Cl− channel (CaCC) is critical for fluid secretion in exocrine glands like the salivary gland. Also in the mammary gland, it has been hypothesized that CaCC plays an important role in the secretion of Cl− and aqueous phase of milk. However, there has been no evidence for the functional expression of CaCC in native mammary secretory (MS) cells of lactating animals. We therefore assessed membrane current in MS cells that were freshly isolated from lactating mice using whole cell patch-clamp techniques. In MS cells, we detected CaCC current that exhibited the following characteristics: 1) Ca2+-dependent activation at the concentrations of submicromolar range; 2) voltage-dependent activation; 3) slow kinetics for activation and deactivation; 4) outward rectification of the steady-state current; 5) anion permeability in the sequence of I− > NO3− > Br− > Cl− >> glutamate; 6) inhibition by Cl− channel blockers (niflumic acid, DIDS, and CaCCinh-A01). These characteristics of native CaCC current were similar to reported characteristics of heterologously expressed TMEM16A. RT-PCR analyses showed the expression of multiple CaCC channels including TMEM16A, Best1, and Best3 in the mammary glands of lactating mice. Immunohistochemical staining revealed the localization of TMEM16A protein at the apical membrane of the MS cells. Collectively, our data strongly suggest that MS cells functionally express CaCC, which is at least partly constituted by TMEM16A. The CaCC such as TMEM16A at the apical membrane of the MS cells may influence the quantity and/or quality of milk.

Role of CFTR in epithelial physiology

Salt and fluid absorption and secretion are two processes that are fundamental to epithelial function and whole body fluid homeostasis, and as such are tightly regulated in epithelial tissues. The CFTR anion channel plays a major role in regulating both secretion and absorption in a diverse range of epithelial tissues, including the airways, the GI and reproductive tracts, sweat and salivary glands. It is not surprising then that defects in CFTR function are linked to disease, including life-threatening secretory diarrhoeas, such as cholera, as well as the inherited disease, cystic fibrosis (CF), one of the most common life-limiting genetic diseases in Caucasian populations. More recently, CFTR dysfunction has also been implicated in the pathogenesis of acute pancreatitis, chronic obstructive pulmonary disease (COPD), and the hyper-responsiveness in asthma, underscoring its fundamental role in whole body health and disease. CFTR regulates many mechanisms in epithelial physiology, such as maintaining epithelial surface hydration and regulating luminal pH. Indeed, recent studies have identified luminal pH as an important arbiter of epithelial barrier function and innate defence, particularly in the airways and GI tract. In this chapter, we will illustrate the different operational roles of CFTR in epithelial function by describing its characteristics in three different tissues: the airways, the pancreas, and the sweat gland.

Calcium signalling in salivary gland physiology and dysfunction

Studies over the past four decades have established that Ca(2+) is a critical factor in control of salivary gland function and have led to identification of the critical components of this process. The major ion transport mechanisms and ion channels that are involved in fluid secretion have also been established. The key event in activation of fluid secretion is an increase in [Ca(2+) ]i triggered by inositol 1,4,5-trisphosphate (IP3 )-induced release of Ca(2+) from ER via the IP3 receptor (IP3 R). IP3 Rs determine the site of initiation and the pattern of the [Ca(2+) ]i signal in the cell. However, Ca(2+) entry into the cell is required to sustain the elevation of [Ca(2+) ]i and fluid secretion and is mediated by the store-operated Ca(2+) entry (SOCE) mechanism. Orai1, TRPC1, TRPC3 and STIM1 have been identified as critical components of SOCE in these cells. Cells finely tune the generation and amplification of [Ca(2+) ]i signals for regulation of cell function. An important emerging area is the concept that unregulated [Ca(2+) ]i signals in cells can directly cause cell damage, dysfunction and disease. Alternatively, aberrant [Ca(2+) ]i signals can also amplify and increase the rates of cell damage. Such defects in Ca(2+) signalling have been described in salivary glands in conjunction with radiation-induced loss of salivary gland function as well as in the salivary defects associated with the autoimmune exocrinopathy Sjögren's syndrome. Such defects have been associated with altered function or expression of key Ca(2+) signalling components, such as STIM proteins and TRP channels. These studies offer new avenues for examining the mechanisms underlying the disease and development of novel clinical targets and therapeutic strategies.

The Physiology and Pathophysiology of Pancreatic Ductal Secretion: The Background for Clinicians

The human exocrine pancreas consists of 2 main cell types: acinar and ductal cells. These exocrine cells interact closely to contribute to the secretion of pancreatic juice. The most important ion in terms of the pancreatic ductal secretion is HCO3. In fact, duct cells produce an alkaline fluid that may contain up to 140 mM NaHCO3, which is essential for normal digestion. This article provides an overview of the basics of pancreatic ductal physiology and pathophysiology. In the first part of the article, we discuss the ductal electrolyte and fluid transporters and their regulation. The central role of cystic fibrosis transmembrane conductance regulator (CFTR) is highlighted, which is much more than just a Cl channel. We also review the role of pancreatic ducts in severe debilitating diseases such as cystic fibrosis (caused by various genetic defects of cftr), pancreatitis, and diabetes mellitus. Stimulation of ductal secretion in cystic fibrosis and pancreatitis may have beneficial effects in their treatment.

Evidence for Active Electrolyte Transport by Two-Dimensional Monolayers of Human Salivary Epithelial Cells

Functional reconstruction of lost tissue by regenerative therapy of salivary glands would be of immense benefit following radiotherapy or in the treatment of Sjogren's syndrome. The purpose of this study was to develop primary cultures of human salivary gland cells as potential regenerative resources and to characterize their acinar/ductal phenotype using electrophysiological measurements of ion transport. Human salivary gland cultures were prepared either from adherent submandibular gland cells (huSMG) or from mixed adherent and nonadherent cells (PTHSG) and were cultivated in Hepato-STIM or minimum essential medium (MEM). Expression of key epithelial marker proteins was determined by quantitative reverse transcription polymerase chain reaction (RT-PCR). Transepithelial electrical resistance (TER) was monitored following seeding the cells on Transwell membranes. Transepithelial ion transport was estimated by short-circuit current (Isc) measurements in an Ussing chamber. Both huSMG and PTHSG cells showed epithelial characteristics when cultivated in Hepato-STIM, while fibroblast-like elements dominated in MEM. Compared to intact tissue, cultivation of the cells resulted in substantial decreases in AQP5 and NKCC1 expression and moderate increases in claudin-1 and ENaC expression. Both cultures achieved high TER and transepithelial electrolyte movement in Hepato-STIM, but not in MEM. The Isc was substantially reduced by basolateral Cl(-) and bicarbonate withdrawal, indicating the involvement of basolateral-to-apical anion transport, and by the blockade of apical ENaC by amiloride, indicating the involvement of apical-to-basolateral Na(+) transport. An almost complete inhibition was observed following simultaneous ENaC block and withdrawal of the two anions. Isc was enhanced by either apical adenosine triphosphate (ATP) or basolateral carbachol application, but not by forskolin, confirming the expected role of Ca(2+)-activated regulatory pathways in electrolyte secretion. Inhibition of basolateral NKCC1 by bumetanide reduced the response to ATP, indicating the active involvement of this transporter in Cl(-) secretion. In conclusion, we have demonstrated that both PTHSG and huSMG primary cultures cultivated in Hepato-STIM form two-dimensional monolayers in vitro on permeable supports and achieve active vectorial transepithelial electrolyte transport. The presence of both basolateral-to-apical anion fluxes and an apical-to-basolateral Na(+) flux indicates both acinar and ductal characteristics. With further refinement, this model should provide a firm basis for new interventions to correct salivary gland dysfunction.

Function and repair of dental enamel - Potential role of epithelial transport processes of ameloblasts

The hardest mammalian tissue, dental enamel is produced by ameloblasts, which are electrolyte-transporting epithelial cells. Although the end product is very different, they show many similarities to transporting epithelia of the pancreas, salivary glands and kidney. Enamel is produced in a multi-step epithelial secretory process that features biomineralization which is an interplay of secreted ameloblast specific proteins and the time-specific transport of minerals, protons and bicarbonate. First, "secretory" ameloblasts form the entire thickness of the enamel layer, but with low mineral content. Then they differentiate into "maturation" ameloblasts, which remove organic matrix from the enamel and in turn further build up hydroxyapatite crystals. The protons generated by hydroxyapatite formation need to be buffered, otherwise enamel will not attain full mineralization. Buffering requires a tight pH regulation and secretion of bicarbonate by ameloblasts. The whole process has been the focus of many immunohistochemical and gene knock-out studies, but, perhaps surprisingly, no functional data existed for mineral ion transport by ameloblasts. However, recent studies including ours provided a better insight for molecular mechanism of mineral formation. The secretory regulation is not completely known as yet, but its significance is crucial. Impairing regulation retards or prevents completion of enamel mineralization and results in the development of hypomineralized enamel that easily erodes after dental eruption. Factors that impair this function are fluoride and disruption of pH regulators. Revealing these factors may eventually lead to the treatment of enamel hypomineralization related to genetic or environmentally induced malformation.

Insulin is involved in transcriptional regulation of NKCC and the CFTR Cl(-) channel through PI3K activation and ERK inactivation in renal epithelial cells

It is is well known that insulin stimulates glucose transport and epithelial Na(+) channel (ENaC)-mediated Na(+) reabsorption; however, the action of insulin on Cl(-) secretion is not fully understood. In this study, we investigated the action of insulin on Na(+)-K(+)-2Cl(-) cotransporter (NKCC)-mediated Cl(-) secretion in epithelial A6 cells. Interestingly, insulin treatment remarkably enhanced the forskolin-stimulated Cl(-) secretion associated with an increase in apical Cl(-) conductance by upregulating mRNA expression of both CFTR and NKCC, although insulin treatment alone had no effect on the basal Cl(-) secretion or apical Cl(-) conductance without forskolin application. We next elucidated a role of phosphoinositide 3-kinase (PI3K) in the insulin-induced enhancement of the Cl(-) secretion, since insulin actually activated PI3K, resulting in activation of Akt, a downstream molecule of PI3K. LY294002 (a PI3K inhibitor) reduced the Cl(-) secretion by suppressing mRNA expression of NKCC, whereas insulin still had a stimulatory action on mRNA expression of CFTR even in the presence of LY294002. On the other hand, we found that a MEK inhibitor (PD98059) further enhanced the insulin-stimulated CFTR mRNA expression and the Cl(-) secretion in forskolin-stimulated A6 cells and that insulin induced slight, transient activation of ERK followed by significant inactivation of ERK. These observations suggest that: (1) insulin respectively upregulates mRNA expression of NKCC and CFTR through activation of PI3K and inactivation of ERK; (2) insulin signals on mRNA expression of NKCC and CFTR are not enough to stimulate transepithelial Cl(-) secretion, but enhance the stimulatory action of cAMP on transepithelial Cl(-) secretion.

Distinct action of flavonoids, myricetin and quercetin, on epithelial Cl⁻ secretion: useful tools as regulators of Cl⁻ secretion

Epithelial Cl⁻ secretion plays important roles in water secretion preventing bacterial/viral infection and regulation of body fluid. We previously suggested that quercetin would be a useful compound for maintaining epithelial Cl⁻ secretion at a moderate level irrespective of cAMP-induced stimulation. However, we need a compound that stimulates epithelial Cl⁻ secretion even under cAMP-stimulated conditions, since in some cases epithelial Cl⁻ secretion is not large enough even under cAMP-stimulated conditions. We demonstrated that quercetin and myricetin, flavonoids, stimulated epithelial Cl⁻ secretion under basal conditions in epithelial A6 cells. We used forskolin, which activates adenylyl cyclase increasing cytosolic cAMP concentrations, to study the effects of quercetin and myricetin on cAMP-stimulated epithelial Cl⁻ secretion. In the presence of forskolin, quercetin diminished epithelial Cl⁻ secretion to a level similar to that with quercetin alone without forskolin. Conversely, myricetin further stimulated epithelial Cl⁻ secretion even under forskolin-stimulated conditions. This suggests that the action of myricetin is via a cAMP-independent pathway. Therefore, myricetin may be a potentially useful compound to increase epithelial Cl⁻ secretion under cAMP-stimulated conditions. In conclusion, myricetin would be a useful compound for prevention from bacterial/viral infection even under conditions that the amount of water secretion driven by cAMP-stimulated epithelial Cl⁻ secretion is insufficient.

cAMP and Ca²⁺ signaling in secretory epithelia: crosstalk and synergism

The Ca(2+) and cAMP/PKA pathways are the primary signaling systems in secretory epithelia that control virtually all secretory gland functions. Interaction and crosstalk in Ca(2+) and cAMP signaling occur at multiple levels to control and tune the activity of each other. Physiologically, Ca(2+) and cAMP signaling operate at 5-10% of maximal strength, but synergize to generate the maximal response. Although synergistic action of the Ca(2+) and cAMP signaling is the common mode of signaling and has been known for many years, we know very little of the molecular mechanism and mediators of the synergism. In this review, we discuss crosstalk between the Ca(2+) and cAMP signaling and the function of IRBIT (IP3 receptors binding protein release with IP3) as a third messenger that mediates the synergistic action of the Ca(2+) and cAMP signaling.