Regulation of membrane potential and fluid secretion by Ca2+-activated K+ channels in mouse submandibular glands

Application of anti-vascular endothelial growth factor antibody restores the function of saliva secretion in a type 2 diabetes mouse model

OBJECTIVES

Xerostomia, a common complication of type 2 diabetes, leads to an increased risk of caries, dysphagia, and dysgeusia. Although anti-vascular endothelial growth factor (VEGF) antibodies, such as ranibizumab (RBZ), have been used to treat diabetic retinopathy, their effects on the salivary glands are unknown. This study evaluated the effects of RBZ on salivary glands to reduce inflammation and restore salivary function in a mouse model of type 2 diabetes.

METHODS

Male KK-Ay mice with type 2 diabetes (10-12 weeks old) were used. The diabetes mellitus (DM) group received phosphate-buffered saline, while the DM + RBZ group received an intraperitoneal administration of RBZ (100 μg/kg) 24 h before the experiment.

RESULTS

Ex vivo perfusion experiments showed a substantial increase in salivary secretion from the submandibular gland (SMG) in the DM + RBZ group. In addition, the mRNA expression levels of TNF-α and IL-1β were considerably lower in this group. In contrast, those of aquaporin 5 were substantially higher in the DM + RBZ group, as revealed by quantitative reverse transcription PCR. Furthermore, the number of lymphocyte infiltration spots in the SMG was notably lower in the DM + RBZ group. Finally, intracellular Ca2+ signaling in acinar cells was considerably higher in the DM + RBZ group than that in the DM group.

CONCLUSION

Treating a type 2 diabetic mouse model with RBZ restored salivary secretion through its anti-inflammatory effects.

Structural and functional analysis of salivary intercalated duct cells reveals a secretory phenotype

Currently, all salivary ducts (intercalated, striated and collecting) are assumed to function broadly in a similar manner, reclaiming ions that were secreted by the secretory acinar cells while preserving fluid volume and delivering saliva to the oral cavity. Nevertheless, there has been minimal investigation into the structural and functional differences between distinct types of salivary duct cells. Therefore, in this study, the expression profile of proteins involved in stimulus-secretion coupling, as well as the function of the intercalated duct (ID) and striated duct cells, was examined. Particular focus was placed on defining differences between distinct duct cell populations. To accomplish this, immunohistochemistry and in situ hybridization were utilized to examine the localization and expression of proteins involved in reabsorption and secretion of ions and fluid. Further, in vivo calcium imaging was employed to investigate cellular function. Based on the protein expression profile and functional data, marked differences between the IDs and striated ducts were observed. Specifically, the ID cells express proteins native to the secretory acinar cells while lacking proteins specifically expressed in the striated ducts. Further, the ID and striated duct cells display different calcium signalling characteristics, with the IDs responding to a neural stimulus in a manner similar to the acinar cells. Overall, our data suggest that the IDs have a distinct role in the secretory process, separate from the reabsorptive striated ducts. Instead, based on our evidence, the IDs express proteins found in secretory cells, generate calcium signals in a manner similar to acinar cells, and, therefore, are likely secretory cells. KEY POINTS: Current studies examining salivary intercalated duct cells are limited, with minimal documentation of the ion transport machinery and the overall role of the cells in fluid generation. Salivary intercalated duct cells are presumed to function in the same manner as other duct cells, reclaiming ions, maintaining fluid volume and delivering the final saliva to the oral cavity. Here we systematically examine the structure and function of the salivary intercalated duct cells using immunohistochemistry, in situ hybridization and by monitoring in vivo Ca2+ dynamics. Structural data revealed that the intercalated duct cells lack proteins vital for reabsorption and express proteins necessary for secretion. Ca2+ dynamics in the intercalated duct cells were consistent with those observed in secretory cells and resulted from GPCR-mediated IP3 production.

Major depression-related factor NEGR1 controls salivary secretion in mouse submandibular glands

Salivary gland cells, which secrete water in response to neuronal stimulation, are closely connected to other neurons. Transcriptomic studies show that salivary glands also express some proteins responsible for neuronal function. However, the physiological functions of these common neuro-exocrine factors in salivary glands are largely unknown. Here, we studied the function of Neuronal growth regulator 1 (NEGR1) in the salivary gland cells. NEGR1 was also expressed in mouse and human salivary glands. The structure of salivary glands of Negr1 knockout (KO) mice was normal. Negr1 KO mice showed tempered carbachol- or thapsigargin-induced intracellular Ca²⁺ increases and store-operated Ca²⁺ entry. Of interest, the activity of the large-conductance Ca²⁺-activated K⁺ channel (BK channel) was increased, whereas Ca²⁺-activated Cl^- channel ANO1 channel activity was not altered in Negr1 KO mice. Pilocarpine- and carbachol-induced salivation was decreased in Negr1 KO mice. These results suggest that NEGR1 influence salivary secretion though the muscarinic Ca²⁺ signaling.

In vivo Ca2+ Imaging in Mouse Salivary Glands

Changes in intracellular calcium drive exocrine cell activity. In the salivary gland, acetylcholine released from parasympathetic neurons mobilizes endoplasmic reticulum calcium stores in acinar cells, which consequently initiates saliva secretion. However, our understanding of the signaling cascade is mainly based on ex vivo studies performed in enzymatically isolated cells. The dissociation process likely disrupts the extracellular matrix, removes neurons as the source of signal input, and disturbs the integrity of tight and gap junctional acinar connections. These alterations may affect the spatiotemporal properties of calcium signaling events. In vivo observations of calcium signals, where tissue organization is intact, are therefore important to establish the characteristics of physiological calcium signals that are crucial for the stimulation of fluid secretion. Here, we present a detailed protocol for in vivo imaging of calcium signaling events, following nervous stimulation by multi-photon microscopy in mouse salivary gland acinar cells, expressing the genetically encoded calcium indicator GCamp6F.

Highly localized intracellular Ca2+ signals promote optimal salivary gland fluid secretion

Salivary fluid secretion involves an intricate choreography of membrane transporters to result in the trans-epithelial movement of NaCl and water into the acinus lumen. Current models are largely based on experimental observations in enzymatically isolated cells where the Ca²⁺ signal invariably propagates globally and thus appears ideally suited to activate spatially separated Cl and K channels, present on the apical and basolateral plasma membrane, respectively. We monitored Ca²⁺ signals and salivary secretion in live mice expressing GCamp6F, following stimulation of the nerves innervating the submandibular gland. Consistent with in vitro studies, Ca²⁺ signals were initiated in the apical endoplasmic reticulum. In marked contrast to in vitro data, highly localized trains of Ca²⁺ transients that failed to fully propagate from the apical region were observed. Following stimuli optimum for secretion, large apical-basal gradients were elicited. A new mathematical model, incorporating these data was constructed to probe how salivary secretion can be optimally stimulated by apical Ca²⁺ signals.

Muscarinic Receptors and BK Channels Are Affected by Lipid Raft Disruption of Salivary Gland Cells

Activity-dependent fluid secretion is the most important physiological function of salivary glands and is regulated via muscarinic receptor signaling. Lipid rafts are important for G-protein coupled receptor (GPCR) signaling and ion channels in plasma membranes. However, it is not well understood whether lipid raft disruption affects all membrane events or only specific functions in muscarinic receptor-mediated water secretion in salivary gland cells. We investigated the effects of lipid raft disruption on the major membrane events of muscarinic transcellular water movement in human salivary gland (HSG) cells. We found that incubation with methyl-β-cyclodextrin (MβCD), which depletes lipid rafts, inhibited muscarinic receptor-mediated Ca²⁺ signaling in HSG cells and isolated mouse submandibular acinar cells. However, MβCD did not inhibit a Ca²⁺ increase induced by thapsigargin, which activates store-operated Ca²⁺ entry (SOCE). Interestingly, MβCD increased the activity of the large-conductance Ca²⁺-activated K⁺ channel (BK channel). Finally, we found that MβCD did not directly affect the translocation of aquaporin-5 (AQP5) into the plasma membrane. Our results suggest that lipid rafts maintain muscarinic Ca²⁺ signaling at the receptor level without directly affecting the activation of SOCE induced by intracellular Ca²⁺ pool depletion or the translocation of AQP5 into the plasma membrane.

In silico and in vitro analysis of cation-activated potassium channels in human corneal endothelial cells

The corneal endothelium is the inner cell monolayer involved in the maintenance of corneal transparence by the generation of homeostatic dehydration. The glycosaminoglycans of the corneal stroma develop a continuous swelling pressure that should be counteracted by the corneal endothelial cells through active transport mechanisms to move the water to the anterior chamber. Protein transporters for sodium (Na⁺), potassium (K⁺), chloride (Cl^-) and bicarbonate (HCO₃^-) are involved in this endothelial "pump function", however despite its physiological importance, the efflux mechanism is not completely understood. There is experimental evidence describing transendothelial diffusion of water in the absence of osmotic gradients. Therefore, it is important to get a deeper understanding of alternative models that drive the fluid transport across the endothelium such as the electrochemical gradients. Three transcriptomic datasets of the corneal endothelium were used in this study to analyze the expression of genes that encode proteins that participate in the transport and the reestablishment of the membrane potential across the semipermeable endothelium. Subsequently, the expression of the identified channels was validated in vitro both at mRNA and protein levels. The results of this study provide the first evidence of the expression of KCNN2, KCNN3 and KCNT2 genes in the corneal endothelium. Differences among the level of expression of KCNN2, KCNT2 and KCNN4 genes were found in a differentially expressed gene analysis of the dataset. Taken together these results underscore the potential importance of the ionic channels in the pathophysiology of corneal diseases. Moreover, we elucidate novel mechanisms that might be involved in the pivotal dehydrating function of the endothelium and in others physiologic functions of these cells using in silico pathways analysis.

Regulation of BK Channels by Beta and Gamma Subunits

Ca²⁺- and voltage-gated K⁺ channels of large conductance (BK channels) are expressed in a diverse variety of both excitable and inexcitable cells, with functional properties presumably uniquely calibrated for the cells in which they are found. Although some diversity in BK channel function, localization, and regulation apparently arises from cell-specific alternative splice variants of the single pore-forming α subunit ( KCa1.1, Kcnma1, Slo1) gene, two families of regulatory subunits, β and γ, define BK channels that span a diverse range of functional properties. We are just beginning to unravel the cell-specific, physiological roles served by BK channels of different subunit composition.

Ca2+ Signaling in Exocrine Cells

Calcium (Ca²⁺) and cyclic AMP (cAMP) signaling cross talk and synergize to stimulate the cardinal functions of exocrine cells, regulated exocytosis, and fluid and electrolyte secretion. This physiological process requires the organization of the two signaling pathways into complexes at defined cellular domains and close placement. Such domains are formed by membrane contact sites (MCS). This review discusses the basic properties of Ca²⁺ signaling in exocrine cells, the role of MCS in the organization of cell signaling and in cross talk and synergism between the Ca²⁺ and cAMP signaling pathways and, finally, the mechanism by which the Ca²⁺ and cAMP pathways synergize to stimulate epithelial fluid and electrolyte secretion.

Ano6 disruption impairs acinar cell regulatory volume decrease and protein secretion in murine submandibular salivary glands

The widely expressed Anoctamin 6 (Ano6) supports different Ca²⁺ -dependent functions, but little is known about its role in salivary glands. Mouse submandibular gland (SMG) acinar cells exhibited a robust regulatory volume decrease (RVD) following cell swelling that was reduced approximately 70% in Ano6^-/- mice. Ca²⁺ -free conditions nearly eliminated the RVD response suggesting that Ano6 is an obligatory component of the cell volume-activated, Ca²⁺ -dependent RVD pathway in salivary gland acinar cells. Ex vivo agonist-stimulated secretion of water and ions was unaffected by Ano6 disruption under both isotonic and hypotonic conditions suggesting that Ano6 does not play a major role in fluid and electrolyte secretion. In contrast, the total amount of β-adrenergic-dependent protein secretion by the SMG was significantly reduced in Ano6^-/- mice. Closer inspection of these latter results revealed that protein secretion was affected only in the female SMG by Ano6 disruption. These results indicate that Ano6 modulates the RVD response and protein secretion by salivary gland acinar cells.

Loss of the disease-associated glycosyltransferase Galnt3 alters Muc10 glycosylation and the composition of the oral microbiome

The importance of the microbiome in health and its disruption in disease is continuing to be elucidated. However, the multitude of host and environmental factors that influence the microbiome are still largely unknown. Here, we examined UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase 3 (Galnt3)-deficient mice, which serve as a model for the disease hyperphosphatemic familial tumoral calcinosis (HFTC). In HFTC, loss of GALNT3 activity in the bone is thought to lead to altered glycosylation of the phosphate-regulating hormone fibroblast growth factor 23 (FGF23), resulting in hyperphosphatemia and subdermal calcified tumors. However, GALNT3 is expressed in other tissues in addition to bone, suggesting that systemic loss could result in other pathologies. Using semiquantitative real-time PCR, we found that Galnt3 is the major O-glycosyltransferase expressed in the secretory cells of salivary glands. Additionally, 16S rRNA gene sequencing revealed that the loss of Galnt3 resulted in changes in the structure, composition, and stability of the oral microbiome. Moreover, we identified the major secreted salivary mucin, Muc10, as an in vivo substrate of Galnt3. Given that mucins and their O-glycans are known to interact with various microbes, our results suggest that loss of Galnt3 decreases glycosylation of Muc10, which alters the composition and stability of the oral microbiome. Considering that oral findings have been documented in HFTC patients, our study suggests that investigating GALNT3-mediated changes in the oral microbiome may be warranted.

Physiological cAMP-elevating secretagogues differentially regulate fluid and protein secretions in mouse submandibular and sublingual glands

The mechanisms underlying the functional differences in sympathetic and parasympathetic regulation of the major salivary glands have received little attention. The acute effects of parasympathetic muscarinic (carbachol)-dependent and combined parasympathetic-dependent plus cAMP-dependent pathways on fluid secretion rates, ion composition, and protein content were assessed using a newly developed ex vivo preparation that allows the simultaneous perfusion of the mouse submandibular (SMGs) and sublingual glands (SLGs). Our results confirm that the muscarinic-dependent pathway accounts for the bulk of salivation in SMGs and SLGs, whereas costimulation with a cAMP-increasing agent (forskolin, isoproterenol, or vasoactive intestinal peptide) did not increase the flow rate. Costimulation with carbachol plus the β-adrenergic agonist isoproterenol decreased the concentration of NaCl and produced a substantial increase in the protein and Ca²⁺ content of SMG but not SLG saliva, consistent with a sparse sympathetic innervation of the SLGs. On the other hand, forskolin, which bypasses receptors to increase intracellular cAMP by directly activating the enzyme adenylate cyclase, enhanced the secretion of protein and Ca²⁺ by both the SMGs and SLGs. In contrast, isoproterenol and vasoactive intestinal peptide specifically stimulated protein secretion in SMG and SLG salivas, respectively. In summary, cAMP-dependent signaling does not play a major role in the stimulation of fluid secretion in SMGs and SLGs, whereas each cAMP-increasing agonist behaves differently in a gland-specific manner suggesting differential expression of G protein-coupled receptors in the epithelial cells of SMGs and SLGs.

Loss of Fam20c causes defects in the acinar and duct structure of salivary glands in mice

Family with sequence similarity 20-member C (FAM20C), a recently characterized Golgi kinase, performs numerous biological functions by phosphorylating more than 100 secreted proteins. However, the role of FAM20C in the salivary glands remains undefined. The present study demonstrated that FAM20C is mainly located in the cytoplasm of duct epithelial cells in the salivary glands. Fam20c^f/f; Mmtv-Cre mice were created in which Fam20c was inactivated in the salivary gland cells and observed that the number of ducts and the ductal cross-sectional area increased significantly, while the number of acinar cells was reduced. The granular convoluted tubules (GCTs) exhibited an accumulation of aberrant secretory granules, along with a reduced expression and altered distribution patterns of β nerve growth factor, α-amylase and bone morphogenetic protein (BMP) 4. This abnormality suggested that the GCT cells were immature and exhibited defects in developmental and secretory functions. In accordance with the morphological alterations and the reduced number of acinar cells, FAM20C deficiency in the salivary glands significantly decreased the salivary flow rate. The Na⁺, Cl⁻ and K⁺ concentrations in the saliva were all significantly increased due to dysfunction of the ducts. Furthermore, Fam20c deficiency significantly increased BMP2 and BMP7 expression, decreased BMP4 expression, and attenuated the canonical and noncanonical BMP signaling pathways in the salivary glands. Collectively, the results of the present study demonstrate that FAM20C is a key regulator of acinar and duct structure and duct maturation and provide a novel avenue for investigating novel therapeutic targets for oral diseases including xerostomia.

Regulation of BK Channels by Beta and Gamma Subunits

Ca²⁺- and voltage-gated K⁺ channels of large conductance (BK channels) are expressed in a diverse variety of both excitable and inexcitable cells, with functional properties presumably uniquely calibrated for the cells in which they are found. Although some diversity in BK channel function, localization, and regulation apparently arises from cell-specific alternative splice variants of the single pore-forming α subunit ( KCa1.1, Kcnma1, Slo1) gene, two families of regulatory subunits, β and γ, define BK channels that span a diverse range of functional properties. We are just beginning to unravel the cell-specific, physiological roles served by BK channels of different subunit composition.

Inward rectifier potassium (Kir) channels mediate salivary gland function and blood feeding in the lone star tick, Amblyomma americanum

Background

Tick feeding causes extreme morbidity and mortality to humans through transmission of pathogens and causes severe economic losses to the agricultural industry by reducing livestock yield. Salivary gland secretions are essential for tick feeding and thus, reducing or preventing saliva secretions into the vertebrate host is likely to reduce feeding and hinder pathogen life cycles. Unfortunately, the membrane physiology of tick salivary glands is underexplored and this gap in knowledge limits the development of novel therapeutics for inducing cessation of tick feeding.

Methodology

We studied the influence of inward rectifier potassium (Kir) channel subtypes to the functional capacity of the isolated tick salivary gland through the use of a modified Ramsay assay. The secreted saliva was subsequently used for quantification of the elemental composition of the secreted saliva after the glands were exposed to K⁺ channel modulators as a measure of osmoregulatory capacity. Lastly, changes to blood feeding behavior and mortality were measured with the use of a membrane feeding system.

Principal findings

In this study, we characterized the fundamental role of Kir channel subtypes in tick salivary gland function and provide evidence that pharmacological inhibition of these ion channels reduces the secretory activity of the Amblyomma americanum salivary gland. The reduced secretory capacity of the salivary gland was directly correlated with a dramatic reduction of blood ingestion during feeding. Further, exposure to small-molecule modulators of Kir channel subtypes induced mortality to ticks that is likely resultant from an altered osmoregulatory capacity.

Conclusions

Our data contribute to understanding of tick salivary gland function and could guide future campaigns aiming to develop chemical or reverse vaccinology technologies to reduce the worldwide burden of tick feeding and tick-vectored pathogens.

Tick feeding results in negative health and economic consequences worldwide and there has been continued interest in the development of products with novel mechanisms of action for control of tick populations. Kir channels have been shown to be a significant ion conductance pathway in arthropods and are critical for proper functioning of multiple biological processes. Previous work on insect Kir channels has focused on their physiological roles in renal system of mosquitoes and the data suggest that these channels represent a viable pathway to induce renal failure that leads to mortality. Based on the functional and cellular similarities of arthropod salivary glands and Malpighian tubules, we hypothesized that Kir channels constitute a critical conductance pathway within arthropod salivary glands and inhibition of this pathway will preclude feeding. Data presented in this study show that pharmacological modulators of Kir channels elicited a significant reduction in the fluid and ion secretory activity of tick salivary glands that resulted in reduced feeding, altered osmoregulation, and lead to mortality. These data could guide the future development of novel acaricides, RNAi, or genetically modified ticks to mitigate health and economic damages resulting from their feeding. Further, these data indicate a conserved function of Kir channels within multiple tissues of taxonomically diverse organisms, such as ticks and humans.

Long-term dexamethasone treatment diminishes store-operated Ca2+ entry in salivary acinar cells

Corticosteroids are used in the treatment of many diseases; however, they also induce various side effects. Dexamethasone is one of the most potent corticosteroids, and it has been reported to induce the side effect of impaired salivary gland function. This study aimed to evaluate the effects of dexamethasone on mouse submandibular gland function to gain insight into the mechanism of dexamethasone-induced salivary hypofunction. The muscarinic agonist carbachol (CCh) induced salivary secretion and was not affected by short-term dexamethasone treatment but was decreased following long-term dexamethasone administration. The expression levels of the membrane proteins Na⁺-K⁺-2Cl^- cotransporter, transmembrane member 16A, and aquaporin 5 were comparable between the control and long-term dexamethasone treatment groups. The CCh-induced increase in calcium concentration was significantly lower in the presence of extracellular Ca²⁺ in the long-term dexamethasone treatment group compared to that in the control group. Furthermore, CCh-induced salivation in the absence of extracellular Ca²⁺ and Ca²⁺ ionophore A23187-induced salivation was comparable between the control and long-term dexamethasone treatment groups. Moreover, salivation induced by the Ca²⁺-ATPase inhibitor thapsigargin was diminished in the long-term dexamethasone treatment group. In summary, these results demonstrate that short-term dexamethasone treatment did not impair salivary gland function, whereas long-term dexamethasone treatment diminished store-operated Ca²⁺ entry, resulting in hyposalivation in mouse submandibular glands.

Defective NaCl Reabsorption in Salivary Glands of Eda-Null X-LHED Mice

Mutations in the ectodysplasin A gene ( EDA) cause X-LHED (X-linked hypohidrotic ectodermal dysplasia), the most common human form of ectodermal dysplasia. Defective EDA signaling is linked to hypoplastic development of epithelial tissues, resulting in hypotrichosis, hypodontia, hypohidrosis, and xerostomia. The primary objective of the present study was to better understand the salivary gland dysfunction associated with ectodermal dysplasia using the analogous murine disorder. The salivary flow rate and ion composition of the 3 major salivary glands were determined in adult Eda-deficient Tabby hemizygous male (Ta/Y) and heterozygous female (Ta/X) mice. Submandibular and sublingual glands of Eda-mutant mice were smaller than wild-type littermates, while parotid gland weight was not significantly altered. Fluid secretion by the 3 major salivary glands was essentially unchanged, but the decrease in submandibular gland size was associated with a dramatic loss of ducts in Ta/Y and Ta/X mice. Reabsorption of Na+ and Cl-, previously linked in salivary glands to Scnn1 Na+ channels and Cftr Cl- channels, respectively, was markedly reduced at high flow rates in the ex vivo submandibular glands of Ta/Y mice (~60%) and, to a lesser extent, Ta/X mice (Na+ by 14%). Consistent with decreased Na+ reabsorption in Ta/Y mice, quantitative polymerase chain reaction analysis detected decreased mRNA expression for Scnn1b and Scnn1g, genes encoding the β and γ subunits, respectively. Moreover, the Na+ channel blocker amiloride significantly inhibited Na+ and Cl- reabsorption by wild-type male submandibular glands to levels comparable to those observed in Ta/Y mice. In summary, fluid secretion was intact in the salivary glands of Eda-deficient mice but displayed marked Na+ and Cl- reabsorption defects that correlated with the loss of duct cells and decreased Scnn1 Na+ channel expression. These results provide a likely mechanism for the elevated NaCl concentration observed in the saliva of affected male and female patients with X-LHED.

Salivary secretion in health and disease

Saliva is a complex fluid produced by 3 pairs of major salivary glands and by hundreds of minor salivary glands. It comprises a large variety of constituents and physicochemical properties, which are important for the maintenance of oral health. Saliva not only protects the teeth and the oropharyngeal mucosa, it also facilitates articulation of speech, and is imperative for mastication and swallowing. Furthermore, saliva plays an important role in maintaining a balanced microbiota. Thus, the multiple functions provided by saliva are essential for proper protection and functioning of the body as a whole and for the general health. A large number of diseases and medications can affect salivary secretion through different mechanisms, leading to salivary gland dysfunction and associated oral problems, including xerostomia, dental caries and fungal infections. The first part of this review article provides an updated insight into our understanding of salivary gland structure, the neural regulation of salivary gland secretion, the mechanisms underlying the formation of saliva, the various functions of saliva and factors that influence salivary secretion under normal physiological conditions. The second part focuses on how various diseases and medical treatment including commonly prescribed medications and cancer therapies can affect salivary gland structure and function. We also provide a brief insight into how to diagnose salivary gland dysfunction.

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.

Drosophila CRY Entrains Clocks in Body Tissues to Light and Maintains Passive Membrane Properties in a Non-clock Body Tissue Independent of Light

Circadian (∼24 hr) clocks regulate daily rhythms in physiology, metabolism, and behavior via cell-autonomous transcriptional feedback loops. In Drosophila, the blue-light photoreceptor CRYPTOCHROME (CRY) synchronizes these feedback loops to light:dark cycles by binding to and degrading TIMELESS (TIM) protein. CRY also acts independently of TIM in Drosophila to alter potassium channel conductance in arousal neurons after light exposure, and in many animals CRY acts independently of light to repress rhythmic transcription. CRY expression has been characterized in the Drosophila brain and eyes, but not in peripheral clock and non-clock tissues in the body. To investigate CRY expression and function in body tissues, we generated a GFP-tagged-cry transgene that rescues light-induced behavioral phase resetting in cry⁰³ mutant flies and sensitively reports GFP-CRY expression. In bodies, CRY is detected in clock-containing tissues including Malpighian tubules, where it mediates both light-dependent TIM degradation and clock function. In larval salivary glands, which lack clock function but are amenable to electrophysiological recording, CRY prevents membrane input resistance from falling to low levels in a light-independent manner. The ability of CRY to maintain high input resistance in these non-excitable cells also requires the K⁺ channel subunits Hyperkinetic, Shaker, and ether-a-go-go. These findings for the first time define CRY expression in Drosophila peripheral tissues and reveal that CRY acts together with K⁺ channels to maintain passive membrane properties in a non-clock-containing peripheral tissue independent of light.

Knockout of the LRRC26 subunit reveals a primary role of LRRC26-containing BK channels in secretory epithelial cells

Leucine-rich-repeat-containing protein 26 (LRRC26) is the regulatory γ1 subunit of Ca²⁺- and voltage-dependent BK-type K⁺ channels. BK channels that contain LRRC26 subunits are active near normal resting potentials even without Ca²⁺, suggesting they play unique physiological roles, likely limited to very specific cell types and cellular functions. By using Lrrc26 KO mice with a β-gal reporter, Lrrc26 promoter activity is found in secretory epithelial cells, especially acinar epithelial cells in lacrimal and salivary glands, and also goblet and Paneth cells in intestine and colon, although absent from neurons. We establish the presence of LRRC26 protein in eight secretory tissues or tissues with significant secretory epithelium and show that LRRC26 protein coassembles with the pore-forming BK α-subunit in at least three tissues: lacrimal gland, parotid gland, and colon. In lacrimal, parotid, and submandibular gland acinar cells, LRRC26 KO shifts BK gating to be like α-subunit-only BK channels. Finally, LRRC26 KO mimics the effect of SLO1/BK KO in reducing [K⁺] in saliva. LRRC26-containing BK channels are competent to contribute to resting K⁺ efflux at normal cell membrane potentials with resting cytosolic Ca²⁺ concentrations and likely play a critical physiological role in supporting normal secretory function in all secretory epithelial cells.

A mathematical model of calcium dynamics in HSY cells

Saliva is an essential part of activities such as speaking, masticating and swallowing. Enzymes in salivary fluid protect teeth and gums from infectious diseases, and also initiate the digestion process. Intracellular calcium (Ca2+) plays a critical role in saliva secretion and regulation. Experimental measurements of Ca2+ and inositol trisphosphate (IP3) concentrations in HSY cells, a human salivary duct cell line, show that when the cells are stimulated with adenosine triphosphate (ATP) or carbachol (CCh), they exhibit coupled oscillations with Ca2+ spike peaks preceding IP3 spike peaks. Based on these data, we construct a mathematical model of coupled Ca2+ and IP3 oscillations in HSY cells and perform model simulations of three different experimental settings to forecast Ca2+ responses. The model predicts that when Ca2+ influx from the extracellular space is removed, oscillations gradually slow down until they stop. The model simulation of applying a pulse of IP3 predicts that photolysis of caged IP3 causes a transient increase in the frequency of the Ca2+ oscillations. Lastly, when Ca2+-dependent activation of PLC is inhibited, we see an increase in the oscillation frequency and a decrease in the amplitude. These model predictions are confirmed by experimental data. We conclude that, although concentrations of Ca2+ and IP3 oscillate, Ca2+ oscillations in HSY cells are the result of modulation of the IP3 receptor by intracellular Ca2+, and that the period is modulated by the accompanying IP3 oscillations.

Expression of a Diverse Array of Ca2+-Activated K+ Channels (SK1/3, IK1, BK) that Functionally Couple to the Mechanosensitive TRPV4 Channel in the Collecting Duct System of Kidney

The voltage- and Ca²⁺-activated, large conductance K⁺ channel (BK, maxi-K) is expressed in the collecting duct system of kidney where it underlies flow- and Ca²⁺-dependent K⁺ excretion. To determine if other Ca²⁺-activated K⁺ channels (KCa) may participate in this process, mouse kidney and the K⁺-secreting mouse cortical collecting duct (CCD) cell line, mCCDcl1, were assessed for TRPV4 and KCa channel expression and cross-talk. qPCR mRNA analysis and immunocytochemical staining demonstrated TRPV4 and KCa expression in mCCDcl1 cells and kidney connecting tubule (CNT) and CCD. Three subfamilies of KCa channels were revealed: the high Ca²⁺-binding affinity small-conductance SK channels, SK1and SK3, the intermediate conductance channel, IK1, and the low Ca²⁺-binding affinity, BK channel (BKα subunit). Apparent expression levels varied in CNT/CCD where analysis of CCD principal cells (PC) and intercalated cells (IC) demonstrated differential staining: SK1:PC<IC, and SK3:PC>IC, IK1:PC>IC, BKα:PC = IC, and TRPV4:PC>IC. Patch clamp analysis and fluorescence Ca²⁺ imaging of mCCDcl1 cells demonstrated potent TRPV4-mediated Ca²⁺ entry and strong functional cross-talk between TRPV4 and KCa channels. TRPV4-mediated Ca²⁺ influx activated each KCa channel, as evidenced by selective inhibition of KCa channels, with each active KCa channel enhancing Ca²⁺ entry (due to membrane hyperpolarization). Transepithelial electrical resistance (TEER) analysis of confluent mCCDcl1 cells grown on permeable supports further demonstrated this cross-talk where TRPV4 activation induce a decrease in TEER which was partially restored upon selective inhibition of each KCa channel. It is concluded that SK1/SK3 and IK1 are highly expressed along with BKα in CNT and CCD and are closely coupled to TRPV4 activation as observed in mCCDcl1 cells. The data support a model in CNT/CCD segments where strong cross talk between TRPV4-mediated Ca²⁺ influx and each KCa channel leads to enhance Ca²⁺ entry which will support activation of the low Ca²⁺-binding affinity BK channel to promote BK-mediated K⁺ secretion.

Orai1 and STIM1 in ER/PM junctions: roles in pancreatic cell function and dysfunction

Membrane contact sites (MCS) are critical junctions that form between the endoplasmic reticulum (ER) and membranes of various organelles, including the plasma membrane (PM). Signaling complexes, including mediators of Ca(2+) signaling, are assembled within MCS, such as the ER/PM junction. This is most evident in polarized epithelial cells, such as pancreatic cells. Core Ca(2+) signaling proteins cluster at the apical pole, the site of inositol 1,4,5-trisphosphate-mediated Ca(2+) release and Orai1/transient receptor potential canonical-mediated store-dependent Ca(2+) entry. Recent advances have characterized the proteins that tether the membranes at MCS and the role of these proteins in modulating physiological and pathological intracellular signaling. This review discusses recent advances in the characterization of Ca(2+) signaling at ER/PM junctions and the relation of these junctions to physiological and pathological Ca(2+) signaling in pancreatic acini.

Calcium signaling and cell volume regulation are altered in Sjögren's Syndrome

OBJECTIVE

Sjögren's Syndrome (SS) is a chronic autoimmune disease, leading to deficient secretion from salivary and lacrimal glands. Saliva production is normally increased by cholinergic innervation, giving rise to intracellular calcium signaling and water transport through water channels (aquaporins, AQPs). The aim of this study was to investigate possible pathophysiological changes in cell volume regulation, AQP expression and localization, and intracellular calcium signaling in glandular cells from SS patients compared to controls.

MATERIALS AND METHODS

A total of 35 SS patients and 41 non-SS controls were included. Real time qPCR was combined with immunohistochemistry to analyze the mRNA expression and cellular distribution of AQP1, 3 and 5. Cell volume regulation and intracellular calcium signaling were examined in fresh acinar cells.

RESULTS

We show for the first time a reduced mRNA expression of AQP1 and 5 in SS compared to controls, accompanied by a decrease in staining intensity of AQP1, 3 and 5 in areas adjacent to local lymphocytic infiltration. Furthermore, we observed that the SS cells' capacity for volume regulation was abnormal. Similarly, the calcium response after parasympathetic agonist (carbachol) stimulation was markedly decreased in SS cells.

CONCLUSIONS

It is concluded that mRNA expression of AQP1 and 5, protein distribution of AQP1, 3 and 5, glandular cell volume regulation and intracellular calcium signaling are all altered in SS, pointing to possible pathophysiological mechanisms in SS.

Role of calcium signaling in epithelial bicarbonate secretion

Transepithelial bicarbonate secretion plays a key role in the maintenance of fluid and protein secretion from epithelial cells and the protection of the epithelial cell surface from various pathogens. Epithelial bicarbonate secretion is mainly under the control of cAMP and calcium signaling. While the physiological roles and molecular mechanisms of cAMP-induced bicarbonate secretion are relatively well defined, those induced by calcium signaling remain poorly understood in most epithelia. The present review summarizes the current status of knowledge on the role of calcium signaling in epithelial bicarbonate secretion. Specifically, this review introduces how cytosolic calcium signaling can increase bicarbonate secretion by regulating membrane transport proteins and how it synergizes with cAMP-induced mechanisms in epithelial cells. In addition, tissue-specific variations in the pancreas, salivary glands, intestines, bile ducts, and airways are discussed. We hope that the present report will stimulate further research into this important topic. These studies will provide the basis for future medicines for a wide spectrum of epithelial disorders including cystic fibrosis, Sjögren's syndrome, and chronic pancreatitis.

Ca²⁺-dependent K⁺ channels in exocrine salivary glands

In the last 15 years, remarkable progress has been realized in identifying the genes that encode the ion-transporting proteins involved in exocrine gland function, including salivary glands. Among these proteins, Ca(2+)-dependent K(+) channels take part in key functions including membrane potential regulation, fluid movement and K(+) secretion in exocrine glands. Two K(+) channels have been identified in exocrine salivary glands: (1) a Ca(2+)-activated K(+) channel of intermediate single channel conductance encoded by the KCNN4 gene, and (2) a voltage- and Ca(2+)-dependent K(+) channel of large single channel conductance encoded by the KCNMA1 gene. This review focuses on the physiological roles of Ca(2+)-dependent K(+) channels in exocrine salivary glands. We also discuss interesting recent findings on the regulation of Ca(2+)-dependent K(+) channels by protein-protein interactions that may significantly impact exocrine gland physiology.

Mechanism and synergism in epithelial fluid and electrolyte secretion

A central function of epithelia is the control of the volume and electrolyte composition of bodily fluids through vectorial transport of electrolytes and the obligatory H2O. In exocrine glands, fluid and electrolyte secretion is carried out by both acinar and duct cells, with the portion of fluid secreted by each cell type varying among glands. All acinar cells secrete isotonic, plasma-like fluid, while the duct determines the final electrolyte composition of the fluid by absorbing most of the Cl(-) and secreting HCO3 (-). The key transporters mediating acinar fluid and electrolyte secretion are the basolateral Na(+)/K(+) /2Cl(-) cotransporter, the luminal Ca(2+)-activated Cl(-) channel ANO1 and basolateral and luminal Ca(2+)-activated K(+) channels. Ductal fluid and HCO3 (-) secretion are mediated by the basolateral membrane Na(+)-HCO3 (-) cotransporter NBCe1-B and the luminal membrane Cl(-)/HCO3 (-) exchanger slc26a6 and the Cl(-) channel CFTR. The function of the transporters is regulated by multiple inputs, which in the duct include major regulation by the WNK/SPAK pathway that inhibit secretion and the IRBIT/PP1 pathway that antagonize the effects of the WNK/SPAK pathway to both stimulate and coordinate the secretion. The function of these regulatory pathways in secretory glands acinar cells is yet to be examined. An important concept in biology is synergism among signaling pathways to generate the final physiological response that ensures regulation with high fidelity and guards against cell toxicity. While synergism is observed in all epithelial functions, the molecular mechanism mediating the synergism is not known. Recent work reveals a central role for IRBIT as a third messenger that integrates and synergizes the function of the Ca(2+) and cAMP signaling pathways in activation of epithelial fluid and electrolyte secretion. These concepts are discussed in this review using secretion by the pancreatic and salivary gland ducts as model systems.

Implantable three-dimensional salivary spheroid assemblies demonstrate fluid and protein secretory responses to neurotransmitters

Radiation treatment in patients with head and neck tumors commonly results in hyposalivation and xerostomia due to the loss of fluid-secreting salivary acinar cells. Patients develop susceptibility to oral infections, dental caries, impaired speech and swallowing, reducing the quality of life. Clinical management is largely unsatisfactory. The development of a tissue-engineered, implantable salivary gland will greatly benefit patients suffering from xerostomia. This report compares the ability of a 2.5-dimensional (2.5D) and a three-dimensional (3D) hyaluronic acid (HA)-based culture system to support functional salivary units capable of producing fluid and phenotypic proteins. Parotid cells seeded on 2.5D, as well as those encapsulated in 3D HA hydrogels, self-assembled into acini-like structures and expressed functional neurotransmitter receptors. Structures in 3D hydrogels merged to form organized 50 μm spheroids that could be maintained in culture for over 100 days and merged to form structures over 500 μm in size. Treatment of acini-like structures with the β-adrenergic agonists norepinephrine or isoproterenol increased granule production and α-amylase staining in treated structures, demonstrating regain of protein secretion. Upon treatment with the M3 muscarinic agonist acetylcholine, acini-like structures activated the fluid production pathway by increasing intracellular calcium levels. The increase in intracellular calcium seen in structures in the 3D hydrogel culture system was more robust and prolonged than that in 2.5D. To compare the long-term survival and retention of acini-like structures in vivo, cell-seeded 2.5D and 3D hydrogels were implanted into an athymic rat model. Cells in 2.5D failed to maintain organized acini-like structures and dispersed in the surrounding tissue. Encapsulated cells in 3D retained their spheroid structure and structural integrity, along with the salivary biomarkers and maintained viability for over 3 weeks in vivo. This report identifies a novel hydrogel culture system capable of creating and maintaining functional 3D salivary spheroid structures for long periods in vitro that regain both fluid and protein secreting functions and are suitable for tissue restoration.

Autoantibodies against muscarinic type 3 receptor in Sjögren's syndrome inhibit aquaporin 5 trafficking

Sjögren's syndrome (SjS) is a chronic autoimmune disease that mainly targets the salivary and lacrimal glands. It has been controversial whether anti-muscarinic type 3 receptor (α-M3R) autoantibodies in patients with SjS inhibit intracellular trafficking of aquaporin-5 (AQP5), water transport protein, leading to secretory dysfunction. To address this issue, GFP-tagged human AQP5 was overexpressed in human salivary gland cells (HSG-hAQP5) and monitored AQP5 trafficking to the plasma membrane following carbachol (CCh, M3R agonist) stimulation. AQP5 trafficking was indeed mediated by M3R stimulation, shown in partial blockage of trafficking by M3R-antagonist 4-DAMP. HSG-hAQP5 pre-incubated with SjS plasma for 24 hours significantly reduced AQP5 trafficking with CCh, compared with HSG-hAQP5 pre-incubated with healthy control (HC) plasma. This inhibition was confirmed by monoclonal α-M3R antibody and pre-absorbed plasma. Interestingly, HSG-hAQP5 pre-incubated with SjS plasma showed no change in cell volume, compared to the cells incubated with HC plasma showing shrinkage by twenty percent after CCh-stimulation. Our findings clearly indicate that binding of anti-M3R autoantibodies to the receptor, which was verified by immunoprecipitation, suppresses AQP5 trafficking to the membrane and contribute to impaired fluid secretion in SjS. Our current study urges further investigations of clinical associations between SjS symptoms, such as degree of secretory dysfunction, cognitive impairment, and/or bladder irritation, and different profiles (titers, isotypes, and/or specificity) of anti-M3R autoantibodies in individuals with SjS.

A quantitative analysis of electrolyte exchange in the salivary duct

A healthy salivary gland secretes saliva in two stages. First, acinar cells generate primary saliva, a plasma-like, isotonic fluid high in Na(+) and Cl(-). In the second stage, the ducts exchange Na(+) and Cl(-) for K(+) and HCO(3)(-), producing a hypotonic final saliva with no apparent loss in volume. We have developed a tool that aims to understand how the ducts achieve this electrolyte exchange while maintaining the same volume. This tool is part of a larger multiscale model of the salivary gland and can be used at the duct or gland level to investigate the effects of genetic and chemical alterations. In this study, we construct a radially symmetric mathematical model of the mouse salivary gland duct, representing the lumen, the cell, and the interstitium. For a given flow and primary saliva composition, we predict the potential differences and the luminal and cytosolic concentrations along a duct. Our model accounts well for experimental data obtained in wild-type animals as well as knockouts and chemical inhibitors. Additionally, the luminal membrane potential of the duct cells is predicted to be very depolarized compared with acinar cells. We investigate the effects of an electrogenic vs. electroneutral anion exchanger in the luminal membrane on concentration and the potential difference across the luminal membrane as well as how impairing the cystic fibrosis transmembrane conductance regulator channel affects other ion transporting mechanisms. Our model suggests the electrogenicity of the anion exchanger has little effect in the submandibular duct.

Extracellular Ca(2+) sensing in salivary ductal cells

Ca(2+) is secreted from the salivary acinar cells as an ionic constituent of primary saliva. Ions such as Na(+) and Cl(-) get reabsorbed whereas primary saliva flows through the salivary ductal system. Although earlier studies have shown that salivary [Ca(2+)] decreases as it flows down the ductal tree into the oral cavity, ductal reabsorption of Ca(2+) remains enigmatic. Here we report a potential role for the G protein-coupled receptor, calcium-sensing receptor (CSR), in the regulation of Ca(2+) reabsorption by salivary gland ducts. Our data show that CSR is present in the apical region of ductal cells where it is co-localized with transient receptor potential canonical 3 (TRPC3). CSR is activated in isolated salivary gland ducts as well as a ductal cell line (SMIE) by altering extracellular [Ca(2+)] or by aromatic amino acid, L-phenylalanine (L-Phe, endogenous component of saliva), as well as neomycin. CSR activation leads to Ca(2+) influx that, in polarized cells grown on a filter support, is initiated in the luminal region. We show that TRPC3 contributes to Ca(2+) entry triggered by CSR activation. Further, stimulation of CSR in SMIE cells enhances the CSR-TRPC3 association as well as surface expression of TRPC3. Together our findings suggest that CSR could serve as a Ca(2+) sensor in the luminal membrane of salivary gland ducts and regulate reabsorption of [Ca(2+)] from the saliva via TRPC3, thus contributing to maintenance of salivary [Ca(2+)]. CSR could therefore be a potentially important protective mechanism against formation of salivary gland stones (sialolithiasis) and infection (sialoadenitis).

Extracellular Ca(2+) sensing in salivary ductal cells

Ca(2+) is secreted from the salivary acinar cells as an ionic constituent of primary saliva. Ions such as Na(+) and Cl(-) get reabsorbed whereas primary saliva flows through the salivary ductal system. Although earlier studies have shown that salivary [Ca(2+)] decreases as it flows down the ductal tree into the oral cavity, ductal reabsorption of Ca(2+) remains enigmatic. Here we report a potential role for the G protein-coupled receptor, calcium-sensing receptor (CSR), in the regulation of Ca(2+) reabsorption by salivary gland ducts. Our data show that CSR is present in the apical region of ductal cells where it is co-localized with transient receptor potential canonical 3 (TRPC3). CSR is activated in isolated salivary gland ducts as well as a ductal cell line (SMIE) by altering extracellular [Ca(2+)] or by aromatic amino acid, L-phenylalanine (L-Phe, endogenous component of saliva), as well as neomycin. CSR activation leads to Ca(2+) influx that, in polarized cells grown on a filter support, is initiated in the luminal region. We show that TRPC3 contributes to Ca(2+) entry triggered by CSR activation. Further, stimulation of CSR in SMIE cells enhances the CSR-TRPC3 association as well as surface expression of TRPC3. Together our findings suggest that CSR could serve as a Ca(2+) sensor in the luminal membrane of salivary gland ducts and regulate reabsorption of [Ca(2+)] from the saliva via TRPC3, thus contributing to maintenance of salivary [Ca(2+)]. CSR could therefore be a potentially important protective mechanism against formation of salivary gland stones (sialolithiasis) and infection (sialoadenitis).

Apical Ca2+-activated potassium channels in mouse parotid acinar cells

Ca²⁺ activation of Cl and K channels is a key event underlying stimulated fluid secretion from parotid salivary glands. Cl channels are exclusively present on the apical plasma membrane (PM), whereas the localization of K channels has not been established. Mathematical models have suggested that localization of some K channels to the apical PM is optimum for fluid secretion. A combination of whole cell electrophysiology and temporally resolved digital imaging with local manipulation of intracellular [Ca²⁺] was used to investigate if Ca²⁺-activated K channels are present in the apical PM of parotid acinar cells. Initial experiments established Ca²⁺-buffering conditions that produced brief, localized increases in [Ca²⁺] after focal laser photolysis of caged Ca²⁺. Conditions were used to isolate K⁺ and Cl⁻ conductances. Photolysis at the apical PM resulted in a robust increase in K⁺ and Cl⁻ currents. A localized reduction in [Ca²⁺] at the apical PM after photolysis of Diazo-2, a caged Ca²⁺ chelator, resulted in a decrease in both K⁺ and Cl⁻ currents. The K⁺ currents evoked by apical photolysis were partially blocked by both paxilline and TRAM-34, specific blockers of large-conductance “maxi-K” (BK) and intermediate K (IK), respectively, and almost abolished by incubation with both antagonists. Apical TRAM-34–sensitive K⁺ currents were also observed in BK-null parotid acini. In contrast, when the [Ca²⁺] was increased at the basal or lateral PM, no increase in either K⁺ or Cl⁻ currents was evoked. These data provide strong evidence that K and Cl channels are similarly distributed in the apical PM. Furthermore, both IK and BK channels are present in this domain, and the density of these channels appears higher in the apical versus basolateral PM. Collectively, this study provides support for a model in which fluid secretion is optimized after expression of K channels specifically in the apical PM.

Molecular mechanism of pancreatic and salivary gland fluid and HCO3 secretion

Fluid and HCO(3)(-) secretion is a vital function of all epithelia and is required for the survival of the tissue. Aberrant fluid and HCO(3)(-) secretion is associated with many epithelial diseases, such as cystic fibrosis, pancreatitis, Sjögren's syndrome, and other epithelial inflammatory and autoimmune diseases. Significant progress has been made over the last 20 years in our understanding of epithelial fluid and HCO(3)(-) secretion, in particular by secretory glands. Fluid and HCO(3)(-) secretion by secretory glands is a two-step process. Acinar cells secrete isotonic fluid in which the major salt is NaCl. Subsequently, the duct modifies the volume and electrolyte composition of the fluid to absorb the Cl(-) and secrete HCO(3)(-). The relative volume secreted by acinar and duct cells and modification of electrolyte composition of the secreted fluids varies among secretory glands to meet their physiological functions. In the pancreas, acinar cells secrete a small amount of NaCl-rich fluid, while the duct absorbs the Cl(-) and secretes HCO(3)(-) and the bulk of the fluid in the pancreatic juice. Fluid secretion appears to be driven by active HCO(3)(-) secretion. In the salivary glands, acinar cells secrete the bulk of the fluid in the saliva that is driven by active Cl(-) secretion and contains high concentrations of Na(+) and Cl(-). The salivary glands duct absorbs both the Na(+) and Cl(-) and secretes K(+) and HCO(3)(-). In this review, we focus on the molecular mechanism of fluid and HCO(3)(-) secretion by the pancreas and salivary glands, to highlight the similarities of the fundamental mechanisms of acinar and duct cell functions, and to point out the differences to meet gland-specific secretions.

Role of BK channels in hypertension and potassium secretion

PURPOSE OF REVIEW

This review summarizes recent studies of hypertension associated with a defect in renal K excretion due to genetic deletions of various components of the large, Ca-activated K channel (BK), and describes new evidence and theories regarding K secretory roles of BK in intercalated cells.

RECENT FINDINGS

Isolated perfused tubule methods have revealed the importance of BK in flow-induced K secretion. Subsequently, mice with genetically deleted BK subunits revealed the complexities of BK-mediated K secretion. Deletion of BKα results in extreme aldosteronism, hypertension, and an absence of flow-induced K secretion. Deletion of the BKβ1 ancillary subunit results in decreased handling of a K load, increased plasma K, mild aldosteronism and hypertension that is exacerbated by a high K diet. Deletion of BKβ4 (β4KO) leads to insufficient K handling, high plasma K, fluid retention, but with milder hypertension. Fluid retention in β4KO may be the result of insufficient flow-induced secretion of adenosine triphosphate (ATP), which normally inhibits epithelial Na channels (ENaCs).

SUMMARY

Classical physiological analysis of electrolyte handling in knockout mice has enlightened our understanding of the mechanism of handling K loads by renal K channels. Studies have focused on the different roles of BK-α/β1 and BK-α/β4 in the kidney. BKβ1 hypertension may be a 'three-hit' hypertension, involving a K secretory defect, elevated production of aldosterone, and increased vascular tone. The disorders observed in BK knockout mice have shed new insights on the importance of proper renal K handling for maintaining volume balance and blood pressure.

The LRRC26 protein selectively alters the efficacy of BK channel activators

Large conductance, Ca(2+)-activated K channel proteins are involved in a wide range of physiological activities, so there is considerable interest in the pharmacology of large conductance calcium-activated K (BK) channels. One potent activator of BK channels is mallotoxin (MTX), which produces a very large hyperpolarizing shift of the voltage gating of heterologously expressed BK channels and causes a dramatic increase in the activity of BK channels in human smooth muscle cells. However, we found that MTX shifted the steady-state activation of BK channels in native parotid acinar cells by only 6 mV. This was not because the parotid BK isoform (parSlo) is inherently insensitive to MTX as MTX shifted the activation of heterologously expressed parSlo channels by 70 mV. Even though MTX had a minimal effect on steady-state activation of parotid BK channels, it produced an approximate 2-fold speeding of the channel-gating kinetics. The BK channels in parotid acinar cells have a much more hyperpolarized voltage activation range than BK channels in most other cell types. We found that this is probably attributable to an accessory protein, LRRC26, which is expressed in parotid glands: expressed parSlo + LRRC26 channels were resistant to the actions of MTX. Another class of BK activators is the benzimidazalones that includes 1,3-dihydro-1-(2-hydroxy-5-(trifluoromethyl)phenyl)-5-(trifluoromethyl)-2H-benzimidazol-2-one (NS-1619). Although the LRRC26 accessory protein strongly inhibited the ability of MTX to activate BK channels, we found that it had only a small effect on the action of NS-1619 on BK channels. Thus, the LRRC26 BK channel accessory protein selectively alters the pharmacology of BK channels.

Ascl3 knockout and cell ablation models reveal complexity of salivary gland maintenance and regeneration

Expression of the transcription factor, Ascl3, marks a population of adult progenitor cells, which can give rise to both acinar and duct cell types in the murine salivary glands. Using a previously reported Ascl3(EGFP-Cre/+) knock-in strain, we demonstrate that Ascl3-expressing cells represent a molecularly distinct, and proliferating population of progenitor cells located in salivary gland ducts. To investigate both the role of the Ascl3 transcription factor, and the role of the cells in which it is expressed, we generated knockout and cell-specific ablation models. Ascl3 knockout mice develop smaller salivary glands than wild type littermates, but secrete saliva normally. They display a lower level of cell proliferation, consistent with their smaller size. In the absence of Ascl3, the cells maintain their progenitor function and continue to generate both acinar and duct cells. To directly test the role of the progenitor cells, themselves, in salivary gland development and regeneration, we used Cre-activated expression of diphtheria toxin (DTA) in the Ascl3-expressing (Ascl3+) cell population, resulting in specific cell ablation of Ascl3+ cells. In the absence of the Ascl3+ progenitor cells, the mice developed morphologically normal, albeit smaller, salivary glands able to secrete saliva. Furthermore, in a ductal ligation model of salivary gland injury, the glands of these mice were able to regenerate acinar cells. Our results indicate that Ascl3+ cells are active proliferating progenitors, but they are not the only precursors for salivary gland development or regeneration. We conclude that maintenance of tissue homeostasis in the salivary gland must involve more than one progenitor cell population.

Cevimeline-induced monophasic salivation from the mouse submandibular gland: decreased Na+ content in saliva results from specific and early activation of Na+/H+ exchange

Cevimeline and pilocarpine are muscarinic agonists used clinically to treat dry mouth. In this study, we explored fluid secretion from mouse submandibular glands to determine the mechanism of cevimeline, pilocarpine, and an experimentally used agent carbachol. Cevimeline evoked almost the same amount of secretion at concentrations from 30 μM to 1 mM. Pilocarpine also induced secretion at a concentration as low as 1 μM and was the most powerful secretagogue at 10 μM. Secretion was induced by carbachol at 0.1 μM, with maximum secretion at 1.0 μM. Cevimeline induced monophasic secretion at all concentrations tested, whereas higher concentrations of pilocarpine and carbachol induced secretion with variable kinetics, i.e., an initial transient high flow rate, followed by decreased secretion after 2 to 3 min. In the presence of an epithelial Na(+) channel blocker, amiloride, neither carbachol nor pilocarpine affected the Na(+) level of secreted saliva; however, it significantly increased the Na(+) content of cevimeline-induced saliva. The intracellular Ca(2+) response of acinar cells was almost identical among all three agents, although recovery after drug removal was slower for cevimeline and pilocarpine. A profound decrease in intracellular pH was observed during pilocarpine and carbachol treatment, whereas intracellular acidification induced by cevimeline was only seen in the presence of a Na(+)/H(+) exchange inhibitor. When external HCO(3)(-) was removed, cevimeline-induced saliva significantly decreased. These findings suggest that cevimeline specifically activates Na(+)/H(+) exchange and may promote Na(+) reabsorption by stabilizing epithelial sodium channel activity.

Channels and transporters in salivary glands

According to the two-stage hypothesis, primary saliva, a NaCl-rich plasma-like isotonic fluid is secreted by salivary acinar cells and its ionic composition becomes modified in the duct system. The ducts secrete K(+) and HCO (3) (-) and reabsorb Na(+) and Cl(-) without any water movement, thus establishing a hypotonic final saliva. Salivary secretion depends on the coordinated action of several channels and transporters localized in the apical and basolateral membrane of acinar and duct cells. Early functional studies in perfused glands, followed by the molecular cloning of several transport proteins and the subsequent analysis of mutant mice, have greatly contributed to our understanding of salivary fluid and the electrolyte secretion process. With a few exceptions, most of the key channels and transporters involved in salivary secretion have now been identified and characterized. However, the picture that has emerged from all these studies is one of a complex molecular network characterized by redundancy for several transport proteins, compensatory mechanisms, and adaptive changes in health and disease. Current research is directed to the molecular interactions between the determinants and the ways in which they are regulated by extracellular signals and intracellular mediators. This review focuses on the functionally and molecularly best-characterized channels and transporters that are considered to be involved in transepithelial fluid and electrolyte transport in salivary glands.

Modulation of calcium-activated potassium channels induces cardiogenesis of pluripotent stem cells and enrichment of pacemaker-like cells

BACKGROUND

Ion channels are key determinants for the function of excitable cells, but little is known about their role and involvement during cardiac development. Earlier work identified Ca(2+)-activated potassium channels of small and intermediate conductance (SKCas) as important regulators of neural stem cell fate. Here we have investigated their impact on the differentiation of pluripotent cells toward the cardiac lineage.

METHODS AND RESULTS

We have applied the SKCa activator 1-ethyl-2-benzimidazolinone on embryonic stem cells and identified this particular ion channel family as a new critical target involved in the generation of cardiac pacemaker-like cells: SKCa activation led to rapid remodeling of the actin cytoskeleton, inhibition of proliferation, induction of differentiation, and diminished teratoma formation. Time-restricted SKCa activation induced cardiac mesoderm and commitment to the cardiac lineage as shown by gene regulation, protein, and functional electrophysiological studies. In addition, the differentiation into cardiomyocytes was modulated in a qualitative fashion, resulting in a strong enrichment of pacemaker-like cells. This was accompanied by induction of the sino-atrial gene program and in parallel by a loss of the chamber-specific myocardium. In addition, SKCa activity induced activation of the Ras-Mek-Erk signaling cascade, a signaling pathway involved in the 1-ethyl-2-benzimidazolinone-induced effects.

CONCLUSIONS

SKCa activation drives the fate of pluripotent cells toward mesoderm commitment and cardiomyocyte specification, preferentially into nodal-like cardiomyocytes. This provides a novel strategy for the enrichment of cardiomyocytes and in particular, the generation of a specific subtype of cardiomyocytes, pacemaker-like cells, without genetic modification.

Specific expression of salivary maxi-K channel variant is augmented in diabetic mice

BACKGROUND AND OBJECTIVE

A genetically diabetic mouse strain (db/db) exhibits severe obesity and a syndrome resembling human non-insulin-dependent diabetes mellitus. Our histological study of submandibular glands revealed that the size and area of the granular convoluted tubules was substantially decreased in db/db mice. We hypothesized that this structural difference reflected a specific alteration in salivary duct function.

METHODS

The saliva evoked by pilocarpine was used for the measurement of ion concentrations, and submandibular glands were dissected out for the immunohistochemistry and real-time PCR study.

RESULTS

The K(+) concentration of the salivary secretion was higher in db/db than in control m+/m+ mice, while neither saliva volume nor the concentrations of Na(+) or Cl⁻ differed between these strains. In db/db mice (vs. m+/m+ mice): quantitative PCR analysis revealed an increased mRNA expression of large-conductance Ca²(+)-activated K(+) (maxi-K) channels, immunohistochemistry revealed an increase in the luminal surface expression of the maxi-K channel protein, and a particularly interesting finding was that there was a substantial increase in the salivary tissue-specific splice variant ParSlo.

CONCLUSION

These results suggest that in db/db mice, the K(+) content of saliva may be elevated due to an expression of a maxi-K channel variant, which results from a modification of ductal structure.

GENERAL SIGNIFICANCE

Our data may shed some light on the mechanism responsible for determining the dynamics of salivary K(+) concentration increased in diabetic patients.

Ca2+-activated K channels in parotid acinar cells: The functional basis for the hyperpolarized activation of BK channels

Fluid secretion relies on a close interplay between Ca(2+)-activated Cl and K channels. Salivary acinar cells contain both large conductance, BK, and intermediate conductance, IK1, K channels. Physiological fluid secretion occurs with only modest (<500 nM) increases in intracellular Ca(2+) levels but BK channels in many cell types and in heterologous expression systems require very high concentrations for significant activation. We report here our efforts to understand this apparent contradiction. We determined the Ca(2+) dependence of IK1 and BK channels in mouse parotid acinar cells. IK1 channels activated with an apparent Ca(2+) affinity of about 350 nM and a Hill coefficient near 3. Native parotid BK channels activated at similar Ca(2+) levels unlike the BK channels in other cell types. Since the parotid BK channel is encoded by an uncommon splice variant, we examined this clone in a heterologous expression system. In contrast to the native parotid channel, activation of this expressed "parSlo" channel required very high levels of Ca(2+). In order to understand the functional basis for the special properties of the native channels, we analyzed the parotid BK channel in the context of the Horrigan-Aldrich model of BK channel gating. We found that the shifted activation of parotid BK channels resulted from a hyperpolarizing shift of the voltage dependence of voltage sensor activation and channel opening and included a large change in the coupling of these two processes.

The salivary gland fluid secretion mechanism

Fluid secretion by exocrine glands requires the coordinated activity of multiple water and ion transporter and channel proteins. The molecular cloning of many of the transporter molecules involved in fluid secretion has yielded a better understanding of the fluid secretion process. Mouse salivary glands are easily accessible model systems for the study of exocrine gland secretion at the cellular and organ level. Indeed, the characterization of mice with null mutations in many of the water and ion transporter and channel genes has demonstrated the physiological roles of individual proteins. This overview will focus on recent developments in determining the molecular identification of the proteins that are involved in the fluid secretion process.