Cell-permeable fluorescent indicator for cytosolic chloride

Tunable Cytosolic Chloride Indicators for Real-Time Chloride Imaging in Live Cells

Chloride plays a crucial role in various cellular functions, and its level is regulated by a variety of chloride transporters and channels. However, to date, we still lack the capability to image instantaneous ion flux through chloride channels at single-cell level. Here, we developed a series of cell-permeable, pH-independent, chloride-sensitive fluorophores for real-time cytosolic chloride imaging, which we call CytoCl dyes. We demonstrated the ability of CytoCl dyes to monitor cytosolic chloride and used it to uncover the rapid changes and transient events of halide flux, which cannot be captured by steady-state imaging. Finally, we successfully imaged the proton-activated chloride channel-mediated ion flux at single-cell level, which is, to our knowledge, the first real-time imaging of ion flux through a chloride channel in unmodified cells. By enabling the imaging of single-cell level ion influx through chloride channels and transporters, CytoCl dyes can expand our understanding of ion flux dynamics, which is critical for characterization and modulator screening of these membrane proteins. A conjugable version of CytoCl dyes was also developed for its customization across different applications.

Chloride ions in health and disease

Chloride is a key anion involved in cellular physiology by regulating its homeostasis and rheostatic processes. Changes in cellular Cl- concentration result in differential regulation of cellular functions such as transcription and translation, post-translation modifications, cell cycle and proliferation, cell volume, and pH levels. In intracellular compartments, Cl- modulates the function of lysosomes, mitochondria, endosomes, phagosomes, the nucleus, and the endoplasmic reticulum. In extracellular fluid (ECF), Cl- is present in blood/plasma and interstitial fluid compartments. A reduction in Cl- levels in ECF can result in cell volume contraction. Cl- is the key physiological anion and is a principal compensatory ion for the movement of the major cations such as Na+, K+, and Ca2+. Over the past 25 years, we have increased our understanding of cellular signaling mediated by Cl-, which has helped in understanding the molecular and metabolic changes observed in pathologies with altered Cl- levels. Here, we review the concentration of Cl- in various organs and cellular compartments, ion channels responsible for its transportation, and recent information on its physiological roles.

Engineering the ChlorON Series: Turn-On Fluorescent Protein Sensors for Imaging Labile Chloride in Living Cells

Beyond its role as the "queen of electrolytes", chloride can also serve as an allosteric regulator or even a signaling ion. To illuminate this essential anion across such a spectrum of biological processes, researchers have relied on fluorescence imaging with genetically encoded sensors. In large part, these have been derived from the green fluorescent protein found in the jellyfish Aequorea victoria. However, a standalone sensor with a turn-on intensiometric response at physiological pH has yet to be reported. Here, we address this technology gap by building on our discovery of the anion-sensitive fluorescent protein mNeonGreen (mNG). The targeted engineering of two non-coordinating residues, namely K143 and R195, in the chloride binding pocket of mNG coupled with an anion walking screening and selection strategy resulted in the ChlorON sensors: ChlorON-1 (K143W/R195L), ChlorON-2 (K143R/R195I), and ChlorON-3 (K143R/R195L). In vitro spectroscopy revealed that all three sensors display a robust turn-on fluorescence response to chloride (20- to 45-fold) across a wide range of affinities (Kd ≈ 30-285 mM). We further showcase how this unique sensing mechanism can be exploited to directly image labile chloride transport with spatial and temporal resolution in a cell model overexpressing the cystic fibrosis transmembrane conductance regulator. Building from this initial demonstration, we anticipate that the ChlorON technology will have broad utility, accelerating the path forward for fundamental and translational aspects of chloride biology.

Synthesis and Characterization of a Novel Concentration-Independent Fluorescent Chloride Indicator, ABP-Dextran, Optimized for Extracellular Chloride Measurement

GABA, the primary inhibitory neurotransmitter, stimulates GABAA receptors (GABAARs) to increase the chloride conductance of the cytosolic membrane. The driving forces for membrane chloride currents are determined by the local differences between intracellular and extracellular chloride concentrations (Cli and Clo, respectively). While several strategies exist for the measurement of Cli, the field lacks tools for non-invasive measurement of Clo. We present the design and development of a fluorescent lifetime imaging (FLIM)-compatible small molecule, N(4-aminobutyl)phenanthridiunium (ABP) with the brightness, spectral features, sensitivity to chloride, and selectivity versus other anions to serve as a useful probe of Clo. ABP can be conjugated to dextran to ensure extracellular compartmentalization, and a second chloride-insensitive counter-label can be added for ratiometric imaging. We validate the utility of this novel sensor series in two sensor concentration-independent modes: FLIM or ratiometric intensity-based imaging.

Chloride transporters controlling neuronal excitability

Synaptic inhibition plays a crucial role in regulating neuronal excitability, which is the foundation of nervous system function. This inhibition is largely mediated by the neurotransmitters GABA and glycine that activate Cl^--permeable ion channels, which means that the strength of inhibition depends on the Cl^- gradient across the membrane. In neurons, the Cl^- gradient is primarily mediated by two secondarily active cation-chloride cotransporters (CCCs), NKCC1 and KCC2. CCC-mediated regulation of the neuronal Cl^- gradient is critical for healthy brain function, as dysregulation of CCCs has emerged as a key mechanism underlying neurological disorders including epilepsy, neuropathic pain, and autism spectrum disorder. This review begins with an overview of neuronal chloride transporters before explaining the dependent relationship between these CCCs, Cl^- regulation, and inhibitory synaptic transmission. We then discuss the evidence for how CCCs can be regulated, including by activity and their protein interactions, which underlie inhibitory synaptic plasticity. For readers who may be interested in conducting experiments on CCCs and neuronal excitability, we have included a section on techniques for estimating and recording intracellular Cl^-, including their advantages and limitations. Although the focus of this review is on neurons, we also examine how Cl^- is regulated in glial cells, which in turn regulate neuronal excitability through the tight relationship between this nonneuronal cell type and synapses. Finally, we discuss the relatively extensive and growing literature on how CCC-mediated neuronal excitability contributes to neurological disorders.

Citrate-based fluorometric sensor for multi-halide sensing

Halides play important roles in human health and environmental monitoring. However, different halides interfere with each other in current measurement methods. Simultaneous sensing of multiple halides in a fast and low-cost manner remains a challenge. Here, we report a fluorometric multi-halide sensing method by using a single citrate-based fluorophore, CA-Cys, on a custom-made portable device. The fluorescence emitted by CA-Cys is quenched due to the dynamic quenching of halide ions; the sensitivities vary from halide types and pH, providing the capability to obtain multiple Stern-Volmer equations at various pH values. The concentration of each halide can then be obtained by solving the resultant set of equations. A mM scale detection limit is demonstrated, which is suitable for halide wastewater monitoring. A proof-of-concept smartphone-based portable device is also fabricated and tested. The results from the fluorometer and portable device indicated that our multi-halide system is promising for real-world multi-halide sensing applications. This work represents a new direction in developing portable, low-cost, and simultaneous multi-halide sensing technologies.

Fluorescent Indicators For Biological Imaging of Monatomic Ions

Monatomic ions play critical biological roles including maintaining the cellular osmotic pressure, transmitting signals, and catalyzing redox reactions as cofactors in enzymes. The ability to visualize monatomic ion concentration, and dynamic changes in the concentration, is essential to understanding their many biological functions. A growing number of genetically encodable and synthetic indicators enable the visualization and detection of monatomic ions in biological systems. With this review, we aim to provide a survey of the current landscape of reported indicators. We hope this review will be a useful guide to researchers who are interested in using indicators for biological applications and to tool developers seeking opportunities to create new and improved indicators.

Increased intracellular Cl- concentration in pulmonary arterial myocytes is associated with chronic hypoxic pulmonary hypertension

Chloride channels play an important role in regulating smooth muscle contraction and proliferation, and contribute to the enhanced constriction of pulmonary arteries (PAs) in pulmonary hypertension (PH). The intracellular Cl^- concentration ([Cl^-]_i), tightly regulated by various Cl^- transporters, determines the driving force for Cl^- conductance, thereby the functional outcome of Cl^- channel activation. This study characterizes for the first time the expression profile of Cl^- transporters/exchangers in PA smooth muscle and provides the first evidence that the intracellular Cl^- homeostasis is altered in PA smooth muscle cells (PASMCs) associated with chronic hypoxic PH (CHPH). Quantitative RT-PCR revealed that the endothelium-denuded intralobar PA of rats expressed Slc12a gene family-encoded Na-K-2Cl cotransporter 1 (NKCC1), K-Cl cotransporters (KCC) 1, 3, and 4, and Slc4a gene family-encoded Na⁺-independent and Na⁺-dependent Cl^-/HCO₃^- exchangers. Exposure of rats to chronic hypoxia (10% O₂, 3 wk) caused CHPH and selectively increased the expression of Cl^--accumulating NKCC1 and reduced the Cl^--extruding KCC4. The intracellular Cl^- concentration ([Cl^-]_i) averaged at 45 mM and 47 mM in normoxic PASMCs as determined by fluorescent indicator MEQ and by gramicidin-perforated patch-clamp technique, respectively. The ([Cl^-]_i was increased by ∼10 mM in PASMCs of rats with CHPH. Future studies are warranted to further establish the hypothesis that the altered intracellular Cl^- homeostasis contributes to the pathogenesis of CHPH.

Unique Actions of GABA Arising from Cytoplasmic Chloride Microdomains

Developmental, cellular, and subcellular variations in the direction of neuronal Cl- currents elicited by GABAA receptor activation have been frequently reported. We found a corresponding variance in the GABAA receptor reversal potential (EGABA) for synapses originating from individual interneurons onto a single pyramidal cell. These findings suggest a similar heterogeneity in the cytoplasmic intracellular concentration of chloride ([Cl-]i) in individual dendrites. We determined [Cl-]i in the murine hippocampus and cerebral cortex of both sexes by (1) two-photon imaging of the Cl--sensitive, ratiometric fluorescent protein SuperClomeleon; (2) Fluorescence Lifetime IMaging (FLIM) of the Cl--sensitive fluorophore MEQ (6-methoxy-N-ethylquinolinium); and (3) electrophysiological measurements of EGABA by pressure application of GABA and RuBi-GABA uncaging. Fluorometric and electrophysiological estimates of local [Cl-]i were highly correlated. [Cl-]i microdomains persisted after pharmacological inhibition of cation-chloride cotransporters, but were progressively modified after inhibiting the polymerization of the anionic biopolymer actin. These methods collectively demonstrated stable [Cl-]i microdomains in individual neurons in vitro and in vivo and the role of immobile anions in its stability. Our results highlight the existence of functionally significant neuronal Cl- microdomains that modify the impact of GABAergic inputs.SIGNIFICANCE STATEMENT Microdomains of varying chloride concentrations in the neuronal cytoplasm are a predictable consequence of the inhomogeneous distribution of anionic polymers such as actin, tubulin, and nucleic acids. Here, we demonstrate the existence and stability of these microdomains, as well as the consequence for GABAergic synaptic signaling: each interneuron produces a postsynaptic GABAA response with a unique reversal potential. In individual hippocampal pyramidal cells, the range of GABAA reversal potentials evoked by stimulating different interneurons was >20 mV. Some interneurons generated postsynaptic responses in pyramidal cells that reversed at potentials beyond what would be considered purely inhibitory. Cytoplasmic chloride microdomains enable each pyramidal cell to maintain a compendium of unique postsynaptic responses to the activity of individual interneurons.

Advances in the development of novel compounds targeting cation-chloride cotransporter physiology

For about half a century, the pharmacology of electroneutral cation-chloride cotransporters has been dominated by a few drugs that are widely used in clinical medicine. Because these diuretic drugs are so good at what they do, there has been little incentive in expanding their pharmacology. The increasing realization that cation-chloride cotransporters are involved in many other key physiological processes and the knowledge that different tissues express homologous proteins with matching transport functions have rekindled interest in drug discovery. This review summarizes the methods available to assess the function of these transporters and describe the multiple efforts that have made to identify new compounds. We describe multiple screens targeting KCC2 function and one screen designed to find compounds that discriminate between NKCC1 and NKCC2. Two of the KCC2 screens identified new inhibitors that are 3-4 orders of magnitude more potent than furosemide. Additional screens identified compounds that purportedly increase cell surface expression of the cotransporter, as well as several FDA-approved drugs that increase KCC2 transcription and expression. The technical details of each screen biased them toward specific processes in the life cycle of the transporter, making these efforts independent and complementary. In addition, each drug discovery effort contributes to our understanding of the biology of the cotransporters.

Long-Term Quantitatively Imaging Intracellular Chloride Concentration Using a Core-/Shell-Structured Nanosensor and Time-Domain Dual-Lifetime Referencing Method

Luminescence lifetime-based nanosensors for chloride ions were designed by incorporating a luminescent ruthenium dye [Ru(1,10-phenanthroline)₃] inside silica nanoparticles and chemically labelling their outer surface with chloride ion-sensitive fluorescent dyes (N,N'-bis(carboxypropyl)-9,9'-biacridine). The nanosensor surface was further functionalized with positively charged amino groups to facilitate intracellular uptake via endocytosis and target lysosomes. The nanosensors have an average diameter of 52 nm and are monodispersed in aqueous solutions. Because of the long lifetime of the reference ruthenium dye, the sensor response can be analyzed using the time-domain dual-lifetime referencing (td-DLR) approach. The use of pulsed excitation in td-DLR rather than intense continuous illumination in ratiometric measurements greatly prevents the dye from photobleaching which significantly improves its measurement stability and reproducibility for long-term monitoring. At optimum conditions, the sensor can measure chloride concentration in the range of 0-200 mM with a large ratiometric signal change from 140.9 to 40.2. Combined with our custom-built microscopic td-DLR system, variations of intracellular chloride concentration in lysosomes were imaged quantitatively with a high spatial resolution and accuracy.

Live Cell Microscopy and Flow Cytometry to Study Streptolysin S-Mediated Erythrocyte Hemolysis

The ability to induce hemolysis, the rupturing of erythrocytes with the consequent release of their intracellular contents, is a phenotypic hallmark of a number of microbial toxins. Streptococcus pyogenes or Group A Streptococcus (GAS) is a human pathogen responsible for a wide range of diseases from mild pharyngitis to severe conditions such as toxic shock syndrome. GAS produces a powerful hemolytic toxin called streptolysin S (SLS). Herein, we describe a procedure for the preparation of SLS toxin and the use of two complementary approaches, live microscopy and flow cytometry, to study the effects of the SLS toxin on erythrocytes. In addition to providing insights into SLS-mediated hemolysis, these assays have the potential to be modified for the study of other hemolytic toxins and compounds.

Olesoxime, a cholesterol-like neuroprotectant restrains synaptic vesicle exocytosis in the mice motor nerve terminals: Possible role of VDACs

Olesoxime is a cholesterol-like neuroprotective compound that targets to mitochondrial voltage dependent anion channels (VDACs). VDACs were also found in the plasma membrane and highly expressed in the presynaptic compartment. Here, we studied the effects of olesoxime and VDAC inhibitors on neurotransmission in the mouse neuromuscular junction. Electrophysiological analysis revealed that olesoxime suppressed selectively evoked neurotransmitter release in response to a single stimulus and 20 Hz activity. Also olesoxime decreased the rate of FM1-43 dye loss (an indicator of synaptic vesicle exocytosis) at low frequency stimulation and 20 Hz. Furthermore, an increase in extracellular Cl^- enhanced the action of olesoxime on the exocytosis and olesoxime increased intracellular Cl^- levels. The effects of olesoxime on the evoked synaptic vesicle exocytosis and [Cl^-]_i were blocked by membrane-permeable and impermeable VDAC inhibitors. Immunofluorescent labeling pointed on the presence of VDACs on the synaptic membranes. Rotenone-induced mitochondrial dysfunction perturbed the exocytotic release of FM1-43 and cell-permeable VDAC inhibitor (but not olesoxime or impermeable VDAC inhibitor) partially mitigated the rotenone-driven alterations in the FM1-43 unloading and mitochondrial superoxide production. Thus, olesoxime restrains neurotransmission by acting on plasmalemmal VDACs whose activation can limit synaptic vesicle exocytosis probably via increasing anion flux into the nerve terminals.

What biologists want from their chloride reporters – a conversation between chemists and biologists

Impaired chloride transport affects diverse processes ranging from neuron excitability to water secretion, which underlie epilepsy and cystic fibrosis, respectively. The ability to image chloride fluxes with fluorescent probes has been essential for the investigation of the roles of chloride channels and transporters in health and disease. Therefore, developing effective fluorescent chloride reporters is critical to characterizing chloride transporters and discovering new ones. However, each chloride channel or transporter has a unique functional context that demands a suite of chloride probes with appropriate sensing characteristics. This Review seeks to juxtapose the biology of chloride transport with the chemistries underlying chloride sensors by exploring the various biological roles of chloride and highlighting the insights delivered by studies using chloride reporters. We then delineate the evolution of small-molecule sensors and genetically encoded chloride reporters. Finally, we analyze discussions with chloride biologists to identify the advantages and limitations of sensors in each biological context, as well as to recognize the key design challenges that must be overcome for developing the next generation of chloride sensors.

KCC2 Manipulation Alters Features of Migrating Interneurons in Ferret Neocortex

KCC2 is a brain specific chloride-potassium cotransporter affecting neuronal development including migration and cellular maturation. It modulates chloride homeostasis influencing the switch of GABA from depolarizing to hyperpolarizing, which contributes to the cues that influence the termination of neuronal migration. The expression of KCC2 during migration of interneurons, therefore, correlates with the ability of these cells to respond to GABA as a stop signal. Manipulation of KCC2 in development can affect various aspects of migrating neurons, including the speed. We describe the effect of KCC2 downregulation and inhibition on features of migrating interneurons of normal ferret kits and those treated with methylazoxymethanol acetate, which increases KCC2. Treatment of organotypic cultures with Bisphenol A, an environmental toxin that alters gene expression, also downregulates KCC2 protein. In organotypic slices treated with the KCC2 antagonist VU0240551, chloride imaging shows inhibition of KCC2 via blockade of chloride flux. Time-lapse video imaging of organotypic cultures treated with either drug, shows a significant increase in the average speed, step size, and number of turns made by migrating neurons leaving the ganglionic eminence. Our findings demonstrate the harmful effect of environmental toxins on brain development and potential consequences in the pathogenesis of neurodevelopmental disorders.

Fluorescent probes for chloride ions in biological samples

As one of the most widespread anions, chloride ion largely existed in the water sources as well as living organism. Therefore, determination of chloride ions in biological samples is evidently important. Herein, we developed two analogous fluorescence probes BeQ1 and BeQ2 for the sensitive detection of chloride ions. The chloride ions in biological samples were determined by a direct and simple method with the detection limit of 46 and 66 μM respectively. In addition, the probes were found having the two-photon excitation property.

CLIC1 null mice demonstrate a role for CLIC1 in macrophage superoxide production and tissue injury

We generated and studied CLIC1 null (C1KO) mice to investigate the physiological role of this protein. C1KO and matched wild-type (WT) mice were studied in two models of acute toxic tissue injury. CLIC1 expression is upregulated following acute injury of WT kidney and pancreas and is absent in C1KOs. Acute tissue injury is attenuated in the C1KOs and this correlates with an absence of the rise in tissue reactive oxygen species (ROS) that is seen in WT mice. Infiltration of injured tissue by inflammatory cells was comparable between WT and C1KOs. Absence of CLIC1 increased PMA-induced superoxide production by isolated peritoneal neutrophils but dramatically decreased PMA-induced superoxide production by peritoneal macrophages. CLIC1 is expressed in both neutrophils and macrophages in a peripheral pattern consistent with either plasma membrane or the cortical cytoskeleton in resting cells and redistributes away from the periphery following PMA stimulation in both cell types. Absence of CLIC1 had no effect on redistribution or dephosphorylation of Ezrin/ERM cytoskeleton in macrophages. Plasma membrane chloride conductance is altered in the absence of CLIC1, but not in a way that would be expected to block superoxide production. NADPH oxidase redistributes from an intracellular compartment to the plasma membrane when WT macrophages are stimulated to produce superoxide and this redistribution fails to occur in C1KO macrophages. We conclude that the role of CLIC1 in macrophage superoxide production is to support redistribution of NADPH oxidase to the plasma membrane, and not through major effects on ERM cytoskeleton or by acting as a plasma membrane chloride channel.

Mechanical perturbations trigger endothelial nitric oxide synthase activity in human red blood cells

Nitric oxide (NO), a vascular signaling molecule, is primarily produced by endothelial NO synthase. Recently, a functional endothelial NO synthase (eNOS) was described in red blood cells (RBC). The RBC-eNOS contributes to the intravascular NO pool and regulates physiological functions. However the regulatory mechanisms and clinical implications of RBC-eNOS are unknown. The present study investigated regulation and functions of RBC-eNOS under mechanical stimulation. This study shows that mechanical stimuli perturb RBC membrane, which triggers a signaling cascade to activate the eNOS. Extracellular NO level, estimated by the 4-Amino-5-Methylamino-2', 7'-Difluorofluorescein Diacetate probe, was significantly increased under mechanical stimuli. Immunostaining and western blot studies confirmed that the mechanical stimuli phosphorylate the serine 1177 moiety of RBC-eNOS, and activates the enzyme. The NO produced by activation of RBC-eNOS in vortexed RBCs promoted important endothelial functions such as migration and vascular sprouting. We also show that mechanical perturbation facilitates nitrosylation of RBC proteins via eNOS activation. The results of the study confirm that mechanical perturbations sensitize RBC-eNOS to produce NO, which ultimately defines physiological boundaries of RBC structure and functions. Therefore, we propose that mild physical perturbations before, after, or during storage can improve viability of RBCs in blood banks.

Activation of band 3 mediates group A Streptococcus streptolysin S-based beta-haemolysis

Streptococcus pyogenes, or group A Streptococcus (GAS), is a human bacterial pathogen that can manifest as a range of diseases from pharyngitis and impetigo to severe outcomes such as necrotizing fasciitis and toxic shock syndrome. GAS disease remains a global health burden with cases estimated at over 700 million annually and over half a million deaths due to severe infections(1). For over 100 years, a clinical hallmark of diagnosis has been the appearance of complete (beta) haemolysis when grown in the presence of blood. This activity is due to the production of a small peptide toxin by GAS known as streptolysin S. Although it has been widely held that streptolysin S exerts its lytic activity through membrane disruption, its exact mode of action has remained unknown. Here, we show, using high-resolution live cell imaging, that streptolysin S induces a dramatic osmotic change in red blood cells, leading to cell lysis. This osmotic change was characterized by the rapid influx of Cl(-) ions into the red blood cells through SLS-mediated disruption of the major erythrocyte anion exchange protein, band 3. Chemical inhibition of band 3 function significantly reduced the haemolytic activity of streptolysin S, and dramatically reduced the pathology in an in vivo skin model of GAS infection. These results provide key insights into the mechanism of streptolysin S-mediated haemolysis and have implications for the development of treatments against GAS.

H(+)/K(+) ATPase activity is required for biomineralization in sea urchin embryos

The bioelectrical signatures associated with regeneration, wound healing, development, and cancer are changes in the polarization state of the cell that persist over long durations, and are mediated by ion channel activity. To identify physiologically relevant bioelectrical changes that occur during normal development of the sea urchin Lytechinus variegatus, we tested a range of ion channel inhibitors, and thereby identified SCH28080, a chemical inhibitor of the H(+)/K(+) ATPase (HKA), as an inhibitor of skeletogenesis. In sea urchin embryos, the primary mesodermal lineage, the PMCs, produce biomineral in response to signals from the ectoderm. However, in SCH28080-treated embryos, aside from randomization of the left-right axis, the ectoderm is normally specified and differentiated, indicating that the block to skeletogenesis observed in SCH28080-treated embryos is PMC-specific. HKA inhibition did not interfere with PMC specification, and was sufficient to block continuing biomineralization when embryos were treated with SCH28080 after the initiation of skeletogenesis, indicating that HKA activity is continuously required during biomineralization. Ion concentrations and voltage potential were abnormal in the PMCs in SCH28080-treated embryos, suggesting that these bioelectrical abnormalities prevent biomineralization. Our results indicate that this effect is due to the inhibition of amorphous calcium carbonate precipitation within PMC vesicles.

Novel lipophilic probe for detecting near-membrane reactive oxygen species responses and its application for studies of pancreatic acinar cells: effects of pyocyanin and L-ornithine

AIMS

The aim of this study was to develop a fluorescent reactive oxygen species (ROS) probe, which is preferentially localized in cellular membranes and displays a strong change in fluorescence upon oxidation. We also aimed to test the performance of this probe for detecting pathophysiologically relevant ROS responses in isolated cells.

RESULTS

We introduced a novel lipophilic ROS probe dihydrorhodamine B octadecyl ester (H2RB-C18). We then applied the new probe to characterize the ROS changes triggered by inducers of acute pancreatitis in pancreatic acinar cells. We resolved ROS changes produced by L-ornithine, L-arginine, cholecystokinin-8, acetylcholine, taurolithocholic acid 3-sulfate, palmitoleic acid ethyl ester, and the bacterial toxin pyocyanin. Particularly prominent ROS responses were induced by pyocyanin and L-ornithine. These ROS responses were accompanied by changes in cytosolic Ca(2+)concentration ([Ca(2+)]i), mitochondrial membrane potential (ΔΨ), and NAD(P)H concentration.

INNOVATION

The study describes a novel sensitive lipophilic ROS probe. The probe is particularly suitable for detecting ROS in near-membrane regions and therefore for reporting the ROS environment of plasma membrane channels and pumps.

CONCLUSIONS

In our experimental conditions, the novel probe was more sensitive than 5-(and-6)-chloromethyl-2',7'-dichlorodihydrofluorescein (CM-H2DCF) and dihydrorhodamine123 (H2R123) and allowed us to resolve ROS responses to secretagogues, pyocyanin, and L-ornithine. Changes in the fluorescence of the new probe were particularly prominent in the peripheral plasma membrane-associated regions. Our findings suggest that the new probe will be a useful tool in studies of the contribution of ROS to the pathophysiology of exocrine pancreas and other organs/tissues.

Synthesis, characterization and applications of selenocysteine-responsive nanoprobe based on dinitrobenzene sulfonyl-modified poly(carbonate) micelles

Dinitrobenzenesulfonyl-modified micelles can be used for selenocysteine detection in cells and tissues.

Luminescent probes for the bioimaging of small anionic species in vitro and in vivo

This comprehensive review examines recent developments in the use of fluorescent/luminescent probes for the bioimaging of anionic species. Images in cover art reproduced with permission from ref. 290 and 306.

Asphyxia induced by umbilical cord occlusion alters glutamatergic and GABAergic synaptic transmission in neurons of the superior colliculus in fetal rats

Using optical recordings, we studied the effects of asphyxia on intracellular Cl(-) and Ca(2+) concentrations ([Cl(-)]i; [Ca(2+)]i) in the superior colliculus of fetal rats, which were connected via the umbilical cord to the dam. Acute asphyxia was induced by umbilical cord occlusion. The number of fetal superior colliculus neurons showing GABA-mediated increases in [Cl(-)]i (leading to hyperpolarization) following local synaptic electrical stimulation had decreased by 3 h post-asphyxiation, while the number showing GABA-mediated decreases in [Cl(-)]i (leading to depolarization) increased. [Ca(2+)]i rise, which occurred after acute asphyxiation, was antagonized by both non-NMDA and NMDA receptor antagonists. The increase in [Ca(2+)]i following focal superior colliculus stimulation was markedly attenuated at 3 h post-asphyxiation. These findings suggest that asphyxia induced by umbilical occlusion induces changes in glutamatergic and GABAergic synaptic transmission in the fetal brain.

Differences between femoral artery and vein smooth muscle cells in volume-regulated chloride channels

The purpose of the present study was to compare the differences between the role of volume-regulated Cl⁻ channels (VRCCs) in veins and arteries. We used the whole cell patch clamp and fluorescence imaging techniques to evaluate swelling-induced Cl⁻ current (I(Cl,vol)) and changes in the intracellular concentrations of Cl⁻ ([Cl⁻](i)) induced by hypotonic solutions in rat femoral artery cells (FASMCs) and vein smooth muscle cells (FVSMCs). I(Cl,vol) and [Cl⁻](i) decline induced by hypotonic solution were more prominent in FASMCs than in FVSMCs. I(Cl,vol) and the alterations in [Cl⁻](i) were gradually increased as the number of cell passages increased. However, the regulatory function of tyrosine protein phosphorylation in volume-regulated chloride movement is prominent in veins. The expression of ClC-3 was higher in FASMCs than in FVSMCs. VRCC activity is more pronounced in rat femoral arteries than in veins. VRCC activity and tyrosine protein phosphorylation regulative function increase gradually as vascular cells switch from contractile to proliferative phenotypes.

Two-photon chloride imaging using MQAE in vitro and in vivo

This protocol describes a technique for high-resolution chloride imaging of living cells using a quinoline-based chloride (Cl(-)) indicator dye, MQAE (N-[6-methoxyquinolyl] acetoethyl ester). Bath-applied to acute brain slices, MQAE provides high-quality labeling of neuronal cells and their processes. In living anesthetized mice, cortical cells are labeled using the multicell bolus loading procedure. In combination with two-photon microscopy, this procedure enables in vivo visualization of cell bodies of neurons and astrocytes as well as some astrocytic processes and allows one to monitor changes in the intracellular chloride concentration in dozens of individual cells.

A cell-penetrating ratiometric nanoprobe for intracellular chloride

NanoChlor, a nanoparticle-based fluorescent probe for chloride that is both ratiometric and capable of spontaneously penetrating neuronal cells at submillimolar concentrations, was designed and studied. NanoChlor is built on silica nanoparticles grafted with 6-methoxyquinolinium as the chloride-sensitive component and fluorescein as the reference dye. A Stern-Volmer constant of 50 M(-1) was measured in Ringer's buffer at pH 7.2, and the response to chemically induced chloride currents was recorded in real time in hippocampal cells.

An intramolecular transport metabolon: fusion of carbonic anhydrase II to the COOH terminus of the Cl(-)/HCO(3)(-)exchanger, AE1

Anion exchanger 1 (AE1) is the plasma membrane Cl(-)/HCO(3)(-) exchanger of erythrocytes. Carbonic anhydrases (CA) provide substrate for AE1 by catalyzing the reaction, H(2)O + CO(2) ↔ HCO(3)(-) + H(+). The physical complex of CAII with AE1 has been proposed to maximize anion exchange activity. To examine the effect of CAII catalysis on AE1 transport rate, we fused either CAII-wild type or catalytically inactive CAII-V143Y to the cytoplasmic COOH terminus of AE1 to form AE1.CAII and AE1.CAII-V143Y, respectively. When expressed in transfected human embryonic kidney 293 cells, AE1.CAII had a similar Cl(-)/HCO(3)(-) exchange activity to AE1 alone, as assessed by the flux of H(+) equivalents (87 ± 4% vs. AE1) or rate of change of intracellular Cl(-) concentration (93 ± 4% vs. AE1), suggesting that CAII does not activate AE1. In contrast, AE1.CAII-V143Y displayed transport rates for H(+) equivalents and Cl(-) of 55 ± 2% and of 40 ± 2%, versus AE1. Fusion of CAII to AE1 therefore reduces anion transport activity, but this reduction is compensated for during Cl(-)/HCO(3)(-) exchange by the presence of catalytically active CAII. Overexpression of free CAII-V143Y acts in a dominant negative manner to reduce AE1-mediated HCO(3)(-) transport by displacement of endogenous CAII-wild type from its binding site on AE1. To examine whether AE1.CAII bound endogenous CAII, we coexpressed CAII-V143Y along with AE1 or AE1.CAII. The bicarbonate transport activity of AE1 was inhibited by CAII-V143Y, whereas the activity of AE1.CAII was unaffected by CAII-V143Y, suggesting impaired transport activity upon displacement of functional CAII from AE1 but not AE1.CAII. Taken together, these data suggest that association of functional CAII with AE1 increases Cl(-)/HCO(3)(-) exchange activity, consistent with the HCO(3)(-) transport metabolon model.

Nonsynaptic communication through ATP release from volume-activated anion channels in axons

The release of neuronal messengers outside synapses has broad biological implications, particularly with regard to communication between axons and glia. We identify a mechanism for nonsynaptic, nonvesicular release of adenosine triphosphate (ATP) from axons through volume-activated anion channels (VAACs) activated by microscopic axon swelling during action potential firing. We used a combination of single-photon imaging of ATP release, together with imaging for intrinsic optical signals, intracellular calcium ions (Ca(2+)), time-lapse video, and confocal microscopy, to investigate action potential-induced nonsynaptic release of this neurotransmitter. ATP release from cultured embryonic dorsal root ganglion axons persisted when bafilomycin or botulinum toxin was used to block vesicular release, whereas pharmacological inhibition of VAACs or prevention of action potential-induced axon swelling inhibited ATP release and disrupted activity-dependent signaling between axons and astrocytes. This nonvesicular, nonsynaptic communication could mediate various activity-dependent interactions between axons and nervous system cells in normal conditions, development, and disease.

Genetically encoded optical sensors for monitoring of intracellular chloride and chloride-selective channel activity

This review briefly discusses the main approaches for monitoring chloride (Cl⁻), the most abundant physiological anion. Noninvasive monitoring of intracellular Cl⁻ ([Cl⁻]i) is a challenging task owing to two main difficulties: (i) the low transmembrane ratio for Cl⁻, approximately 10:1; and (ii) the small driving force for Cl⁻, as the Cl⁻ reversal potential (E_Cl) is usually close to the resting potential of the cells. Thus, for reliable monitoring of intracellular Cl⁻, one has to use highly sensitive probes. From several methods for intracellular Cl⁻ analysis, genetically encoded chloride indicators represent the most promising tools. Recent achievements in the development of genetically encoded chloride probes are based on the fact that yellow fluorescent protein (YFP) exhibits Cl⁻-sensitivity. YFP-based probes have been successfully used for quantitative analysis of Cl⁻ transport in different cells and for high-throughput screening of modulators of Cl⁻-selective channels. Development of a ratiometric genetically encoded probe, Clomeleon, has provided a tool for noninvasive estimation of intracellular Cl⁻ concentrations. While the sensitivity of this protein to Cl⁻ is low (EC₅₀ about 160 mM), it has been successfully used for monitoring intracellular Cl⁻ in different cell types. Recently a CFP–YFP-based probe with a relatively high sensitivity to Cl⁻ (EC₅₀ about 30 mM) has been developed. This construct, termed Cl-Sensor, allows ratiometric monitoring using the fluorescence excitation ratio. Of particular interest are genetically encoded probes for monitoring of ion channel distribution and activity. A new molecular probe has been constructed by introducing into the cytoplasmic domain of the Cl⁻-selective glycine receptor (GlyR) channel the CFP–YFP-based Cl-Sensor. This construct, termed BioSensor-GlyR, has been successfully expressed in cell lines. The new genetically encoded chloride probes offer means of screening pharmacological agents, analysis of Cl⁻ homeostasis and functions of Cl⁻-selective channels under different physiological and pathological conditions.

Imaging nervous system activity

This unit describes methods for loading ion- and voltage-sensitive dyes into neurons, with a particular focus on the spinal cord as a model system. In addition, we describe the use of these dyes to visualize neural activity. Although the protocols described here concern spinal networks in culture or an intact in vitro preparation, they can be, and have been, widely used in other parts of the nervous system.

Regulation of intestinal Cl−/HCO3− exchanger SLC26A3 by intracellular pH

SLC26A3, a Cl−/HCO3− exchanger, is highly expressed in intestinal epithelial cells, and its mutations cause congenital chloride diarrhea. This suggests that SLC26A3 plays a key role in NaCl absorption in the intestine. Electroneutral NaCl absorption in the intestine is mediated by functional coupling of the Na+/H+ exchanger and Cl−/HCO3− exchanger. It is proposed that the coupling of these exchangers may occur as a result of indirect linkage by changes of intracellular pH (pHi). We therefore investigated whether SLC26A3 is regulated by pHi. We generated a hemagglutinin epitope-tagged human SLC26A3 construct and expressed it in Chinese hamster ovary cells. Transport activities were measured with a fluorescent chloride-sensitive dye dihydro-6-methoxy- N-ethylquinolinium iodide (diH-MEQ). pHi was clamped at a range of values from 6.0 to 7.4. We monitored the transport activity of SLC26A3 by reverse mode of Cl−/HCO3− and Cl−/NO3− exchange. None of these exchange modes induced membrane potential changes. At constant external pH 7.4, Cl−/HCO3− exchange was steeply inhibited with pHi decrease between 7.3 and 6.8 as opposed to thermodynamic prediction. In contrast, however, Cl−/NO3− exchange was essentially insensitive to pHi within physiological ranges. We also characterized the pHi dependency of COOH-terminal truncation mutants. Removal of the entire COOH-terminal resulted in decrease of the transport activity but did not noticeably affect pHi sensitivity. These results suggest that Cl−/HCO3− exchange mode of human SLC26A3 is controlled by a pH-sensitive intracellular modifier site, which is likely in the transmembrane domain. These observations raise the possibility that SLC26A3 activity may be regulated via Na+/H+ exchanger 3 (NHE3) through the alteration of pHi under physiological conditions.

Expression and function of NFAT5 in medullary thick ascending limb (mTAL) cells

The contribution of nuclear factor of activated T cells 5 (NFAT5) to the regulation of tumor necrosis factor-alpha (TNF) production in medullary thick ascending limb (mTAL) cells is unclear. RT-PCR analysis was performed on primary cultures of mouse mTAL cells and freshly isolated mTAL tubules to determine which NFAT isoforms are present in this nephron segment. Primer pairs were designed, based on published sequences for mouse NFAT1-5, to produce fragments of approximately 200 bp. Analysis of PCR products by gel electrophoresis and subsequent DNA sequencing indicated that cells and tubules contained mRNA for all five NFAT isoforms. The relative expression of NFAT isoforms was then determined using quantitative real-time RT-PCR. The data indicate that NFAT isoforms 5 >/= 1 are the predominant isoforms present in mTAL cells and tubules. Western blot analysis demonstrated constitutive expression of NFAT5 in nuclear extracts from mTAL tubules and primary culture cells; expression in mTAL cells also was detected by immunofluorescence. Expression of NFAT5 was increased in mTAL cells transiently transfected with an NFAT5 overexpression vector (pcDNA3.1-NFAT5), resulting in increased basal and calcium-sensing receptor (CaR)-mediated TNF production. Transient transfection of mTAL cells with a small hairpin RNA vector that targeted exon 8 of NFAT5 (U6-N5 ex8) significantly inhibited TNF promoter activity. Transient transfection with U6-N5 ex8 also reduced nuclear expression of NFAT5, TNF mRNA accumulation, and attenuated CaR-mediated activation of Cl(-) entry into polarized mTAL cells. Collectively, these data suggest that activation of NFAT5 is part of a TNF-dependent pathway that inhibits apical Cl(-) influx in the mTAL after activation of CaR.

The Vibrio cholerae cytolysin promotes chloride secretion from intact human intestinal mucosa

Background

The pathogenicity of the Vibrio cholerae strains belonging to serogroup O1 and O139 is due to the production of virulence factors such as cholera toxin (CT) and the toxin-coregulated pilus (TCP). The remaining serogroups, which mostly lack CT and TCP, are more frequently isolated from aquatic environmental sources than from clinical samples; nevertheless, these strains have been reported to cause human disease, such as sporadic outbreaks of watery diarrhoea and inflammatory enterocolitis. This evidence suggested the possibility that other virulence factor(s) than cholera toxin might be crucial in the pathogenesis of Vibrio cholerae-induced diarrhoea, but their nature remains unknown. VCC, the hemolysin produced by virtually all Vibrio cholerae strains, has been proposed as a possible candidate, though a clear-cut demonstration attesting VCC as crucial in the pathogenesis of Vibrio cholerae-induced diarrhoea is still lacking.

Methodology/Principal Findings

Electrophysiological parameters and paracellular permeability of stripped human healthy colon tissues, obtained at subtotal colectomy, mounted in Ussing chamber were studied in the presence or absence of VCC purified from culture supernatants of V. cholerae O1 El Tor strain. Short circuit current (I_SC) and transepithelial resistance (R_T) were measured by a computerized voltage clamp system. The exposure of sigmoid colon specimens to 1 nM VCC resulted in an increase of I_SC by 20.7%, with respect to the basal values, while R_T was reduced by 12.3%. Moreover, increase in I_SC was abolished by bilateral Cl⁻ reduction.

Conclusion/Significance

Our results demonstrate that VCC, by forming anion channels on the apical membrane of enterocytes, triggers an outward transcellular flux of chloride. Such an ion movement, associated with the outward movement of Na⁺ and water, might be responsible for the diarrhoea caused by the non-toxigenic strains of Vibrio cholerae.

Monitoring of chloride and activity of glycine receptor channels using genetically encoded fluorescent sensors

Genetically encoded probes have become powerful tools for non-invasive monitoring of ions, distributions of proteins and the migration and formation of cellular components. We describe the functional expression of two molecular probes for non-invasive fluorescent monitoring of intracellular Cl ([Cl]i) and the functioning of glycine receptor (GlyR) channels. The first probe is a recently developed cyan fluorescent protein-yellow fluorescent protein-based construct, termed Cl-Sensor, with relatively high sensitivity to Cl (Kapp approximately 30 mM). In this study, we describe its expression in retina cells using in vivo electroporation and analyse changes in [Cl]i at depolarization and during the first three weeks of post-natal development. An application of 40 mM K+ causes an elevation in [Cl]i of approximately 40 mM. In photoreceptors from retina slices of a 6-day-old rat (P6 rat), the mean [Cl]i is approximately 50 mM, and for P16 and P21 rats it is approximately 30-35 mM. The second construct, termed BioSensor-GlyR, is a GlyR channel with Cl-Sensor incorporated into the cytoplasmic domain. This is the first molecular probe for spectroscopic monitoring of the functioning of receptor-operated channels. These types of probes offer a means of screening pharmacological agents and monitoring Cl under different physiological and pathological conditions and permit spectroscopic monitoring of the activity of GlyRs expressed in heterologous systems and neurons.

On the measurement of the functional properties of the CFTR

A number of methods are currently employed to assess the functional properties of CFTR channels and their response to pharmacological potentiators, correction of the defective CFTR trafficking, and vectorial introduction of new proteins. Here we review the most common methods used to assess CFTR channel function. The suitability of each technique to various experimental conditions is discussed.

Imaging the spatiotemporal organization of neural activity in the developing spinal cord

In this review, we discuss the use of imaging to visualize the spatiotemporal organization of network activity in the developing spinal cord of the chick embryo and the neonatal mouse. We describe several different methods for loading ion- and voltage-sensitive dyes into spinal neurons and consider the advantages and limitations of each one. We review work in the chick embryo, suggesting that motoneurons play a critical role in the initiation of each cycle of spontaneous network activity and describe how imaging has been used to identify a class of spinal interneuron that appears to be the avian homolog of mammalian Renshaw cells or 1a-inhibitory interneurons. Imaging of locomotor-like activity in the neonatal mouse revealed a wave-like activation of motoneurons during each cycle of discharge. We discuss the significance of this finding and its implications for understanding how locomotor-like activity is coordinated across different segments of the cord. In the last part of the review, we discuss some of the exciting new prospects for the future.

Measurement of chloride movement in neuronal preparations

In this unit, protocols are described for biochemical and optical techniques that have been used by investigators to measure ligand-gated chloride movement in vesicular structures called synaptoneurosomes (also referred to as microsacs), in cultured neurons, and in the acute brain slice. These techniques can be applied to other ions as well. The measurement of uptake and efflux of radioisotopic chloride in synaptoneurosomes is used to study the responses of gamma-aminobutyric acid (GABA) receptors, which are coupled to chloride channels. Similar chloride flux assays for primary neuronal cultures are also presented. Alternatively, the efflux of chloride from synaptoneurosomes and primary neuronal cultures can be studied using fluorescent dyes and photometry. Finally, the measurement of chloride uptake can be studied in individual neurons in brain slices using fluorescent dyes and optical imaging by nonconfocal and confocal microscopy. Several support protocols are provided as well, outlining the preparation of synaptoneurosomes from specific brain regions, and the preparation, loading, and calibration of chloride-sensitive fluorescent dyes.

Ca2+-induced Cl- efflux at rat distal colonic epithelium

With the aid of the halide-sensitive dye 6-methoxy-N-ethylquinolinium iodide (MEQ), changes in intracellular Cl(-) concentration were measured to characterize the role of Ca(2+)-dependent Cl(-) channels at the rat distal colon. In order to avoid indirect effects of secretagogues mediated by changes in the driving force for Cl(-) exit (i.e., mediated by opening of Ca(2+)-dependent K(+) channels), all experiments were performed under depolarized conditions, i.e., in the presence of high extracellular K(+) concentrations. The Ca(2+)-dependent secretagogue carbachol induced a stilbene-sensitive Cl(-) efflux, which was mimicked by the Ca(2+) ionophore ionomycin. Surprisingly, the activation of Ca(2+)-dependent Cl(-) efflux was resistant against blockers of classical Ca(2+) signaling pathways such as phospholipase C, protein kinase C and calmodulin. Hence, alternative pathways must be involved in the signaling cascade. One possible signaling molecule seems to be nitric oxide (NO) as the NO donor sodium nitroprusside could induce Cl(- )efflux. Vice versa, the NO synthase inhibitor N-omega-monomethyl-arginine (L: -NMMA) reduced the carbachol-induced Cl(- )efflux. This indicates that NO may be involved in part of the signaling cascade. In order to test the ability of the epithelium to produce NO, the expression of different isoforms of NO synthase was verified by immunohistochemistry. In addition, the cytoskeleton seems to play a role in the activation of Ca(2+)-dependent Cl(-) channels. Inhibitors of microtubule association such as nocodazole and colchicine as well as jasplakinolide, a drug that enhances actin polymerization, inhibited the carbachol-induced Cl(-) efflux. Consequently, the activation of apical Cl(-) channels by muscarinic receptor stimulation differs in signal transduction from the classical phospholipase C/protein kinase C way.

Genetically encoded chloride indicator with improved sensitivity

Chloride (Cl) is the most abundant physiological anion. Abnormalities in Cl regulation are instrumental in the development of several important diseases including motor disorders and epilepsy. Because of difficulties in the spectroscopic measurement of Cl in live tissues there is little knowledge available regarding the mechanisms of regulation of intracellular Cl concentration. Several years ago, a CFP-YFP based ratiometric Cl indicator (Clomeleon) was introduced [Kuner, T., Augustine, G.J. A genetically encoded ratiometric indicator for chloride: capturing chloride transients in cultured hippocampal neurons. Neuron 2000; 27: 447-59]. This construct with relatively low sensitivity to Cl (K(app) approximately 160 mM) allows ratiometric monitoring of Cl using fluorescence emission ratio. Here, we propose a new CFP-YFP-based construct (Cl-sensor) with relatively high sensitivity to Cl (K(app) approximately 30 mM) due to triple YFP mutant. The construct also exhibits good pH sensitivity with pK(alpha) ranging from 7.1 to 8.0 pH units at different Cl concentrations. Using Cl-sensor we determined non-invasively the distribution of [Cl](i) in cultured CHO cells, in neurons of primary hippocampal cultures and in photoreceptors of rat retina. This genetically encoded indicator offers a means for monitoring Cl and pH under different physiological conditions and high-throughput screening of pharmacological agents.

Ursodeoxycholic acid stimulates cholangiocyte fluid secretion in mice via CFTR-dependent ATP secretion

BACKGROUND & AIMS

Cholangiopathies are characterized by impaired cholangiocyte secretion. Ursodeoxycholic acid (UDCA) is widely used for cholangiopathy treatment, but its effects on cholangiocyte secretory functions remain unclear and are the subject of this study.

METHODS

Polarized mouse cholangiocytes in tubular (isolated bile-duct units [IBDU]) or monolayer configuration were obtained from wild-type (WT) and B6-129-Cftr(tm1Kth) and Cftr(tm1Unc) mice that are defective in CFTR, an adenosine 3',5'-cyclic monophosphate (cAMP)-stimulated Cl(-) channel expressed in cholangiocytes. Fluid secretion was assessed by video-optical planimetry, Cl(-) and Ca(2+) efflux by microfluorimetry (6-methoxy-N-ethylquinolinium chloride, fura-2, and fluo-4), adenosine triphosphate (ATP) secretion by luciferin-luciferase assay, and protein kinase C (PKC) by Western blot.

RESULTS

UDCA stimulated fluid secretion and Cl(-) efflux in WT-IBDU but not in CFTR-KO-IBDU or in WT-IBDU exposed to CFTR inhibitors. UDCA did not affect intracellular cAMP levels but increased [Ca(2+)]i in WT and not in CFTR-KO cholangiocytes. UDCA stimulated apical ATP secretion in WT but not in CFTR-KO cholangiocytes. UDCA-stimulated [Ca(2+)]i increase was inhibited by suramin, a purinergic 2Y-receptor inhibitor. UDCA stimulated the translocation of PKC-alpha and PKC-epsilon to the plasma membrane. UDCA-stimulated secretion was inhibited by 2-bis(2-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid and by phospholipase C and PKC inhibitors. UDCA increased ATP output in isolated perfused livers from WT but not from CFTR-KO mice.

CONCLUSIONS

Our data indicate that UDCA stimulates a CFTR-dependent apical ATP release in cholangiocytes. Secreted ATP activates purinergic 2Y receptors, and, through [Ca(2+)]i increase and PKC activation stimulates Cl(-) efflux and fluid secretion. These data support the concept that CFTR plays a role in modulating purinergic signaling in secretory epithelia and suggest a novel mechanism explaining the choleretic effect of UDCA.

Characterization of a long-term rat mTAL cell line

A medullary thick ascending limb (mTAL) cell line, termed raTAL, has been established from freshly isolated rat mTAL tubules and cultured continuously for up to 75 passages; it retains characteristics of mTAL cells even after retrieval from storage in liquid nitrogen for several months. The cells express Tamm-Horsfall glycoprotein (THP), a TAL-specific marker, grow to confluence, exhibit a polygonal morphology characteristic of epithelial cells, and form “domes.” Detection of THP, Na+-K+-2Cl−cotransporter (NKCC2), Na+-K+-ATPase, and renal outer medullary K+channel (ROMK) was achieved using indirect immunofluorescence and confocal microscopy. Western blot analysis of NKCC2 expression using two different antibodies revealed a band of ∼160 kDa, and RT-PCR analysis demonstrated the presence of NKCC2 isoforms A and F, which was confirmed by DNA sequencing; transport of Cl−into raTAL cells was inhibited by furosemide. Ouabain- and bumetanide-sensitive oxygen consumption, an index of ion transport activity in the mTAL, was observed in raTAL cells, and the number of domes present was reduced significantly when cells were incubated in the presence of ouabain or bumetanide. The specific activity of Na+-K+-ATPase activity was determined in raTAL cells (0.67 ± 0.18 nmol Pi·μg protein−1·min−1), primary cultures of mTAL cells (0.39 ± 0.08 nmol Pi·μg protein−1·min−1), and freshly isolated mTAL tubules (1.10 ± 0.29 nmol Pi·μg protein−1·min−1), and ∼30–50% of total cellular ATPase activity was inhibited by ouabain, in accord with other mTAL preparations. This cell line will be used in studies that address biochemical, molecular, and physiological mechanisms in the mTAL.