Fluorescence measurement of chloride transport in monolayer cultured cells. Mechanisms of chloride transport in fibroblasts

K+-Driven Cl-/HCO3- Exchange Mediated by Slc4a8 and Slc4a10

Slc4a genes encode various types of transporters, including Na+-HCO3- cotransporters, Cl-/HCO3- exchangers, or Na+-driven Cl-/HCO3- exchangers. Previous research has revealed that Slc4a9 (Ae4) functions as a Cl-/HCO3- exchanger, which can be driven by either Na+ or K+, prompting investigation into whether other Slc4a members facilitate cation-dependent anion transport. In the present study, we show that either Na+ or K+ drive Cl-/HCO3- exchanger activity in cells overexpressing Slc4a8 or Slc4a10. Further characterization of cation-driven Cl-/HCO3- exchange demonstrated that Slc4a8 and Slc4a10 also mediate Cl- and HCO3--dependent K+ transport. Full-atom molecular dynamics simulation on the recently solved structure of Slc4a8 supports the coordination of K+ at the Na+ binding site in S1. Sequence analysis shows that the critical residues coordinating monovalent cations are conserved among mouse Slc4a8 and Slc4a10 proteins. Together, our results suggest that Slc4a8 and Slc4a10 might transport K+ in the same direction as HCO3- ions in a similar fashion to that described for Na+ transport in the rat Slc4a8 structure.

Simultaneous Monitoring of pH and Chloride (Cl-) in Brain Slices of Transgenic Mice

Optosensorics is the direction of research possessing the possibility of non-invasive monitoring of the concentration of intracellular ions or activity of intracellular components using specific biosensors. In recent years, genetically encoded proteins have been used as effective optosensory means. These probes possess fluorophore groups capable of changing fluorescence when interacting with certain ions or molecules. For monitoring of intracellular concentrations of chloride ([Cl-]i) and hydrogen ([H+] i) the construct, called ClopHensor, which consists of a H+- and Cl--sensitive variant of the enhanced green fluorescent protein (E2GFP) fused with a monomeric red fluorescent protein (mDsRed) has been proposed. We recently developed a line of transgenic mice expressing ClopHensor in neurons and obtained the map of its expression in different areas of the brain. The purpose of this study was to examine the effectiveness of transgenic mice expressing ClopHensor for estimation of [H+]i and [Cl-]i concentrations in neurons of brain slices. We performed simultaneous monitoring of [H+]i and [Cl-]i under different experimental conditions including changing of external concentrations of ions (Ca2+, Cl-, K+, Na+) and synaptic stimulation of Shaffer's collaterals of hippocampal slices. The results obtained illuminate different pathways of regulation of Cl- and pH equilibrium in neurons and demonstrate that transgenic mice expressing ClopHensor represent a reliable tool for non-invasive simultaneous monitoring of intracellular Cl- and pH.

Activation of the Ae4 (Slc4a9) cation-driven Cl-/HCO3- exchanger by the cAMP-dependent protein kinase in salivary gland acinar cells

Ae4 transporters are critical for Cl- uptake across the basolateral membrane of acinar cells in the submandibular gland (SMG). Although required for fluid secretion, little is known about the physiological regulation of Ae4. To investigate whether Ae4 is regulated by the cAMP-dependent signaling pathway, we measured Cl-/HCO3- exchanger activity in SMG acinar cells from Ae2-/- mice, which only express Ae4, and found that the Ae4-mediated activity was increased in response to β-adrenergic receptor stimulation. Moreover, pretreatment with H89, an inhibitor of the cAMP-activated kinase (PKA), prevented the stimulation of Ae4 exchangers. We then expressed Ae4 in CHO-K1 cells and found that the Ae4-mediated activity was increased when Ae4 is coexpressed with the catalytic subunit of PKA (PKAc), which is constitutively active. Ae4 sequence analysis showed two potential PKA phosphorylation serine residues located at the intracellular NH2-terminal domain according to a homology model of Ae4. NH2-terminal domain Ser residues were mutated to alanine (S173A and S273A, respectively), where the Cl-/HCO3- exchanger activity displayed by the mutant S173A was not activated by PKA. Conversely, S273A mutant kept the PKA dependency. Together, we conclude that Ae4 is stimulated by PKA in SMG acinar cells by a mechanism that probably depends on the phosphorylation of S173.NEW & NOTEWORTHY We found that Ae4 exchanger activity in secretory salivary gland acinar cells is increased upon β-adrenergic receptor stimulation. The activation of Ae4 was prevented by H89, a nonselective PKA inhibitor. Protein sequence analysis revealed two residues (S173 and S273) that are potential targets of cAMP-dependent protein kinase (PKA). Experiments in CHO-K1 cells expressing S173A and S273A mutants showed that S173A, but not S273A, is not activated by PKA.

Glial Chloride Homeostasis Under Transient Ischemic Stress

High water permeabilities permit rapid adjustments of glial volume upon changes in external and internal osmolarity, and pathologically altered intracellular chloride concentrations ([Cl^–]_int) and glial cell swelling are often assumed to represent early events in ischemia, infections, or traumatic brain injury. Experimental data for glial [Cl^–]_int are lacking for most brain regions, under normal as well as under pathological conditions. We measured [Cl^–]_int in hippocampal and neocortical astrocytes and in hippocampal radial glia-like (RGL) cells in acute murine brain slices using fluorescence lifetime imaging microscopy with the chloride-sensitive dye MQAE at room temperature. We observed substantial heterogeneity in baseline [Cl^–]_int, ranging from 14.0 ± 2.0 mM in neocortical astrocytes to 28.4 ± 3.0 mM in dentate gyrus astrocytes. Chloride accumulation by the Na⁺-K⁺-2Cl^– cotransporter (NKCC1) and chloride outward transport (efflux) through K⁺-Cl^– cotransporters (KCC1 and KCC3) or excitatory amino acid transporter (EAAT) anion channels control [Cl^–]_int to variable extent in distinct brain regions. In hippocampal astrocytes, blocking NKCC1 decreased [Cl^–]_int, whereas KCC or EAAT anion channel inhibition had little effect. In contrast, neocortical astrocytic or RGL [Cl^–]_int was very sensitive to block of chloride outward transport, but not to NKCC1 inhibition. Mathematical modeling demonstrated that higher numbers of NKCC1 and KCC transporters can account for lower [Cl^–]_int in neocortical than in hippocampal astrocytes. Energy depletion mimicking ischemia for up to 10 min did not result in pronounced changes in [Cl^–]_int in any of the tested glial cell types. However, [Cl^–]_int changes occurred under ischemic conditions after blocking selected anion transporters. We conclude that stimulated chloride accumulation and chloride efflux compensate for each other and prevent glial swelling under transient energy deprivation.

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.

Increased intracellular Cl- concentration improves airway epithelial migration by activating the RhoA/ROCK Pathway

In the airway, Cl- is the most abundant anion and is critically involved in transepithelial transport. The correlation of the abnormal expression and activation of chloride channels (CLCs), such as cystic fibrosis transmembrane conductance regulators (CFTRs), anoctamin-1, and CLC-2, with cell migration capability suggests a relationship between defective Cl- transport and epithelial wound repair. However, whether a correlation exists between intracellular Cl- and airway wound repair capability has not been explored thus far, and the underlying mechanisms involved in this relationship are not fully defined. Methods: In this work, the alteration of intracellular chloride concentration ([Cl-]i) was measured by using a chloride-sensitive fluorescent probe (N-[ethoxycarbonylmethyl]-6-methoxyquinolium bromide). Results: We found that clamping with high [Cl-]i and 1 h of treatment with the CLC inhibitor CFTR blocker CFTRinh-172 and chloride intracellular channel inhibitor IAA94 increased intracellular Cl- concentration ([Cl-]i) in airway epithelial cells. This effect improved epithelial cell migration. In addition, increased [Cl-]i in cells promoted F-actin reorganization, decreased cell stiffness, and improved RhoA activation and LIMK1/2 phosphorylation. Treatment with the ROCK inhibitor of Y-27632 and ROCK1 siRNA significantly attenuated the effects of increased [Cl-]i on LIMK1/2 activation and cell migration. In addition, intracellular Ca2+ concentration was unaffected by [Cl-]i clamping buffers and CFTRinh-172 and IAA94. Conclusion: Taken together, these results suggested that Cl- accumulation in airway epithelial cells could activate the RhoA/ROCK/LIMK cascade to induce F-actin reorganization, down-regulate cell stiffness, and improve epithelial migration.

Bright Dots and Smart Optical Microscopy to Probe Intracellular Events in Single Cells

Probing intracellular events is a key step in developing new biomedical methodologies. Optical microscopy has been one of the best options to observe biological samples at single cell and sub-cellular resolutions. Morphological changes are readily detectable in brightfield images. When stained with fluorescent molecules, distributions of intracellular organelles, and biological molecules are made visible using fluorescence microscopes. In addition to these morphological views of cells, optical microscopy can reveal the chemical and physical status of defined intracellular spaces. This review begins with a brief overview of genetically encoded fluorescent probes and small fluorescent chemical dyes. Although these are the most common approaches, probing is also made possible by using tiny materials that are incorporated into cells. When these tiny materials emit enough photons, it is possible to draw conclusions about the environment in which the tiny material resides. Recent advances in these tiny but sufficiently bright fluorescent materials are nextly reviewed to show their applications in tracking target molecules and in temperature imaging of intracellular spots. The last section of this review addresses purely optical methods for reading intracellular status without staining with probes. These non-labeling methods are especially essential when biospecimens are thereafter required for in vivo uses, such as in regenerative medicine.

CLCN2 chloride channel mutations in familial hyperaldosteronism type II

Primary aldosteronism, a common cause of severe hypertension¹, features constitutive production of the adrenal steroid aldosterone. We analyzed a multiplex family with familial hyperaldosteronism type II (FH-II)² and 80 additional probands with unsolved early-onset primary aldosteronism. Eight probands had novel heterozygous variants in CLCN2, including two de novo mutations and four independent occurrences of the identical p.Arg172Gln mutation; all relatives with early-onset primary aldosteronism carried the CLCN2 variant found in probands. CLCN2 encodes a voltage-gated chloride channel expressed in adrenal glomerulosa that opens at hyperpolarized membrane potentials. Channel opening depolarizes glomerulosa cells and induces expression of aldosterone synthase, the rate-limiting enzyme for aldosterone biosynthesis. Mutant channels cause gain of function, with higher open probabilities at the glomerulosa resting potential. These findings for the first time demonstrate a role of anion channels in glomerulosa membrane potential determination, aldosterone production and hypertension. They establish the cause of a substantial fraction of early-onset primary aldosteronism.

Ae4 (Slc4a9) is an electroneutral monovalent cation-dependent Cl-/HCO3- exchanger

Ae4 (Slc4a9) belongs to the Slc4a family of Cl(-)/HCO3 (-) exchangers and Na(+)-HCO3 (-) cotransporters, but its ion transport cycle is poorly understood. In this study, we find that native Ae4 activity in mouse salivary gland acinar cells supports Na(+)-dependent Cl(-)/HCO3 (-) exchange that is comparable with that obtained upon heterologous expression of mouse Ae4 and human AE4 in CHO-K1 cells. Additionally, whole cell recordings and ion concentration measurements demonstrate that Na(+) is transported by Ae4 in the same direction as HCO3 (-) (and opposite to that of Cl(-)) and that ion transport is not associated with changes in membrane potential. We also find that Ae4 can mediate Na(+)-HCO3 (-) cotransport-like activity under Cl(-)-free conditions. However, whole cell recordings show that this apparent Na(+)-HCO3 (-) cotransport activity is in fact electroneutral HCO3 (-)/Na(+)-HCO3 (-) exchange. Although the Ae4 anion exchanger is thought to regulate intracellular Cl(-) concentration in exocrine gland acinar cells, our thermodynamic calculations predict that the intracellular Na(+), Cl(-), and HCO3 (-) concentrations required for Ae4-mediated Cl(-) influx differ markedly from those reported for acinar secretory cells at rest or under sustained stimulation. Given that K(+) ions share many properties with Na(+) ions and reach intracellular concentrations of 140-150 mM (essentially the same as extracellular [Na(+)]), we hypothesize that Ae4 could mediate K(+)-dependent Cl(-)/HCO3 (-) exchange. Indeed, we find that Ae4 mediates Cl(-)/HCO3 (-) exchange activity in the presence of K(+) as well as Cs(+), Li(+), and Rb(+) In summary, our results strongly suggest that Ae4 is an electroneutral Cl(-)/nonselective cation-HCO3 (-) exchanger. We postulate that the physiological role of Ae4 in secretory cells is to promote Cl(-) influx in exchange for K(+)(Na(+)) and HCO3 (-) ions.

Glutamate transporter-associated anion channels adjust intracellular chloride concentrations during glial maturation

Astrocytic volume regulation and neurotransmitter uptake are critically dependent on the intracellular anion concentration, but little is known about the mechanisms controlling internal anion homeostasis in these cells. Here we used fluorescence lifetime imaging microscopy (FLIM) with the chloride-sensitive dye MQAE to measure intracellular chloride concentrations in murine Bergmann glial cells in acute cerebellar slices. We found Bergmann glial [Cl^- ]_int to be controlled by two opposing transport processes: chloride is actively accumulated by the Na⁺ -K⁺ -2Cl^- cotransporter NKCC1, and chloride efflux through anion channels associated with excitatory amino acid transporters (EAATs) reduces [Cl^- ]_int to values that vary upon changes in expression levels or activity of these channels. EAATs transiently form anion-selective channels during glutamate transport, and thus represent a class of ligand-gated anion channels. Age-dependent upregulation of EAATs results in a developmental chloride switch from high internal chloride concentrations (51.6 ± 2.2 mM, mean ± 95% confidence interval) during early development to adult levels (35.3 ± 0.3 mM). Simultaneous blockade of EAAT1/GLAST and EAAT2/GLT-1 increased [Cl^- ]_int in adult glia to neonatal values. Moreover, EAAT activation by synaptic stimulations rapidly decreased [Cl^- ]_int . Other tested chloride channels or chloride transporters do not contribute to [Cl^- ]_int under our experimental conditions. Neither genetic removal of ClC-2 nor pharmacological block of K⁺ -Cl^- cotransporter change resting Bergmann glial [Cl^- ]_int in acute cerebellar slices. We conclude that EAAT anion channels play an important and unexpected role in adjusting glial intracellular anion concentration during maturation and in response to cerebellar activity. GLIA 2017;65:388-400.

Intracellular chloride concentration of the mouse vomeronasal neuron

Background

The vomeronasal organ (VNO) is specialized in detecting pheromone and heterospecific cues in the environment. Recent studies demonstrate the involvement of multiple ion channels in VNO signal transduction, including the calcium-activated chloride channels (CACCs). Opening of CACCs appears to result in activation of VNO neuron through outflow of Cl⁻ ions. However, the intracellular Cl⁻ concentration remains undetermined.

Results

We used the chloride ion quenching dye, MQAE, to measure the intracellular Cl⁻ concentration of VNO neuron in live VNO slices. The resting Cl⁻ concentration in the VNO neurons is measured at 84.73 mM. Urine activation of the VNO neurons causes a drop in Cl⁻ concentration, consistent with the notion of an efflux of Cl⁻ to depolarize the cells. Similar observation is made for VNO neurons from mice with deletion of the transient receptor potential canonical channel 2 (TRPC2), which have a resting Cl⁻ concentrations at 81 mM.

Conclusions

The VNO neurons rest at high intracellular Cl⁻ concentration, which can lead to depolarization of the cell when chloride channels open. These results also provide additional support of TRPC2-independent pathway of VNO activation.

Annexin A4 induces platinum resistance in a chloride-and calcium-dependent manner

Platinum resistance has long been a major issue in the treatment of various cancers. We previously reported that enhanced annexin A4 (ANXA4) expression, a Ca²⁺-regulated phospholipid-binding protein, induces chemoresistance to platinum-based drugs. In this study, we investigated the role of annexin repeats, a conserved structure of all the annexin family, responsible for platinum-resistance as well as the effect of knockdown of ANXA4. ANXA4 knockdown increased sensitivity to platinum-based drugs both in vitro and in vivo. To identify the domain responsible for chemoresistance, ANXA4 deletion mutants were constructed by deleting annexin repeats one by one from the C terminus. Platinum resistance was induced both in vitro and in vivo in cells expressing either full-length ANXA4 or the deletion mutants, containing at least one intact annexin repeat. However, cells expressing the mutant without any calcium-binding sites in the annexin repeated sequence, which is essential for ANXA4 translocation from the cytosol to plasma membrane, failed to acquire platinum resistance. After cisplatin treatment, the intracellular chloride ion concentration, whose channel is partly regulated by ANXA4, significantly increased in the platinum-resistant cells. These findings indicate that the calcium-binding site in the annexin repeat induces chemoresistance to the platinum-based drug by elevating the intracellular chloride concentration.

Ionic imbalance, in addition to molecular crowding, abates cytoskeletal dynamics and vesicle motility during hypertonic stress

Cell volume homeostasis is vital for the maintenance of optimal protein density and cellular function. Numerous mammalian cell types are routinely exposed to acute hypertonic challenge and shrink. Molecular crowding modifies biochemical reaction rates and decreases macromolecule diffusion. Cell volume is restored rapidly by ion influx but at the expense of elevated intracellular sodium and chloride levels that persist long after challenge. Although recent studies have highlighted the role of molecular crowding on the effects of hypertonicity, the effects of ionic imbalance on cellular trafficking dynamics in living cells are largely unexplored. By tracking distinct fluorescently labeled endosome/vesicle populations by live-cell imaging, we show that vesicle motility is reduced dramatically in a variety of cell types at the onset of hypertonic challenge. Live-cell imaging of actin and tubulin revealed similar arrested microfilament motility upon challenge. Vesicle motility recovered long after cell volume, a process that required functional regulatory volume increase and was accelerated by a return of extracellular osmolality to isosmotic levels. This delay suggests that, although volume-induced molecular crowding contributes to trafficking defects, it alone cannot explain the observed effects. Using fluorescent indicators and FRET-based probes, we found that intracellular ATP abundance and mitochondrial potential were reduced by hypertonicity and recovered after longer periods of time. Similar to the effects of osmotic challenge, isovolumetric elevation of intracellular chloride concentration by ionophores transiently decreased ATP production by mitochondria and abated microfilament and vesicle motility. These data illustrate how perturbed ionic balance, in addition to molecular crowding, affects membrane trafficking.

Ae4 (Slc4a9) Anion Exchanger Drives Cl- Uptake-dependent Fluid Secretion by Mouse Submandibular Gland Acinar Cells

Transcellular Cl(-) movement across acinar cells is the rate-limiting step for salivary gland fluid secretion. Basolateral Nkcc1 Na(+)-K(+)-2Cl(-) cotransporters play a critical role in fluid secretion by promoting the intracellular accumulation of Cl(-) above its equilibrium potential. However, salivation is only partially abolished in the absence of Nkcc1 cotransporter activity, suggesting that another Cl(-) uptake pathway concentrates Cl(-) ions in acinar cells. To identify alternative molecular mechanisms, we studied mice lacking Ae2 and Ae4 Cl(-)/HCO3 (-) exchangers. We found that salivation stimulated by muscarinic and β-adrenergic receptor agonists was normal in the submandibular glands of Ae2(-/-) mice. In contrast, saliva secretion was reduced by 35% in Ae4(-/-) mice. The decrease in salivation was not related to loss of Na(+)-K(+)-2Cl(-) cotransporter or Na(+)/H(+) exchanger activity in Ae4(-/-) mice but correlated with reduced Cl(-) uptake during β-adrenergic receptor activation of cAMP signaling. Direct measurements of Cl(-)/HCO3 (-) exchanger activity revealed that HCO3 (-)-dependent Cl(-) uptake was reduced in the acinar cells of Ae2(-/-) and Ae4(-/-) mice. Moreover, Cl(-)/HCO3 (-) exchanger activity was nearly abolished in double Ae4/Ae2 knock-out mice, suggesting that most of the Cl(-)/HCO3 (-) exchanger activity in submandibular acinar cells depends on Ae2 and Ae4 expression. In conclusion, both Ae2 and Ae4 anion exchangers are functionally expressed in submandibular acinar cells; however, only Ae4 expression appears to be important for cAMP-dependent regulation of fluid secretion.

Phosphorylation-dependent 14-3-3 protein interactions regulate CFTR biogenesis

cAMP/PKA stimulation elicited posttranslational increases in CFTR expression and the interaction of specific 14-3-3 proteins with phosphorylated sites within the R region. This improved the efficiency of nascent CFTR biogenesis and reduced its interaction with the COPI retrograde retrieval mechanism, making more CFTR available for anion secretion.

Cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP/protein kinase A (PKA)–regulated chloride channel whose phosphorylation controls anion secretion across epithelial cell apical membranes. We examined the hypothesis that cAMP/PKA stimulation regulates CFTR biogenesis posttranslationally, based on predicted 14-3-3 binding motifs within CFTR and forskolin-induced CFTR expression. The 14-3-3β, γ, and ε isoforms were expressed in airway cells and interacted with CFTR in coimmunoprecipitation assays. Forskolin stimulation (15 min) increased 14-3-3β and ε binding to immature and mature CFTR (bands B and C), and 14-3-3 overexpression increased CFTR bands B and C and cell surface band C. In pulse-chase experiments, 14-3-3β increased the synthesis of immature CFTR, reduced its degradation rate, and increased conversion of immature to mature CFTR. Conversely, 14-3-3β knockdown decreased CFTR B and C bands (70 and 55%) and elicited parallel reductions in cell surface CFTR and forskolin-stimulated anion efflux. In vitro, 14-3-3β interacted with the CFTR regulatory region, and by nuclear magnetic resonance analysis, this interaction occurred at known PKA phosphorylated sites. In coimmunoprecipitation assays, forskolin stimulated the CFTR/14-3-3β interaction while reducing CFTR's interaction with coat protein complex 1 (COP1). Thus 14-3-3 binding to phosphorylated CFTR augments its biogenesis by reducing retrograde retrieval of CFTR to the endoplasmic reticulum. This mechanism permits cAMP/PKA stimulation to make more CFTR available for anion secretion.

Direct evidence of chloride ion efflux in ischaemic and pharmacological preconditioning of cultured cardiomyocytes

AIMS

We have previously shown that reduction of ischaemic cell swelling via enhanced cell volume regulation is a key mechanism of ischaemic preconditioning (IPC) in cardiomyocytes. We have also shown that pharmacological blockade of Cl(-) channels abolishes cardioprotection achieved by IPC in Langendorff-perfused hearts and freshly isolated cardiomyocytes, thus suggesting that Cl(-) plays a key role in IPC cardioprotection. However, direct evidence of Cl(-) channel activation resulting in transsarcolemmal Cl(-) efflux by IPC had been lacking. To address this issue, 24 h cultured rabbit cardiomyocytes were loaded with 5 mM 6-methoxy-N-(3-sulfopropyl)quinolinium (SPQ), a specific fluorescence indicator that is quenched by Cl(-) so that cellular efflux of Cl(-) results in an increase in SPQ fluorescence.

METHODS AND RESULTS

After stabilization for 10 min, cardiomyocytes were preconditioned either with 10 min simulated ischaemia/10 min simulated reperfusion or with 10 min treatment with 1 microM N(6)-2-(4-aminophenyl)ethyladenosine (APNEA). IPC and APNEA significantly (P < 0.001) reduced the intracellular Cl(-) concentration ([Cl(-)](i)) to 31.9 +/- 3.2 mM (mean +/- SEM) and 32.5 +/- 2.8 mM, respectively, from an initial [Cl(-)](i) (pooled stabilization 61.5 +/- 7.1 mM). [Cl(-)](i) did not change in control (non-preconditioned) cardiomyocytes (control 58.1 +/- 1.9 mM and control + vehicle 62.6 +/- 4.9 mM, P = 0.98 and 0.99 vs. pooled pre-treatment baseline, respectively). Inhibition of Cl(-) channels with 50 microM indanyloxyacetic acid 94 completely blocked preconditioning-induced Cl(-) efflux. Thus, a net Cl(-) efflux of 29.6 and 29.0 mM was triggered by IPC and APNEA.

CONCLUSION

These findings provide the first direct evidence of activation of sarcolemmal Cl(-) channels by ischaemic and pharmacological preconditioning in cardiomyocytes.

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.

Mutation of Aspartate 555 of the Sodium/Bicarbonate Transporter SLC4A4/NBCe1 Induces Chloride Transport

To understand the mechanism for ion transport through the sodium/bicarbonate transporter SLC4A4 (NBCe1), we examined amino acid residues, within transmembrane domains, that are conserved among electrogenic Na/HCO(3) transporters but are substituted with residues at the corresponding site of all electroneutral Na/HCO(3) transporters. Point mutants were constructed and expressed in Xenopus oocytes to assess function using two-electrode voltage clamp. Among the mutants, D555E (charge-conserved substitution of the aspartate at position 555 with a glutamate) produced decreasing HCO(3)(-) currents at more positive membrane voltages. Immunohistochemistry showed D555E protein expression in oocyte membranes. D555E induced Na/HCO(3)-dependent pH recovery from a CO(2)-induced acidification. Current-voltage relationships revealed that D555E produced an outwardly rectifying current in the nominally CO(2)/HCO(3)(-)-free solution that was abolished by Cl(-) removal from the bath. In the presence of CO(2)/HCO(3)(-), however, the outward current produced by D555E decreased only slightly after Cl(-) removal. Starting from a Cl(-)-free condition, D555E produced dose-dependent outward currents in response to a series of chloride additions. The D555E-mediated chloride current decreased by 70% in the presence of CO(2)/HCO(3)(-). The substitution of Asp(555) with an asparagine also produced a Cl(-) current. Anion selectivity experiments revealed that D555E was broadly permissive to other anions including NO(3)(-). Fluorescence measurements of chloride transport were done with human embryonic kidney HEK 293 cells expressing NBCe1 and D555E. A marked increase in chloride transport was detected in cells expressing D555E. We conclude that Asp(555) plays a role in HCO(3)(-) selectivity.

Rab11b regulates the apical recycling of the cystic fibrosis transmembrane conductance regulator in polarized intestinal epithelial cells

The cystic fibrosis transmembrane conductance regulator (CFTR), a cAMP/PKA-activated anion channel, undergoes efficient apical recycling in polarized epithelia. The regulatory mechanisms underlying CFTR recycling are understood poorly, yet this process is required for proper channel copy number at the apical membrane, and it is defective in the common CFTR mutant, DeltaF508. Herein, we investigated the function of Rab11 isoforms in regulating CFTR trafficking in T84 cells, a colonic epithelial line that expresses CFTR endogenously. Western blotting of immunoisolated Rab11a or Rab11b vesicles revealed localization of endogenous CFTR within both compartments. CFTR function assays performed on T84 cells expressing the Rab11a or Rab11b GDP-locked S25N mutants demonstrated that only the Rab11b mutant inhibited 80% of the cAMP-activated halide efflux and that only the constitutively active Rab11b-Q70L increased the rate constant for stimulated halide efflux. Similarly, RNAi knockdown of Rab11b, but not Rab11a, reduced by 50% the CFTR-mediated anion conductance response. In polarized T84 monolayers, adenoviral expression of Rab11b-S25N resulted in a 70% inhibition of forskolin-stimulated transepithelial anion secretion and a 50% decrease in apical membrane CFTR as assessed by cell surface biotinylation. Biotin protection assays revealed a robust inhibition of CFTR recycling in polarized T84 cells expressing Rab11b-S25N, demonstrating the selective requirement for the Rab11b isoform. This is the first report detailing apical CFTR recycling in a native expression system and to demonstrate that Rab11b regulates apical recycling in polarized epithelial cells.

Differential maturation of chloride homeostasis in primary afferent neurons of the somatosensory system

Recent research into the generation of hyperalgesia has revealed that both the excitability of peripheral nociceptors and the transmission of their afferent signals in the spinal cord are subject to modulation by Cl(-) currents. The underlying Cl(-) homeostasis of nociceptive neurons, in particular its postnatal maturation, is, however, poorly understood. Here we measure the intracellular Cl(-) concentration, [Cl(-)]i, of somatosensory neurons in intact dorsal root ganglia of mice. Using two-photon fluorescence-lifetime imaging microscopy, we determined [Cl(-)]i in newborn and adult animals. We found that the somatosensory neurons undergo a transition of Cl(-) homeostasis during the first three postnatal weeks that leads to a decline of [Cl(-)]i in most neurons. Immunohistochemistry showed that a major fraction of neurons in the dorsal root ganglia express the cation-chloride co-transporters NKCC1 and KCC2, indicating that the molecular equipment for Cl(-) accumulation and extrusion is present. RT-PCR analysis showed that the transcription pattern of electroneutral Cl(-) co-transporters does not change during the maturation process. These findings demonstrate that dorsal root ganglion neurons undergo a developmental transition of chloride homeostasis during the first three postnatal weeks. This process parallels the developmental "chloride switch" in the central nervous system. However, while most CNS neurons achieve homogeneously low [Cl(-)]i levels - which is the basis of GABAergic and glycinergic inhibition - somatosensory neurons maintain a heterogeneous pattern of [Cl(-)]i values. This suggests that Cl(-) currents are excitatory in some somatosensory neurons, but inhibitory in others. Our results are consistent with the hypothesis that Cl(-) homeostasis in somatosensory neurons is regulated through posttranslational modification of cation-chloride co-transporters.

Cl−-dependent upregulation of human organic anion transporters: different effects on transport kinetics between hOAT1 and hOAT3

Chloride ion has a stimulatory effect on the transport of organic anions across renal basolateral membranes. However, the exact mechanisms at molecular levels have been unclear as of yet. Human organic anion transporters hOAT1 and hOAT3 play important roles in renal basolateral membranes. In this study, the effects of Cl− on the activities of these transporters were evaluated by using HEK293 cells stably expressing hOAT1 or hOAT3 (HEK-hOAT1 or HEK-hOAT3). The uptake of p-[14C]aminohippurate by HEK-hOAT1 and [3H]estrone sulfate by HEK-hOAT3 was greater in the presence of Cl− than in the presence of SO42− or gluconate. Additionally, the uptake of various compounds by HEK-hOAT1 and HEK-hOAT3 was significantly higher in the Cl−-containing medium than the gluconate-containing medium, suggesting that the influences of Cl− are not dependent on substrate and that Cl− directly stimulates the functions of hOAT1 and hOAT3. The substitution of gluconate with Cl− did not change the Km value for the uptake of p-[14C]aminohippurate by HEK-hOAT1 but caused an approximately threefold increase in the maximal uptake rate (Vmax) value. On the other hand, replacement of gluconate with Cl− decreased the Km value for the uptake of [3H]estrone sulfate and cefotiam by HEK-hOAT3 to about one-third, while it did not change the Vmax value. In summary, Cl− upregulates the activities of both hOAT1 and hOAT3, but its effects on transport kinetics differ between these transporters. It was suggested that Cl− participates in the trans-location process for hOAT1, and the substrate recognition process for hOAT3.

Optical imaging of Ca2+-evoked fluid secretion by murine nasal submucosal gland serous acinar cells

Airway submucosal glands are sites of high expression of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel and contribute to fluid homeostasis in the lung. However, the molecular mechanisms of gland ion and fluid transport are poorly defined. Here, submucosal gland serous acinar cells were isolated from murine airway, identified by immunofluorescence and gene expression profiling, and used in physiological studies. Stimulation of isolated acinar cells with carbachol (CCh), histamine or ATP was associated with marked decreases in cell volume (20 +/- 2% within 62 +/- 5 s) that were tightly correlated with increases in cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) as revealed by simultaneous DIC and fluorescent indicator dye microscopy. Simultaneous imaging of cell volume and the Cl(-)-sensitive fluorophore SPQ indicated that the 20% shrinkage was associated with a fall of [Cl(-)](i) from 65 mm to 28 mm, reflecting loss of 67% of cell Cl(-) content, accompanied by parallel efflux of K(+). Upon agonist removal, [Ca(2+)](i) relaxed and the cells swelled back to resting volume via a bumetanide-sensitive Cl(-) influx pathway, likely to be NKCC1. Accordingly, agonist-induced serous acinar cell shrinkage and swelling are caused by activation of solute efflux and influx pathways, respectively, and cell volume reflects the secretory state of these cells. In contrast, elevation of cAMP failed to elicit detectible volume responses, or enhance those induced by submaximal [CCh], because the magnitude of the changes were likely to be below the threshold of detection using optical imaging. Finally, when stimulated with cholinergic or cAMP agonists, cells from mice that lacked CFTR, as well as wild-type cells treated with a CFTR inhibitor, exhibited identical rates and magnitudes of shrinkage and Cl(-) efflux compared with control cells. These results provide insights into the molecular mechanisms of salt and water secretion by lung submucosal glands, and they suggest that while murine submucosal gland fluid secretion in response to cholinergic stimulation can originate from CFTR-expressing serous acinar cells, it is not dependent upon CFTR function.

Chloride accumulation in mammalian olfactory sensory neurons

The generation of an excitatory receptor current in mammalian olfactory sensory neurons (OSNs) involves the sequential activation of two distinct types of ion channels: cAMP-gated Ca(2+)-permeable cation channels and Ca(2+)-gated Cl(-) channels, which conduct a depolarizing Cl(-) efflux. This unusual transduction mechanism requires an outward-directed driving force for Cl(-), established by active accumulation of Cl(-) within the lumen of the sensory cilia. We used two-photon fluorescence lifetime imaging microscopy of the Cl(-)-sensitive dye 6-methoxy-quinolyl acetoethyl ester to measure the intracellular Cl(-) concentration in dendritic knobs of OSNs from mice and rats. We found a uniform intracellular Cl(-) concentration in the range of 40-50 mm, which is indicative of active Cl(-) accumulation. Functional assays and PCR experiments revealed that NKCC1-mediated Cl(-) uptake through the apical membrane counteracts Cl(-) depletion in the sensory cilia, and thus maintains the responsiveness of OSNs to odor stimulation. To permit Cl(-) accumulation, OSNs avoid the "chloride switch": they do not express KCC2, the main Cl(-) extrusion cotransporter operating in neurons of the adult CNS. Cl(-) accumulation provides OSNs with the driving force for the depolarizing Cl(-) current that is the basis of the low-noise receptor current in these neurons.

Inhibition of ENaC by intracellular Cl- in an MDCK clone with high ENaC expression

We examined the effects of intracellular Cl- concentration ([Cl-]i) on the epithelial Na channel (ENaC) in a line of Madin-Darby canine kidney (MDCK) cells (FL-MDCK) with a high rate of Na+ transport produced by stable retroviral transfection with rENaC subunits (Morris RG and Schafer JA. J Gen Physiol 120: 71-85, 2002). Treatment with cAMP (100 microM 8-cpt-cAMP plus 100 microM IBMX) stimulated ENaC-mediated Na+ absorption as well as Cl- secretion via cystic fibrosis transmembrane conductance regulator, which was characterized in alpha-toxin-permeabilized monolayers to have the anion selectivity sequence NO3- > Br- > Cl- > I-. With the use of FL-MDCK monolayers in which the basolateral membrane was permeabilized by nystatin, the ENaC conductance of the apical membrane [determined from the amiloride-sensitive short-circuit current (AS-Isc) driven by an apical-to-basolateral Na+ concentration gradient] was progressively inhibited by increasing the [Cl-] in the basolateral solution (and hence in the cytosol), but it was insensitive to the [Cl-] in the apical solution. This inhibitory effect of [Cl-]i occurred regardless of the presence or absence of net Cl- transport. However, from fluorometric measurements using the Cl(-)-sensitive dye 6-methoxy-N-(3-sulfopropyl)-quinolinium in intact FL-MDCK monolayers on permeable supports, cAMP, which activates both Na+ absorption and Cl- secretion, produced a decrease of [Cl-]i from 76 +/- 14 to 36 +/- 8 mM (P = 0.03). Thus it might be expected that activation of Cl- secretion by cAMP would lead to stimulation rather than inhibition of ENaC. In the nystatin-treated monolayers, an increase in [Cl-]i from 15 to 145 mM decreased AS-Isc from 24.5 +/- 1.0 to 10.2 +/- 1.6 microA/cm2. This inhibition of ENaC could be attributed to nearly proportional decreases in the density of ENaC in the apical membrane from 1.91 +/- 0.16 to 1.32 +/- 0.17 fmol/cm2 and in the intrinsic channel activity (the average current per ENaC subunit) from 13.3 +/- 1.2 to 8.2 +/- 1.4 microA/fmol.

Halide fluxes in epithelial cells measured with an automated cell plate reader

A method is introduced to measure chloride permeability in cultured epithelial cells using 6-methoxy-N-(3-sulfopropyl)quinolinium (SPQ) and 6-methoxy-N-ethylquinolinium iodide quinolinium (MEQ) as fluorescent chloride-sensitive probes. The method involves growing cells in multiwell plates, incubating cells with SPQ or MEQ, and then exchanging intracellular or extracellular halide ions with nitrate. The resulting time course of SPQ or MEQ fluorescence is followed by repetitive readings with a multiwell fluorescence plate reader. Exchange times are extracted by fitting the time course with a single exponential function of time. The method was validated by measuring the effect of chloride channel activators and blockers in A6 and MDCK cells. The baseline iodide/nitrate exchange time was 200-300 s. Isoproterenol (a modulator of cAMP-activated chloride channels) increased the exchange rate by a factor of 1.4+/-0.1; A23187 (a modulator of calcium-activated chloride channels) increased the rate by 3.4+/-0.4; bradykinin (also a modulator of calcium-activated chloride channels) increased the rate by 2.0+/-0.4; forskolin (a direct stimulator of adenylate cyclase) increased the rate by 2.7+/-0.3. Diphenylamine-2-carboxylate (a chloride channel blocker) decreased the rate by 0.12+/-0.03. These results indicate that our method is a valid indicator of halide-nitrate exchange in cultured epithelial cells.

Cell volume response to hyposmotic shock and elevated cAMP in bovine trabecular meshwork cells

PURPOSE

Hyposmolar perfusion of intact trabecular meshwork (TM) induces a decrease in its hydraulic conductivity (Lp). However, exposure to agents that elevate intracellular cAMP in TM cells increases Lp. Since volume of TM cells could directly influence porosity of the TM and hence Lp, this study has investigated changes in volume in response to acute hyposmotic shock (i.e. regulatory volume decrease or RVD) and elevated cAMP in cultured TM cells.

METHODS

Bovine trabecular meshwork cells (BTMC), grown on glass coverslips and loaded with the fluorescent dye MQAE, were used to measure rapid changes in cell volume using the principle of dynamic fluorescence quenching. Activation of volume-regulated anion channels (VRAC) was assessed by measuring volume-sensitive Cl(-) currents (I(Cl,swell)) in the whole cell configuration of the patch clamp technique and by determining the swelling-induced enhancement in I(-) permeability using the halide-sensitivity of MQAE. Expressions of ClC (chloride channels of the ClC gene family), P-glycoprotein (Pgp), and cystic fibrosis transmembrane regulator (CFTR) Cl(-) channels were examined by RT-PCR. Elevation of cAMP in response to forskolin was confirmed by determining the phosphorylation of cAMP response element-binding protein and activating transcription factor-1 (CREB, ATF-1), which form the downstream targets of protein kinase A.

RESULTS

As a response to hyposmotic shock, there was an acute increase in cell volume but there was no robust RVD. Patch clamp experiments showed activation of a characteristic Cl(-) current in response to cell swelling. This Cl(-) current was inhibited by NPPB (100microM) and fluoxetine (50microM), both of which are known blockers of VRAC. Experiments, which used the halide-sensitivity of MQAE, also indicated a 9-fold increase in I(-) influx upon cell swelling (8.9+/-4.6; n=9), consistent with activation of a VRAC-like Cl(-) current. To examine whether RVD is limited by K(+) conductance, the swollen cells were exposed to gramicidin, which is known to induce cation channel activity. Such a maneuver led to secondary swelling with [Na(+)](o)=140mM but a rapid shrinkage [Na(+)](o)=8mM indicating that the RVD is limited by cationic conductance necessary for K(+) efflux. Exposure to forskolin, which resulted in CREB and ATF-1 phosphorylation, caused a reversible decrease in cell volume (14.5+/-5%; n=20) under isosmotic and hyposmotic conditions. RT-PCR analysis confirmed expression of ClC-2, ClC-5, and Pgp Cl(-) channels in bovine TM cells. However, ClC-3 and CFTR were not expressed.

CONCLUSIONS

TM cells respond to acute hyposmotic shock in an osmometric manner, but their RVD is limited by K(+) conductance. The lack of CFTR expression and decrease in cell volume in response to forskolin concomitant with hyposmolarity suggest that elevated cAMP activates a K(+) conductance. Thus, the altered resistance to aqueous outflow in response to hyposmotic perfusion of the TM and elevated cAMP may be attributed to persistent cell swelling and cell shrinkage, respectively.

Measurement of rapid changes in cell volume by forward light scattering

Light scattering is an empirical technique employed to measure rapid changes in cell volume. This study describes a new configuration for the method of light scattering and its corroboration by measurements of cell height (as a measure of cell volume). Corneal endothelial cells cultured on glass cover-slips were mounted in a perfusion chamber on the stage of an inverted microscope. A beam of light was focused on the cells from above the stage at an angle of 40 degrees to the plane of the stage. The scattered light intensity (SLI), captured by the objective and referred to as forward light scatter (FLS), increased and decreased in response to hyposmotic and hyperosmotic shocks, respectively. The rapid increase and decrease in SLI corresponded to cell swelling and shrinkage, respectively. Subsequently, SLI decreased and increased as expected for a regulatory volume decrease (RVD) and increase (RVI), respectively. These data are in agreement with measurements of cell height, demonstrating that the method of light scatter in FLS mode is useful for monitoring rapid changes in cell volume of cultured cells. Changes in SLI caused by gramicidin were consistent with cell volume changes induced by equilibration of NaCl and KCl concentrations across the cell membrane. Similarly, an additional decrease in SLI was recorded during RVD upon increasing K+ conductance by valinomycin. Decreasing K+ conductance of the cell membrane with Ba2+ changed the time course of SLI consistent with the effect of the K+ channel blocker on RVD. Bumetanide and dihydro-ouabain inhibited increases in SLI during RVI. In conclusion, FLS is a valid method for qualitative analysis of cell volume changes with a high time resolution.

The RUNX1 Runt domain at 1.25A resolution: a structural switch and specifically bound chloride ions modulate DNA binding

The evolutionarily conserved Runt homology domain is characteristic of the RUNX family of heterodimeric eukaryotic transcription factors, including RUNX1, RUNX2 and RUNX3. The genes for RUNX1, also termed acute myeloid leukemia protein 1, AML1, and its dimerization partner core-binding factor beta, CBFbeta, are essential for hematopoietic development and are together the most common targets for gene rearrangements in acute human leukemias. Here, we describe the crystal structure of the uncomplexed RUNX1 Runt domain at 1.25A resolution and compare its conformation to previously published structures in complex with DNA, CBFbeta or both. We find that complex formation induces significant structural rearrangements in this immunoglobulin (Ig)-like DNA-binding domain. Most pronounced is the movement of loop L11, which changes from a closed conformation in the free Runt structure to an open conformation in the CBFbeta-bound and DNA-bound forms. This transition, which we refer to as the S-switch, and accompanying structural movements that affect other parts of the Runt domain are crucial for sustained DNA binding. The closed to open transition can be induced by CBFbeta alone; suggesting that one role of CBFbeta is to trigger the S-switch and to stabilize the Runt domain in a conformation enhanced for DNA binding.A feature of the Runt domain hitherto unobserved in any Ig-like DNA-binding domain is the presence of two specifically bound chloride ions. One chloride ion is coordinated by amino acid residues that make direct DNA contact. In a series of electrophoretic mobility-shift analyses, we demonstrate a chloride ion concentration-dependent stimulation of the DNA-binding activity of Runt in the physiological range. A comparable DNA-binding stimulation was observed for negatively charged amino acid residues. This suggests a regulatory mechanism of RUNX proteins through acidic amino acid residues provided by activation domains during cooperative interaction with other transcription factors.

Chloride concentration in endosomes measured using a ratioable fluorescent Cl- indicator: evidence for chloride accumulation during acidification

A novel long wavelength fluorescent Cl(-) indicator was used to test whether endosomal Cl(-) conductance provides the principal electrical shunt to permit endosomal acidification. The green fluorescent Cl(-)-sensitive chromophore 10,10'-bis[3-carboxypropyl]-9,9'-biacridinium dinitrate (BAC) was conjugated to aminodextran together with the red fluorescent Cl(-)-insensitive chromophore tetramethylrhodamine (TMR). BAC fluorescence is pH-insensitive and quenched by Cl(-) with a Stern-Volmer constant of 36 m(-1). Endosomes in J774 and Chinese hamster ovary (CHO) cells were pulse-labeled with BAC-TMR-dextran by fluid-phase endocytosis. Endosomal [Cl(-)] increased over 45 min from 17 to 53 mm in J774 cells and from 28 to 73 mm in CHO cells, during which time endosomal pH decreased from 6.95 to 5.30 (J774) and 6.92 to 5.60 (CHO). The acidification and increased [Cl(-)] were blocked by bafilomycin. Together with ion substitution and buffer capacity measurements, we conclude that Cl(-) transport accounts quantitatively for the electrical shunt during vacuolar acidification. Measurements of relative endosomal volume by a novel ratio imaging method involving fluorescence self-quenching indicated a 2.5-fold increase in volume during early acidification and Cl(-) accumulation, which was blocked by bafilomycin. These experiments provide the first direct measurement of endosomal [Cl(-)] and indicate that endosomal acidification is accompanied by significant Cl(-) entry and volume increase.

Chloride binding by the AML1/Runx1 transcription factor studied by NMR

It is known that the DNA binding Runt domain of the AML1/Runx1 transcription factor coordinates Cl(-) ions. In this paper we have determined Cl(-) binding affinities of AML1 by (35)Cl nuclear magnetic resonance (NMR) linewidth analysis. The Runt domain binds Cl(-) with a dissociation constant (K(d,Cl)) of 34 mM. If CBFbeta is added to form a 1:1 complex, the K(d,Cl) value increases to 56 mM. Homology modeling suggests that a high occupancy Cl(-) binding site overlaps with the DNA binding surface. NMR data show that DNA displaces this Cl(-) ion. Possible biological roles of Cl(-) binding are discussed based on these findings.

Microfluorometric analysis of Cl- permeability and its relation to oscillatory Ca2+ signalling in glucose-stimulated pancreatic beta-cells

The cytoplasmic concentrations of Cl-([Cl-]i) and Ca2+ ([Ca2+]i) were measured with the fluorescent indicators N-(ethoxycarbonylmethyl)-6-methoxyquinilinum bromide (MQAE) and fura-2 in pancreatic beta-cells isolated from ob/ob mice. Steady-state [Cl-]i in unstimulated beta-cells was 34 mM, which is higher than expected from a passive distribution. Increase of the glucose concentration from 3 to 20 mM resulted in an accelerated entry of Cl- into beta-cells depleted of this ion. The exposure to 20 mM glucose did not affect steady-state [Cl-]i either in the absence or presence of furosemide inhibition of Na+, K+, 2 Cl- co-transport. Glucose-induced oscillations of [Ca2+]i were transformed into sustained elevation in the presence of 4,4' diisothiocyanato-dihydrostilbene-2,2'-disulfonic acid (H2DIDS). A similar effect was noted when replacing 25% of extracellular Cl- with the more easily permeating anions SCN-, I-, NO3- or Br-. It is concluded that glucose stimulation of the beta-cells is coupled to an increase in their Cl- permeability and that the oscillatory Ca2+ signalling is critically dependent on transmembrane Cl- fluxes.

Molecular cloning and characterization of a novel chloride intracellular channel-related protein, parchorin, expressed in water-secreting cells

We previously reported a 120-kDa phosphoprotein that translocated from cytosol to the apical membrane of gastric parietal cells in association with stimulation of HCl secretion. To determine the molecular identity of the protein, we performed molecular cloning and expression of the protein. Immunoblot analysis showed that this protein was highly enriched in tissues that secrete water, such as parietal cell, choroid plexus, salivary duct, lacrimal gland, kidney, airway epithelia, and chorioretinal epithelia. We named this protein "parchorin" based on its highest enrichment in parietal cells and choroid plexus. We obtained cDNA for parchorin from rabbit choroid plexus coding a protein consisting of 637 amino acids with a predicted molecular mass of 65 kDa. The discrepancy in size on 6% SDS-polyacrylamide gel electrophoresis is considered to be due to its highly acidic nature (pI = 4.18), because COS-7 cells transfected with parchorin cDNA produced a protein with apparent molecular mass of 120 kDa on 6% SDS-polyacrylamide gel electrophoresis. Parchorin is a novel protein that has significant homology to the family of chloride intracellular channels (CLIC), especially the chloride channel from bovine kidney, p64, in the C-terminal 235 amino acids. When expressed as a fusion protein with green fluorescent protein (GFP) in the LLC-PK1 kidney cell line, GFP-parchorin, unlike other CLIC family members, existed mainly in the cytosol. Furthermore, when Cl(-) efflux from the cell was elicited, GFP-parchorin translocated to the plasma membrane. These results suggest that parchorin generally plays a critical role in water-secreting cells, possibly through the regulation of chloride ion transport.

Chloride fluxes activated by parathyroid hormone in human erythrocytes

We used the chloride fluorescent probe, 6-methoxy-N-(3-sulfopropyl)quinolinium (SPQ), to study chloride fluxes in human erythrocytes. The SPQ load was made by hypotonic buffer (150 mOsm, 10 min). Intracellular fluorescence was monitored continuously at 360 nm excitation and 410 nm emission wavelengths. The leakage of SPQ out of cells was <5% h(-1) and the Stern-Volmer constant for quenching of intracellular SPQ by Cl was 0.023 mM(-1). The time course of intracellular [Cl] was measured and the influence of PTH, forskolin, and phorbol 12-myristate 13-acetate (PMA) on erythrocyte Cl transport was examined. The results establish a direct method to measure intracellular [Cl] continuously in erythrocytes and show that PTH induces a Cl efflux inhibited by 4, 4'-diisothiocyanatostilbene-2,2'-disulfonate. This effect was similar to those induced by forskolin, which stimulates cAMP generation, and by PMA, which stimulates protein kinase C.

Long-wavelength iodide-sensitive fluorescent indicators for measurement of functional CFTR expression in cells

Limitations of available indicators [such as 6-methoxy-N-(3-sulfopropyl)quinolinium (SPQ)] for measurement of intracellular Cl(-) are their relatively dim fluorescence and need for ultraviolet excitation. A series of long-wavelength polar fluorophores was screened to identify compounds with Cl(-) and/or I(-) sensitivity, bright fluorescence, low toxicity, uniform loading of cytoplasm with minimal leakage, and chemical stability in cells. The best compound found was 7-(beta-D-ribofuranosylamino)-pyrido[2, 1-h]-pteridin-11-ium-5-olate (LZQ). LZQ is brightly fluorescent with excitation and emission maxima at 400-470 and 490-560 nm, molar extinction 11,100 M(-1). cm(-1) (424 nm), and quantum yield 0.53. LZQ fluorescence is quenched by I(-) by a collisional mechanism (Stern-Volmer constant 60 M(-1)) and is not affected by other halides, nitrate, cations, or pH changes (pH 5-8). After LZQ loading into cytoplasm by hypotonic shock or overnight incubation, LZQ remained trapped in cells (leakage <3%/h). LZQ stained cytoplasm uniformly, remained chemically inert, did not bind to cytoplasmic components, and was photobleached by <1% during 1 h of continuous illumination. Cytoplasmic LZQ fluorescence was quenched selectively by I(-) (50% quenching at 38 mM I(-)). LZQ was used to measure forskolin-stimulated I(-)/Cl(-) and I(-)/NO(-)(3) exchange in cystic fibrosis transmembrane conductance regulator (CFTR)-expressing cell lines by fluorescence microscopy and microplate reader instrumentation using 96-well plates. The substantially improved optical and cellular properties of LZQ over existing indicators should permit the quantitative analysis of CFTR function in gene delivery trials and high-throughput screening of compounds for correction of the cystic fibrosis phenotype.

Intracellular chloride and calcium transients evoked by gamma-aminobutyric acid and glycine in neurons of the rat inferior colliculus

Microfluorometric recordings showed that the inhibitory neurotransmitters gamma-aminobutyric acid (GABA) and glycine activated transient increases in the intracellular Cl- concentration in neurons of the inferior colliculus (IC) from acutely isolated slices of the rat auditory midbrain. Current recordings in gramicidin-perforated patch mode disclosed that GABA and glycine mainly evoked inward or biphasic currents. These currents were dependent on HCO3- and characterized by a continuous shift of their reversal potential (E(GABA/gly)) in the positive direction. In HCO3- -buffered saline, GABA and glycine could also evoke an increase in the intracellular Ca2+ concentration. Ca2+ transients occurred only with large depolarizations and were blocked by Cd2+, suggesting an activation of voltage-gated Ca2+ channels. However, in the absence of HCO3-, only a small rise, if any, in the intracellular Ca2+ concentration could be evoked by GABA or glycine. We suggest that the activation of GABAA or glycine receptors results in an acute accumulation of Cl- that is enhanced by the depolarization owing to HCO3- efflux, thus shifting E(GABA/gly) to more positive values. A subsequent activation of these receptors would result in a strenghtened depolarization and an enlarged Ca2+ influx that might play a role in the stabilization of inhibitory synapses in the auditory pathway.

Engineering charge selectivity in alamethicin channels

The peptide alamethicin provides a system for engineering ion channel charge selectivity. To define alamethicin charge selectivity experimentally, we measured single-channel current-voltage relationships in KCl gradients using covalently linked peptide dimers. Two factors were found to contribute to the charge selectivity of these channels: (i) the ionic strength of the surrounding solutions; and (ii) the distribution of fixed charge on the peptide. Native alamethicin channels exhibited either cation selectivity or anion selectivity depending on which end of the channel was at the low salt side of the membrane. When the glutamine residue at position 18 in the sequence was replaced with a lysine residue, an anion-selective channel was obtained regardless of which end of the channel was at the low salt side of the membrane.

Fluorescent chloride indicators to assess the efficacy of CFTR cDNA delivery

Cl(-)-sensitive fluorescent indicators have been used extensively in cell culture systems to measure the Cl(-)-transporting function of the cystic fibrosis transmembrane conductance regulator protein CFTR. These indicators have been used in establishing a surrogate end point to assess the efficacy of CFTR cDNA delivery in human gene therapy trials. The ability to measure Cl- transport with high sensitivity in small and heterogeneous tissue samples makes the use of Cl- indicators potentially attractive in gene delivery studies. In this review article, the important technical aspects of Cl- transport measurements by fluorescent indicators such as SPQ are described, applications of Cl- indicators to assay CFTR function are critically evaluated, and new methodological developments are discussed. The available Cl- indicators have been effective in quantifying Cl- transport rates in cell culture models and in vitro systems such as isolated membrane vesicles and liposomes. However, the imperfect photophysical properties of existing Cl- indicators limit their utility in performing measurements in airway tissues, where gene transfer vectors are delivered in CF gene therapy trials. The low efficiency of gene transfer and the cellular heterogeneity in airway samples pose substantial obstacles to functional measurements of CFTR expression. Significant new developments in generating long-wavelength and dual-wavelength halide indicators are described, and recommendations are proposed for the use of the indicators in gene therapy trials.

Synthesis and characterization of dual-wavelength Cl--sensitive fluorescent indicators for ratio imaging

The fluorescence of quinolinium-based Cl- indicators such as 6-methoxy-N-(3-sulfopropyl)quinolinium (SPQ) is quenched by Cl- by a collisional mechanism without change in spectral shape. A series of "chimeric" dual-wavelength Cl- indicators were synthesized by conjugating Cl--sensitive and -insensitive chromophores with spacers. The SPQ chromophore (N-substituted 6-methoxyquinolinium; MQ) was selected as the Cl--sensitive moiety [excitation wavelength (lambdaex) 350 nm, emission wavelength (lambdaem) 450 nm]. N-substituted 6-aminoquinolinium (AQ) was chosen as the Cl--insensitive moiety because of its different spectral characteristics (lambdaex 380 nm, lambdaem 546 nm), insensitivity to Cl-, positive charge (to minimize quenching by chromophore stacking/electron transfer), and reducibility (for noninvasive cell loading). The dual-wavelength indicators were stable and nontoxic in cells and were distributed uniformly in cytoplasm, with occasional staining of the nucleus. The brightest and most Cl--sensitive indicators were alpha-MQ-alpha'-dimethyl-AQ-xylene dichloride and trans-1, 2-bis(4-[1-alpha'-MQ-1'-alpha'-dimethyl-AQ-xylyl]-pyridinium)ethyl ene (bis-DMXPQ). At 365-nm excitation, emission maxima were at 450 nm (Cl- sensitive; Stern-Volmer constants 82 and 98 M-1) and 565 nm (Cl- insensitive). Cystic fibrosis transmembrane conductance regulator-expressing Swiss 3T3 fibroblasts were labeled with bis-DMXPQ by hypotonic shock or were labeled with its uncharged reduced form (octahydro-bis-DMXPQ) by brief incubation (20 microM, 10 min). Changes in Cl- concentration in response to Cl-/nitrate exchange were recorded by emission ratio imaging (450/565 nm) at 365-nm excitation wavelength. These results establish a first-generation set of chimeric bisquinolinium Cl- indicators for ratiometric measurement of Cl- concentration.

Na+-K+-Cl- cotransport in human fibroblasts is inhibited by cytomegalovirus infection

We examined the effects of human cytomegalovirus (HCMV) infection on the Na+-K+-Cl- cotransporter (NKCC) in a human fibroblast cell line. Using the Cl--sensitive dye MQAE, we showed that the mock-infected MRC-5 cells express a functional NKCC. 1) Intracellular Cl- concentration ([Cl-]i) was significantly reduced from 53.4 +/- 3.4 mM to 35.1 +/- 3.6 mM following bumetanide treatment. 2) Net Cl- efflux caused by replacement of external Cl- with gluconate was bumetanide sensitive. 3) In Cl--depleted mock-infected cells, the Cl- reuptake rate (in HCO-3-free media) was reduced in the absence of external Na+ and by treatment with bumetanide. After HCMV infection, we found that although [Cl-]i increased progressively [24 h postexposure (PE), 65.2 +/- 4.5 mM; 72 h PE, 80.4 +/- 5.0 mM], the bumetanide and Na+ sensitivities of [Cl-]i and net Cl- uptake and loss were reduced by 24 h PE and abolished by 72 h PE. Western blots using the NKCC-specific monoclonal antibody T4 showed an approximately ninefold decrease in the amount of NKCC protein after 72 h of infection. Thus HCMV infection resulted in the abolition of NKCC function coincident with the severe reduction in the amount of NKCC protein expressed.

Human cytomegalovirus infection stimulates Cl-/HCO-3 exchanger activity in human fibroblasts

The effects of human cytomegalovirus (HCMV) infection on Cl-/HCO-3 exchanger activity in human lung fibroblasts (MRC-5 cells) were studied using fluorescent, ion-sensitive dyes. The intracellular pH (pHi) of mock- and HCMV-infected cells bathed in a solution containing 5% CO2-25 mM HCO-3 were nearly the same. However, replacement of external Cl- with gluconate caused an H2DIDS-inhibitable (100 microM) increase in the pHi of HCMV-infected cells but not in mock-infected cells. Continuous exposure to hyperosmotic external media containing CO2/HCO-3 caused the pHi of both cell types to increase. The pHi remained elevated in mock-infected cells. However, in HCMV-infected cells, the pHi peaked and then recovered toward control values. This pHi recovery phase was completely blocked by 100 microM H2DIDS. In the presence of CO2/HCO-3, there was an H2DIDS-sensitive component of net Cl- efflux (external Cl- was substituted with gluconate) that was less in mock- than in HCMV-infected cells. When nitrate was substituted for external Cl- (in the nominal absence of CO2/HCO-3), the H2DIDS-sensitive net Cl- efflux was much greater from HCMV- than from mock-infected cells. In mock-infected cells, H2DIDS-sensitive, net Cl- efflux decreased as pHi increased, whereas for HCMV-infected cells, efflux increased as pHi increased. All these results are consistent with an HCMV-induced enhancement of Cl-/HCO-3 exchanger activity.

Assessment of swelling-activated Cl- channels using the halide-sensitive fluorescent indicator 6-methoxy-N-(3-sulfopropyl)quinolinium

This study describes a quantitative analysis of the enhancement in anion permeability through swelling-activated Cl- channels, using the halide-sensitive fluorescent dye 6-methoxy-N-(3-sulfopropyl)quinolinium (SPQ). Cultured bovine corneal endothelial monolayers perfused with NO3- Ringer's were exposed to I- pulses under isosmotic and, subsequently, hyposmotic conditions. Changes in SPQ fluorescence due to I- influx were significantly faster under hyposmotic than under isosmotic conditions. Plasma membrane potential (Em) was -58 and -32 mV under isosmotic and hyposmotic conditions, respectively. An expression for the ratio of I- permeability under hyposmotic condition to that under isosmotic condition (termed enhancement ratio or ER) was derived by combining the Stern-Volmer equation (for modeling SPQ fluorescence quenching by I-) and the Goldman flux equation (for modeling the electrodiffusive unidirectional I- influx). The fluorescence values and slopes at the inflection points of the SPQ fluorescence profile during I- influx, together with Em under isosmotic and hyposmotic conditions, were used to calculate ER. Based on this approach, endothelial cells were shown to express swelling-activated Cl- channels with ER = 4.9 when the hyposmotic shock was 110 +/- 10 mosM. These results illustrate the application of the SPQ-based method for quantitative characterization of swelling-activated Cl- channels in monolayers.

Intracellular pH regulation in cultured astrocytes from rat hippocampus. II. Electrogenic Na/HCO3 cotransport

In the preceding paper (Bevensee, M.O., R.A. Weed, and W.F. Boron. 1997. 110: 453-465.), we showed that a Na-driven influx of HCO causes the increase in intracellular pH (pH) observed when astrocytes cultured from rat hippocampus are exposed to 5% CO/17 mM HCO. In the present study, we used the pH-sensitive fluorescent indicator 2',7'-biscarboxyethyl-5,6-carboxyfluorescein (BCECF) and the perforated patch-clamp technique to determine whether this transporter is a Na-driven Cl-HCO exchanger, an electrogenic Na/HCO cotransporter, or an electroneutral Na/HCO cotransporter. To determine if the transporter is a Na-driven Cl-HCO exchanger, we depleted the cells of intracellular Cl by incubating them in a Cl-free solution for an average of approximately 11 min. We verified the depletion with the Cl-sensitive dye -(6-methoxyquinolyl)acetoethyl ester (MQAE). In Cl-depleted cells, the pH still increases after one or more exposures to CO/HCO. Furthermore, the pH decrease elicited by external Na removal does not require external Cl. Therefore, the transporter cannot be a Na-driven Cl-HCO exchanger. To determine if the transporter is an electrogenic Na/ HCO cotransporter, we measured pH and plasma membrane voltage (V) while removing external Na, in the presence/absence of CO/HCO and in the presence/absence of 400 microM 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS). The CO/HCO solutions contained 20% CO and 68 mM HCO, pH 7.3, to maximize the HCO flux. In pH experiments, removing external Na in the presence of CO/HCO elicited an equivalent HCO efflux of 281 microM s. The HCO influx elicited by returning external Na was inhibited 63% by DIDS, so that the predicted DIDS-sensitive V change was 3.3 mV. Indeed, we found that removing external Na elicited a DIDS-sensitive depolarization that was 2.6 mV larger in the presence than in the absence of CO/ HCO. Thus, the Na/HCO cotransporter is electrogenic. Because a cotransporter with a Na:HCO stoichiometry of 1:3 or higher would predict a net HCO efflux, rather than the required influx, we conclude that rat hippocampal astrocytes have an electrogenic Na/HCO cotransporter with a stoichiometry of 1:2.

Chloride secretion in the trachea of null cystic fibrosis mice: the effects of transfection with pTrial10-CFTR2

1. An improved novel plasmid backbone, pTrial10, has been developed. We have used this vector to deliver the cDNA for the cystic fibrosis transmembrane conductance regulator (CFTR) to cells, both in vitro and in vivo, complexed with cationic liposomes. 2. Human 293 kidney epithelial cells (HEK 293) showed expression of an immunoprecipitable 165 kDa protein corresponding to CFTR when transfected in vitro with pTrial10-CFTR2, but not when the vector pTrial10 was used. 3. HEK 293 cells transfected with pTrial10-CFTR2, but not pTrial10, demonstrated a cAMP-dependent anion conductance, measured by fluorescence microscopy using a halide-sensitive probe, SPQ. 4. The CFTR-dependent, cAMP-sensitive chloride secretory response in murine tracheal epithelium could be measured if the calcium-dependent chloride secretory process was first maximally stimulated with a mixture of the Ca(2+)-ATPase inhibitor, TBHQ, and the calcium ionophore, A23187. With these conditions wild-type and CF-null (transgenic animals in which the cystic fibrosis (CF) gene has been disrupted so that no CFTR is produced) murine tracheas could be distinguished. The difference between the current elicited by forskolin in wild-type and CF tracheas was highly significantly different (P < 0.001), giving a CFTR-dependent current of 11.2 microA cm-2. 5. Transfection of the airways with pTrial10-CFTR2, but not pTrial10, significantly (P < 0.01) increased the CFTR-dependent chloride secretory current in CF tracheas. The degree of correction was greater when intra-tracheal installation rather than nasal insufflation was used to deliver the plasmids.

Rapid decrease in cellular sodium and chloride content during cold incubation of cultured liver endothelial cells and hepatocytes

Hypothermia, as used for organ preservation in transplantation medicine, is generally supposed to lead to an intracellular accumulation of sodium, and subsequently of chloride, via inhibition of the Na+/K+-ATPase. However, on studying the cellular sodium concentration of cultured liver endothelial cells using fluorescence microscopy, we found a 55% decrease in the cellular sodium concentration after 30 min of cold incubation in University of Wisconsin (UW) solution. To confirm this surprising result, we set up a capillary electrophoresis method that allowed us to determine the cellular contents of inorganic cations and of inorganic anions. Using this method we measured a decrease in the cellular sodium content from 104+/-11 to 55+/-4 nmol/mg of protein, accompanied by a decrease in the chloride content from 71+/-9 to 25+/-5 nmol/mg of protein, after 30 min of cold incubation in UW solution. When the endothelial cells were incubated in cold Krebs-Henseleit buffer or in cold cell culture medium instead of UW solution, similar early decreases in cellular sodium and chloride contents were observed, thus excluding the possibility of the decreases being dependent on the preservation solution used. Furthermore, experiments with cultured rat hepatocytes yielded a similar decrease in sodium content during initiation of cold incubation in UW solution, so the decrease does not appear to be cell-specific either. These results suggest that, contrary to current opinion, sodium efflux predominates over sodium influx during the early phase of cold incubation of cells.