Cross talk between the GABA(A) receptor and the Na-K-Cl cotransporter is mediated by intracellular Cl-

Cell Volume Control in Healthy Brain and Neuropathologies

Regulation of cellular volume is a critical homeostatic process that is intimately linked to ionic and osmotic balance in the brain tissue. Because the brain is encased in the rigid skull and has a very complex cellular architecture, even minute changes in the volume of extracellular and intracellular compartments have a very strong impact on tissue excitability and function. The failure of cell volume control is a major feature of several neuropathologies, such as hyponatremia, stroke, epilepsy, hyperammonemia, and others. There is strong evidence that such dysregulation, especially uncontrolled cell swelling, plays a major role in adverse pathological outcomes. To protect themselves, brain cells utilize a variety of mechanisms to maintain their optimal volume, primarily by releasing or taking in ions and small organic molecules through diverse volume-sensitive ion channels and transporters. In principle, the mechanisms of cell volume regulation are not unique to the brain and share many commonalities with other tissues. However, because ions and some organic osmolytes (e.g., major amino acid neurotransmitters) have a strong impact on neuronal excitability, cell volume regulation in the brain is a surprisingly treacherous process, which may cause more harm than good. This topical review covers the established and emerging information in this rapidly developing area of physiology.

TRPM7 is regulated by halides through its kinase domain

Transient receptor potential melastatin 7 (TRPM7) is a divalent-selective cation channel fused to an atypical α-kinase. TRPM7 is a key regulator of cell growth and proliferation, processes accompanied by mandatory cell volume changes. Osmolarity-induced cell volume alterations regulate TRPM7 through molecular crowding of solutes that affect channel activity, including magnesium (Mg(2+)), Mg-nucleotides and a further unidentified factor. Here, we assess whether chloride and related halides can act as negative feedback regulators of TRPM7. We find that chloride and bromide inhibit heterologously expressed TRPM7 in synergy with intracellular Mg(2+) ([Mg(2+)]i) and this is facilitated through the ATP-binding site of the channel's kinase domain. The synergistic block of TRPM7 by chloride and Mg(2+) is not reversed during divalent-free or acidic conditions, indicating a change in protein conformation that leads to channel inactivation. Iodide has the strongest inhibitory effect on TRPM7 at physiological [Mg(2+)]i. Iodide also inhibits endogenous TRPM7-like currents as assessed in MCF-7 breast cancer cells, where upregulation of SLC5A5 sodium-iodide symporter enhances iodide uptake and inhibits cell proliferation. These results indicate that chloride could be an important factor in modulating TRPM7 during osmotic stress and implicate TRPM7 as a possible molecular mechanism contributing to the anti-proliferative characteristics of intracellular iodide accumulation in cancer cells.

WNK2 kinase is a novel regulator of essential neuronal cation-chloride cotransporters

NKCC1 and KCC2, related cation-chloride cotransporters (CCC), regulate cell volume and γ-aminobutyric acid (GABA)-ergic neurotranmission by modulating the intracellular concentration of chloride [Cl(-)]. These CCCs are oppositely regulated by serine-threonine phosphorylation, which activates NKCC1 but inhibits KCC2. The kinase(s) that performs this function in the nervous system are not known with certainty. WNK1 and WNK4, members of the WNK (with no lysine [K]) kinase family, either directly or via the downstream SPAK/OSR1 Ste20-type kinases, regulate the furosemide-sensitive NKCC2 and the thiazide-sensitive NCC, kidney-specific CCCs. What role the novel WNK2 kinase plays in this regulatory cascade, if any, is unknown. Here, we show that WNK2, unlike other WNKs, is not expressed in kidney; rather, it is a neuron-enriched kinase primarily expressed in neocortical pyramidal cells, thalamic relay cells, and cerebellar granule and Purkinje cells in both the developing and adult brain. Bumetanide-sensitive and Cl(-)-dependent (86)Rb(+) uptake assays in Xenopus laevis oocytes revealed that WNK2 promotes Cl(-) accumulation by reciprocally activating NKCC1 and inhibiting KCC2 in a kinase-dependent manner, effectively bypassing normal tonicity requirements for cotransporter regulation. TiO(2) enrichment and tandem mass spectrometry studies demonstrate WNK2 forms a protein complex in the mammalian brain with SPAK, a known phosphoregulator of NKCC1. In this complex, SPAK is phosphorylated at Ser-383, a consensus WNK recognition site. These findings suggest a role for WNK2 in the regulation of CCCs in the mammalian brain, with implications for both cell volume regulation and/or GABAergic signaling.

Determination of transmembrane water fluxes in neurons elicited by glutamate ionotropic receptors and by the cotransporters KCC2 and NKCC1: a digital holographic microscopy study

Digital holographic microscopy (DHM) is a noninvasive optical imaging technique that provides quantitative phase images of living cells. In a recent study, we showed that the quantitative monitoring of the phase signal by DHM was a simple label-free method to study the effects of glutamate on neuronal optical responses (Pavillon et al., 2010). Here, we refine these observations and show that glutamate produces the following three distinct optical responses in mouse primary cortical neurons in culture, predominantly mediated by NMDA receptors: biphasic, reversible decrease (RD) and irreversible decrease (ID) responses. The shape and amplitude of the optical signal were not associated with a particular cellular phenotype but reflected the physiopathological status of neurons linked to the degree of NMDA activity. Thus, the biphasic, RD, and ID responses indicated, respectively, a low-level, a high-level, and an "excitotoxic" level of NMDA activation. Moreover, furosemide and bumetanide, two inhibitors of sodium-coupled and/or potassium-coupled chloride movement strongly modified the phase shift, suggesting an involvement of two neuronal cotransporters, NKCC1 (Na-K-Cl) and KCC2 (K-Cl) in the genesis of the optical signal. This observation is of particular interest since it shows that DHM is the first imaging technique able to monitor dynamically and in situ the activity of these cotransporters during physiological and/or pathological neuronal conditions.

Resting intracellular calcium concentration, depolarizing Gamma-Aminobutyric Acid and possible role of local estradiol synthesis in the developing male and female hippocampus

The maturation of the hippocampus is impacted by a multitude of factors, including the regulation of intracellular calcium levels. Depolarizing actions of Gamma-Aminobutyric Acid (GABA) can profoundly alter intracellular calcium in immature hippocampal neurons via influx through voltage-gated calcium channels. We here report fundamental sex differences in properties of depolarizing GABA responses and in resting intracellular calcium in neonatal cultured hippocampal neurons. The effects of the estrogen receptor antagonist, ICI 182,780, and the estradiol-synthesis inhibitor, formestane, indicate the sex differences in depolarizing GABA responses are at least in part due to de novo estradiol synthesis by female neurons, whereas a sex difference in resting calcium is independent of steroids. We postulate that local estradiol synthesis in cultured female hippocampal neurons affects the kinetics of either the GABA(A) receptor or voltage sensitive calcium channels. These data highlight the fact that immature hippocampal neurons exhibit fundamentally different physiological properties in males versus females. Elucidating how and where immature male and female neurons differ is essential for a complete understanding of normal rodent brain development.

Na-K-Cl Cotransporter-1 in the mechanism of ammonia-induced astrocyte swelling

Brain edema and the consequent increase in intracranial pressure and brain herniation are major complications of acute liver failure (fulminant hepatic failure) and a major cause of death in this condition. Ammonia has been strongly implicated as an important factor, and astrocyte swelling appears to be primarily responsible for the edema. Ammonia is known to cause cell swelling in cultured astrocytes, although the means by which this occurs has not been fully elucidated. A disturbance in one or more of these systems may result in loss of ion homeostasis and cell swelling. In particular, activation of the Na-K-Cl cotransporter (NKCC1) has been shown to be involved in cell swelling in several neurological disorders. We therefore examined the effect of ammonia on NKCC activity and its potential role in the swelling of astrocytes. Cultured astrocytes were exposed to ammonia (NH(4)Cl; 5 mm), and NKCC activity was measured. Ammonia increased NKCC activity at 24 h. Inhibition of this activity by bumetanide diminished ammonia-induced astrocyte swelling. Ammonia also increased total as well as phosphorylated NKCC1. Treatment with cyclohexamide, a potent inhibitor of protein synthesis, diminished NKCC1 protein expression and NKCC activity. Since ammonia is known to induce oxidative/nitrosative stress, and antioxidants and nitric-oxide synthase inhibition diminish astrocyte swelling, we also examined whether ammonia caused oxidation and/or nitration of NKCC1. Cultures exposed to ammonia increased the state of oxidation and nitration of NKCC1, whereas the antioxidants N-nitro-l-arginine methyl ester and uric acid all significantly diminished NKCC activity. These agents also reduced phosphorylated NKCC1 expression. These results suggest that activation of NKCC1 is an important factor in the mediation of astrocyte swelling by ammonia and that such activation appears to be mediated by NKCC1 abundance as well as by its oxidation/nitration and phosphorylation.

Impact of estradiol on gamma-aminobutyric acid- and glutamate-mediated calcium responses of fetal baboon (Papio anubis) hippocampal and cortical neurons

High levels of maternal estrogens are likely to gain access to the fetal brain, yet little is known regarding the role of the steroid hormone 17beta-estradiol in neuronal differentiation and maturation of primate neurons. Previous research documented the presence of estrogen receptors during development in the hippocampus and cortex of the primate brain, but the functional significance of steroid exposure has not been widely investigated. Using both an in vitro preparation of primary hippocampal and frontal cortex neurons and Western blot analysis of fetal hippocampal and frontal cortex tissue, we documented the effects of in utero and acute in vitro exposure to 17beta-estradiol on the development of neuronal responsiveness to the amino acid transmitters gamma-aminobutyric acid (GABA) and glutamate in fetal baboon, Papio anubis, hippocampal, and cortical neurons. We found that in utero 17beta-estradiol exposure enhanced the excitatory action of the GABAergic system on immature cortical and hippocampal neurons, as manifest by increases in intracellular calcium after transient muscimol application and changes in the relevant ion cotransporters. Acute exposure to 17beta-estradiol in vitro had limited effect on GABAergic responses in cultured hippocampal and frontal cortex neurons. Moreover, there was limited effect of both prolonged in utero and acute estradiol on the response to glutamatergic system activation, consistent with previous findings in the rat. Along with documenting a prominent role for 17beta-estradiol in maturation of the GABAergic system, these findings increase our understanding of neuronal differentiation and maturation in the fetal primate brain.

Modulation of chloride homeostasis by inflammatory mediators in dorsal root ganglion neurons

Background

Chloride currents in peripheral nociceptive neurons have been implicated in the generation of afferent nociceptive signals, as Cl^- accumulation in sensory endings establishes the driving force for depolarizing, and even excitatory, Cl^- currents. The intracellular Cl^- concentration can, however, vary considerably between individual DRG neurons. This raises the question, whether the contribution of Cl^- currents to signal generation differs between individual afferent neurons, and whether the specific Cl^- levels in these neurons are subject to modulation. Based on the hypothesis that modulation of the peripheral Cl^- homeostasis is involved in the generation of inflammatory hyperalgesia, we examined the effects of inflammatory mediators on intracellular Cl^- concentrations and on the expression levels of Cl^- transporters in rat DRG neurons.

Results

We developed an in vitro assay for testing how inflammatory mediators influence Cl^- concentration and the expression of Cl^- transporters. Intact DRGs were treated with 100 ng/ml NGF, 1.8 μM ATP, 0.9 μM bradykinin, and 1.4 μM PGE₂ for 1–3 hours. Two-photon fluorescence lifetime imaging with the Cl^--sensitive dye MQAE revealed an increase of the intracellular Cl^- concentration within 2 hours of treatment. This effect coincided with enhanced phosphorylation of the Na⁺-K⁺-2Cl^- cotransporter NKCC1, suggesting that an increased activity of that transporter caused the early rise of intracellular Cl^- levels. Immunohistochemistry of NKCC1 and KCC2, the main neuronal Cl^- importer and exporter, respectively, exposed an inverse regulation by the inflammatory mediators. While the NKCC1 immunosignal increased, that of KCC2 declined after 3 hours of treatment. In contrast, the mRNA levels of the two transporters did not change markedly during this time. These data demonstrate a fundamental transition in Cl^- homeostasis toward a state of augmented Cl^- accumulation, which is induced by a 1–3 hour treatment with inflammatory mediators.

Conclusion

Our findings indicate that inflammatory mediators impact on Cl^- homeostasis in DRG neurons. Inflammatory mediators raise intracellular Cl^- levels and, hence, the driving force for depolarizing Cl^- efflux. These findings corroborate current concepts for the role of Cl^- regulation in the generation of inflammatory hyperalgesia and allodynia. As the intracellular Cl^- concentration rises in DRG neurons, afferent signals can be boosted by excitatory Cl^- currents in the presynaptic terminals. Moreover, excitatory Cl^- currents in peripheral sensory endings may also contribute to the generation or modulation of afferent signals, especially in inflamed tissue.

Na+,K+,2Cl- cotransport and intracellular chloride regulation in rat primary sensory neurons: thermodynamic and kinetic aspects

Adult primary afferent neurons are depolarized by GABA throughout their entire surface, including their somata located in dorsal root ganglia (DRG). Primary afferent depolarization (PAD) mediated by GABA released from spinal interneurons determines presynaptic inhibition, a key mechanism in somatosensory processing. The depolarization is due to Cl(-) efflux through GABA(A) channels; the outward Cl(-) gradient is generated by a Na+,K+,2Cl(-) cotransporter (NKCC) as first established in amphibians. Using fluorescence imaging microscopy we measured [Cl(-)]i and cell water volume (CWV) in dissociated rat DRG cells (P0-P21) loaded with N-(ethoxycarbonylmethyl)-6-methoxyquinolinium bromide and calcein, respectively. Basal [Cl(-)]i was 44.2 +/- 1.2 mM (mean +/- SE), Cl(-) equilibrium potential (E Cl) was -27.0 +/- 0.7 mV (n = 75). This [Cl(-)]i is about four times higher than electrochemical equilibrium. On isosmotic removal of external Cl(-), cells lost Cl(-) and shrank. On returning to control solution, cells reaccumulated Cl(-) and recovered CWV. Cl(-) reaccumulation had Na+-dependent (SDC) and Na+-independent (SIC) components. The SIC stabilized at [Cl(-)]i = 13.2 +/- 1.2 mM, suggesting that it was passive (E(Cl) = -60.5 +/- 3 mV). Bumetanide blocked CWV recovery and most (65%) of the SDC (IC50 = 5.7 microM), indicating that both were mediated by NKCC. Active Cl(-) uptake fell with increasing [Cl(-)]i and became negligible when [Cl(-)]i reached basal levels. The kinetics of active Cl(-) uptake suggests a negative feedback system in which intracellular Cl(-)regulates its own influx thereby keeping [Cl(-)]i constant, above electrochemical equilibrium but below the value that would attain if NKCC reached thermodynamic equilibrium.

Sodium valproate stimulates potassium and chloride urinary excretion in rats: gender differences

BACKGROUND

The diuretic effect of valproates and its relation to urinary potassium (K+) and chloride (Cl-) excretion have not yet been investigated, so the aim of this study was to evaluate the influence of a single dose of sodium valproate (NaVPA) on 24-h urinary K+ and Cl- excretion in young adult Wistar rats of both genders. For measurement of K+ in urine, the same animals and samples as in our earlier publication were used (Pharmacology 2005 Nov, 75:111-115). The authors propose a new approach to the pathophysiological mechanisms of NaVPA effect on K+ and Cl- metabolism. Twenty six Wistar rats were examined after a single intragastric administration of 300 mg/kg NaVPA (13 NaVPA-male and 13 NaVPA-female), 28 control intact Wistar rats (14 males and 14 females) were studied as a control group. The 24-h urinary K+, Cl-, creatinine and pH levels were measured.

RESULTS

Total 24-h diuresis and 24-h diuresis per 100 g of body weight were found to be significantly higher in NaVPA-rats of both genders than in rats of the control group (p < 0.05). The data showed NaVPA to enhance 24-h K+ excretion in NaVPA-males and NaVPA-females with significant gender-related differences: 24-h K+ excretion in NaVPA-male rats was significantly higher than in control males (p = 0.003) and NaVPA-female rats (p < 0.001). Regarding the 24-h K+ excretion, NaVPA-female rats did not show a statistically significant difference versus females of the control group (p > 0.05). 24-h urinary K+ excretion per 100 g of body weight in NaVPA-male rats was significantly higher than in control males (p = 0.025). NaVPA enhanced Cl- urinary excretion: 24-h Cl- urinary excretion, 24-h urinary Cl- excretion per 100 g of body weight and the Cl-/creatinine ratio were significantly higher in NaVPA-male and NaVPA-female rats than in gender-matched controls (p < 0.05). 24-h chloriduretic response to NaVPA in male rats was significantly higher than in female rats (p < 0.05).

CONCLUSION

NaVPA causes kaliuretic and chloriduretic effects with gender-related differences in rats. Further investigations are necessary to elucidate the mechanism of such pharmacological effects of NaVPA.

Subtractive hybridization unravels a role for the ion cotransporter NKCC1 in the murine intestinal pacemaker

In the small intestine, interstitial cells of Cajal (ICC) surrounding the myenteric plexus generate the pacemaking slow waves that are essential for an efficient intestinal transit. The underlying molecular mechanisms of the slow wave are poorly known. Our aim was to identify ICC-specific genes and their function in the mouse jejunum. Suppression subtractive hybridization using two independent ICC-deficient mouse models identified 56 genes putatively downregulated in the muscularis propria compared with wild-type littermates. Differential expression was confirmed by real-time quantitative PCR for the tyrosine kinase receptor KIT, the established marker for ICC, and for the Na(+)-K(+)-2Cl(-) cotransporter (NKCC1). Immunoreactivity for NKCC1 was detected in myenteric ICC but not in the ICC population located at the deep muscular plexus. NKCC1 was also expressed in enteric neurons and mucosal crypts. Bumetanide, an NKCC1 inhibitor, reversibly affected the shape, amplitude, and frequency of the slow waves. Similar alterations were observed in NKCC1 knockout mice. These data support the hypothesis that NKCC1 expressed in myenteric ICC is involved in the mechanism of slow waves in the murine jejunum.

Serotoninergic modulation of chloride homeostasis during maturation of the locomotor network in zebrafish

During development, neural networks progress through important functional changes such as the generation of spontaneous activity, the expression of a depolarizing chloride gradient, and the appearance of neuromodulation. Little is known about how these processes are integrated to yield mature behaviors. We showed previously that, during the maturation of the locomotor network of the zebrafish, endogenous serotonin (5HT) increased motor activity by reducing intervals of inactivity, without affecting the active swim periods that are the target of 5HT in other and more mature preparations. Because membrane properties were constant during the rest intervals, we examined here whether 5HT modulates chloride homeostasis. We compared the effects of blocking (inward) chloride cotransport with bumetanide to the effects of 5HT and its antagonists, both behaviorally by video imaging and cellularly by whole-cell and gramicidin-perforated patch recordings. Bumetanide mimicked the effects of 5HT antagonists, by prolonging rest intervals without affecting the properties of swim episodes (duration; frequency; extent of depolarization) either behaviorally or during fictive swimming. Furthermore, bumetanide and 5HT antagonists suppressed the amplitude of depolarizing responses evoked by ionophoresis of glycine onto spinal neurons in the presence of tetrodotoxin and transiently suppressed the amplitude of responses to glycine measured after fictive swimming. The effects of bumetanide contrasted with and occluded the effects of 5HT. We suggest that, during development, endogenous 5HT modulates chloride homeostasis during the quiescent intervals and thereby offsets the long periods of quiescence commonly observed in developing networks to allow expression of sustained and behaviorally relevant activity.

Physiology and pathophysiology of Na+/H+ exchange and Na+ -K+ -2Cl- cotransport in the heart, brain, and blood

Maintenance of a stable cell volume and intracellular pH is critical for normal cell function. Arguably, two of the most important ion transporters involved in these processes are the Na+/H+ exchanger isoform 1 (NHE1) and Na+ -K+ -2Cl- cotransporter isoform 1 (NKCC1). Both NHE1 and NKCC1 are stimulated by cell shrinkage and by numerous other stimuli, including a wide range of hormones and growth factors, and for NHE1, intracellular acidification. Both transporters can be important regulators of cell volume, yet their activity also, directly or indirectly, affects the intracellular concentrations of Na+, Ca2+, Cl-, K+, and H+. Conversely, when either transporter responds to a stimulus other than cell shrinkage and when the driving force is directed to promote Na+ entry, one consequence may be cell swelling. Thus stimulation of NHE1 and/or NKCC1 by a deviation from homeostasis of a given parameter may regulate that parameter at the expense of compromising others, a coupling that may contribute to irreversible cell damage in a number of pathophysiological conditions. This review addresses the roles of NHE1 and NKCC1 in the cellular responses to physiological and pathophysiological stress. The aim is to provide a comprehensive overview of the mechanisms and consequences of stress-induced stimulation of these transporters with focus on the heart, brain, and blood. The physiological stressors reviewed are metabolic/exercise stress, osmotic stress, and mechanical stress, conditions in which NHE1 and NKCC1 play important physiological roles. With respect to pathophysiology, the focus is on ischemia and severe hypoxia where the roles of NHE1 and NKCC1 have been widely studied yet remain controversial and incompletely elucidated.

The chloride transporter Na(+)-K(+)-Cl- cotransporter isoform-1 contributes to intracellular chloride increases after in vitro ischemia

Ischemic episodes in the CNS cause significant disturbances in neuronal ionic homeostasis. To directly measure changes in intracellular Cl- concentration ([Cl-]i) during and after ischemia, we used Clomeleon, a novel ratiometric optical indicator for Cl-. Hippocampal slices from adult transgenic mice expressing Clomeleon in hippocampal neurons were subjected to 8 min of oxygen-glucose deprivation (OGD) (an in vitro model for ischemia) and reoxygenated in the presence of glucose. This produced mild neuronal damage 3 h later that was prevented when the extracellular [Cl-] was maintained at 10 mm during reoxygenation. OGD induced a transient decrease in fluorescence resonance energy transfer within Clomeleon, indicating an increase in [Cl-]i. During reoxygenation, there was a partial recovery in [Cl-]i, but [Cl-]i rose again 45 min later. To investigate sources of Cl- accumulation, we examined the effects of Cl- transport inhibitors on the rises in [Cl-]i during and after OGD. Bumetanide and furosemide, which inhibit Cl- influx through the Na(+)-K(+)-Cl- cotransporter isoform-1 (NKCC-1) and efflux through the K(+)-Cl- cotransporter isoform-2, were unable to inhibit the first rise in [Cl-]i, yet entirely prevented the secondary rise in [Cl-]i during reoxygenation. In contrast, picrotoxin, which blocks the GABA-gated Cl- channel, did not inhibit the secondary rise in [Cl-]i after OGD. [Cl-]i increases during reoxygenation were accompanied by an increase in phosphorylation of NKCC-1, an indication of increased NKCC-1 activity after OGD. We conclude that NKCC-1 plays an important role in OGD-induced Cl- accumulation and subsequent neuronal damage.

A C-terminal domain in KCC2 confers constitutive K+-Cl- cotransport

The neuron-specific K(+)-Cl(-) cotransporter KCC2 plays a crucial role in determining intracellular chloride activity and thus the neuronal response to gamma-aminobutyric acid and glycine. Of the four KCCs, KCC2 is unique in mediating constitutive K(+)-Cl(-) cotransport under isotonic conditions; the other three KCCs are exclusively swelling-activated, with no isotonic activity. We have utilized a series of chimeric cDNAs to localize the determinant of isotonic transport in KCC2. Two generations of chimeric KCC4-KCC2 cDNAs initially localized this characteristic to within a KCC2-specific expansion of the cytoplasmic C terminus, between residues 929 and 1043. This region of KCC2 is rich in prolines, serines, and charged residues and encompasses two predicted PEST sequences. Substitution of this region in KCC2 with the equivalent sequence of KCC4 resulted in a chimeric KCC that was devoid of isotonic activity, with intact swelling-activated transport. A third generation of chimeras demonstrated that a domain just distal to the PEST sequences confers isotonic transport on KCC4. Mutagenesis of this region revealed that residues 1021-1035 of KCC2 are sufficient for isotonic transport. Swelling-activated K(+)-Cl(-) cotransport is abrogated by calyculin A, whereas isotonic transport mediated by KCC chimeras and KCC2 is completely resistant to this serine-threonine phosphatase inhibitor. In summary, a 15-residue C-terminal domain in KCC2 is both necessary and sufficient for constitutive K(+)-Cl(-) cotransport under isotonic conditions. Furthermore, unlike swelling-activated transport, constitutive K(+)-Cl(-) cotransport mediated by KCC2 is completely independent of serine-threonine phosphatase activity, suggesting that these two modes of transport are activated by distinct mechanisms.

The role of Na-K-Cl co-transporter in cerebral ischemia

The electroneutral Na-K-Cl co-transporter (NKCC) protein transports Na(+), K(+) and Cl(-) into cells under physiological conditions with a stoichiometry of 1Na(+) :1K(+) :2Cl(-). NKCC is characteristically inhibited by the sulfamoylbenzoic acid "loop'' diuretics, such as bumetanide and furosemide. To date, only two distinct isoforms, NKCC1 and NKCC2, have been identified. NKCC1 has a broad tissue distribution, while the NKCC2 isoform is only found in vertebrate kidney. NKCC serves multiple functions, including ion and fluid movements in secreting or reabsorbing epithelia and cell volume regulation. However, understanding the role of NKCC1 in the central nervous system has just begun. NKCC1 protein is expressed in neurons throughout the brain. Dendritic localization of NKCC1 is found in both pyramidal and non-pyramidal neurons. NKCC1 is important in the maintenance of intracellular Cl(-) in neurons and contributes to GABA-mediated depolarization in immature neurons. Thus, NKCC1 may affect neuronal excitability through regulation of intracellular Cl(-) concentration. Expression of NKCC1 protein has also been found in astrocytes and oligodendrocytes. In addition to its role in the accumulation of Cl(-), NKCC1 may also contribute to K(+) clearance and maintenance of intracellular Na(+) in glia. Our recent studies suggest that NKCC1 activation leads to high [K(+)](o(-)) induced astrocyte swelling and glutamate release, as well as neuronal Na(+) , and Cl(-) influx during acute excitotoxicity. Inhibition of NKCC1 activity significantly reduces infarct volume and cerebral edema following cerebral focal ischemia.

Na-K-Cl cotransporter-mediated intracellular Na+ accumulation affects Ca2+ signaling in astrocytes in an in vitro ischemic model

Na-K-Cl cotransporter isoform 1 (NKCC1) plays an important role in maintenance of intracellular Na+, K+, and Cl- levels in astrocytes. We propose that NKCC1 may contribute to perturbations of ionic homeostasis in astrocytes under ischemic conditions. After 3-8 hr of oxygen and glucose deprivation (OGD), NKCC1-mediated 86Rb influx was significantly increased in astrocytes from NKCC1 wild-type (NKCC1+/+) and heterozygous mutant (NKCC1+/-) mice. Phosphorylated NKCC1 protein was increased in NKCC1+/+ astrocytes at 2 hr of OGD. Two hours of OGD and 1 hr of reoxygenation (OGD/REOX) triggered an 3.6-fold increase in intracellular Na+ concentration ([Na+]i) in NKCC1+/+ astrocytes. Inhibition of NKCC1 activity by bumetanide or ablation of the NKCC1 gene significantly attenuated the rise in [Na+]i. Moreover, NKCC1+/+ astrocytes swelled by 10-30% during 20-60 min of OGD. Either genetic ablation of NKCC1 or inhibition of NKCC1 by bumetanide-attenuated OGD-mediated swelling. An NKCC1-mediated increase in [Na+]i may subsequently affect Ca2+ signaling through the Na+/Ca2+ exchanger (NCX). A rise in [Ca2+]i was detected after OGD/REOX in the presence of a sarcoplasmic-endoplasmic reticulum (ER) Ca2+-ATPase inhibitor thapsigargin. Moreover, OGD/REOX led to a significant increase in Ca2+ release from ER Ca2+ stores. Furthermore, KB-R7943 (2-[2-[4(4-nitrobenzyloxy)phenyl]ethyl]isothiourea mesylate), an inhibitor of reverse-mode operation of NCX, abolished the OGD/REOX-induced enhancement in filling of ER Ca2+ stores. OGD/REOX-mediated Ca2+ accumulation in ER Ca2+ stores was absent when NKCC1 activity was ablated or pharmacologically inhibited. These findings imply that stimulation of NKCC1 activity leads to Na+ accumulation after OGD/REOX and that subsequent reverse-mode operation of NCX contributes to increased Ca2+ accumulation by intracellular Ca2+ stores.

KCC2 expression in immature rat cortical neurons is sufficient to switch the polarity of GABA responses

During brain development, GABA and glycine switch from being depolarizing to being hyperpolarizing neurotransmitters. This conversion results from a gradual decrease in the chloride electrochemical equilibrium potential (ECl) of developing neurons, which correlates to an increase in the expression or activity of the potassium chloride cotransporter, KCC2. However, evidence as to whether KCC2 expression is sufficient, in and of itself, to induce this switch is lacking. In order to address this question, we used a gain-of-function approach by over-expressing human KCC2 (hKCC2) in immature cortical neurons, before endogenous up-regulation of KCC2. We found that premature expression of hKCC2 produced a substantial negative shift in the GABA reversal potential and decreased or abolished GABA-elicited calcium responses in cultured neurons. We conclude that KCC2 expression is not only necessary but is also sufficient for ending the depolarizing period of GABA in developing cortical neurons.

Cortical neurons lacking KCC2 expression show impaired regulation of intracellular chloride

As excitable cells, neurons experience constant changes in their membrane potential due to ion flux through plasma membrane channels. They maintain their transmembrane cation concentrations through robust Na(+)/K(+)-ATPase pump activity. During synaptic transmission and spread of action potentials, the concentration of the major anion, Cl-, is also under constant challenge from membrane potential changes. Moreover, intracellular Cl- is also affected by ligand-gated Cl- channels such as GABA(A) and glycine receptors. To regulate intracellular Cl- in an electrically silent manner, neurons couple the movement of Cl- with K+. In this study, we have used gene-targeted KCC2-/- mice to provide strong evidence that KCC2, the neuronal-specific K-Cl co-transporter, drives neuronal Cl- to low concentrations, shifting the GABA reversal potential toward more negative potentials, thus promoting hyperpolarizing GABA responses. Cortical neurons lacking KCC2, not only fail to show a developmental decrease in [Cl-]i, but also are unable to regulate [Cl-]i on Cl- loading or maintain [Cl]i during membrane depolarization. These data are consistent with the central role of KCC2 in promoting inhibition and preventing hyperexcitability.

Presynaptic Inactivation of Action Potentials and Postsynaptic Inhibition of GABAA Currents Contribute to KA-Induced Disinhibition in CA1 Pyramidal Neurons

Kainate-type glutamate ionotropic receptors (KAR) mediate either depression or potentiation of inhibitory transmission. The mechanisms underlying the depressant effect of KAR agonists have been controversial. Under dual patch-clamp recording techniques in synaptically coupled pairs of CA1 interneurons and pyramidal neurons in hippocampal slices, micromolar concentrations of KAR agonists, kainic acid (KA, 10 μM) and ATPA (10 μM), induced inactivation of action potentials (APs) in 58 and 50% of presynaptic interneurons, respectively. Inactivation of interneuronal APs might have significantly contributed to KA-induced decreases in evoked inhibitory postsynaptic currents (eIPSCs) that are obtained by stimulating the stratum radiatum. With controlled interneuronal APs, KAR agonists induced a decrease in the potency (mean amplitude of successful events) and mean amplitude (including failures) of unitary inhibitory postsynaptic currents (uIPSCs) without significantly changing the success rate (Ps) at perisomatic high-Ps synapses. In contrast, KAR agonists induced a decrease in both the Ps and potency of uIPSCs at dendritic high-Ps synapses. KAR agonists induced an inhibition of GABAA currents by activating postsynaptic KARs in pyramidal neurons; this was more prominent at dendrites than at soma. Both the exogenous GABA-induced current and the amplitude of miniature IPSCs (mIPSCs) were attenuated by KAR agonists. Thus the postsynaptic KAR-mediated inhibition of GABAA currents may contribute to the KAR agonist-induced decrease in the potency of uIPSCs and KA-induced disinhibition.

Synaptically activated Cl- accumulation responsible for depolarizing GABAergic responses in mature hippocampal neurons

It is known that GABA, a major inhibitory transmitter in the CNS, acts as an excitatory (or depolarizing) transmitter transiently after intense GABAA receptor activation in adult brains. The depolarizing effect is considered to be dependent on two GABAA receptor-permeable anions, chloride (Cl-) and bicarbonate (HCO3-). However, little is known about their spatial and temporal profiles during the GABAergic depolarization in postsynaptic neurons. In the present study, we show that the amplitude of synaptically induced depolarizing response was correlated with intracellular Cl- accumulation in the soma of mature hippocampal CA1 pyramidal cells, by using whole cell patch-clamp recording and Cl- imaging technique with a Cl- indicator 6-methoxy-N-ethylquinolinium iodide (MEQ). The synaptically activated Cl- accumulation was mediated dominantly through GABAA receptors. Basket cells, a subclass of fast-spiking interneurons innervating the somatic portion of the pyramidal cells, actually fired at high frequency during the Cl- accumulation accompanying the depolarizing responses. These results suggest synaptically activated GABAA-mediated Cl- accumulation may play a critical role in generation of an excitatory GABAergic response in the mature pyramidal cells receiving intense synaptic inputs. This may be the first demonstration of microscopic visualization of intracellular Cl- accumulation during synaptic activation.

Na-K-Cl cotransporter contributes to glutamate-mediated excitotoxicity

We hypothesized that cation-dependent Cl- transport protein Na-K-Cl cotransporter isoform 1 (NKCC1) plays a role in the disruption of ion homeostasis in cerebral ischemia. In the current study, a role for NKCC1 in neuronal death was elucidated in neurotoxicity induced by glutamate and oxygen and glucose deprivation (OGD). Incubation of cortical neurons cultured for 14-15 d in vitro (DIV) with 100 microm glutamate for 24 hr resulted in 50% cell death. Three hours of OGD followed by 21 hr of reoxygenation led to 70% cell death. Inhibition of NMDA receptors with dizocilpine hydrogen maleate (1 microm) prevented both OGD- and glutamate-mediated cell death. Moreover, blocking of NKCC1 activity with bumetanide (5-10 microm) abolished glutamate- or OGD-induced neurotoxicity. Bumetanide was ineffective if added after 10-120 min of glutamate incubation or 3-6 hr of OGD treatment. Accumulation of intracellular Na+ and 36Cl content after NMDA receptor activation was inhibited by bumetanide. Blockage of NKCC1 significantly attenuated cell swelling after OGD or NMDA receptor activation. This neuroprotection was age dependent. Inhibition of NKCC1 did not protect DIV 7-8 neurons against OGD-mediated cell death. In contrast, cell death in DIV 7-8 neurons was prevented by the protein-synthesis inhibitor, cycloheximide. Taken together, the results suggest that NKCC1 activity is involved in the acute excitotoxicity as a result of excessive Na+ and Cl- entry and disruption of ion homeostasis.

GABA-mediated trophic effect on oligodendrocytes requires Na-K-2Cl cotransport activity

The Na-K-2Cl cotransporter isoform1 (NKCC1) is present in many animal cells where it plays prominent roles in regulating cell volume and maintaining intracellular Cl(-) concentration ([Cl(-)]i) above electrochemical equilibrium. We show here that NKCC1 is present and active in cultured oligodendrocytes. Expression of NKCC1 in the rat spinal cord increased during development from postnatal day 6 through 21 in parallel with that of myelin basic protein. In cultured oligodendrocytes, 39% of the total K+ (86Rb+) influx represented NKCC1 activity. Activation of GABA(A) receptors with muscimol produced a reduction in intracellular Cl(-) content, cell shrinkage, and a stimulation of NKCC1 activity. Muscimol also triggered an increase in intracellular Ca2+, which depended on NKCC1 activity. Survival of oligodendrocytes following withdrawal of growth factors was enhanced by muscimol and this effect also required NKCC1 activity. Our results suggest that NKCC1 functions in oligodendrocytes to maintain [Cl(-)]i above electrochemical equilibrium and that NKCC1 is required for GABAergic trophic effects.