Abstract
GABAA receptors form Cl− permeable channels that mediate the majority of fast synaptic inhibition in the brain. The K+/Cl− cotransporter KCC2 is the main mechanism by which neurons establish low intracellular Cl− levels, which is thought to enable GABAergic inhibitory control of neuronal activity. However, the widely used KCC2 inhibitor furosemide is nonselective with antiseizure efficacy in slices and in vivo, leading to a conflicting scheme of how KCC2 influences GABAergic control of neuronal synchronization. Here we used the selective KCC2 inhibitor VU0463271 [N-cyclopropyl-N-(4-methyl-2-thiazolyl)-2-[(6-phenyl-3-pyridazinyl)thio]acetamide] to investigate the influence of KCC2 function. Application of VU0463271 caused a reversible depolarizing shift in EGABA values and increased spiking of cultured hippocampal neurons. Application of VU0463271 to mouse hippocampal slices under low-Mg2+ conditions induced unremitting recurrent epileptiform discharges. Finally, microinfusion of VU0463271 alone directly into the mouse dorsal hippocampus rapidly caused epileptiform discharges. Our findings indicated that KCC2 function was a critical inhibitory factor ex vivo and in vivo.
Introduction
Neuronal activity patterns in the CNS are shaped by inhibitory synaptic signaling (Freund and Buzsáki, 1996; Isaacson and Scanziani, 2011). Because of the excitatory effects of Na+ currents and the lack of neurotransmitter-gated selectively permeable K+ channels (Alexander et al., 2011), fast synaptic inhibition in the mammalian CNS is mediated by the anion permeable GABAA and glycine ionotropic receptors. Synaptic GABAA currents mediate shunting and hyperpolarizing inhibition. Although both forms of inhibition are affected by the [Cl−]i, sustained hyperpolarizing GABAA-mediated currents are only possible if neurons have a mechanism that persistently extrudes Cl−. This persistent isotonically active Cl− extrusion mechanism was ascribed to KCC2 after it was tested in vitro (Payne, 1997). KCC2 exhibits the predicted effects on Cl− homeostasis in neurons (Rivera et al., 1999) and a temporal expression pattern that matches the postnatal development of hyperpolarizing IPSPs in the hippocampus (Schwartzkroin, 1981; Ben-Ari et al., 1989).
Genetic knock-out of KCC2 expression is lethal at birth (Hübner et al., 2001), and genetic knock-out of the KCC2b isoform leads to spontaneous seizures and death 2–3 weeks postnatally (Woo et al., 2002; Uvarov et al., 2007). However, KCC2 exhibits several transport-independent properties at excitatory synapses: (1) it binds scaffolding proteins within dendritic spines (Li et al., 2007); (2) it affects dendritic spine morphology (Fiumelli et al., 2013); (3) it influences the lateral membrane diffusion of AMPA receptors (Gauvain et al., 2011); and (4) it forms complexes with kainate receptors (Mahadevan et al., 2014). Because of these transporter-independent properties, it is unclear whether the vital and anticonvulsant roles of KCC2 are caused by its K+/Cl− cotransport function.
Moreover, pharmacological inhibition of KCC2 has yielded contradictory results. In cultured hippocampal neurons, the nonselective KCC2 inhibitor furosemide positively shifts the reversal potential of GABAA-mediated currents (EGABA) and increases spontaneous firing rates (Jarolimek et al., 1996; Deeb et al., 2013). In contrast, furosemide applied to hippocampal slices suppresses synchronized activity in several models of seizures, including the low-Mg2+ model (Hochman et al., 1995; Gutschmidt et al., 1999). Furosemide also exhibits antiseizure actions in rodents (Hochman et al., 1995) and humans (Ahmad et al., 1976; Haglund and Hochman, 2005). Therefore, we sought to clarify these discrepancies by using the selective KCC2 inhibitor VU0463271 [N-cyclopropyl-N-(4-methyl-2-thiazolyl)-2-[(6-phenyl-3-pyridazinyl)thio]acetamide (Delpire et al., 2012)] to test whether it would increase network activity in cultured neurons, aggravate low-Mg2+-induced epileptiform discharges in hippocampal slices, and provoke seizures in awake behaving mice.
Materials and Methods
Animals.
Cultured neurons were obtained from E18 embryos from Sprague Dawley rats (Charles River). C57BL/6 male mice aged 3–5 weeks were used for slice recordings, and 10- to 12-week-old mice were used for EEG experiments. All animal procedures were performed in accordance with the National Institutes of Health and approved by the Institutional Animal Care and Use Committee of Tufts University.
Cell culture.
Hippocampal neurons were plated at 250,000 cells per dish and maintained in Neurobasal media containing B27 (2%), glucose (0.6%), Glutamax (1%), and penicillin/streptomycin (1%), and grown for 18–21 d before experimentation. HEK cells were grown in DMEM containing 10% FBS and 1% penicillin/streptomycin. HEK cells were transfected with cDNA encoding human KCC2, rat glycine α1 subunit, and eGFP using Lipofectamine and were grown for 48 h before experimentation. Cell culture materials were purchased from Invitrogen.
Compounds.
VU0463271 was generously provided by Craig Lindsley (Vanderbilt University, Nashville, TN) and dissolved in DMSO at a concentration of 10 mm. The selectivity of VU0463271 against a broad range of targets was tested in binding and functional assay panels (Cerep). The assay methodology can be found online at www.cerep.fr. We used gramicidin (50 μg/ml; Sigma) for perforated patch experiments. Glycine (100 mm), muscimol (50 mm), and TTX (1 mm) were dissolved in deionized H20 and purchased from Tocris Bioscience.
Electrophysiology.
Neuron culture recordings were performed at 34°C, and HEK cells were recorded at room temperature in the bath saline. For perforated patch experiments, pipettes contained saline (in mm): 140 KCl and 10 HEPES, pH 7.4 KOH. For whole-cell experiments, pipettes contained saline (in mm): 130 K-gluconate, 10 KCl, 0.1 CaCl2, 2 Mg-ATP, 1.1 EGTA, and 10 HEPES, pH 7.4 KOH. Bath saline contained the following (in mm): 140 NaCl, 2.5 KCl, 2.5 CaCl2, 1.2 MgCl2, 10 HEPES, and 11 glucose, pH 7.4 NaOH. We performed an equimolar substitution of NaCl by KCl for the 10 mm [K+]o experiments. All solutions were applied onto cells using a three-barrelled 700 μm pipe positioned just above the cell (Warner Instruments). We used 20 mV voltage-ramp protocols over 1 s periods to determine the reversal potentials of the leak-subtracted muscimol-activated or glycine-activated currents. The voltages from whole-cell experiments were corrected offline using a calculated liquid junction potential value (16.3 mV) in Clampex (Molecular Devices). Transverse hippocampal slices (400 μm) were immersed in ice-cold cutting solution containing the following (in mm): 87 NaCl, 2.5 KCl, 0.5 CaCl2, 25 NaHCO3, 1.25 NaH2PO4, 7 MgCl2, 50 sucrose, and 25 glucose (equilibrated with 95% O2 and 5% CO2). Slices recovered for 1 h in ACSF containing the following: (in mm) 126 NaCl, 26 NaHCO3, 1.5 NaH2PO4, 2.5 KCl, 2 CaCl2, 2 MgCl2, and 10 glucose at 34°C. Electrodes filled with normal ACSF (1–5 MΩ resistance) and positioned in layer III of the medial entorhinal cortex were used to record epileptiform activity in normal ACSF with elevated KCl (5 mm) and lacking MgCl2. Data were acquired at 10 kHz with PowerLab hardware (ADInstruments) or an Axopatch 200B amplifier (Molecular Devices).
Intrahippocampal EEG recordings.
Mice were implanted with differential depth recording electrodes fixed to a guide cannula (Plastics One) 1 week before experimentation. EEG recordings were acquired using 100× gain amplification, high-pass filtered at 1 Hz. Epileptiform activity was measured in the dorsal hippocampus after unilateral intrahippocampal administration of 500 nl of 100 μm VU0463271 in freely moving unanesthetized mice. EEGs were assessed for ictal events and abnormal activity characterized by periods of rhythmic spiking lasting longer than 30 s (Lee and Maguire, 2013). Spectrum analysis was performed to assess changes in the power of the FFT. LabChart Pro software was used for data acquisition and analysis.
Statistics.
Statistical analysis was performed using Prism 4 software (GraphPad Software). Paired t tests (two-tailed) were used throughout except when indicated, and p < 0.05 was considered significant. I–V relationships were fit by linear regression analysis using GraphPad software. All data are reported as the mean ± SEM.
Results
VU0463271 inhibited KCC2 function in HEK cells
We performed gramicidin perforated patch recordings in HEK cells transfected with glycine receptors and KCC2. These cells exhibited outward glycine-activated currents at a holding potential of −30 mV and basal EGly values of −71 ± 2 mV (n = 7 cells; Fig. 1A,B). A 5 min perfusion of VU0463271 (10 μm) decreased the amplitude of glycine-activated currents (Fig. 1A), which was caused by a positive shift in EGly values to −35 ± 1 mV (n = 7, p = 0.0002), corresponding to a [Cl−]i shift from 10.2 ± 0.7 to 40.3 ± 1.6 mm (Fig. 1B). VU0463271 did not reduce the slope conductance of glycine-activated currents (data not shown). After a 5 min washout, EGly returned to basal values (−63 ± 2 mV, n = 7, p = 0.0718).
VU0463271 caused a depolarizing shift in EGly values and EGABA values. A, Glycine-activated currents in KCC2-expressing HEK cells decreased during onset of 10 μm VU0463271 (left) and then recovered during washout (right). The time of each glycine pulse is indicated to the left of each trace, and the holding potential was −30 mV. B, Graph of EGly values of KCC2-expressing HEK cells before, during, and after VU0463271 exposure at different concentrations. Lines connect the values obtained for each cell. C, Graph of EGABA values of cultured neurons before and during VU0463271 exposure at the indicated concentrations. Lines connect the values obtained for each cell. D, Traces show the shift in polarity of muscimol responses (arrowheads) under basal conditions (top trace), during exposure to VU0463271 (10 μm; middle trace), and after washout (bottom trace). E, Traces of neuronal activity under basal conditions (blue), 4 min after onset of VU0463271 plus bumetanide (black), and 4 min after onset of VU0463271 plus bumetanide plus TTX (red). Note the reduction in muscimol amplitude (arrowhead) during addition of TTX. F, Graphs of the EGABA and driving force on GABAA-mediated currents (DFGABA) values under basal conditions, after a 4 min exposure to VU0463271 (VU) plus bumetanide (Bume), and after a 4 min exposure to VU0463271 plus bumetanide plus TTX. G, Traces illustrate the increased spontaneous firing rates after exposure to VU0463271 at 100 nm (middle) and 10 μm (bottom). *Indicates statistical significance of p < 0.05 (see text for specific values).
Application of 100 nm VU0463271 on the same cells for 5 min increased EGly values to −43 ± 1 mV and calculated [Cl−]i to 28.7 ± 1.3 mm, which was significantly lower than values obtained in the presence of 10 μm (n = 7, p = 0.0245; Fig. 1B). Washout of VU0463271 resulted in a rapid recovery of EGly to −71 ± 2 mV (p = 0.9602, compared with basal levels). Using the calculated [Cl−]i values, the shift of 100 nm relative to 10 μm VU0463271 was 68 ± 4%, which is similar to the relative efficacy of 100 nm VU0463271 obtained by Rb+ flux assays (Delpire et al., 2012). In contrast, cells not transfected with KCC2 were insensitive to 10 μm VU0463271 (EGly values: −24 ± 1 mV before, −26 ± 1 mV after 5 min exposure, n = 7, p = 0.3869) but were sensitive to the NKCC1 inhibitor bumetanide (10 μm; EGly values: −21 ± 1 mV before, −27 ± 1 mV after 5 min exposure, n = 5, p = 0.0059). To evaluate the selectivity of VU0463271 beyond its initial characterization, a secondary pharmacology screen was performed that identified several high-potency hits, including the mitochondrial translocator protein TSPO (IC50 of ∼200 nm; Rupprecht et al., 2010) and the α1B adrenergic receptor (IC50 of ∼350 nm; Pizzanelli et al., 2009; Table 1). Importantly, these proteins are not known to affect Cl− homeostasis. These data indicated that VU0463271 inhibited KCC2 function in HEK cells in a reversible and concentration-dependent manner.
Off-target hits of VU0463271
VU0463271 inhibited KCC2 function in cultured neurons
We examined the effects of VU0463271 in cultured hippocampal neurons using the gramicidin perforated patch technique. We used the GABAA agonist muscimol (5 μm) to measure EGABA values, which was −76 ± 5 mV (n = 11) under basal conditions (Fig. 1C). A 5 min perfusion of 10 μm VU0463271 converted hyperpolarizing muscimol responses into depolarizing potentials and shifted EGABA values to −36 ± 2 mV (n = 11, p < 0.0001), corresponding to a [Cl−]i shift from 9.8 ± 1.6 to 39.1 ± 2.6 mm (Fig. 1C,D). During washout of VU0463271, neurons reestablished hyperpolarizing muscimol responses and EGABA values of −80 ± 5 mV (p = 0.2280, compared with basal levels; Fig. 1D). On separate neurons, a 5 min perfusion of 100 nm VU0463271 shifted EGABA values from −73 ± 4 to −42 ± 3 mV (n = 10, p = 0.0011), corresponding to a [Cl−]i shift from 10.4 ± 1.3 to 32.4 ± 4.4 mm (Fig. 1C). Washout of VU0463271 allowed a recovery of hyperpolarizing muscimol responses and EGABA values of −78 ± 7 mV (n = 10, p = 0.7707, compared with basal levels). In addition, the effects of VU0463271 (10 μm) were occluded in the presence of 10 mm [K+]o (EGABA in high [K+]o, −37 ± 5 mV; EGABA in high [K+]o plus VU0463271, −40 ± 3 mV, n = 5, p = 0.4544).
To further characterize VU0463271, we performed whole-cell experiments on cultured neurons using recording pipettes containing 10 mm Cl−. Basal EGABA values (−83 ± 2 mV, n = 13) and the calculated [Cl−]i (6.6 ± 0.5 mm) were below the predicted Nernst potential value of approximately −72 mV and the imposed pipette [Cl−], indicating that these neurons expressed a persistent Cl− extrusion mechanism. Consistent with inhibition of KCC2, exposure to VU0463271 (10 μm) rapidly and reversibly increased EGABA to −62 ± 1 mV, corresponding to [Cl−]i values of 14.3 ± 0.5 mm (n = 13, p < 0.0001). The imposed Cl− load from the pipette revealed that KCC2 was completely inhibited within 2 min. In parallel, we examined VU0463271 on the resting membrane potential and input resistance, which were significantly increased from −69.8 ± 1.5 to −68.2 ± 1.5 mV (n = 13, p = 0.0002) and 149 ± 16 to 161 ± 18 MΩ (p = 0.0192). These changes in the membrane properties are consistent with decreased Cl− leak currents caused by elevated [Cl−]i.
The small resting membrane potential shift cannot account for the high EGABA values obtained in the presence of VU0463271; therefore, we examined the two most likely sources of Cl− loading. Coapplication of VU0463271 and bumetanide (both 10 μm) caused a depolarizing EGABA shift to −39 ± 2 mV (n = 8, p = 0.2937, compared with VU0463271 alone, unpaired t test), indicating that NKCC1 was not a major source in these cultured cells (Fig. 1E,F). However, addition of TTX to this mixture reduced the driving force on GABAA-mediated currents and caused a progressive hyperpolarizing EGABA shift over 4 min to −52 ± 3 mV (n = 8, p = 0.0002, compared with VU0463271 plus bumetanide), indicating that activity-dependent depolarizations contributed a substantial portion of the Cl− load (Fig. 1E,F; Buzsáki et al., 2007). In separate experiments, application of 10 μm VU0463271 increased the number of spontaneous action potentials (APs) from 18 ± 5 to 78 ± 26 AP/min (n = 15, p = 0.0330; Fig. 1G). Similarly, application of 100 nm VU0463271 increased the firing rate from 22 ± 6 to 83 ± 23 AP/min (n = 10, p = 0.0165). Together, these data indicated that VU0463271 impaired KCC2 function, resulting in increased network activity that exacerbated the initial Cl− extrusion deficit.
VU0463271 caused epileptiform activity in hippocampal slices and in vivo
Previous studies indicated that the nonselective KCC2 inhibitor furosemide blocked seizure-like events (SLEs) in the low-Mg2+ in vitro model of epilepsy (Hochman et al., 1995; Gutschmidt et al., 1999). Therefore, we tested whether VU0463271 had similar effects. Field recordings from layer III of the medial entorhinal cortex demonstrated that, during perfusion of Mg2+-free ACSF, repetitive SLEs appeared over the course of 120 min (Fig. 2A). SLEs were characterized by slow-wave deflections with superimposed high-frequency events, followed by an afterdischarge phase (Fig. 2A,B). After the development of the first SLE, addition of VU0463271 (10 μm) quickly disrupted the pattern of SLEs transitioning to a persistent afterdischarge phase or recurrent epileptiform discharges in all slices tested (n = 5 slices; Fig. 2A,B). We further examined SLEs in slices that were preincubated for 15 min with 100 nm VU0463271. Perfusion of Mg2+-free ACSF in the continued presence of VU0463271 caused a series of initial SLEs that transitioned to persistent recurrent discharges at 24 ± 4 min after Mg2+ washout in all slices tested (n = 8 slices; Fig. 2C). At the same time points, all control slices in Mg2+-free ACSF displayed SLEs but no recurrent discharges (n = 9 slices). These data indicated that selective inhibition of KCC2 resulted in unremitting epileptiform activity in brain slices exposed to low-Mg2+ conditions.
VU0463271 caused unremitting recurrent discharges in the low-Mg2+ model of epileptiform activity. A, Representative traces of 0 Mg2+-induced SLEs recorded from layer III of the medial entorhinal cortex in acutely prepared slices in the absence (top trace) and presence (10 μm) of VU0463271. The arrow in the bottom trace indicates the onset of VU0463271 perfusion. B, Higher-magnification traces of the first and last SLEs from the bottom trace in A (black bars). Note that the last SLE did not terminate. C, Representative traces demonstrating the effect of VU0463271 (100 nm, preincubated for 15 min) on 0 Mg2+-induced epileptiform activity.
Because of the rapid metabolism of systemically administered VU0463271 (Delpire et al., 2012), we examined its effects on EEG activity after unilateral intrahippocampal administration. Microinfusion of VU0463271 (100 μm, 1 min) resulted in immediate behavioral arrest that was associated with rhythmic spiking activity beginning 12 ± 7 s after infusion, which lasted 23 ± 10 min in duration (n = 4 mice; Fig. 3A,B). A representative power spectra of EEG activity before and immediately after intrahippocampal administration revealed the occurrence of high-powered, low-frequency (1–16 Hz), rhythmic epileptiform activity with a peak frequency of 3.9 Hz (Fig. 3C). Consistent with our slice recordings, our in vivo data revealed that KCC2 inhibition resulted in a persistent recurrent discharge pattern without ictal-like activity. These data indicated that inhibition of KCC2 in a small region of the hippocampus produced aberrant epileptiform activity.
Intrahippocampal administration of VU0463271 caused epileptiform activity in vivo. A, Representative recording of EEG activity before and after microinfusion of 100 μm VU0463271 (asterisk) into the hippocampus. B, An expanded trace of activity before and after microinfusion of VU0463271. C, Left, A representative spectrogram of the power of the EEG activity before and after VU0463271 microinfusion (asterisk). Right, The average power of the EEG activity demonstrates an increase in the low-frequency activity after VU0463271 microinfusion (red) compared with baseline (black).
Discussion
Our data demonstrated that selective pharmacological inhibition of KCC2 function affects neuronal activity in vitro, ex vivo, and in vivo. Although the nonselective KCC2 inhibitor furosemide has similar effects as VU0463271 on KCC2 function in cultured neurons and KCC2-expressing cell lines (Payne, 1997; Lee et al., 2011; Friedel et al., 2013), furosemide suppresses synchronized activity under epileptiform conditions in slices and in vivo (Ahmad et al., 1976; Hochman et al., 1995; Gutschmidt et al., 1999; Haglund and Hochman, 2005). Reviews of the literature led to the conclusions that the antiseizure efficacy of furosemide is caused by inhibition of NKCC1 (Hochman, 2012) and/or targets outside the CNS (Löscher et al., 2013). Our slice and whole animal experiments support these hypotheses, because selective inhibition of KCC2 in the hippocampus resulted in a rapid appearance of epileptiform activity.
It is well known that GABAergic interneurons can cause activity-dependent shifts of intracellular Cl− (Thompson and Gähwiler, 1989). Positive EGABA shifts and the underlying Cl− loads can occur even in the presence of intact KCC2 function, but our results demonstrated that cultured neurons could not return EGABA back to baseline levels in the presence of the KCC2 inhibitor. Furthermore, in ACSF where bicarbonate is used as the buffer, GABAA-mediated HCO3− currents can directly depolarize neurons and push [Cl−]i above equilibrium (Burg et al., 1998; Viitanen et al., 2010). Thus, the combination of GABAA-mediated Cl−/HCO3− currents, direct membrane depolarization, and KCC2 inhibition can cause a protracted loss of GABAA-mediated hyperpolarizing potentials.
Our results with VU0463271 demonstrated that inhibition of KCC2 throughout the slice converted the SLEs into persistent discharge activity that is analogous to status epilepticus (Dreier and Heinemann, 1991), which is characterized as a breakdown of the endogenous seizure termination mechanism (Engel, 2006). It is unclear from our results where this breakdown occurred. Importantly, elevations of intracellular Cl− occur in several slice models of epilepsy (Fujiwara-Tsukamoto et al., 2010; Ilie et al., 2012; Lillis et al., 2012). All of these studies indicated that the [Cl−]i of hippocampal principle cells surges to high levels during SLE onset and remains high until after spontaneous SLE termination. However, it is not known whether [Cl−]i levels rise within the local population of interneurons that are also subjected to GABAergic control. Thus, the effects of VU0463271 on SLE termination could be attributable to the impaired Cl− extrusion mechanism in the principle cells and/or in the upstream interneurons that normally drive the clonic afterdischarge phase of SLEs (Ellender et al., 2014). Our data on KCC2 function is consistent with theoretical models that predicted that an impaired Cl− extrusion mechanism would prolong SLEs (Krishnan and Bazhenov, 2011).
Given the large amount of data supporting the critical role of KCC2 in affecting Cl− homeostasis and the exclusive role played by synaptic Cl− channels in nervous systems, our results are somewhat unsurprising. However, the influence of KCC2 ion cotransport could not be elucidated before because of its transporter-independent properties and the previous lack of selective KCC2 inhibitors (Delpire et al., 2012). Here we have demonstrated that the selective KCC2 inhibitor VU0463271 can be used in cell-based assays, in brain slices under active conditions, and in vivo for additional exploration of the role of KCC2 during seizures.
Footnotes
This work was supported by the Academy of Finland (S.S.), Simons Foundation Grant 206026 (S.J.M.), National Institutes of Health/National Institute of Neurological Disorders and Stroke Grants, NS051195, NS056359, NS081735, NS087662 (S.J.M.), and NS073574 (J.Ma.), and funding from AstraZeneca (S.J.M.).
The authors declare no competing financial interests.
- Correspondence should be addressed to Drs. Tarek Z. Deeb and Stephen J. Moss, AstraZeneca Tufts Laboratory for Basic and Translational Neuroscience, Department of Neuroscience, Tufts Medical School, 136 Harrison Avenue, Boston, MA 02111. Tarek.Deeb{at}tufts.edu, Stephen.Moss{at}tufts.edu
