Potassium channel activity and glutamate uptake are impaired in astrocytes of seizure-susceptible DBA/2 mice (original) (raw)
Related papers
Epilepsia, 2010
Purpose: KCNJ10 encodes subunits of inward rectifying potassium (Kir) channel Kir4.1 found predominantly in glial cells within the brain. Genetic inactivation of these channels in glia impairs extracellular K+ and glutamate clearance and produces a seizure phenotype. In both mice and humans, polymorphisms and mutations in the KCNJ10 gene have been associated with seizure susceptibility. The purpose of the present study was to determine whether there are differences in Kir channel activity and potassium- and glutamate-buffering capabilities between astrocytes from seizure resistant C57BL/6 (B6) and seizure susceptible DBA/2 (D2) mice that are consistent with an altered K+ channel activity as a result of genetic polymorphism of KCNJ10.Methods: Using cultured astrocytes and hippocampal brain slices together with whole-cell patch-clamp, we determined the electrophysiologic properties, particularly K+ conductances, of B6 and D2 mouse astrocytes. Using a colorimetric assay, we determined glutamate clearance capacity by B6 and D2 astrocytes.Results: Barium-sensitive Kir currents elicited from B6 astrocytes are substantially larger than those elicited from D2 astrocytes. In addition, potassium and glutamate buffering by D2 cortical astrocytes is impaired, relative to buffering by B6 astrocytes.Discussion: In summary, the activity of Kir4.1 channels differs between seizure-susceptible D2 and seizure-resistant B6 mice. Reduced activity of Kir4.1 channels in astrocytes of D2 mice is associated with deficits in potassium and glutamate buffering. These deficits may, in part, explain the relatively low seizure threshold of D2 mice.
The Journal of neuroscience : the official journal of the Society for Neuroscience, 2007
During neuronal activity, extracellular potassium concentration ([K+]out) becomes elevated and, if uncorrected, causes neuronal depolarization, hyperexcitability, and seizures. Clearance of K+ from the extracellular space, termed K+ spatial buffering, is considered to be an important function of astrocytes. Results from a number of studies suggest that maintenance of [K+]out by astrocytes is mediated by K+ uptake through the inward-rectifying Kir4.1 channels. To study the role of this channel in astrocyte physiology and neuronal excitability, we generated a conditional knock-out (cKO) of Kir4.1 directed to astrocytes via the human glial fibrillary acidic protein promoter gfa2. Kir4.1 cKO mice die prematurely and display severe ataxia and stress-induced seizures. Electrophysiological recordings revealed severe depolarization of both passive astrocytes and complex glia in Kir4.1 cKO hippocampal slices. Complex cell depolarization appears to be a direct consequence of Kir4.1 removal, w...
Role of astroglial Kir4.1 channels in the pathogenesis and treatment of epilepsy
The inwardly rectifying potassium (Kir) channel subunit Kir4.1 is specifically expressed in brain astrocytes and Kir4.1-containing channels (Kir4.1 channels) mediate astroglial spatial potassium (K +) buffering. Recent advances in Kir4.1 research revealed that Kir4.1 channels can serve as a novel therapeutic target for epilepsy. Specifically, reduced expression or dysfunction of Kir4.1 channels seems to be involved in generation of generalized tonic-clonic seizures (GTCS) in animal models of epilepsy and patients with temporal lobe epilepsy. In addition, recent clinical studies showed that loss-of-function mutations of human gene (KCNJ10) encoding Kir4.1 elicit " EAST " or " SeSAME " syndrome which manifests as GTCS and ataxia. Although the precise mechanisms remain to be clarified, it is suggested that dysfunction of Kir4.1 channels disrupts spatial K + buffering by astrocytes, elevates extracellular levels of K + and/or glutamate and causes abnormal excitation ...
Glia, 2007
Glial cell-mediated potassium and glutamate homeostases play important roles in the regulation of neuronal excitability. Diminished potassium and glutamate buffering capabilities of astrocytes result in hyperexcitability of neurons and abnormal synaptic transmission. The role of the different K 1 channels in maintaining the membrane potential and buffering capabilities of cortical astrocytes has not yet been definitively determined due to the lack of specific K 1 channel blockers. The purpose of the present study was to assess the role of the inward-rectifying K 1 channel subunit Kir4.1 on potassium fluxes, glutamate uptake and membrane potential in cultured rat cortical astrocytes using RNAi, whole-cell patch clamp and a colorimetric assay. The membrane potentials of control cortical astrocytes had a bimodal distribution with peaks at 268 and 241 mV. This distribution became unimodal after knockdown of Kir4.1, with the mean membrane potential being shifted in the depolarizing direction (peak at 245 mV). The ability of Kir4.1-suppressed cells to mediate transmembrane potassium flow, as measured by the current response to voltage ramps or sequential application of different extracellular [K 1 ], was dramatically impaired. In addition, glutamate uptake was inhibited by knock-down of Kir4.1-containing channels by RNA interference as well as by blockade of Kir channels with barium (100 lM). Together, these data indicate that Kir4.1 channels are primarily responsible for significant hyperpolarization of cortical astrocytes and are likely to play a major role in potassium buffering. Significant inhibition of glutamate clearance in astrocytes with knockdown of Kir4.1 highlights the role of membrane hyperpolarization in this process. V
Mutations in the human Kir4.1 potassium channel gene (KCNJ10) are associated with epilepsy. Using a mouse model with glia-specific deletion of Kcnj10, we have explored the mechanistic underpinning of the epilepsy phenotype. The gene deletion was shown to delay K(+) clearance after synaptic activation in stratum radiatum of hippocampal slices. The activity-dependent changes in extracellular space volume did not differ between Kcnj10 mutant and wild-type mice, indicating that the Kcnj10 gene product Kir4.1 mediates osmotically neutral K(+) clearance. Combined, our K(+) and extracellular volume recordings indicate that compromised K(+) spatial buffering in brain underlies the epilepsy phenotype associated with human KCNJ10 mutations.
Hippocampus, 2007
Reactive glial cells, for example, from patients with temporal lope epilepsy have a reduced density of inward rectifying K 1 (Kir) channels and thus a reduced K 1 buffering capacity. Evidence is accumulating that this downregulation of Kir channels could be implicated in epileptogenesis. In rat hippocampal brain slices, prolonged exposure to the nonselective Kir channel antagonist, Cs 1 (5 mM), gives rise to an epileptiform field potential (Cs-FP) in area CA1 composed of an initial positive (interictal-like) phase followed by a prolonged negative (ictallike) phase. We have previously shown that the interictal-like phase depends on synaptic activation. The present study extends these findings by showing that the ictal-like phase of the Cs-FP is (i) sensitive to osmotic expansion of the extracellular space, (ii) reversed very quickly during wash out of Cs 1 , and (iii) re-established in the presence of Ba 21 (30-200 lM) or isosmotic low extracellular concentration of Na 1 ([Na 1 ] o , 51.25 mM). The interictal-like phase showed less or no sensitivity to these treatments. In the complete absence of Cs 1 , the Cs-FP could be fully reconstructed by the combined application of 4-aminopyridine (0.5 mM), an isosmotic high extracellular concentration of K 1 ([K 1 ] o , 7 mM), and low [Na 1 ] o (51.25 mM). These results suggest that the interictal-like phase is initiated through synaptic activation and results from an unspecific increase in neuronal excitability, whereas the ictal-like phase is entirely dependent on blockade of Kir channels in CA1. We propose that glial dysfunction-related loss of Kir channels may not alone be sufficient for starting the induction process, but will likely increase the tendency of an epileptogenic process to proceed into seizure activity. V V C 2007 Wiley-Liss, Inc.
Journal of Neuropathology & Experimental Neurology, 2012
Recent experimental data in mice have shown that the inwardly rectifying K + channel Kir4.1 mediates K + spatial buffering in the hippocampus. Here we used immunohistochemistry to examine the distribution of Kir4.1 in hippocampi from patients with medicationrefractory temporal lobe epilepsy. The selectivity of the antibody was confirmed in mice with a glial conditional deletion of the gene encoding Kir4.1. These mice showed a complete loss of labeled cells, indicating that Kir4.1 is restricted to glia. In human cases, Kir4.1 immunoreactivity observed in cells morphologically consistent with astrocytes was significantly reduced in 12 patients with hippocampal sclerosis versus 11 patients without sclerosis and 4 normal autopsy controls. Loss of astrocytic Kir4.1 immunoreactivity was most pronounced around vessels and was restricted to gliotic areas. Loss of Kir4.1 expression was associated with loss of dystrophin and >syntrophin, but not with loss of A-dystroglycan, suggesting partial disruption of the dystrophin-associated protein complex. The changes identified in patients with hippocampal sclerosis likely interfere with K + homeostasis and may contribute to the epileptogenicity of the sclerotic hippocampus.
Targeting BK (big potassium) channels in epilepsy
Expert Opinion on Therapeutic Targets, 2011
Introduction-Epilepsies are disorders of neuronal excitability characterized by spontaneous and recurrent seizures. Ion channels are critical for regulating neuronal excitability and, therefore, can contribute significantly to epilepsy pathophysiology. In particular, large conductance, Ca 2+activated K + (BK Ca) channels play an important role in seizure etiology. These channels are activated by both membrane depolarization and increased intracellular Ca 2+. This unique coupling of Ca 2+ signaling to membrane depolarization is important in controlling neuronal hyperexcitability, as outward K + current through BK Ca channels hyperpolarizes neurons. Areas covered-This review focuses on BK Ca channel structure-function and discusses the role of these channels in epilepsy pathophysiology. Expert opinion-Loss-of-function BK Ca channels contribute neuronal hyperexcitability that can lead to temporal lobe epilepsy, tonic-clonic seizures and alcohol withdrawal seizures. Similarly, BK Ca channel blockade can trigger seizures and status epilepticus. Paradoxically, some mutations in BK Ca channel subunit can give rise to the channel gain-of-function that leads to development of idiopathic epilepsy (primarily absence epilepsy). Seizures themselves also enhance BK Ca channel currents associated with neuronal hyperexcitability, and blocking BK Ca channels suppresses generalized tonic-clonic seizures. Thus, both loss-of-function and gain-of-function BK Ca channels might serve as molecular targets for drugs to suppress certain seizure phenotypes including temporal lobe seizures and absence seizures, respectively.
K+ channelepsy: Progress in the neurobiology of potassium channels and epilepsy
2013
K + channels are important determinants of seizure susceptibility. These membrane proteins, encoded by more than 70 genes, make the largest group of ion channels that fine-tune the electrical activity of neuronal and non-neuronal cells in the brain. Their ubiquity and extremely high genetic and functional diversity, unmatched by any other ion channel type, place K + channels as primary targets of genetic variations or perturbations in K + -dependent homeostasis, even in the absence of a primary channel defect. It is therefore not surprising that numerous inherited or acquired K + channels dysfunctions have been associated with several neurologic syndromes, including epilepsy, which often generate confusion in the classification of the associated diseases. Therefore, we propose to name the K + channels defects underlying distinct epilepsies as "K + channelepsies," and introduce a new nomenclature (e.g., Kx.y-channelepsy), following the widely used K + channel classification, which could be also adopted to easily identify other channelopathies involving Na + (e.g., Na v x.y-phenotype), Ca 2+ (e.g., Ca v x.y-phenotype), and Cl − channels. Furthermore, we discuss novel genetic defects in K + channels and associated proteins that underlie distinct epileptic phenotypes in humans, and analyze critically the recent progress in the neurobiology of this disease that has also been provided by investigations on valuable animal models of epilepsy. The abundant and varied lines of evidence discussed here strongly foster assessments for variations in genes encoding for K + channels and associated proteins in patients with idiopathic epilepsy, provide new avenues for future investigations, and highlight these proteins as critical pharmacological targets.