K+ inward rectifier currents in reactive astrocytes from adult rat brain

Dysregulation of extracellular potassium distinguishes healthy ageing from neurodegeneration

Progressive neuronal loss is a hallmark feature distinguishing neurodegenerative diseases from normal ageing. However, the underlying mechanisms remain unknown. Extracellular K+ homeostasis is a potential mediator of neuronal injury as K+ elevations increase excitatory activity. The dysregulation of extracellular K+ and potassium channel expressions during neurodegeneration could contribute to this distinction. Here we measured the cortical extracellular K+ concentration ([K+]e) in awake wild-type mice as well as murine models of neurodegeneration using K+-sensitive microelectrodes. Unexpectedly, aged wild-type mice exhibited significantly lower cortical [K+]e than young mice. In contrast, cortical [K+]e was consistently elevated in Alzheimer's disease (APP/PS1), amyotrophic lateral sclerosis (ALS) (SOD1G93A) and Huntington's disease (R6/2) models. Cortical resting [K+]e correlated inversely with neuronal density and the [K+]e buffering rate but correlated positively with the predicted neuronal firing rate. Screening of astrocyte-selective genomic datasets revealed a number of potassium channel genes that were downregulated in these disease models but not in normal ageing. In particular, the inwardly rectifying potassium channel Kcnj10 was downregulated in ALS and Huntington's disease models but not in normal ageing, while Fxyd1 and Slc1a3, each of which acts as a negative regulator of potassium uptake, were each upregulated by astrocytes in both Alzheimer's disease and ALS models. Chronic elevation of [K+]e in response to changes in gene expression and the attendant neuronal hyperexcitability may drive the neuronal loss characteristic of these neurodegenerative diseases. These observations suggest that the dysregulation of extracellular K+ homeostasis in a number of neurodegenerative diseases could be due to aberrant astrocytic K+ buffering and as such, highlight a fundamental role for glial dysfunction in neurodegeneration.

Neuroinflammation alters voltage-dependent conductance in striatal astrocytes

Neuroinflammation has the capacity to alter normal central nervous system (CNS) homeostasis and function. The objective of the present study was to examine the effects of an inflammatory milieu on the electrophysiological properties of striatal astrocyte subpopulations with a mouse bacterial brain abscess model. Whole cell patch-clamp recordings were performed in striatal glial fibrillary acidic protein (GFAP)-green fluorescent protein (GFP)(+) astrocytes neighboring abscesses at postinfection days 3 or 7 in adult mice. Cell input conductance (G(i)) measurements spanning a membrane potential (V(m)) surrounding resting membrane potential (RMP) revealed two prevalent astrocyte subsets. A1 and A2 astrocytes were identified by negative and positive G(i) increments vs. V(m), respectively. A1 and A2 astrocytes displayed significantly different RMP, G(i), and cell membrane capacitance that were influenced by both time after bacterial exposure and astrocyte proximity to the inflammatory site. Specifically, the percentage of A1 astrocytes was decreased immediately surrounding the inflammatory lesion, whereas A2 cells were increased. These changes were particularly evident at postinfection day 7, revealing increased cell numbers with an outward current component. Furthermore, RMP was inversely modified in A1 and A2 astrocytes during neuroinflammation, and resting G(i) was increased from 21 to 30 nS in the latter. In contrast, gap junction communication was significantly decreased in all astrocyte populations associated with inflamed tissues. Collectively, these findings demonstrate the heterogeneity of striatal astrocyte populations, which experience distinct electrophysiological modifications in response to CNS inflammation.

Mild brain ischemia induces unique physiological properties in striatal astrocytes

We studied the properties of GFAP-expressing cells in adult mouse striatum using acute brain slices from transgenic animals expressing EGFP under GFAP promoter. Under physiological conditions, two distinct populations of GFAP-EGFP cells could be identified: (1) brightly fluorescent cells had bushy processes, passive membrane properties, glutamate transporter activity, and high gap junction coupling rate typical for classical astrocytes; (2) weakly fluorescent cells were characterized by thin, clearly distinguishable processes, voltage-gated currents, complex responses to kainate, and low coupling rate reminiscent of an astrocyte subtype recently described in the hippocampus. Mild focal cerebral ischemia confers delayed neuronal cell death and astrogliosis in the striatum. Following middle cerebral artery occlusion and reperfusion, brightly fluorescent cells were the dominant GFAP-EGFP population observed within the ischemic lesion. Interestingly, the majority of these cells expressed voltage-gated channels, showed complex responses to kainate, and a high coupling rate exceeding that of brightly fluorescent control cells. A minority of cells had passive membrane properties and was coupled less compared with passive control cells. We conclude that, in the adult striatum, astrocytes undergo distinct pathophysiological changes after ischemic insults. The dominant population in the ischemic lesion constitutes a novel physiological phenotype unlike any normal astrocyte and generates a large syncytium which might be a neuroprotective response of reactive astrocytes.

High extracellular K(+) evokes changes in voltage-dependent K(+) and Na (+) currents and volume regulation in astrocytes

[K(+)](e) increase accompanies many pathological states in the CNS and evokes changes in astrocyte morphology and glial fibrillary acidic protein expression, leading to astrogliosis. Changes in the electrophysiological properties and volume regulation of astrocytes during the early stages of astrocytic activation were studied using the patch-clamp technique in spinal cords from 10-day-old rats after incubation in 50 mM K(+). In complex astrocytes, incubation in high K(+) caused depolarization, an input resistance increase, a decrease in membrane capacitance, and an increase in the current densities (CDs) of voltage-dependent K(+) and Na(+) currents. In passive astrocytes, the reversal potential shifted to more positive values and CDs decreased. No changes were observed in astrocyte precursors. Under hypotonic stress, astrocytes in spinal cords pre-exposed to high K(+) revealed a decreased K(+) accumulation around the cell membrane after a depolarizing prepulse, suggesting altered volume regulation. 3D confocal morphometry and the direct visualization of astrocytes in enhanced green fluorescent protein/glial fibrillary acidic protein mice showed a smaller degree of cell swelling in spinal cords pre-exposed to high K(+) compared to controls. We conclude that exposure to high K(+), an early event leading to astrogliosis, caused not only morphological changes in astrocytes but also changes in their membrane properties and cell volume regulation.

Expression and function of inwardly rectifying potassium channels after experimental subarachnoid hemorrhage

Cerebral vasospasm after subarachnoid hemorrhage (SAH) is because of smooth muscle contraction, although the mechanism of this contraction remains unresolved. Membrane potential controls the contractile state of arterial myocytes by gating voltage-sensitive calcium channels and is in turn primarily controlled by K(+) ion conductance through several classes of K(+) channels. We characterized the role of inwardly rectifying K(+) (K(IR)) channels in vasospasm. Vasospasm was created in dogs using the double-hemorrhage model of SAH. Electrophysiological, real-time quantitative reverse-transcriptase polymerase chain reaction, Western blotting, immunohistochemistry, and isometric tension techniques were used to characterize the expression and function of K(IR) channels in normal and vasospastic basilar artery 7 days after SAH. Subarachnoid hemorrhage resulted in severe vasospasm of the basilar artery (mean of 61% +/- 5% reduction in diameter). Membrane potential of pressurized vasospastic basilar arteries was significantly depolarized compared with control arteries (-46 +/- 1.4 mV versus -29.8 +/- 1.8 mV, respectively, P < 0.01). In whole-cell patch clamp of enzymatically isolated basilar artery myocytes, average K(IR) conductance was 1.6 +/- 0.5 pS/pF in control cells and 9.2 +/- 2.2 pS/pF in SAH cells (P = 0.007). Blocking K(IR) channels with BaCl(2) (0.1 mmol/L) resulted in significantly greater membrane depolarization in vasospastic compared with normal myocytes. Expression of K(IR) 2.1 messenger ribonucleic acid (mRNA) was increased after SAH. Western blotting and immunohistochemistry also showed increased expression of K(IR) protein in vasospastic smooth muscle. Blockage of K(IR) channels in arteries under isometric tension produced a greater contraction in SAH than in control arteries. These results document increased expression of K(IR) 2.1 mRNA and protein during vasospasm after experimental SAH and suggest that this increase is a functionally significant adaptive response acting to reduce vasospasm.

Doppel-induced apoptosis and counteraction by cellular prion protein in neuroblastoma and astrocytes

Expression of a prion-like protein, doppel, induces apoptosis-like changes in cerebellar neuronal granule and Purkinje cells of prion-knockout mice and this effect can be rescued by re-introduction of cellular prion. Since most of those studies were done in transgenic mice, in the present study, we have established a murine neuro-2a cell line and the primary rat adult reactive astrocyte model for studying doppel-induced apoptosis and possible prion counteraction. We demonstrate that expression of doppel in neuro-2a cells causes apoptosis, during which DNA fragmentation occurs as visualized by terminal deoxynucleotidyl transferase biotin-dUTP nick end labeling staining and other intracellular changes characteristic of apoptosis are observed in the electron microscope. Using immunoblot analyses, we further demonstrate that doppel expression activates caspase-10 as well as caspase-3, but does not activate caspase-9. Addition of purified doppel to cultures of neuro-2a cells and the primary astrocytes causes similar apoptotic changes. Significantly, apoptosis induced by doppel is enhanced when cellular prion protein is depleted by RNA interference, suggesting a protective effect of cellular prion against doppel-induced apoptosis. The antagonistic interaction between cellular prion and doppel appears to involve direct protein-protein interaction possibly on cell membrane as cellular prion and doppel physically interact with each other and co-localize on cell membranes. Together, our data show that doppel induces apoptosis in neuroblastoma neuro-2a and rat primary astrocytes via a caspase-10 mediated pathway and that this effect is counteracted by cellular prion through direct interaction with doppel possibly on cell membrane.

Voltage-dependent potassium currents in hypertrophied rat astrocytes after a cortical stab wound

Changes in the membrane properties of reactive astrocytes in gliotic cortex induced by a stab wound were studied in brain slices of 21-28-day-old rats, using the patch-clamp technique and were correlated with changes in resting extracellular K+ concentration ([K+]e) measured in vivo using K+-selective microelectrodes. Based on K+ current expression, three types of astrocytes were identified in gliotic cortex: A1 astrocytes expressing a time- and voltage-independent K+ current component and additional inwardly rectifying K+ currents (K(IR)); A2 astrocytes expressing a time- and voltage-independent K+ current component and additional delayed outwardly rectifying K+ currents (K(DR)); and complex astrocytes expressing K(DR), K(IR), and A-type K+ (K(A)) currents and Na+ currents (I(Na)). Nestin/bromodeoxyuridine (BrdU)-negative A1 astrocytes were found further than approximately 100 microm from the stab wound and showed an upregulation of K(IR) currents within the first day post-injury (PI), correlating with an increased resting [K+]e. Their number declined from 62% of total astrocytes in control rats to 41% in rats at 7 days PI. Nestin/BrdU-positive A2 astrocytes were found only within a distance of approximately 100 microm from the stab wound and, in comparison to those in control rats, showed an upregulation of K(DR) currents. Their number increased from 8% of the total number of astrocytes in control rats to 39% 7 days PI. Both A1 and A2 astrocytes showed hypertrophied processes and increased GFAP staining, but an examination of cell morphology revealed greater changes in the surface/volume ratio in A2 astrocytes than in A1 astrocytes. Complex astrocytes did not display a hypertophied morphology; K(IR) currents in these cells were upregulated within 1 day PI, while the K(DR), K(A), and I(Na) currents were increased only 6 h PI. We conclude that two electrophysiologically, immunohistochemically, and morphologically distinct types of hypertrophied astrocytes are present at the site of a stab wound, depending on the distance from the lesion, and may have different functions in ionic homeostasis and/or regeneration.

The role of glial membrane ion channels in seizures and epileptogenesis

Epilepsy is one of the most common neurological disorders, but the cellular basis of human epilepsy remains largely a mystery, and about 30% of all epilepsies remain uncontrolled. The vast bulk of epilepsy research has focused on neuronal and synaptic mechanisms, but the hypersynchronous firing that is the hallmark of epilepsy could also result from the abnormal function of glial cells by virtue of their critical role in the homeostasis of the brain's extracellular milieu. Therefore, increasing our understanding of glial pro-epileptic and epileptogenic mechanisms holds promise for the development of improved pharmacological treatments for epilepsy. Reactive astrocytes, a prominent feature of the human epileptic brain, undergo changes in their membrane properties and electrophysiology, in particular in the expression of membrane K(+) and Na(+) channels, which result in pro-epileptic changes in their homeostatic control of the extracellular space. Nonetheless, a causal role for reactive astrocytosis in epilepsy has been difficult to determine because glial reactivity can be induced by a wide range of central nervous system insults, including epileptic seizures themselves. A complicating factor is that different insults to the central nervous system result in reactive astrocytes with different membrane properties. Therefore, most animal models of epilepsy preselect the properties of the reactive glia studied. Finally, a causal role for reactive glia in epilepsy cannot be firmly established by examining human epileptic tissue because of its chronic and pharmacoresistant pathological condition that warranted the surgical intervention. Therefore, the development of clinically relevant models of reactive astrocytosis, and of symptomatic epileptogenesis, is needed to investigate the issue. A recently developed model of post-traumatic epileptogenesis in the rat, where chronic spontaneous recurrent seizures develop after a single event of a clinically relevant form of closed head injury, the fluid percussion injury, offers hope to help understand the role of reactive glia in seizures and epileptogenesis and lead to the development of improved therapies.

Chlorotoxin-sensitive Ca2+-activated Cl- channel in type R2 reactive astrocytes from adult rat brain

Astrocytes express four types of Cl(-) or anion channels, but Ca(2+)-activated Cl(-) (Cl(Ca)) channels have not been described. We studied Cl(-) channels in a morphologically distinct subpopulation ( approximately 5% of cells) of small (10-12 micro m, 11.8 +/- 0.6 pF), phase-dark, GFAP-positive native reactive astrocytes (NRAs) freshly isolated from injured adult rat brains. Their resting potential, -57.1 +/- 4.0 mV, polarized to -72.7 +/- 4.5 mV with BAPTA-AM, an intracellular Ca(2+) chelator, and depolarized to -30.7 +/- 6.1 mV with thapsigargin, which mobilizes Ca(2+) from intracellular stores. With nystatin-perforated patch clamp, thapsigargin activated a current that reversed near the Cl(-) reversal potential, which was blocked by Cl(-) channel blockers, 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB) and Zn(2+), by I(-) (10 mM), and by chlorotoxin (EC(50) = 47 nM). With conventional whole-cell clamp, NPPB- and Zn(2+)-sensitive currents became larger with increasing [Ca(2+)](i) (10, 150, 300 nM). Single-channel recordings of inside-out patches confirmed Ca(2+) sensitivity of the channel and showed open-state conductances of 40, 80, 130, and 180 pS, and outside-out patches confirmed sensitivity to chlorotoxin. In primary culture, small phase-dark NRAs developed into small GFAP-positive bipolar cells with chlorotoxin-sensitive Cl(Ca) channels. Imaging with biotinylated chlorotoxin confirmed the presence of label in GFAP-positive cells from regions of brain injury, but not from uninjured brain. Chlorotoxin-tagged cells isolated by flow cytometry and cultured up to two passages exhibit positive labeling for GFAP and vimentin, but not for prolyl 4-hydroxylase (fibroblast), A2B5 (O2A progenitor), or OX-42 (microglia). Expression of a novel chlorotoxin-sensitive Cl(Ca) channel in a morphologically distinct subpopulation of NRAs distinguishes these cells as a new subtype of reactive astrocyte.

Synaptically evoked GABA transporter currents in neocortical glia

The presence, magnitude, and time course of GABA transporter currents were investigated in electrophysiologically characterized neocortical astrocytes in an in vitro slice preparation. On stimulation with a bipolar-tungsten stimulating electrode placed nearby, the majority of cells tested displayed long-lasting GABA transporter currents using both single and repetitive stimulation protocols. Using subtype-specific GABA transporter antagonists, long-lasting GABA transporter currents were identified in neocortical astrocytes that originated from at least two subtypes of GABA transporters: GAT-1 and GAT-2/3. These transporter currents displayed slow rise times and long decay times, contrasting the time course observed for glutamate transporter currents, and are indicative of a long extracellular time course of GABA as well as a role for glial GABA transporters during synaptic transmission.

Transforming growth factor-beta 1 regulates Kir2.3 inward rectifier K+ channels via phospholipase C and protein kinase C-delta in reactive astrocytes from adult rat brain

The multifunctional cytokine, transforming growth factor beta(1) (TGF-beta(1)), exerts complex effects on astrocytes with early signaling events being less well characterized than transcriptional mechanisms. We examined the effect of TGF-beta(1) on the 14-pS Kir2.3 inward rectifier K(+) channel in rat primary cultured reactive astrocytes. Immunofluorescence study showed that cells co-expressed TGF-beta(1) receptors 1 and 2, Kir2.3, and glial fibrillary acidic protein (GFAP). Patch clamp study showed that TGF-beta(1) (0.1-100 ng/ml) caused a rapid (<5 min) depolarization because of dose-dependent down-regulation of Kir2.3 channels, which was mimicked by the protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (10-500 nm) and which was inhibited by the PKC inhibitor calphostin C (100 nm), by PKC desensitization produced by 3 h of exposure to phorbol 12-myristate 13-acetate (100 nm), and by the PKC-delta isoform-specific inhibitor rottlerin (50 microm). Immunoblot analysis and confocal imaging showed that TGF-beta(1) caused PKC-delta translocation to membrane, and co-immunoprecipitation experiments showed that TGF-beta(1) enhanced association between Kir2.3 and PKC-delta. Additional electrophysiological experiments showed that Kir2.3 channel down-regulation was blocked by the phospholipase C inhibitors, neomycin (100 microm) and D609 (200 microm). Given the commonality of signaling involving PLC-PKC-delta, we speculate that TGF-beta(1)-evoked depolarization may be an early signaling event related to gene transcription in astrocytes.

Inward rectifier K(+) channel Kir2.3 (IRK3) in reactive astrocytes from adult rat brain

Astrocytic inward rectifying K(+) channels that participate in K(+) spatial buffering in the central nervous system have been extensively investigated, but specific gene products have not been fully identified. We studied primary cultured reactive astrocytes of stellate and polygonal morphology from adult rat brains, as well as stellate astrocytes from neonatal rat brains. Single-channel recordings of cell-attached patches revealed that polygonal reactive astrocytes expressed only one hyperpolarization-activated single-channel conductance of 11-15 pS whose open probability was independent of voltage, whereas stellate reactive and stellate neonatal astrocytes exhibited two conductances, 11-15 pS and 24-27 pS. All three subtypes of astrocytes exhibited a hyperpolarization-activated macroscopic inward K(+) current that was strongly rectifying and was abrogated by 1 mM intracellular Mg(2+) introduced during conventional but not perforated patch whole-cell recording. This Mg(2+)-sensitive current comprised the total inward rectifier current in polygonal reactive astrocytes, but only a fraction of the inward rectifier current in stellate reactive and stellate neonatal astrocytes. Because a strongly rectifying, inward rectifier K(+) channel with a single-channel conductance of 11-15 pS that is voltage independent is consistent with features of Kir2.3 (IRK3), we performed immunofluorescence experiments with anti-Kir2.3 and anti-glial fibrillary acidic protein antibodies. Both antibodies co-localized to all three subtypes of astrocytes in primary culture and to reactive astrocytes in situ within brain and gelatin sponge implants. Our data indicate that astrocytes of both polygonal and stellate morphology, from both adult and neonatal rat brain, express Kir2.3 both in vivo and in vitro. Constitutive expression of Kir2.3 regardless of cell morphology or age of origin of the source tissue suggests an important functional role for this channel in astrocytes.