Whole-cell chloride currents in rat astrocytes accompany changes in cell morphology

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Inhibitors of Calcium-Activated Anion Channels Modulate Hypnotic Ethanol Responses in Adult Sprague Dawley Rats

BACKGROUND

Ethanol is widely known for its depressant effects; however, the underlying neurobiological mechanisms are not clear. Calcium-activated anion channels (CAACs) contribute to extracellular chloride levels and thus may be involved in regulating inhibitory mechanisms within the central nervous system. Therefore, we hypothesized that CAACs influence ethanol behavioral sensitivity by altering CAAC expression.

METHODS

We assessed the role of CAACs in ethanol-induced loss of righting reflex (LORR) and locomotor activity using intracerebroventricular infusions of several nonselective CAAC blockers. CAAC expression was determined after ethanol exposure.

RESULTS

Ethanol-induced LORR (4.0 g/kg, intraperitoneally [i.p.]) was significantly attenuated by all 4 CAAC blockers. Blocking CAACs did not impact ethanol's low-dose (1.5 g/kg, i.p.) locomotor-impairing effects. Biochemical analysis of CAAC protein expression revealed that cortical Bestrophin1 (Best1) and Tweety1 levels were reduced as early as 30 minutes following a single ethanol injection (3.5 g/kg, intraperitoneally [i.p.]) and remained decreased 24 hours later in P2 fractions. Cortical Best1 levels were also reduced following 1.5 g/kg. However, CAAC expression was unaltered in the striatum following a single ethanol exposure. Ethanol did not affect Tweety2 levels in either brain region.

CONCLUSIONS

These results suggest that CAACs are a major target of ethanol in vivo, and the regulation of these channels contributes to select behavioral actions of ethanol.

Ion Homeostasis in Rhythmogenesis: The Interplay Between Neurons and Astroglia

Proper function of all excitable cells depends on ion homeostasis. Nowhere is this more critical than in the brain where the extracellular concentration of some ions determines neurons' firing pattern and ability to encode information. Several neuronal functions depend on the ability of neurons to change their firing pattern to a rhythmic bursting pattern, whereas, in some circuits, rhythmic firing is, on the contrary, associated to pathologies like epilepsy or Parkinson's disease. In this review, we focus on the four main ions known to fluctuate during rhythmic firing: calcium, potassium, sodium, and chloride. We discuss the synergistic interactions between these elements to promote an oscillatory activity. We also review evidence supporting an important role for astrocytes in the homeostasis of each of these ions and describe mechanisms by which astrocytes may regulate neuronal firing by altering their extracellular concentrations. A particular emphasis is put on the mechanisms underlying rhythmogenesis in the circuit forming the central pattern generator (CPG) for mastication and other CPG systems. Finally, we discuss how an impairment in the ability of glial cells to maintain such homeostasis may result in pathologies like epilepsy and Parkinson's disease.

Schwann cell response on polypyrrole substrates upon electrical stimulation

Current injury models suggest that Schwann cell (SC) migration and guidance are necessary for successful regeneration and synaptic reconnection after peripheral nerve injury. The ability of conducting polymers such as polypyrrole (PPy) to exhibit chemical, contact and electrical stimuli for cells has led to much interest in their use for neural conduits. Despite this interest, there has been very little research on the effect that electrical stimulation (ES) using PPy has on SC behavior. Here we investigate the mechanism by which SCs interact with PPy in the presence of an electric field. Additionally, we explored the effect that the adsorption of different serum proteins on PPy upon the application of an electric field has on SC migration. The results indicate an increase in average displacement of the SC with ES, resulting in a net anodic migration. Moreover, indirect effects of protein adsorption due to the oxidation of the film upon the application of ES were shown to have a larger effect on migration speed than on migration directionality. These results suggest that SC migration speed is governed by an integrin- or receptor-mediated mechanism, whereas SC migration directionality is governed by electrically mediated phenomena. These data will prove invaluable in optimizing conducting polymers for their different biomedical applications such as nerve repair.

Enteric neuropathy can be induced by high fat diet in vivo and palmitic acid exposure in vitro

Objective

Obese and/or diabetic patients have elevated levels of free fatty acids and increased susceptibility to gastrointestinal symptoms. Since the enteric nervous system is pivotal in regulating gastrointestinal functions alterations or neuropathy in the enteric neurons are suspected to occur in these conditions. Lipid induced intestinal changes, in particular on enteric neurons, were investigated in vitro and in vivo using primary cell culture and a high fat diet (HFD) mouse model.

Design

Mice were fed normal or HFD for 6 months. Intestines were analyzed for neuronal numbers, remodeling and lipid accumulation. Co-cultures of myenteric neurons, glia and muscle cells from rat small intestine, were treated with palmitic acid (PA) (0 – 10⁻³ M) and / or oleic acid (OA) (0 – 10⁻³ M), with or without modulators of intracellular lipid metabolism. Analyses were by immunocyto- and histochemistry.

Results

HFD caused substantial loss of myenteric neurons, leaving submucous neurons unaffected, and intramuscular lipid accumulation in ileum and colon. PA exposure in vitro resulted in neuronal shrinkage, chromatin condensation and a significant and concentration-dependent decrease in neuronal survival; OA exposure was neuroprotective. Carnitine palmitoyltransferase 1 inhibition, L-carnitine- or alpha lipoic acid supplementation all counteracted PA-induced neuronal loss. PA or OA alone both caused a significant and concentration-dependent loss of muscle cells in vitro. Simultaneous exposure of PA and OA promoted survival of muscle cells and increased intramuscular lipid droplet accumulation. PA exposure transformed glia from a stellate to a rounded phenotype but had no effect on their survival.

Conclusions

HFD and PA exposure are detrimental to myenteric neurons. Present results indicate excessive palmitoylcarnitine formation and exhausted L-carnitine stores leading to energy depletion, attenuated acetylcholine synthesis and oxidative stress to be main mechanisms behind PA-induced neuronal loss.High PA exposure is suggested to be a factor in causing diabetic neuropathy and gastrointestinal dysregulation.

Megalencephalic leukoencephalopathy with subcortical cysts: chronic white matter oedema due to a defect in brain ion and water homoeostasis

Megalencephalic leukoencephalopathy with subcortical cysts (MLC) is characterised by chronic white matter oedema. The disease has an infantile onset and leads to slow neurological deterioration in most cases, but, surprisingly, some patients recover. The first disease gene, MLC1, identified in 2001, is mutated in 75% of patients. At that time, nothing was known about MLC1 protein function and the pathophysiology of MLC. More recently, HEPACAM (also called GLIALCAM) has been identified as a second disease gene. GlialCAM serves as an escort for MLC1 and the chloride channel CLC2. The defect in MLC1 has been shown to hamper the cell volume regulation of astrocytes. One of the most important consequences involves the potassium siphoning process, which is essential in brain ion and water homoeostasis. An understanding of the mechanisms of white matter oedema in MLC is emerging. Further insight into the specific function of MLC1 is necessary to find treatment targets.

A silk platform that enables electrophysiology and targeted drug delivery in brain astroglial cells

Astroglial cell survival and ion channel activity are relevant molecular targets for the mechanistic study of neural cell interactions with biomaterials and/or electronic interfaces. Astrogliosis is the most typical reaction to in vivo brain implants and needs to be avoided by developing biomaterials that preserve astroglial cell physiological function. This cellular phenomenon is characterized by a proliferative state and altered expression of astroglial potassium (K(+)) channels. Silk is a natural polymer with potential for new biomedical applications due to its ability to support in vitro growth and differentiation of many cell types. We report on silk interactions with cultured neocortical astroglial cells. Astrocytes survival is similar when plated on silk-coated glass and on poly-D-lysine (PDL), a well known polyionic substrate used to promote astroglial cell adhesion to glass surfaces. Comparative analyses of whole-cell patch-clamp experiments reveal that silk- and PDL-coated cells display depolarized resting membrane potentials (-40 mV), very high input resistance, and low specific conductance, with values similar to those of undifferentiated glial cells. Analysis of K(+) channel conductance reveals that silk-astrocytes express large outwardly delayed rectifying K(+) current (K(DR)). The magnitude of K(DR) in PDL- and silk-coated astrocytes is similar, indicating that silk does not alter the resting K(+) current. We also demonstrate that guanosine- (GUO) embedded silk enables the direct modulation of astroglial K(+) conductance in vitro. Astrocytes plated on GUO-embedded silk are more hyperpolarized and express inward rectifying K(+) conductance (K(ir)). The K(+) inward current increases and this is paralleled by upregulation and membrane polarization of K(ir)4.1 protein signal. Collectively these results indicate that silk is a suitable biomaterial platform for the in vitro studies of astroglial ion channel responses and related physiology.

Water transport between CNS compartments: functional and molecular interactions between aquaporins and ion channels

The physiological ability of the mammalian CNS to integrate peripheral stimuli and to convey information to the body is tightly regulated by its capacity to preserve the ion composition and volume of the perineuronal milieu. It is well known that astroglial syncytium plays a crucial role in such process by controlling the homeostasis of ions and water through the selective transmembrane movement of inorganic and organic molecules and the equilibration of osmotic gradients. Astrocytes, in fact, by contacting neurons and cells lining the fluid-filled compartments, are in a strategic position to fulfill this role. They are endowed with ion and water channel proteins that are localized in specific plasma membrane domains facing diverse liquid spaces. Recent data in rodents have demonstrated that the precise dynamics of the astroglia-mediated homeostatic regulation of the CNS is dependent on the interactions between water channels and ion channels, and their anchoring with proteins that allow the formation of macromolecular complexes in specific cellular domains. Interplay can occur with or without direct molecular interactions suggesting the existence of different regulatory mechanisms. The importance of molecular and functional interactions is pinpointed by the numerous observations that as consequence of pathological insults leading to the derangement of ion and volume homeostasis the cell surface expression and/or polarized localization of these proteins is perturbed. Here, we critically discuss the experimental evidence concerning: (1) molecular and functional interplay of aquaporin 4, the major aquaporin protein in astroglial cells, with potassium and gap-junctional channels that are involved in extracellular potassium buffering. (2) the interactions of aquaporin 4 with chloride and calcium channels regulating cell volume homeostasis. The relevance of the crosstalk between water channels and ion channels in the pathogenesis of astroglia-related acute and chronic diseases of the CNS is also briefly discussed.

Microglia processes block the spread of damage in the brain and require functional chloride channels

Microglia cells exhibit two forms of motility, constant movement of filopodia probing surrounding brain tissue, and outgrowth of larger processes in response to nearby damage. The mechanisms and functions of filopodia sensing and process outgrowth are not well characterized but are likely critical for normal immune function in the brain. Using two photon laser scanning microscopy we investigated microglia process outgrowth in response to damage, and explored the relationship between process outgrowth and filopodia movement. Further, we examined the roles of Cl(-) or K(+) channel activation, as well as actin polymerization in these two distinct processes, because mechanistic understanding could provide a strategy to modulate microglia function. We found that volume sensitive Cl(-) channel blockers (NPPB, tamoxifen, DIDS) prevented the rapid process outgrowth of microglia observed in response to damage. In contrast, filopodia extension during sensing was resistant to Cl(-) channel inhibitors, indicating that these motile processes have different cellular mechanisms. However, both filopodia sensing and rapid process outgrowth were blocked by inhibition of actin polymerization. Following lesion formation under control conditions, rapidly outgrowing processes contacted the damaged area and this was associated with a 37% decrease in lesion volume. Inhibition of process outgrowth by Cl(-) channel block, prevention of actin polymerization, or by selectively ablating microglia all allowed lesion volume to increase and spread into the surrounding tissue. Therefore, process outgrowth in response to focal brain damage is beneficial by preventing lesion expansion and suggests microglia represent a front line defence against damage in the brain.

Voltage-dependent ionic conductances in the human malignant astrocytoma cell line U87-MG

Although the human malignant astrocytoma cell line U87-MG has been used in numerous studies, few findings are available on the properties of its membrane ion conductances. Characterization of the ion channels expressed in these cells will make it possible to study membrane ion conductance changes when a receptor is activated by its ligand. This will help to elucidate the functional properties of these receptors and their signal-transduction pathways in pathophysiological events. This work studied the voltage-dependent ionic conductances of U87-MG cells using the Whole-Cell Recording patch-clamp technique. Six types of voltage-dependent ionic currents were identified: (i) a TEA-, 4-AP- and CTX-sensitive Ca2+-dependent K+ current, (ii) a transient K+ current inhibited by 4-AP, (iii) an inwardly rectifying K+ current blocked by Ba2+ and 4-AP, (iv) a DIDS- and SITS-sensitive Cl- current, (v) a TTX-sensitive Na+ conductance and (vi) a L-type Ca2+ conductance activated by BayK-8644 and inhibited by Ni and the L-type Ca2+ channel inhibitor, nifedipine. In addition, electrical depolarizations elicited inward currents due to voltage-independent, Ca2+-dependent K+ influx against the electrochemical gradient, probably via an ouabain-sensitive Na+-K+ pump.

Possible participation of chloride ion channels in ATP release from cancer cells in suspension

1. Cancer cells must detach from the primary focus to initiate the process of metastasis. Previously, we demonstrated that intracellular Ca(2+) levels are increased in endothelial cells in the presence of cancer cells and that ATP derived from these cells causes this increase. The present study clarifies the mechanism of ATP release from cancer cells by investigating the effects of Cl(-) channel inhibitors and other drugs on ATP release from human fibrosarcoma cells (HT-1080 cells). 2. Levels of extracellular ATP and its metabolites were measured using high-performance liquid chromatography (HPLC) with fluorescent detection. 3. Significantly more extracellular ATP was released by suspended than by adherent HT-1080 cells. The Cl(-) channel inhibitors 5-nitro-2-(3-phenylpropylamino) benzoic acid (100 micromol/L), gadolinium (100 micromol/L) and niflumic acid (100 micromol/L) all significantly inhibited ATP release from HT-1080 cells (1 x 10(3) /mL) to 39.7 +/- 6.5, 28.5 +/- 2.5 and 82.5 +/- 4.1% of control, respectively. 4. Neither of the p-glycoprotein inhibitors (i.e. 50 micromol/L quinidine and 90 micromol/L verapamil) had any effect on ATP release from HT-1080 cells. The gap junction hemichannel inhibitor Gap26 (300 micromol/L) slightly, but significantly, decreased ATP release by approximately 20%. The gap junction inhibitor 18-alpha-glycyrrhetinic acid (10 micromol/L) tended to inhibit ATP release from HT-1080 cells, but the difference did not reach statistical significance. 5. These findings indicate that Cl(-) channels play the most important role in ATP release from detached cancer cells and that gap junction hemichannels are also associated with ATP release.

Effect of P2 receptor on the intracellular calcium increase by cancer cells in human umbilical vein endothelial cells

One of the important functions of vascular endothelial cells is as a barrier between blood and vascular tissue. This led us to speculate that cancer cells affect endothelial cells during metastasis. In the present study, we investigated the influence of human fibrosarcoma cells (HT-1080) on human umbilical vein endothelial cells (HUVEC), particularly intracellular calcium ion levels ([Ca2+]i), which are known to be an important intracellular signal transduction factor. HUVEC were treated with a fluorescent marker, and the fluorescence intensity of [Ca2+]i was then measured by phase contrast microscopic imaging. Extracellular adenosine triphosphate (ATP) release was measured using the chemiluminescence of luciferin-luciferase and a photon counting imaging system. HT-1080 (5x10(4) cells per dish) was found to increase [Ca2+]i in HUVEC. This [Ca2+]i rise was significantly reduced by U-73122 (phospholipase C inhibitor, 1 microM) and thapsigargin (calcium pump inhibitor, 1 microM). Interestingly, the [Ca2+]i rise in HUVEC was also significantly reduced by pyridoxalphosphare-6-azophenyl-2', 4'-disulfonic acid, a P2Y receptor antagonist (100 microM) and apyrase, a nucleotidase inhibitor (2 U/ml). In addition, we observed ATP release from HT-1080. These results suggest that [Ca2+]i in HUVEC was increased through the phospholipase C-IP3 pathway via ATP release from cancer cells. We previously reported that extracellular ATP increased [Ca2+]i and enhanced macromolecular permeability via the P2Y receptor. In tumor metastasis, cancer cells may exploit these regulatory mechanisms in the endothelial cell layer.

Functional down-regulation of volume-regulated anion channels in AQP4 knockdown cultured rat cortical astrocytes

In the brain, the astroglial syncytium is crucially involved in the regulation of water homeostasis. Accumulating evidence indicates that a dysregulation of the astrocytic processes controlling water homeostasis has a pathogenetic role in several brain injuries. Here, we have analysed by RNA interference technology the functional interactions occurring between the most abundant water channel in the brain, aquaporin-4 (AQP4), and the swelling-activated Cl(-) current expressed by cultured rat cortical astrocytes. We show that in primary cultured rat cortical astrocytes transfected with control small interfering RNA (siRNA), hypotonic shock promotes an increase in cellular volume accompanied by augmented membrane conductance mediated by volume-regulated anion channels (VRAC). Conversely, astroglia in which AQP4 was knocked down (AQP4 KD) by transfection with AQP4 siRNA changed their morphology from polygonal to process-bearing, and displayed normal cell swelling but reduced VRAC activity. Pharmacological manipulations of actin cytoskeleton in rat astrocytes, and functional analysis in mouse astroglial cells, which retain their morphology upon knockdown of AQP4, suggest that stellation of AQP4 KD rat cortical astrocytes was not causally linked to reduction of VRAC current. Molecular analysis of possible candidates of swelling-activated Cl(-) current provided evidence that in AQP4 KD astrocytes, there was a down-regulation of chloride channel-2 (CIC-2), which, however, was not involved in VRAC conductance. Inclusion of ATP in the intracellular saline restored VRAC activity upon hypotonicity. Collectively, these results support the view that in cultured astroglial cells, plasma membrane proteins involved in cell volume homeostasis are assembled in a functional platform.

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.

Roles of two types of anion channels in glutamate release from mouse astrocytes under ischemic or osmotic stress

Astrocytes release glutamate upon hyperexcitation in the normal brain, and in response to pathologic insults such as ischemia and trauma. In our experiments, both hypotonic and ischemic stimuli caused the release of glutamate from cultured mouse astrocytes, which occurred with little or no contribution of gap junction hemichannels, vesicle-mediated exocytosis, or reversed operation of the Na-dependent glutamate transporter. Cell swelling and chemical ischemia activated, in cell-attached membrane patches, anionic channels with large unitary conductance (approximately 400 pS) and inactivation kinetics at potentials more positive than +20 mV or more negative than -20 mV. These properties are different from those of volume-sensitive outwardly rectifying (VSOR) Cl- channels, which were also expressed in these cells and exhibited intermediate unitary conductance (approximately 80 pS) and inactivation kinetics at large positive potentials of more than +40 mV. Both maxi-anion channels and VSOR Cl- channels were permeable to glutamate with permeability ratios of glutamate to chloride of 0.21 +/- 0.07 and 0.15 +/- 0.01, respectively. However, the release of glutamate was significantly more sensitive to Gd3+, a blocker of maxi-anion channels, than to phloretin, a blocker of VSOR Cl- channels. We conclude that these two channels jointly represent a major conductive pathway for the release of glutamate from swollen and ischemia-challenged astrocytes, with the contribution of maxi-anion channels being predominant.

Cloning and expression of Ca2+-activated chloride channel from rat brain

To clone the gene product responsible for the calcium-activated chloride channel (CLCA) in rat brain cerebrum, we performed a reverse transcription-PCR (RT-PCR) with gene-specific primers of a rat EST clone. We successfully cloned a rat brain CLCA (rbCLCA). The full-length cDNA is 2895 bp long and codes for a 902 amino acid protein. The clone consists of four transmembrane domains and shows a 79.1% of significant homology with previously reported mouse smooth muscle chloride channel sequence. We also performed RT-PCR using single neuron and glia, and various tissues to determine the tissue expression of rbCLCA. We found that rbCLCA was expressed in both neuron and glia. In peripheral organs, rbCLCA showed the predominant expressions in cerebrum, cerebellum, kidney, small intestine, and stomach but not in heart, large intestine, liver, lung, and spleen. Whole-cell patch clamp studies in HEK293 cells transfected with the clone identified a niflumic acid (a CLCA channel blocker)-sensitive and voltage-dependent chloride current but we could not observe this chloride current in mock-transfected cells. The identification of genes belonging to the CLCA family from rat brain and its functional expression will help to evaluate its physiological role in brain as anion channel.

Analysis of K+ accumulation reveals privileged extracellular region in the vicinity of glial cells in situ

Astrocytes and oligodendrocytes in rat and mouse spinal cord slices, characterized by passive membrane currents during de- and hyperpolarizing stimulation pulses, express a high resting K+ conductance. In contrast to the case for astrocytes, a depolarizing prepulse in oligodendrocytes produces a significant shift of reversal potential (Vrev) to positive values, arising from the larger accumulation of K+ in the vicinity of the oligodendrocyte membrane. As a result, oligodendrocytes express large tail currents (Itail) after a depolarizing prepulse due to the shift of K+ into the cell. In the present study, we used a mathematical model to calculate the volume of the extracellular space (ECS) in the vicinity of astrocytes and oligodendrocytes (ESVv), defined as the volume available for K+ accumulation during membrane depolarization. A mathematical analysis of membrane currents revealed no differences between glial cells from mouse (n = 59) or rat (n = 60) spinal cord slices. We found that the Vrev of a cell after a depolarizing pulse increases with increasing Itail, expressed as the ratio of the integral inward current (Qin) after the depolarizing pulse to the total integral outward current (Qout) during the pulse. In astrocytes with small Itail and Vrev ranging from -50 to -70 mV, the Qin was only 3-19% of Qout, whereas, in oligodendrocytes with large Itail and Vrev between -20 and 0 mV, Qin/Qout was 30-75%. On the other hand, ESVv decreased with increasing values of Vrev. In astrocytes, ESVv ranged from 2 to 50 microm3, and, in oligodendrocytes, it ranged from 0.1 to 2.0 microm3. Cell swelling evoked by the application of hypotonic solution shifted Vrev to more positive values by 17.2 +/- 1.8 mV and was accompanied by a decrease in ESVv of 3.6 +/- 1.3 microm3. Our mathematical analysis reveals a 10-100 times smaller region of the extracellular space available for K+ accumulation during cell depolarization in the vicinity of oligodendrocytes than in the vicinity of astrocytes. The presence of such privileged regions around cells in the CNS may affect the accumulation and diffusion of other neuroactive substances and alter communication between cells in the CNS.

Biophysical and pharmacological characterization of hypotonically activated chloride currents in cortical astrocytes

Rat cortical astrocytes regulate their cell volume in response to hypotonic challenge. This regulation is believed to depend largely on the release of chloride or organic osmolytes through anion channels. Using whole-cell recordings, we identified weakly outwardly rectifying chloride currents that could be activated in response to hypotonic challenge. These currents exhibited the following permeability sequence upon replacement of chloride in the bathing solution with various anions: I- > NO3- > Cl- > Gluc- > or = MeS- > Ise-. Interestingly, extracellular I-, albeit showing the greatest permeability, blocked the currents with an IC50 of approximately 50 mM. Currents were almost completely inhibited by 123 microM NPPB and partially inhibited by 200 microM niflumic acid or 200 microM DIDS. Additionally, the total number of Cl- ions effluxed through the hypotonically activated channels was markedly similar to the total solute efflux during volume regulation. We therefore propose the hypotonically activated chloride channel as a major contributor to volume regulation of astrocytes. To examine potential candidate chloride channel genes expressed by astrocytes, we employed RT-PCR to demonstrate the presence of transcripts for ClC-2, 3, 4, 5, and 7, as well as for VDAC and CFTR in cultured astrocytes. Moreover, we performed immunostaining with antibodies against each of these channels and showed the strongest expression of ClC-2 and ClC-3, strong expression of ClC-5 and VDAC, weak expression of ClC-7 and very weak expression of ClC-4 and CFTR. Intriguingly, although we found at least seven Cl- channel proteins from three different gene families in astrocytes, none appeared to be active in resting cells.

Structure and pharmacology of swelling-sensitive chloride channels, I(Cl,swell)

Since several years, the interest for chloride channels and more particularly for the enigmatic swelling-activated chloride channel (I(Cl,swell)) is increasing. Despite its well-characterized electrophysiological properties, the I(Cl,swell) structure and pharmacology are not totally elucidated. These channels are involved in a variety of cell functions, such as cardiac rhythm, cell proliferation and differentiation, cell volume regulation and cell death through apoptosis. This review will consider different aspects regarding structure, electrophysiological properties, pharmacology, modulation and functions of these swelling-activated chloride channels.

Astrocytes from mouse brain slices express ClC-2-mediated Cl- currents regulated during development and after injury

Chloride channels are important for astrocytic volume regulation and K+ buffering. We demonstrate functional expression of a hyperpolarization-activated Cl- current in a subpopulation of astrocytes in acute slices or after fresh isolation from adult brain of GFAP/EGFP transgenic animals in which astrocytes are selectively labeled. When Na+ and K+ were substituted with NMDG+ and Cs+ in extra- and intracellular solutions, an inward current was observed at negative membrane potentials. The current displayed features as described for a Cl- current characterized in cultured astrocytes: it activated time dependently at potentials negative to -40 mV, displayed no inactivation within 1 s, and was inhibited reversibly by submicromolar concentrations of Cd2+. The current was not detectable in astrocytes from ClC-2 knockout mice, indicating that the ClC-2 chloride channel generated the conductance. Current density was significantly lower in a corresponding population of astrocytes isolated from immature brain and in reactive astrocytes within a lesion site.

Expression of voltage-gated chloride channels in human glioma cells

Voltage-gated chloride channels have recently been implicated as being important for cell proliferation and invasive cell migration of primary brain tumors cells. In the present study we provide several lines of evidence that glioma Cl- currents are primarily mediated by ClC-2 and ClC-3, two genes that belong to the ClC superfamily. Transcripts for ClC-2 thru ClC-7 were detected in a human glioma cell line by PCR, whereas only ClC-2, ClC-3, and ClC-5 protein could be identified by Western blot. Prominent ClC-2, -3, and -5 channel expression was also detected in acute patient biopsies from low- and high-grade malignant gliomas. Immunogold electron microscopic studies as well as digital confocal imaging localized a portion of these ClC channels to the plasma membrane. Whole-cell patch-clamp recordings show the presence of two pharmacologically and biophysically distinct Cl- currents that could be specifically reduced by 48 hr exposure of cells to channel-specific antisense oligonucleotides. ClC-3 antisense selectively and significantly reduced the expression of outwardly rectifying current with pronounced voltage-dependent inactivation. Such currents were sensitive to DIDS (200-500 microm) and 5-nitro-2-(3-phenylpropylamino) benzoic acid (165 microm). ClC-2 antisense significantly reduced expression of inwardly rectifying currents, which were potentiated by hyperpolarizing prepulses and inhibited by Cd2+ (200-500 microm). Currents that were mediated by ClC-5 could not be demonstrated. We suggest that ClC-2 and ClC-3 channels are specifically upregulated in glioma membranes and endow glioma cells with an enhanced ability to transport Cl-. This may in turn facilitate rapid changes in cell size and shape as cells divide or invade through tortuous extracellular brain spaces.

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.

ATP released from astrocytes during swelling activates chloride channels

ATP release from astrocytes contributes to calcium ([Ca(2+)]) wave propagation and may modulate neuronal excitability. In epithelial cells and hepatocytes, cell swelling causes ATP release, which leads to the activation of a volume-sensitive Cl(-) current (I(Cl,swell)) through an autocrine pathway involving purinergic receptors. Astrocyte swelling is counterbalanced by a regulatory volume decrease, involving efflux of metabolites and activation of I(Cl,swell) and K(+) currents. We used whole cell patch-clamp recordings in cultured astrocytes to investigate the autocrine role of ATP in the activation of I(Cl,swell) by hypo-osmotic solution (HOS). Apyrase, an ATP/ADP nucleotidase, inhibited HOS-activated I(Cl,swell), whereas ATP and the P2Y agonists, ADPbetaS and ADP, induced Cl(-) currents similar to I(Cl,swell). Neither the P2U agonist, UTP nor the P2X agonist, alpha,beta-methylene ATP, were effective. BzATP was less effective than ATP, suggesting that P2X7 receptors were not involved. P2 purinergic antagonists, suramin, RB2, and pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) reversibly inhibited activation of I(Cl,swell), suggesting that ATP-activated P2Y1 receptors. Thus ATP release mediates I(Cl,swell) in astrocytes through the activation of P2Y1-like receptors. The multidrug resistance protein (MRP) transport inhibitors probenicid, indomethacin, and MK-571 all potently inhibited I(Cl.swell). ATP release from astrocytes in HOS was observed directly using luciferin-luciferase and MK-571 reversibly depressed this HOS-induced ATP efflux. We conclude that ATP release via MRP and subsequent autocrine activation of purinergic receptors contributes to the activation of I(Cl,swell) in astrocytes by HOS-induced swelling.

Glial neuronal signaling in the central nervous system

Glial cells are known to interact extensively with neuronal elements in the brain, influencing their activity. Astrocytes associated with synapses integrate neuronal inputs and release transmitters that modulate synaptic sensitivity. Glial cells participate in formation and rebuilding of synapses and play a prominent role in protection and repair of nervous tissue after damage. For glial cells to take an active part in plastic alterations under physiological conditions and pathological disturbances, extensive specific signaling, both within single cells and between cells, is required. In recent years, intensive research has led to our first insight into this signaling. We know there are active connections between astrocytes in the form of networks promoting Ca2+ and ATP signaling; we also know there is intense signaling between astrocytes, microglia, oligodendrocytes, and neurons, with an array of molecules acting as signaling substances. The cells must be functionally integrated to facilitate the enormous dynamics of and capacity for reconstruction within the nervous system. In this paper, we summarize some basic data on glial neuronal signaling to provide insight into synaptic modulation and reconstruction in physiology and protection and repair after damage.

Chloride/anion channels in glial cell membranes

At least seven different chloride/anion currents have now been identified in astrocytes, oligodendrocytes/Schwann cells, and microglia. Only for two of these currents is the corresponding gene known. One of these genes is not encoding for a chloride channel, but for a class of mitochondria-like pores also found in cell membranes. Astrocytes and oligodendrocytes differ in their resting properties: astrocytes accumulate chloride but do not have a significant permeability. Oligodendrocytes have a close to passive distribution and a significant permeability. Under certain circumstances, astrocytes can express a resting chloride conductance. Reactive and neoplastic astrocytes as well as astrocytes with an altered shape exhibit a resting conductance. The function of these channels certainly involves volume regulation. Other possible functions are potassium homeostasis, migration, proliferation (in microglia), and involvement in spreading depression waves. Of greatest interest are two phenomena discovered in situ: The ClC-2 channel is only found in astrocytic endfeet near blood capillaries adjacent to neuronal GABA(A) receptors. In the supraoptic nucleus of the hypothalamus, there is an osmosensitive astrocytic taurine release. This released taurine interacts with glycine receptors in neighboring neurons, causing inhibition. It is assumed that with the future availability of more in situ, rather than in vitro, studies, an increased number of such complex interactions between glial cells, neurons, and blood vessels will be discovered.

Calcium/calmodulin-modulated chloride and taurine conductances in cultured rat astrocytes

Osmotically swollen rat cerebral astrocytes develop an increased anion conductance which can mediate chloride and taurine release. We used whole cell patch clamp to study mechanisms that modulate this conductance. Astrocyte chloride conductance increased within 4 min of exposure to 200 mOsm medium and was 670+/-123% of its initial value after 15 min (mean+/-S.E.M.). This conductance was substantially reduced in 0.1 mM extracellular calcium with 20 mM BAPTA added to the electrode solution and was completely inhibited with calcium-free perfusion solution containing 1 mM EDTA (n=4). The conductance increase in 200 mOsm medium also was inhibited in a dose-dependent manner by nimodipine with a calculated K(i) of 0.31+/-0.4 microM and mean+/-S.E.M. inhibition of 84.4+/-4% at 100 microM nimodipine. In the presence of 100 microM W-7, a calmodulin antagonist, the mean+/-S.E.M. conductance increase after 15 min was 223+/-40% of the initial value while 300 microM W-7 or 100 microM trifluoperazine inhibited the conductance increase completely (n=6). With taurine as the major anion in electrode and perfusion solutions, a significant conductance increase was observed in 200 mOsm medium. This conductance increase was inhibited by 300 microM W-7 or 100 microM nimodipine. We conclude extracellular calcium influx via L-type calcium channels leads to increased astrocyte anion conductance in 200 mOsm conditions via calmodulin-dependent activation of anion channels. Efflux of anionic taurine from swollen astrocytes also may be affected by calcium influx through a similar calcium/calmodulin-dependent process.

Volume-activated chloride currents contribute to the resting conductance and invasive migration of human glioma cells

We used an in vitro model for glioma cell invasion (transwell migration assay) and patch-clamp techniques to investigate the role of volume-activated Cl(-) currents (I(Cl,Vol)) in glioma cell invasion. Hypotonic solutions ( approximately 230 mOsm) activated outwardly rectifying currents that reversed near the equilibrium potential for Cl(-) ions (E(Cl)). These currents (I(Cl,Vol)) were sensitive to several known Cl(-) channel inhibitors, including DIDS, tamoxifen, and 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB). The IC(50) for NPPB inhibition of I(Cl,Vol) was 21 microm. Under isotonic conditions, NPPB (165 microm) blocked inward currents (at -40 mV) and increased input resistance in both standard whole-cell recordings and amphotericin perforated-patch recordings. Reducing [Cl(-)](o) under isotonic conditions positively shifted the reversal potential of whole-cell currents. These findings suggest a significant resting Cl(-) conductance in glioma cells. Under isotonic and hypotonic conditions, Cl(-) channels displayed voltage- and time-dependent inactivation and had an I(-) > Cl(-) permeability. To assess the potential role of these channels in cell migration, we studied the chemotactic migration of glioma cells toward laminin or vitronectin in a Boyden chamber containing transwell filters with 8 microm pores. Inhibition of I(Cl,Vol) with NPPB reduced chemotactic migration in a dose-dependent fashion with an IC(50) of 27 microm. Time-lapse video microscopy during patch-clamp recordings revealed visible changes in cell shape and/or movement that accompanied spontaneous activation of I(Cl,Vol), suggesting that I(Cl,Vol) is activated during cell movement, consistent with the effects of NPPB in migration assays. We propose that I(Cl,Vol) contributes to cell shape and volume changes required for glioma cell migration through brain tissue.

Effect of elevated K(+), hypotonic stress, and cortical spreading depression on astrocyte swelling in GFAP-deficient mice

Glial fibrillary acidic protein (GFAP) is the main component of intermediate filaments in astrocytes. To assess its function in astrocyte swelling, we compared astrocyte membrane properties and swelling in spinal cord slices of 8- to 10-day-old wild-type control (GFAP(+/+)) and GFAP-knockout (GFAP(-/-)) mice. Membrane currents and K(+) accumulation around astrocytes after a depolarizing pulse were studied using the whole-cell patch-clamp technique. In vivo cell swelling was studied in the cortex during spreading depression (SD) in 3 to 6-month-old animals. Swelling-induced changes of the extracellular space (ECS) diffusion parameters, i.e., volume fraction alpha and tortuosity lambda, were studied by the real-time iontophoretic tetramethylammonium (TMA(+)) method using TMA(+)-selective microelectrodes. Morphological analysis using confocal microscopy and quantification of xy intensity profiles in a confocal plane revealed a lower density of processes in GFAP(-/-) astrocytes than in GFAP(+/+) astrocytes. K(+) accumulation evoked by membrane depolarization was lower in the vicinity of GFAP(-/-) astrocytes than GFAP(+/+) astrocytes, suggesting the presence of a larger ECS around GFAP(-/-) astrocytes. Astrocyte swelling evoked by application of 50 mM K(+) or by hypotonic solution (HS) produced a larger increase in [K(+)](e) around GFAP(+/+) astrocytes than around GFAP(-/-) astrocytes. No differences in alpha and lambda in the spinal cord or cortex of GFAP(+/+) and GFAP(-/-) mice were found; however, the application of either 50 mM K(+) or HS in spinal cord, or SD in cortex, evoked a large decrease in alpha and an increase in lambda in GFAP(+/+) mice only. Slower swelling in GFAP(-/-) astrocytes indicates that GFAP and intermediate filaments play an important role in cell swelling during pathological states.

Mechanical injury alters volume activated ion channels in cortical astrocytes

Although astrocytic swelling is likely to mediate brain edema and high ICP after traumatic brain injury, the mechanism is not understood. We employed whole cell patch clamp electrophysiology and a stretch injury model to understand whether volume regulating ion currents are altered following cell injury. Mixed rat astrocytes and neurons were co-cultured on deformable silastic membranes. Mild-moderate cell injury was produced using a timed pulse of pressurized air to deform the silastic substrates by 6.5 mm. Then, ion currents were recorded with patch clamp methods. Cells were held at -65 mV and were stepped to +10 mV to monitor current changes.

RESULTS

In unstretched astrocytes, small amplitude currents were obtained under isotonic conditions. Hypotonic solution activated an outwardly-rectifying current which reversed near -40 mV. This current resembled a previously reported anion current whose activation may restore cell volume by mediating a net solute efflux. In contrast, stretch injured cells exhibited a large amplitude, nonrectifying current. This current was not due to non-specific ionic leakage, since it was fully suppressed by the cation channel blocker gadolinium. Activation of novel stretch-activated cation currents may exacerbate cell swelling in injured astrocytes. Stretch injured astrocytes thus express a dysfunctional cation current as opposed to an osmoregulatory anion current. This mechanism, if present in vivo, may contribute to the cytotoxic swelling seen after traumatic brain injury.

Induced membrane hypo/hyper-mechanosensitivity: a limitation of patch-clamp recording

Practical limitations of the patch-clamp technique when recording mechanogated membrane ion channels are considered. Mechanical overstimulation of the patch or the cell from excessive suction/pressure protocols induces morphological and functional changes. In particular, the plasma membrane becomes decoupled from the underlying cytoskeleton to form either membrane blebs (cell-attached) or ghosts (whole cell). As a consequence, a membrane ion channel may show either a decrease or an increase in its native mechanosensitivity or even acquire mechanosensitivity. The effect varies with ion channel and cell type and presumably arises because of a disruption of membrane-cytoskeleton interactions. We consider that such disruptions are a pathological consequence of excessive mechanical stress, either during or after seal formation, rather than an immutable consequence of patch-clamp recording. By careful attention to the suction/pressure protocols during sealing and throughout recording, such artifacts can be avoided.

Osmosensitivity of an inwardly rectifying chloride current revealed by whole-cell and perforated-patch recordings in cultured rat cortical astrocytes

The osmosensitivity of the inwardly rectifying Cl(-) current (I(Clh)), expressed by primary cultured rat neocortical astrocytes long-term treated with dibutyryl cyclic AMP, was investigated in the whole-cell and perforated-patch modes. In whole-cell experiments, whereas hypotonic extracellular solution (Delta=100 mOsmol) did not cause any change in I(Clh), hypertonicity produced a slowly developing, approximately 40% reversible decrease in current magnitude. By contrast, in perforated-patch experiments, exposure to a less hypertonic saline (Delta=50 mOsmol) depressed the current to approximately 50%, and hypotonicity induced a approximately 50% slow increase in I(Clh). These differences in osmosensitivity between the two experimental modes suggest that the osmoregulation of I(Clh) may be mediated by complex intracellular mechanism(s), which appear(s) to be partly compromised by the dialysis of the astrocytic cytoplasm.

Distribution of chloride channel-2-immunoreactive neuronal and astrocytic processes in the hippocampus

The chloride homeostasis of neurons and non-neuronal cells is maintained in part by a voltage-sensitive inwardly rectifying chloride conductance through the chloride channel-2. This channel is activated by hyperpolarization and extracellular hypotonicity. In the present study, hippocampal sections were immunostained for chloride channel-2, and somata and dendrites of both pyramidal and non-pyramidal cells were found to be immunoreactive. In addition, glial processes in the vicinity of small blood vessels were also immunostained, whereas the neuropil of strata pyramidale and lacunosum-moleculare contained chloride channel-2-positive punctate structures. Electron microscopy and double immunostaining using antibodies against chloride channel-2 and glial fibrillary acidic protein confirmed that the dense network of chloride channel-2-positive processes corresponds to the end feet of astrocytes. The distribution of chloride channel-2-immunoreactive astrocytes was inhomogeneous throughout the hippocampus: strata oriens, pyramidale and lacunosum-moleculare of CA1-CA3 and the outer molecular layer of the dentate gyrus contained the majority of immunoreactive end feet, whereas the other layers showed sparse labeling. Subcellular studies demonstrated that, in addition to astrocytes, chloride channel-2 was localized in the membrane of dendrites, dendritic spines, cell bodies and axon initial segments of neurons, frequently close to, or within active zones of, symmetrical synapses.Thus, chloride channel-2 appears to be involved in transmembrane chloride movements associated with GABAergic synaptic transmission. The specific laminar distribution of chloride channel-2-positive astroglial processes coinciding with that of GABAergic axon terminals suggests that the network of astrocytes may be able to siphon and deliver Cl(-) ions to layers with intense GABAergic transmission, thereby increasing the efficacy of GABA(A) receptor-mediated inhibition.

pH modulation of an inward rectifier chloride current in cultured rat cortical astrocytes

The effects of changes in extra- and intracellular pH in the pathophysiological range (6.0-8.0) on astroglial plasma membrane ionic currents were investigated with the whole-cell patch-clamp technique. In cultured rat neocortical type-1 astrocytes differentiated by a long-term treatment with dibutyryl cyclic-AMP, exposure to an extracellular pH of 6.4 induced, as compared with the control extracellular pH at 7.3, a sustained and reversible increase in the holding current at -60mV. The rise in current was accompanied by a decrease in the apparent input resistance. Ion substitution experiments indicated that extracellular pH 6.4 upregulated the resting Cl(-) conductance, whereas an opposite effect could be observed at extracellular pH 8.0. Recordings of isolated Cl(-) currents showed that this modulation occurred on the previously identified hyperpolarization-activated, inwardly rectifying Cl(-) current, I(Clh). Extracellular acidification to pH 6.4 shifted the voltage dependence of I(Clh) activation by approximately 20mV towards more positive potentials, whereas a approximately 20mV opposite shift was observed upon exposure to extracellular pH 8.0. These effects were paralleled by an increase (extracellular pH 6.4) or decrease (extracellular pH 8.0) in the maximal conductance. Decreasing (6.0) or increasing (8.0) the intracellular pH shifted the steady-state activation of I(Clh) towards more negative or positive potentials, respectively, leaving unchanged the current sensitivity to extracellular pH modifications. The modulation of the inward rectifier Cl(-) current expressed by differentiated cultured neocortical astrocytes indicates that extra- and intracellular changes in pH occurring in a pathophysiological range may contribute to regulating Cl(-) accumulation in astroglial cells.

Astrocytic clasmatodendrosis in hippocampal organ culture

Mechanisms by which astrocytes are irreversibly injured from ischemic brain injury remain incompletely defined. More than 90 years ago Alzheimer showed that astrocytes lose their distal processes (i.e., undergo "clasmatodendrosis") when irreversibly injured by a reduction in blood flow, a process shown by Friede and van Houten (1961) to be due to energy failure and acidosis. Such alterations in astrocytic morphology can relate directly to changes in cell function. However, astrocytic clasmatodendrosis has largely been lost to the modern literature, perhaps because of a inability to study it under controlled conditions. In the present study, novel four-dimensional (4D)and digital deblurring imaging of glial fibrillary acidic protein (GFAP) immunostaining changes in hippocampal organ cultures (HOTCs) were used to establish an in vitro model of astrocytic clasmatodendrosis. Also, astrocytes in primary culture were transfected with green fluorescent protein (GFP) to show the occurrence of clasmatodendrosis via a parallel and separate means. In HOTCs, a significant reduction in astrocytic process length occurred 15 min (and remained for 60 min) after exposure to acidic Ringer's and mitochondrial inhibition in the pyramidal cell body layer. Time-lapsed images of primary cultures showed thinning of cell processes within 15 min of exposure to acidic Ringer's and mitochondrial inhibition. Distal processes subsequently broke away but retained their fluorescence for minutes before disintegrating along with their parent cell bodies. This report shows the spatiotemporal occurrence of clasmatodendrosis in astrocytes of HOTCs closely parallels that seen in vivo. Thus, HOTCs, where microenvironmental conditions can be controlled and single, identified cells can be followed in space and time, can be applied to study the interrelations between energy metabolism and pH that result in clasmatodendrosis.

Ca2+-sensing receptor-mediated regulation of volume-sensitive Cl- channels in human epithelial cells

Since extracellular Ca2+ or Mg2+ has been reported to modulate swelling-activated Cl- currents, we examined the expression of the G protein-coupled Ca2+-sensing receptor (CaR) and its involvement in the regulation of volume-sensitive Cl- channels in a human epithelial cell line (Intestine 407). Reverse transcriptase-polymerase chain reaction and immunoblotting analysis showed that Intestine 407 cells express CaR mRNA and protein. The swelling-activated whole-cell Cl- current was voltage-independently augmented by extracellular Ca2+ or Mg2+. In addition, Ca2+ or Mg2+ voltage-dependently accelerated the inactivation kinetics of the Cl- current. Neomycin, spermine and La3+ augmented volume-sensitive Cl- currents. However, these CaR agonists failed to affect depolarization-induced inactivation. Intracellular application of GTPgammaS, but not GDPbeta]S, increased the amplitude of the swelling-induced Cl- current without affecting the basal current. The upregulating effect of Ca2+ on the Cl- current amplitude was abolished by either GTPgammaS or GDPbetaS. In contrast, GTPgammaS and GDPbetaS failed to affect the inactivation kinetics of the Cl- current and the accelerating effect of Ca2+ thereon. The Cl- current amplitude was enlarged by stimulation with forskolin, dibutyryl cAMP and IBMX. During the cAMP stimulation, extracellular Ca2+ failed to increase the Cl- current but did accelerate depolarization-induced inactivation. It is concluded that stimulation of the CaR induces upregulation of volume-sensitive Cl- channels via a G protein-mediated increase in intracellular cAMP in the human epithelial cell. However, the accelerating effect of extracellular divalent cations on the inactivation kinetics of the Cl- current is induced by a mechanism independent of the CaR and cAMP.

Single-channel analysis of a ClC-2-like chloride conductance in cultured rat cortical astrocytes

The single-channel behavior of the hyperpolarization-activated, ClC-2-like inwardly rectifying Cl- current (IClh), induced by long-term dibutyryl-cyclic-AMP-treated cultured cortical rat astrocytes, was analyzed with the patch-clamp technique. In outside-out patches in symmetrical 144 mM Cl-solutions, openings of hyperpolarization-activated small-conductance Cl channels revealed burst activity of two equidistant conductance levels of 3 and 6 pS. The unitary openings displayed slow activation kinetics. The probabilities of the closed and conducting states were consistent with a double-barrelled structure of the channel protein. These results suggest that the astrocytic ClC-2-like Cl- current Iclh is mediated by a small-conductance Cl channel, which has the same structural motif as the Cl- channel prototype CIC-0.

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.

Controversy surrounding the existence of discrete functional classes of astrocytes in adult gray matter

Since 1992, it has been possible to record ionic currents from identified astrocytes in situ, using brain slice technology. Brain slice recordings confirm previous in vitro findings that expression of voltage-gated K(+) and Na(+) channels are a feature of this cell type. In contrast to cultured astrocytes, most investigators found that astrocytes in situ did not contain detectable, or at very best only low, levels of glial fibrillary acidic protein (GFAP). Structural and immunocytochemical investigations determined that these cells are different from oligodendrocyte precursors. In addition to cells with this current pattern, many but not all investigators found a second pool of astrocytes with no voltage-gated ion channels and high GFAP content. These two subpopulations of cells were termed complex and passive astrocytes. The existence of passive astrocytes has been questioned because of possible problems with space clamp conditions and spillage of EGTA-buffered pipette solution around the cells before recordings. Another problem is the fact there is a discrepancy regarding the GFAP content of complex astrocytes. It is of interest that recent immunocytochemical studies suggest the existence of two pools of astrocytes, one with a high GFAP content and one with nondetectable GFAP. Given this, it is tempting to correlate the two (controversial) electrophysiological patterns with immunochemical differences (GFAP) in order to demonstrate two functionally discrete classes of astrocytes in adult gray matter. However, despite evidence that some of the K(+) channels may be involved in proliferation, the role of voltage-gated ion channels in this nonexcitable cell type remains unknown. This is despite the fact that astrocytic Na(+) channels show dramatic changes after pathological events, re-enforcing the notion that the expression of this channel is under tight neuronal control. Several studies suggest that there is a great degree of flexibility and that astrocytes can undergo rapid changes in expression of both membrane ion currents and GFAP. Although it is likely that astrocytes exhibit different structural and membrane properties, this heterogeneity might be a reflection of the flexible plasticity of one astrocyte type under influence of environmental factors rather than of the existence of two distinct and permanent subtypes.

Heterogeneity of astroglia cultured from adult human temporal lobe

This study characterized the morphological and electrophysiological diversity of astroglia cultured from adult human cerebral temporal lobe, and explored the influence of the cytokine interleukin-1beta on these cells. The cultures contained astroglia positive for glial fibrillary acidic protein which were flat, bipolar or multipolar in shape and variable in size. A subpopulation of the bipolar and multipolar cells was positive for S100 protein. The most striking feature of these cultures was the presence of glia with long (600 micrometer) processes with few branches or only terminal branches. Patch clamp recordings of the non-stellate process bearing cells revealed prominent inward Na(+) and transient and sustained outward K(+) conductances. Distinct differences in the relative proportion of these conductances were evident among cells but did not appear to be correlated with cell morphology. Treatment of cultures with interleukin-1beta for 96 h did not change total protein content, but increased the content of S100beta protein and decreased the content of glial fibrillary acidic protein. The findings indicate that cultures of adult human cerebrum contain subpopulations of morphologically and electrophysiologically pleomorphic glial fibrillary acidic protein positive astroglia, exhibit increased levels of the neurotrophic factor S100beta when exposed to interleukin-1beta, and may serve as a useful model for investigation of glial involvement in neuropathology.

Ion channels in glial cells

Functional and molecular analysis of glial voltage- and ligand-gated ion channels underwent tremendous boost over the last 15 years. The traditional image of the glial cell as a passive, structural element of the nervous system was transformed into the concept of a plastic cell, capable of expressing a large variety of ion channels and neurotransmitter receptors. These molecules might enable glial cells to sense neuronal activity and to integrate it within glial networks, e.g., by means of spreading calcium waves. In this review we shall give a comprehensive summary of the main functional properties of ion channels and ionotropic receptors expressed by macroglial cells, i.e., by astrocytes, oligodendrocytes and Schwann cells. In particular we will discuss in detail glial sodium, potassium and anion channels, as well as glutamate, GABA and ATP activated ionotropic receptors. A majority of available data was obtained from primary cell culture, these results have been compared with corresponding studies that used acute tissue slices or freshly isolated cells. In view of these data, an active glial participation in information processing seems increasingly likely and a physiological role for some of the glial channels and receptors is gradually emerging.

Role of astrocytes in the clearance of excess extracellular potassium

The development of concepts describing potassium clearance mechanisms in the mammalian central nervous system in the last 35 years is reviewed. The pattern of excess potassium in the extracellular space is discussed as are the implications of these potassium levels for neuronal excitability. There is a systematic description of the available evidence for astrocytic involvement in situ. The three possible astrocytic potassium clearance mechanisms are introduced: spatial buffer mechanism; carrier-operated potassium chloride uptake as well as channel-operated potassium chloride uptake. The three mechanisms are compared and their compatibility is discussed. Evidence is now available showing that at least two of these if not all three mechanisms co-exist and complement each other. Finally, it is concluded that these potassium movements are not used as a signal system, only as a homeostatic feedback mechanisms. Such a genuine signal system involving glial elements exists--but it is based on calcium waves.

cAMP-induced cytoskeleton rearrangement increases calcium transients through the enhancement of capacitative calcium entry

In this report we investigated the correlation between cell morphology and regulation of cytosolic calcium homeostasis. Type I astrocytes were differentiated to stellate process-bearing cells by a 100-min exposure to cAMP. Differentiation of cortical astrocytes increased the magnitude and duration of calcium transients elicited by phospholipase C-activating agents as measured by single cell Fura-2-based imaging. Calcium imaging showed differences in the spatial pattern of the response. In both differentiated and the control cells, the response originated in the periphery and gradually extended into the center of the cell. However, the elevation of cytosolic calcium concentration ([Ca(2+)](i)) was particularly evident within the processes and adjacent to the inner cell membrane of the differentiated astrocytes. In addition, differentiation significantly prolonged the duration of the [Ca(2+)](i) elevation. Potentiation of the calcium transients was mimicked by forskolin-induced differentiation and abolished by a specific protein kinase-A blocker. Conversely, the enhancement of the calcium transients was not mimicked by brief exposure to cAMP not causing morphological differentiation, and in PC12 cells that did not undergo morphological changes after 100 min of cAMP treatment. Impairing cAMP-induced cytoskeleton re-organization, by means of cytochalasin D and nocodazole, prevented the potentiation of the calcium transients in cAMP-treated astrocytes. Phospholipase C activity and sensitivity to inositol (1,4,5)-trisphosphate were not involved in the enhancement of the calcium responses. Also, potentiation of the calcium transients was dependent on extracellular calcium. Calcium storage and thapsigargin-depletable intracellular calcium reservoirs were analogously not increased in differentiated astrocytes. Rearrangement of the cell shape also caused a condensation of the endoplasmic reticulum and altered the spatial relationship between the endoplasmic reticulum and the cell membrane. In conclusion, morphological rearrangements of type I astrocytes increase the magnitude and the duration of agonist-induced calcium transients via enhancement of capacitative calcium entry and is associated with a spatial reorganization of the relationship between cell membrane and the endoplasmic reticulum structures.

Participation of a chloride conductance in the subthreshold behavior of the rat sympathetic neuron

The presence of a novel voltage-dependent chloride current, active in the subthreshold range of membrane potential, was detected in the mature and intact rat sympathetic neuron in vitro by using the two-microelectrode voltage-clamp technique. Hyperpolarizing voltage steps applied to a neuron held at -40/-50 mV elicited inward currents, whose initial magnitude displayed a linear instantaneous current-voltage (I-V) relationship; afterward, the currents decayed exponentially with a single voltage-dependent time constant (63.5 s at -40 mV; 10.8 s at -130 mV). The cell input conductance decreased during the command step with the same time course as the current. On returning to the holding potential, the ensuing outward currents were accompanied by a slow increase in input conductance toward the initial values; the inward charge movement during the transient ON response (a mean of 76 nC in 8 neurons stepped from -50 to -90 mV) was completely balanced by outward charge displacement during the OFF response. The chloride movements accompanying voltage modifications were studied by estimating the chloride equilibrium potential (E(Cl)) at different holding potentials from the reversal of GABA evoked currents. [Cl(-)](i) was strongly affected by membrane potential, and at steady state it was systematically higher than expected from passive ion distribution. The transient current was blocked by substitution of isethionate for chloride and by Cl(-) channel blockers (9AC and DIDS). It proved insensitive to K(+) channel blockers, external Cd(2+), intracellular Ca(2+) chelators [bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA)] and reduction of [Na(+)](e). It is concluded that membrane potential shifts elicit a chloride current that reflects readjustment of [Cl(-)](i). The cell input conductance was measured over the -40/-120-mV voltage range, in control medium, and under conditions in which either the chloride or the potassium current was blocked. A mix of chloride, potassium, and leakage conductances was detected at all potentials. The leakage component was voltage independent and constant at approximately 14 nS. Conversely, gCl decreased with hyperpolarization (80 nS at -40 mV, undetectable below -110 mV), whereas gK displayed a maximum at -80 mV (55.3 nS). Thus the ratio gCl/gK continuously varied with membrane polarization (2.72 at -50 mV; 0.33 at -110 mV). These data were forced in a model of the three current components here described, which accurately simulates the behavior observed in the "resting" neuron during membrane migrations in the subthreshold potential range, thereby confirming that active K and Cl conductances contribute to the genesis of membrane potential and possibly to the control of neuronal excitability.

Glutamate release through volume-activated channels during spreading depression

Volume-sensitive organic anion channels (VSOACs) in astrocytes are activated by cell swelling and are permeable to organic anions, such as glutamate and taurine. We have examined the release of glutamate through VSOACs during the propagation of spreading depression (SD). SD was induced by bath application of ouabain in hippocampal brain slices and was monitored by imaging intrinsic optical signals, a technique that provides a measure of cellular swelling. The onset of SD was associated with increased light transmittance, confirming previous studies that cellular swelling occurs during SD. NMDA receptor antagonists, either noncompetitive (MK-801, 10-50 microM) or competitive (CGS-17355, 100 microM), reduced the rate of propagation of SD, indicating that glutamate release contributes to SD onset. SD still occurred in zero Ca(2+)-EGTA (0-Ca(2+)-EGTA) solution, a manipulation that depresses synaptic transmission. HPLC measurements indicated that, even in this solution, there was significant glutamate release. Two lines of experiments indicated that glutamate was released through VSOACs during SD. First, 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB), a blocker of VSOACs, depressed the rate of propagation of SD in a manner similar to NMDA antagonists. Second, NPPB inhibited the release of glutamate during SD in 0-Ca(2+)-EGTA external solution. These results indicate that cellular swelling during SD causes the activation of VSOACs and the release of glutamate by permeation through this channel. Cellular swelling is a result of neuronal activity and is observed during excitotoxicity. Therefore, glutamate release from VSOAC activation could occur under conditions of cell swelling and contribute to excitotoxic damage.