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

The deletion of AQP4 and TRPV4 affects astrocyte swelling/volume recovery in response to ischemia-mimicking pathologies

Introduction

Astrocytic Transient receptor potential vanilloid 4 (TRPV4) channels, together with Aquaporin 4 (AQP4), are suspected to be the key players in cellular volume regulation, and therefore may affect the development and severity of cerebral edema during ischemia. In this study, we examined astrocytic swelling/volume recovery in mice with TRPV4 and/or AQP4 deletion in response to in vitro ischemic conditions, to determine how the deletion of these channels can affect the development of cerebral edema.

Methods

We used three models of ischemia-related pathological conditions: hypoosmotic stress, hyperkalemia, and oxygenglucose deprivation (OGD), and observed their effect on astrocyte volume changes in acute brain slices of Aqp4-/-, Trpv4-/- and double knockouts. In addition, we employed single-cell RT-qPCR to assess the effect of TRPV4 and AQP4 deletion on the expression of other ion channels and transporters involved in the homeostatic functioning of astrocytes.

Results

Quantification of astrocyte volume changes during OGD revealed that the deletion of AQP4 reduces astrocyte swelling, while simultaneous deletion of both AQP4 and TRPV4 leads to a disruption of astrocyte volume recovery during the subsequent washout. Of note, astrocyte exposure to hypoosmotic stress or hyperkalemia revealed no differences in astrocyte swelling in the absence of AQP4, TRPV4, or both channels. Moreover, under ischemia-mimicking conditions, we identified two distinct subpopulations of astrocytes with low and high volumetric responses (LRA and HRA), and their analyses revealed that mainly HRA are affected by the deletion of AQP4, TRPV4, or both channels. Furthermore, gene expression analysis revealed reduced expression of the ion transporters KCC1 and ClC2 as well as the receptors GABAB and NMDA in Trpv4-/- mice. The deletion of AQP4 instead caused reduced expression of the serine/cysteine peptidase inhibitor Serpina3n.

Discussion

Thus, we showed that in AQP4 or TRPV4 knockouts, not only the specific function of these channels is affected, but also the expression of other proteins, which may modulate the ischemic cascade and thus influence the final impact of ischemia.

Glia as a key factor in cell volume regulation processes of the central nervous system

Brain edema is a pathological condition with potentially fatal consequences, related to cerebral injuries such as ischemia, chronic renal failure, uremia, and diabetes, among others. Under these pathological states, the cell volume control processes are fully compromised, because brain cells are unable to regulate the movement of water, mainly regulated by osmotic gradients. The processes involved in cell volume regulation are homeostatic mechanisms that depend on the mobilization of osmolytes (ions, organic molecules, and polyols) in the necessary direction to counteract changes in osmolyte concentration in response to water movement. The expression and coordinated function of proteins related to the cell volume regulation process, such as water channels, ion channels, and other cotransport systems in the glial cells, and considering the glial cell proportion compared to neuronal cells, leads to consider the astroglial network the main regulatory unit for water homeostasis in the central nervous system (CNS). In the last decade, several studies highlighted the pivotal role of glia in the cell volume regulation process and water homeostasis in the brain, including the retina; any malfunction of this astroglial network generates a lack of the ability to regulate the osmotic changes and water movements and consequently exacerbates the pathological condition.

The molecular evolution of function in the CFTR chloride channel

The ATP-binding cassette (ABC) transporter superfamily includes many proteins of clinical relevance, with genes expressed in all domains of life. Although most members use the energy of ATP binding and hydrolysis to accomplish the active import or export of various substrates across membranes, the cystic fibrosis transmembrane conductance regulator (CFTR) is the only known animal ABC transporter that functions primarily as an ion channel. Defects in CFTR, which is closely related to ABCC subfamily members that bear function as bona fide transporters, underlie the lethal genetic disease cystic fibrosis. This article seeks to integrate structural, functional, and genomic data to begin to answer the critical question of how the function of CFTR evolved to exhibit regulated channel activity. We highlight several examples wherein preexisting features in ABCC transporters were functionally leveraged as is, or altered by molecular evolution, to ultimately support channel function. This includes features that may underlie (1) construction of an anionic channel pore from an anionic substrate transport pathway, (2) establishment and tuning of phosphoregulation, and (3) optimization of channel function by specialized ligand–channel interactions. We also discuss how divergence and conservation may help elucidate the pharmacology of important CFTR modulators.

Dysregulation of Ambient Glutamate and Glutamate Receptors in Epilepsy: An Astrocytic Perspective

Given the important functions that glutamate serves in excitatory neurotransmission, understanding the regulation of glutamate in physiological and pathological states is critical to devising novel therapies to treat epilepsy. Exclusive expression of pyruvate carboxylase and glutamine synthetase in astrocytes positions astrocytes as essential regulators of glutamate in the central nervous system (CNS). Additionally, astrocytes can significantly alter the volume of the extracellular space (ECS) in the CNS due to their expression of the bi-directional water channel, aquaporin-4, which are enriched at perivascular endfeet. Rapid ECS shrinkage has been observed following epileptiform activity and can inherently concentrate ions and neurotransmitters including glutamate. This review highlights our emerging knowledge on the various potential contributions of astrocytes to epilepsy, particularly supporting the notion that astrocytes may be involved in seizure initiation via failure of homeostatic responses that lead to increased ambient glutamate. We also review the mechanisms whereby ambient glutamate can influence neuronal excitability, including via generation of the glutamate receptor subunit GluN2B-mediated slow inward currents, as well as indirectly affect neuronal excitability via actions on metabotropic glutamate receptors that can potentiate GluN2B currents and influence neuronal glutamate release probabilities. Additionally, we discuss evidence for upregulation of System x c - , a cystine/glutamate antiporter expressed on astrocytes, in epileptic tissue and changes in expression patterns of glutamate receptors.

Cell Death Induction and Protection by Activation of Ubiquitously Expressed Anion/Cation Channels. Part 1: Roles of VSOR/VRAC in Cell Volume Regulation, Release of Double-Edged Signals and Apoptotic/Necrotic Cell Death

Cell volume regulation (CVR) is essential for survival and functions of animal cells. Actually, normotonic cell shrinkage and swelling are coupled to apoptotic and necrotic cell death and thus called the apoptotic volume decrease (AVD) and the necrotic volume increase (NVI), respectively. A number of ubiquitously expressed anion and cation channels are involved not only in CVD but also in cell death induction. This series of review articles address the question how cell death is induced or protected with using ubiquitously expressed ion channels such as swelling-activated anion channels, acid-activated anion channels and several types of TRP cation channels including TRPM2 and TRPM7. The Part 1 focuses on the roles of the volume-sensitive outwardly rectifying anion channels (VSOR), also called the volume-regulated anion channel (VRAC), which is activated by cell swelling or reactive oxygen species (ROS) in a manner dependent on intracellular ATP. First we describe phenotypical properties, the molecular identity, and physical pore dimensions of VSOR/VRAC. Second, we highlight the roles of VSOR/VRAC in the release of organic signaling molecules, such as glutamate, glutathione, ATP and cGAMP, that play roles as double-edged swords in cell survival. Third, we discuss how VSOR/VRAC is involved in CVR and cell volume dysregulation as well as in the induction of or protection from apoptosis, necrosis and regulated necrosis under pathophysiological conditions.

Glial Chloride Channels in the Function of the Nervous System Across Species

In the nervous system, the concentration of Cl- in neurons that express GABA receptors plays a key role in establishing whether these neurons are excitatory, mostly during early development, or inhibitory. Thus, much attention has been dedicated to understanding how neurons regulate their intracellular Cl- concentration. However, regulation of the extracellular Cl- concentration by other cells of the nervous system, including glia and microglia, is as important because it ultimately affects the Cl- equilibrium potential across the neuronal plasma membrane. Moreover, Cl- ions are transported in and out of the cell, via either passive or active transporter systems, as counter ions for K+ whose concentration in the extracellular environment of the nervous system is tightly regulated because it directly affects neuronal excitability. In this book chapter, we report on the Cl- channel types expressed in the various types of glial cells focusing on the role they play in the function of the nervous system in health and disease. Furthermore, we describe the types of stimuli that these channels are activated by, the other solutes that they may transport, and the involvement of these channels in processes such as pH regulation and Regulatory Volume Decrease (RVD). The picture that emerges is one of the glial cells expressing a variety of Cl- channels, encoded by members of different gene families, involved both in short- and long-term regulation of the nervous system function. Finally, we report data on invertebrate model organisms, such as C. elegans and Drosophila, that are revealing important and previously unsuspected functions of some of these channels in the context of living and behaving animals.

Physiology of Astroglial Excitability

Classic physiology divides all neural cells into excitable neurons and nonexcitable neuroglia. Neuroglial cells, chiefly responsible for homeostasis and defense of the nervous tissue, coordinate their complex homeostatic responses with neuronal activity. This coordination reflects a specific form of glial excitability mediated by complex changes in intracellular concentration of ions and second messengers organized in both space and time. Astrocytes are equipped with multiple molecular cascades, which are central for regulating homeostasis of neurotransmitters, ionostasis, synaptic connectivity, and metabolic support of the central nervous system. Astrocytes are further provisioned with multiple receptors for neurotransmitters and neurohormones, which upon activation trigger intracellular signals mediated by Ca²⁺, Na⁺, and cyclic AMP. Calcium signals have distinct organization and underlying mechanisms in different astrocytic compartments thus allowing complex spatiotemporal signaling. Signals mediated by fluctuations in cytosolic Na⁺ are instrumental for coordination of Na⁺ dependent astrocytic transporters with tissue state and homeostatic demands. Astroglial ionic excitability may also involve K⁺, H^+, and Cl^-. The cyclic AMP signalling system is, in comparison to ions, much slower in targeting astroglial effector mechanisms. This evidence review summarizes the concept of astroglial intracellular excitability.

Osmolytes: A Possible Therapeutic Molecule for Ameliorating the Neurodegeneration Caused by Protein Misfolding and Aggregation

Most of the neurological disorders in the brain are caused by the abnormal buildup of misfolded or aggregated proteins. Osmolytes are low molecular weight organic molecules usually built up in tissues at a quite high amount during stress or any pathological condition. These molecules help in providing stability to the aggregated proteins and protect these proteins from misfolding. Alzheimer’s disease (AD) is the uttermost universal neurological disorder that can be described by the deposition of neurofibrillary tangles, aggregated/misfolded protein produced by the amyloid β-protein (Aβ). Osmolytes provide stability to the folded, functional form of a protein and alter the folding balance away from aggregation and/or degradation of the protein. Moreover, they are identified as chemical chaperones. Brain osmolytes enhance the pace of Aβ aggregation, combine with the nearby water molecules more promptly, and avert the aggregation/misfolding of proteins by providing stability to them. Therefore, osmolytes can be employed as therapeutic targets and may assist in potential drug design for many neurodegenerative and other diseases.

Physiology of Astroglia

Astrocytes are principal cells responsible for maintaining the brain homeostasis. Additionally, these glial cells are also involved in homocellular (astrocyte-astrocyte) and heterocellular (astrocyte-other cell types) signalling and metabolism. These astroglial functions require an expression of the assortment of molecules, be that transporters or pumps, to maintain ion concentration gradients across the plasmalemma and the membrane of the endoplasmic reticulum. Astrocytes sense and balance their neurochemical environment via variety of transmitter receptors and transporters. As they are electrically non-excitable, astrocytes display intracellular calcium and sodium fluctuations, which are not only used for operative signalling but can also affect metabolism. In this chapter we discuss the molecules that achieve ionic gradients and underlie astrocyte signalling.

Tweety-homolog (Ttyh) Family Encodes the Pore-forming Subunits of the Swelling-dependent Volume-regulated Anion Channel (VRACswell) in the Brain

In the brain, a reduction in extracellular osmolality causes water-influx and swelling, which subsequently triggers Cl^-- and osmolytes-efflux via volume-regulated anion channel (VRAC). Although LRRC8 family has been recently proposed as the pore-forming VRAC which is activated by low cytoplasmic ionic strength but not by swelling, the molecular identity of the pore-forming swelling-dependent VRAC (VRAC_swell) remains unclear. Here we identify and characterize Tweety-homologs (TTYH1, TTYH2, TTYH3) as the major VRAC_swell in astrocytes. Gene-silencing of all Ttyh1/2/3 eliminated hypo-osmotic-solution-induced Cl^- conductance (I_Cl,swell) in cultured and hippocampal astrocytes. When heterologously expressed in HEK293T or CHO-K1 cells, each TTYH isoform showed a significant I_Cl,swell with similar aquaporin-4 dependency, pharmacological properties and glutamate permeability as I_Cl,swell observed in native astrocytes. Mutagenesis-based structure-activity analysis revealed that positively charged arginine residue at 165 in TTYH1 and 164 in TTYH2 is critical for the formation of the channel-pore. Our results demonstrate that TTYH family confers the bona fide VRAC_swell in the brain.

Ionic signalling in astroglia beyond calcium

Astrocytes are homeostatic and protective cells of the central nervous system. Astroglial homeostatic responses are tightly coordinated with neuronal activity. Astrocytes maintain neuronal excitability through regulation of extracellular ion concentrations, as well as assisting and modulating synaptic transmission by uptake and catabolism of major neurotransmitters. Moreover, they support neuronal metabolism and detoxify ammonium and reactive oxygen species. Astroglial homeostatic actions are initiated and controlled by intercellular signalling of ions, including Ca²⁺ , Na⁺ , Cl^- , H⁺ and possibly K⁺ . This review summarises current knowledge on ionic signals mediated by the major monovalent ions, which occur in microdomains, as global events, or as propagating intercellular waves and thereby represent the substrate for astroglial excitability.

Cell Volume-Activated and Volume-Correlated Anion Channels in Mammalian Cells: Their Biophysical, Molecular, and Pharmacological Properties

There are a number of mammalian anion channel types associated with cell volume changes. These channel types are classified into two groups: volume-activated anion channels (VAACs) and volume-correlated anion channels (VCACs). VAACs can be directly activated by cell swelling and include the volume-sensitive outwardly rectifying anion channel (VSOR), which is also called the volume-regulated anion channel; the maxi-anion channel (MAC or Maxi-Cl); and the voltage-gated anion channel, chloride channel (ClC)-2. VCACs can be facultatively implicated in, although not directly activated by, cell volume changes and include the cAMP-activated cystic fibrosis transmembrane conductance regulator (CFTR) anion channel, the Ca²⁺-activated Cl^- channel (CaCC), and the acid-sensitive (or acid-stimulated) outwardly rectifying anion channel. This article describes the phenotypical properties and activation mechanisms of both groups of anion channels, including accumulating pieces of information on the basis of recent molecular understanding. To that end, this review also highlights the molecular identities of both anion channel groups; in addition to the molecular identities of ClC-2 and CFTR, those of CaCC, VSOR, and Maxi-Cl were recently identified by applying genome-wide approaches. In the last section of this review, the most up-to-date information on the pharmacological properties of both anion channel groups, especially their half-maximal inhibitory concentrations (IC₅₀ values) and voltage-dependent blocking, is summarized particularly from the standpoint of pharmacological distinctions among them. Future physiologic and pharmacological studies are definitely warranted for therapeutic targeting of dysfunction of VAACs and VCACs.

Genetic defects disrupting glial ion and water homeostasis in the brain

Electrical activity of neurons in the brain, caused by the movement of ions between intracellular and extracellular compartments, is the basis of all our thoughts and actions. Maintaining the correct ionic concentration gradients is therefore crucial for brain functioning. Ion fluxes are accompanied by the displacement of osmotically obliged water. Since even minor brain swelling leads to severe brain damage and even death, brain ion and water movement has to be tightly regulated. Glial cells, in particular astrocytes, play a key role in ion and water homeostasis. They are endowed with specific channels, pumps and carriers to regulate ion and water flow. Glial cells form a large panglial syncytium to aid the uptake and dispersal of ions and water, and make extensive contacts with brain fluid barriers for disposal of excess ions and water. Genetic defects in glial proteins involved in ion and water homeostasis disrupt brain functioning, thereby leading to neurological diseases. Since white matter edema is often a hallmark disease feature, many of these diseases are characterized as leukodystrophies. In this review we summarize our current understanding of inherited glial diseases characterized by disturbed brain ion and water homeostasis by integrating findings from MRI, genetics, neuropathology and animal models for disease. We discuss how mutations in different glial proteins lead to disease, and highlight the similarities and differences between these diseases. To come to effective therapies for this group of diseases, a better mechanistic understanding of how glial cells shape ion and water movement in the brain is crucial.

Bursting at the Seams: Molecular Mechanisms Mediating Astrocyte Swelling

Brain swelling is one of the most robust predictors of outcome following brain injury, including ischemic, traumatic, hemorrhagic, metabolic or other injury. Depending on the specific type of insult, brain swelling can arise from the combined space-occupying effects of extravasated blood, extracellular edema fluid, cellular swelling, vascular engorgement and hydrocephalus. Of these, arguably the least well appreciated is cellular swelling. Here, we explore current knowledge regarding swelling of astrocytes, the most abundant cell type in the brain, and the one most likely to contribute to pathological brain swelling. We review the major molecular mechanisms identified to date that contribute to or mitigate astrocyte swelling via ion transport, and we touch upon the implications of astrocyte swelling in health and disease.

Physiological and Pathological Functions of Cl- Channels in Chondrocytes

Articular chondrocytes are embedded in the cartilage of diarthrodial joints and responsible for the synthesis and secretion of extracellular matrix. The extracellular matrix mainly contains collagens and proteoglycans, and covers the articular cartilage to protect from mechanical and biochemical stresses. In mammalian chondrocytes, various types of ion channels have been identified: e.g., voltage-dependent K⁺ channels, Ca²⁺-activated K⁺ channels, ATP-sensitive K⁺ channels, two-pore domain K⁺ channels, voltage-dependent Ca²⁺ channels, store-operated Ca²⁺ channels, epithelial Na⁺ channels, acid-sensing ion channels, transient receptor potential channels, and mechanosensitive channels. These channels play important roles for the regulation of resting membrane potential, Ca²⁺ signaling, pH sensing, mechanotransduction, and cell proliferation in articular chondrocytes. In addition to these cation channels, Cl^- channels are known to be expressed in mammalian chondrocytes: e.g., voltage-dependent Cl^- channels, cystic fibrosis transmembrane conductance regulator channels, swelling-activated Cl^- channels, and Ca²⁺-activated Cl^- channels. Although these chondrocyte Cl^- channels are thought to contribute to the regulation of resting membrane potential, Ca²⁺ signaling, cell volume, cell survival, and endochondral bone formation, the physiological functions have not been fully clarified. Osteoarthritis (OA) is caused by the degradation of articular cartilage, resulting in inflammation and pain in the joints. Therefore the pathophysiological roles of Cl^- channels in OA chondrocytes are of considerable interest. Elucidating the physiological and pathological functions of chondrocyte Cl^- channels will provide us a more comprehensive understanding of chondrocyte functions and may suggest novel molecular targets of drug development for OA.

Cell Volume Control in Healthy Brain and Neuropathologies

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

LRRC8A is essential for swelling-activated chloride current and for regulatory volume decrease in astrocytes

Consolidated evidence indicates that astroglial cells are critical in the homeostatic regulation of cellular volume by means of ion channels and aquaporin-4. Volume-regulated anion channel (VRAC) is the chloride channel that is activated upon cell swelling and critically contributes to cell volume regulation in astrocytes. The molecular identity of VRAC has been recently defined, revealing that it belongs to the leucine-rich repeat-containing 8 (LRRC8) protein family. However, there is a lack of evidence demonstrating that LRRC8A underpins VRAC currents in astrocyte. Nonetheless, direct evidence of the role of LRRC8A in astrocytic regulatory volume decrease remains to be proved. Here, we aim to bridge this gap in knowledge by combining RNA interference specific for LRRC8A with patch-clamp analyses and a water-permeability assay. We demonstrated that LRRC8A molecular expression is essential for swelling-activated chloride current via VRAC in primary-cultured cortical astrocytes. The knockdown of LRRC8A with a specific short interference RNA abolished the recovery of the cell volume after swelling induced by hypotonic challenge. In addition, immunoblotting, immunofluorescence, confocal imaging, and immunogold electron microscopy demonstrated that LRRC8A is expressed in the plasma membrane of primary cortical astrocytes and in situ in astrocytes at the perivascular interface with endothelial cells. Collectively, our results suggest that LRRC8A is an essential subunit of VRAC and a key factor for astroglial volume homeostasis.-Formaggio, F., Saracino, E., Mola, M. G., Rao, S. B., Amiry-Moghaddam, M., Muccini, M., Zamboni, R., Nicchia, G. P., Caprini, M., Benfenati, V. LRRC8A is essential for swelling-activated chloride current and for regulatory volume decrease in astrocytes.

The signaling role for chloride in the bidirectional communication between neurons and astrocytes

It is well known that the electrical signaling in neuronal networks is modulated by chloride (Cl^-) fluxes via the inhibitory GABA_A and glycine receptors. Here, we discuss the putative contribution of Cl^- fluxes and intracellular Cl^- to other forms of information transfer in the CNS, namely the bidirectional communication between neurons and astrocytes. The manuscript (i) summarizes the generic functions of Cl^- in cellular physiology, (ii) recaps molecular identities and properties of Cl^- transporters and channels in neurons and astrocytes, and (iii) analyzes emerging studies implicating Cl^- in the modulation of neuroglial communication. The existing literature suggests that neurons can alter astrocytic Cl^- levels in a number of ways; via (a) the release of neurotransmitters and activation of glial transporters that have intrinsic Cl^- conductance, (b) the metabotropic receptor-driven changes in activity of the electroneutral cation-Cl^- cotransporter NKCC1, and (c) the transient, activity-dependent changes in glial cell volume which open the volume-regulated Cl^-/anion channel VRAC. Reciprocally, astrocytes are thought to alter neuronal [Cl^-]_i through either (a) VRAC-mediated release of the inhibitory gliotransmitters, GABA and taurine, which open neuronal GABA_A and glycine receptor/Cl^- channels, or (b) the gliotransmitter-driven stimulation of NKCC1. The most important recent developments in this area are the identification of the molecular composition and functional heterogeneity of brain VRAC channels, and the discovery of a new cytosolic [Cl^-] sensor - the Wnk family protein kinases. With new work in the field, our understanding of the role of Cl^- in information processing within the CNS is expected to be significantly updated.

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.

Preferential association with ClC-3 permits sorting of ClC-4 into endosomal compartments

ClC-4 is an intracellular Cl^-/H⁺ exchanger that is highly expressed in the brain and whose dysfunction has been linked to intellectual disability and epilepsy. Here we studied the subcellular localization of human ClC-4 in heterologous expression systems. ClC-4 is retained in the endoplasmic reticulum (ER) upon overexpression in HEK293T cells. Co-expression with distinct ClC-3 splice variants targets ClC-4 to late endosome/lysosomes (ClC-3a and ClC-3b) or recycling endosome (ClC-3c). When expressed in cultured astrocytes, ClC-4 sorted to endocytic compartments in WT cells but was retained in the ER in Clcn3^-/- cells. To understand the virtual absence of ER-localized ClC-4 in WT astrocytes, we performed association studies by high-resolution clear native gel electrophoresis. Although other CLC channels and transporters form stable dimers, ClC-4 was mostly observed as monomer, with ClC-3-ClC-4 heterodimers being more stable than ClC-4 homodimers. We conclude that unique oligomerization properties of ClC-4 permit regulated targeting of ClC-4 to various endosomal compartment systems via expression of different ClC-3 splice variants.

Inhibition of transmembrane member 16A calcium-activated chloride channels by natural flavonoids contributes to flavonoid anticancer effects

BACKGROUND AND PURPOSE

Natural flavonoids are ubiquitous in dietary plants and vegetables and have been proposed to have antiviral, antioxidant, cardiovascular protective and anticancer effects. Transmembrane member 16A (TMEM16A)-encoded Ca²⁺ -activated Cl^- channels play a variety of physiological roles in many organs and tissues. Overexpression of TMEM16A is also believed to be associated with cancer progression. Therefore, inhibition of TMEM16A current may be a potential target for cancer therapy. In this study, we screened a broad spectrum of flavonoids for their inhibitory activities on TMEM16A currents.

EXPERIMENTAL APPROACH

A whole-cell patch technique was used to record the currents. The BrdU assay and transwell technique were used to investigate cell proliferation and migration.

KEY RESULTS

At a concentration of 100 μM, 10 of 20 compounds caused significant (>50%) inhibition of TMEM16A currents. The four most potent compounds - luteolin, galangin, quercetin and fisetin - had IC₅₀ values ranging from 4.5 to 15 μM). To examine the physiological relevance of these findings, we also studied the effects of these flavonoids on endogenous TMEM16A currents in addition to cell proliferation and migration in LA795 cancer cells. Among the flavonoids tested, we detected a highly significant correlation between TMEM16A current inhibition and cell proliferation or reduction of migration.

CONCLUSIONS AND IMPLICATIONS

This study demonstrates that flavonoids inhibit TMEM16A currents and suggests that flavonoids could have anticancer effects via this mechanism.

Mechanisms of astrocytic K(+) clearance and swelling under high extracellular K(+) concentrations

In response to the elevation of extracellular K(+) concentration ([K(+)]out), astrocytes clear excessive K(+) to maintain conditions necessary for neural activity. K(+) clearance in astrocytes occurs via two processes: K(+) uptake and K(+) spatial buffering. High [K(+)]out also induces swelling in astrocytes, leading to edema and cell death in the brain. Despite the importance of astrocytic K(+) clearance and swelling, the underlying mechanisms remain unclear. Here, we report results from a simulation analysis of astrocytic K(+) clearance and swelling. Astrocyte models were constructed by incorporating various mechanisms such as intra/extracellular ion concentrations of Na(+), K(+), and Cl(-), cell volume, and models of Na,K-ATPase, Na-K-Cl cotransporter (NKCC), K-Cl cotransporter, inwardly-rectifying K(+) (KIR) channel, passive Cl(-) current, and aquaporin channel. The simulated response of astrocyte models under the uniform distribution of high [K(+)]out revealed significant contributions of NKCC and Na,K-ATPase to increases of intracellular K(+) and Cl(-) concentrations, and swelling. Moreover, we found that, under the non-uniform distribution of high [K(+)]out, KIR channels localized at synaptic clefts absorbed excess K(+) by depolarizing the equivalent potential of K(+) (E K) above membrane potential, while K(+) released through perivascular KIR channels was enhanced by hyperpolarizing E K and depolarizing membrane potential. Further analysis of simulated drug effects revealed that astrocyte swelling was modulated by blocking each of the ion channels and transporters. Our simulation analysis revealed controversial mechanisms of astrocytic K(+) clearance and swelling resulting from complex interactions among ion channels and transporters.

What do we know about gliotransmitter release from astrocytes?

Astrocytes participate in information processing by actively modulating synaptic properties via gliotransmitter release. Various mechanisms of astrocytic release have been reported, including release from storage organelles via exocytosis and release from the cytosol via plasma membrane ion channels and pumps. It is still not fully clear which mechanisms operate under which conditions, but some of them, being Ca²⁺-regulated, may be physiologically relevant. The properties of Ca²⁺-dependent transmitter release via exocytosis or via ion channels are different and expected to produce different extracellular transmitter concentrations over time and to have distinct functional consequences. The molecular aspects of these two release pathways are still under active investigation. Here, we discuss the existing morphological and functional evidence in support of either of them. Transgenic mouse models, specific antagonists and localization studies have provided insight into regulated exocytosis, albeit not in a systematic fashion. Even more remains to be uncovered about the details of channel-mediated release. Better functional tools and improved ultrastructural approaches are needed in order fully to define specific modalities and effects of astrocytic gliotransmitter release pathways.

Neuroglial Transmission

Neuroglia, the "glue" that fills the space between neurons in the central nervous system, takes active part in nerve cell signaling. Neuroglial cells, astroglia, oligodendroglia, and microglia, are together about as numerous as neurons in the brain as a whole, and in the cerebral cortex grey matter, but the proportion varies widely among brain regions. Glial volume, however, is less than one-fifth of the tissue volume in grey matter. When stimulated by neurons or other cells, neuroglial cells release gliotransmitters by exocytosis, similar to neurotransmitter release from nerve endings, or by carrier-mediated transport or channel flux through the plasma membrane. Gliotransmitters include the common neurotransmitters glutamate and GABA, the nonstandard amino acid d-serine, the high-energy phosphate ATP, and l-lactate. The latter molecule is a "buffer" between glycolytic and oxidative metabolism as well as a signaling substance recently shown to act on specific lactate receptors in the brain. Complementing neurotransmission at a synapse, neuroglial transmission often implies diffusion of the transmitter over a longer distance and concurs with the concept of volume transmission. Transmission from glia modulates synaptic neurotransmission based on energetic and other local conditions in a volume of tissue surrounding the individual synapse. Neuroglial transmission appears to contribute significantly to brain functions such as memory, as well as to prevalent neuropathologies.

The glutamate aspartate transporter (GLAST) mediates L-glutamate-stimulated ascorbate-release via swelling-activated anion channels in cultured neonatal rodent astrocytes

Vitamin C (ascorbate) plays important neuroprotective and neuromodulatory roles in the mammalian brain. Astrocytes are crucially involved in brain ascorbate homeostasis and may assist in regenerating extracellular ascorbate from its oxidised forms. Ascorbate accumulated by astrocytes can be released rapidly by a process that is stimulated by the excitatory amino acid, L-glutamate. This process is thought to be neuroprotective against excitotoxicity. Although of potential clinical interest, the mechanism of this stimulated ascorbate-release remains unknown. Here, we report that primary cultures of mouse and rat astrocytes release ascorbate following initial uptake of dehydroascorbate and accumulation of intracellular ascorbate. Ascorbate-release was not due to cellular lysis, as assessed by cellular release of the cytosolic enzyme lactate dehydrogenase, and was stimulated by L-glutamate and L-aspartate, but not the non-excitatory amino acid L-glutamine. This stimulation was due to glutamate-induced cellular swelling, as it was both attenuated by hypertonic and emulated by hypotonic media. Glutamate-stimulated ascorbate-release was also sensitive to inhibitors of volume-sensitive anion channels, suggesting that the latter may provide the conduit for ascorbate efflux. Glutamate-stimulated ascorbate-release was not recapitulated by selective agonists of either ionotropic or group I metabotropic glutamate receptors, but was completely blocked by either of two compounds, TFB-TBOA and UCPH-101, which non-selectively and selectively inhibit the glial Na(+)-dependent excitatory amino acid transporter, GLAST, respectively. These results suggest that an impairment of astrocytic ascorbate-release may exacerbate neuronal dysfunction in neurodegenerative disorders and acute brain injury in which excitotoxicity and/or GLAST deregulation have been implicated.

The inhibitor of volume-regulated anion channels DCPIB activates TREK potassium channels in cultured astrocytes

BACKGROUND AND PURPOSE

The ethacrynic acid derivative, 4-(2-butyl-6,7-dichlor-2-cyclopentylindan-1-on-5-yl) oxobutyric acid (DCPIB) is considered to be a specific and potent inhibitor of volume-regulated anion channels (VRACs). In the CNS, DCPIB was shown to be neuroprotective through mechanisms principally associated to its action on VRACs. We hypothesized that DCPIB could also regulate the activity of other astroglial channels involved in cell volume homeostasis.

EXPERIMENTAL APPROACH

Experiments were performed in rat cortical astrocytes in primary culture and in hippocampal astrocytes in situ. The effect of DCPIB was evaluated by patch-clamp electrophysiology and immunocytochemical techniques. Results were verified by comparative analysis with recombinant channels expressed in COS-7 cells.

KEY RESULTS

In cultured astrocytes, DCPIB promoted the activation of a K(+) conductance mediated by two-pore-domain K(+) (K(2P) ) channels. The DCPIB effect occluded that of arachidonic acid, which activates K(2P) channels K(2P) 2.1 (TREK-1) and K(2P) 10.1 (TREK-2) in cultured astrocytes. Immunocytochemical analysis suggests that cultured astrocytes express K(2P) 2.1 and K(2P) 10.1 proteins. Moreover, DCPIB opened recombinant K(2P) 2.1 and K(2P) 10.1 expressed in heterologous system. In brain slices, DCPIB did not augment the large background K(+) conductance in hippocampal astrocytes, but caused an increment in basal K(+) current of neurons.

CONCLUSION AND IMPLICATIONS

Our results indicate that the neuroprotective effect of DCPIB could be due, at least in part, to activation of TREK channels. DCPIB could be used as template to build new pharmacological tools able to increase background K(+) conductance in astroglia and neuronal cells.

Human neural progenitor cells show functional neuronal differentiation and regional preference after engraftment onto hippocampal slice cultures

The transplantation of stem cells offers potential therapies for many neurodegenerative disorders that currently have limited or no treatment options. However, relatively little is known about how the host environment affects the development and integration of these cells. In this study we have engrafted immortalized human midbrain neural progenitor cells (NPCs) onto rat hippocampal brain slice cultures to examine the influence of a neural environment on differentiation. Patch clamp recordings revealed that the transplanted progenitor cells could express neuronal-type voltage-gated currents and rapidly receive synaptic input from the hippocampal brain slice. The distribution of progenitor cells across the hippocampal slices was strongly influenced by the neural architecture, with most cells located in the fissural regions and sending processes parallel to the laminar structure, while in contrast, cells located in the dentate gyrus showed no organized pattern. Almost no cells were found in the stratum radiatum or pyramidal cell layers. Together, these results demonstrate the potential for the architecture of the host environment to regulate the integration of transplanted cells, and highlight the utility of coculture systems for studying the mechanisms underlying the migration, integration, and differentiation of human NPCs in structured neural environments.

GABA(A) receptor and glycine receptor activation by paracrine/autocrine release of endogenous agonists: more than a simple communication pathway

It is a common and widely accepted assumption that glycine and GABA are the main inhibitory transmitters in the central nervous system (CNS). But, in the past 20 years, several studies have clearly demonstrated that these amino acids can also be excitatory in the immature central nervous system. In addition, it is now established that both GABA receptors (GABARs) and glycine receptors (GlyRs) can be located extrasynaptically and can be activated by paracrine release of endogenous agonists, such as GABA, glycine, and taurine. Recently, non-synaptic release of GABA, glycine, and taurine gained further attention with increasing evidence suggesting a developmental role of these neurotransmitters in neuronal network formation before and during synaptogenesis. This review summarizes recent knowledge about the non-synaptic activation of GABA(A)Rs and GlyRs, both in developing and adult CNS. We first present studies that reveal the functional specialization of both non-synaptic GABA(A)Rs and GlyRs and we discuss the neuronal versus non-neuronal origin of the paracrine release of GABA(A)R and GlyR agonists. We then discuss the proposed non-synaptic release mechanisms and/or pathways for GABA, glycine, and taurine. Finally, we summarize recent data about the various roles of non-synaptic GABAergic and glycinergic systems during the development of neuronal networks and in the adult.

Bupivacaine induces apoptosis via mitochondria and p38 MAPK dependent pathways

Mitochondria and the p38 mitogen-activated protein kinase (MAPK) pathways play important roles in apoptosis. Although the effect of bupivacaine on apoptosis is known, it remains unclear whether bupivacaine induces apoptosis via mitochondrial depolarization and the p38 MAPK activity. In this study, SH-SY5Y cells were pretreated respectively with 50μM 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), 10μM 4-(4-Fluorophenyl)-2-[4-(methylsulfinyl)phenyl]-5-(4-pyridyl)-1H-imidazole (SB203580), and 50μM DIDS plus 10μM SB203580 30min prior to the treatment with either 1mM bupivacaine or an equivalent amount of medium. The cell viability, mitochondrial membrane potential, phospho-p38 MAPK (p-p38 MAPK) and cell apoptosis were investigated with MTT assay, western blots, Hoechst 33258 staining and flow cytometry assay. In addition, the roles of chloridion (Cl(-)) channel and reactive oxygen species were studied to explore the molecular mechanism of bupivacaine-induced mitochondrial injury. Pretreatment with DIDS could attenuate reactive oxygen species production, the phosphorylation of p38MAPK, dissipation of mitochondrial membrane potential and apoptosis of SH-SY5Y cells induced by bupivacaine. Pretreatment with SB203580 could attenuate apoptosis, but could not attenuate reactive oxygen species production, or dissipation of mitochondrial membrane potential induced by bupivacaine. These findings indicate that the mitochondrial anion channel and p38 MAPK pathway are implicated in bupicavaine-induced apoptosis. Bupivacaine-induced reactive oxygen species production results in an alteration in the permeability of the mitochondrial membranes and Cl(-) influx into mitochondria, which seems to be responsible for mitochondrial depolarization and the p38 MAPK activation.

Nonsynaptic communication through ATP release from volume-activated anion channels in axons

The release of neuronal messengers outside synapses has broad biological implications, particularly with regard to communication between axons and glia. We identify a mechanism for nonsynaptic, nonvesicular release of adenosine triphosphate (ATP) from axons through volume-activated anion channels (VAACs) activated by microscopic axon swelling during action potential firing. We used a combination of single-photon imaging of ATP release, together with imaging for intrinsic optical signals, intracellular calcium ions (Ca(2+)), time-lapse video, and confocal microscopy, to investigate action potential-induced nonsynaptic release of this neurotransmitter. ATP release from cultured embryonic dorsal root ganglion axons persisted when bafilomycin or botulinum toxin was used to block vesicular release, whereas pharmacological inhibition of VAACs or prevention of action potential-induced axon swelling inhibited ATP release and disrupted activity-dependent signaling between axons and astrocytes. This nonvesicular, nonsynaptic communication could mediate various activity-dependent interactions between axons and nervous system cells in normal conditions, development, and disease.

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.

Bestrophin-1 encodes for the Ca2+-activated anion channel in hippocampal astrocytes

In mammalian brain, neurons and astrocytes are reported to express various chloride and anion channels, but the evidence for functional expression of Ca(2+)-activated anion channel (CAAC) and its molecular identity have been lacking. Here we report electrophysiological evidence for the CAAC expression and its molecular identity by mouse Bestrophin 1 (mBest1) in astrocytes of the mouse brain. Using Ca(2+) imaging and perforated-patch-clamp analysis, we demonstrate that astrocytes displayed an inward current at holding potential of -70 mV that was dependent on an increase in intracellular Ca(2+) after G(alphaq)-coupled receptor activation. This current was mediated mostly by anions and was sensitive to well known anion channel blockers such as niflumic acid, 5-nitro-2(3-phenylpropylamino)-benzoic acid, and flufenamic acid. To find the molecular identity of the anion channel responsible for the CAAC current, we analyzed the expression of candidate genes and found that the mRNA for mouse mBest1 is predominantly expressed in acutely dissociated or cultured astrocytes. Whole-cell patch-clamp analysis using HEK293T cells heterologously expressing full-length mBest1 showed a Ca(2+)-dependent current mediated by mBest1, with a complete impairment of the current by a putative pore mutation, W93C. Furthermore, mBest1-mediated CAAC from cultured astrocytes was significantly reduced by expression of mBest1-specific short hairpin RNA (shRNA), suggesting that the CAAC is mediated by a channel encoded by mBest1. Finally, hippocampal CA1 astrocytes in hippocampal slice also showed mBest1-mediated CAAC because it was inhibited by mBest1-specific shRNA. Collectively, these data provide molecular evidence that the mBest1 channel is responsible for CAAC function in astrocytes.

Pannexins, distant relatives of the connexin family with specific cellular functions?

Intercellular communication (IC) is mediated by gap junctions (GJs) and hemichannels, which consist of proteins. This has been particularly well documented for the connexin (Cx) family. Initially, Cxs were thought to be the only proteins capable of GJ formation in vertebrates. About 10 years ago, however, a new GJ-forming protein family related to invertebrate innexins (Inxs) was discovered in vertebrates, and named the pannexin (Panx) family. Panxs, which are structurally similar to Cxs, but evolutionarily distinct, have been shown to be co-expressed with Cxs in vertebrates. Both protein families show distinct properties and have their own particular function. Identification of the mechanisms that control Panx channel gating is a major challenge for future work. In this review, we focus on the specific properties and role of Panxs in normal and pathological conditions.

Pharmacological "cross-inhibition" of connexin hemichannels and swelling activated anion channels

The study of ion channels has relied heavily on the use of pharmacological blocking agents. However, many of these agents have multiple effects, which may compromise interpretation of results when the affected mechanisms/pathways mediate similar functions. Volume regulated anion channels (VRAC) and connexin hemichannels can both mediate the release of glutamate and taurine, although these channels have distinct activation stimuli and hemichannels, but not VRAC, are permeable to Lucifer Yellow (LY). It has been reported that some anion channel blockers may inhibit connexin hemichannels. We further examined the effects of classic gap junction/hemichannel blockers and anion channel blockers on these channels. The typical VRAC blockers, NPPB, IAA-94, and tamoxifen blocked low divalent cation-induced glutamate and taurine release and LY loading, presumed due to hemichannel opening. The blocking action of these compounds on hemichannels was concentration dependent and fell within the same range where the drugs classically block VRACs. Conversely, carbenoxolone (CBX), the most widely used gap junction/hemichannel blocker, was an effective blocker of VRAC-mediated glutamate and taurine release, and blocked these channels at similar concentrations at which it blocked hemichannels. The CBX effect on VRACs was verified using astrocytes from connexin 43 knock out (Cx43 KO) animals. In these cells, the hypotonic induced amino acid flux was retained whereas the low divalent cation solution-induced flux was lost. These results extend our knowledge about "cross-inhibition" of VRACs and gap junctions/hemichannels by certain pharmacological agents. Given the overlap in function of these two types of channels, great care must be exerted in using pharmacological blockers to identify one channel from the other.

Expression and function of water channels (aquaporins) in migrating malignant astrocytes

Aquaporins (AQP) constitute the principal pathway for water movement across biological membranes. Consequently, their expression and function is important for cell volume regulation. Glioma cells quickly adjust their cell volume in response to osmotic challenges or spontaneously as they invade into the narrow and tortuous extracellular spaces of the brain. These cell volume changes are likely to engage water movements across the cell membrane through AQP. AQP expression in glioma cells is poorly understood. In this study, we examined the expression of AQP in several commonly used human glioma cell lines (D54, D65, STTG1, U87, U251) and in numerous acute patient biopsies by PCR, Western blot, and immunocytochemistry and compared them to nonmalignant astrocytes and normal brain. All glioma patient biopsies expressed AQP1, AQP4 and some expressed AQP5. However, when isolated and grown as cell lines they lose all AQP proteins except a few cell lines that maintain expression of AQP1 (D65, U251, GBM62). Reintroducing either AQP1 or AQP4 stably into glioma cell lines allowed us to show that each AQP is sufficient to restore water permeability. Yet, only the presence of AQP1, but not AQP4, enhanced cell growth and migration, typical properties of gliomas, while AQP4 enhanced cell adhesion suggesting differential biological roles for AQP1 and AQP4 in glioma cell biology.

Volume cytometry: microfluidic sensor for high-throughput screening in real time

Regulation of cell volume was one of the earliest evolutionary demands for life and remains a universal measure of cell metabolism. Since conventional methods to measure cell volume, such as microscopy, are complex and time-consuming, cell volume has not been used as the basis for cell-based screening. We have developed a microfabricated chip that can measure the volume of small numbers of cells in real time with unprecedented resolution. The method is applicable to adherent or suspended populations of cells and membrane-bound organelles. Our prototype device can detect volume changes in a monolayer of tissue-cultured astrocytes responding to anisotonic stimuli of <1mOsm. We determined the sensitivity to antibiotics of different E. coli strains in <10 min at 24 degrees C. This time can be reduced at higher temperatures enabling on-site clinical testing of infectious agents. Using the chip to screen natural products, we found a peptide in spider venom that inhibits eukaryotic volume regulation at approximately 100pM. The prototype chip made in silicon is inexpensive, reusable, and runs on low-voltage electrical power. The technology can be readily transferred to large arrays in plastic.

Anion channels in astrocytes: biophysics, pharmacology, and function

The chloride/anion channels that have been so far identified in cultured astrocytes and those that have been confirmed in situ by a combination of mRNA identification, immunocytochemistry, and biophysical studies are reviewed. It is emphasized that we are just beginning to describe such channels and analyze their functions in astrocytes. The best-studied anion channels studied so far are those known as volume-regulated anion channels (VRACs). These, as for most channels, have been mainly studied in cultured astrocytes, but some correlative studies have been done in situ, because these channels have been emphasized as release routes for transmitters; namely, excitatory amino acids and ATP. They are activated by cell shape changes and cell swelling, and the release of amino acids and ATP and chloride currents, measured by whole cell clamping, by these processes has been well described, as is also their activation by low concentrations of extracellular ATP. However, the identity of these channels in astrocytes, as in all other cells, remains elusive. The potential involvement of VRACs in pathological states such as stroke, metastasis, and spreading depression is also discussed.

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.

Ethanol effects on electrophysiological properties of astrocytes in striatal brain slices

Ethanol (EtOH) is known to alter neuronal physiology, but much less is known about the actions of this drug on glial function. To this end, we examined acute effects of ethanol on resting and voltage-activated membrane currents in striatal astrocytes using rat brain slices. Ten minutes exposure to 50mM EtOH reduced slope conductance by 20%, increased input resistance by 25% and decreased capacitance by 38% but did not affect resting membrane potential. Current generated by a hyperpolarizing pulse was inhibited in a concentration dependent manner in passive astrocytes, while no significant EtOH effect was observed in complex astrocytes or neurons. The EtOH effect was blocked when intracellular KCl was replaced with CsCl, but not during chelation of intracellular calcium with BAPTA. During blockage of gap junction coupling with high intracellular CaCl(2) or extracellular carbenoxolone the EtOH effect persisted but was reduced. Interestingly, EtOH effects were largely irreversible when gap junctions were open, but were fully reversible when gap junctions were closed. Ethanol also reduced the spread to other cells of Lucifer Yellow dye from individual glia filled via the patch pipette. These data suggest that EtOH inhibits a calcium-insensitive potassium channel, most likely a passive potassium channel, but also affects gap junction coupling in a way that is sustained after ethanol withdrawal. Astrocytes play a critical role in brain potassium homeostasis, and therefore EtOH effects on astrocytic function could influence neuronal activity.

Pharmacological comparison of swelling-activated excitatory amino acid release and Cl- currents in cultured rat astrocytes

Ubiquitously expressed volume-regulated anion channels (VRACs) are chloride channels which are permeable to a variety of small organic anions, including the excitatory amino acids (EAAs) glutamate and aspartate. Broad spectrum anion channel blockers strongly reduce EAA release in cerebral ischaemia and other pathological states associated with prominent astrocytic swelling. However, it is uncertain whether VRAC serves as a major pathway for EAA release from swollen cells. In the present study, we measured swelling-activated release of EAAs as D-[3H]aspartate efflux, and VRAC-mediated Cl- currents by whole-cell patch clamp in cultured rat astrocytes. We compared the pharmacological profiles of the swelling-activated EAA release pathway and Cl- currents. The expression of candidate Cl- channels was confirmed by RT-PCR. The maxi Cl- channel (p-VDAC) blocker Gd3+, the ClC-2 inhibitor Cd2+, and the MDR-1 blocker verapamil did not affect EAA release or VRAC currents. An antagonist of calcium-sensitive Cl- channels (CaCC), niflumic acid, had little effect on EAA release and only partially inhibited swelling-activated Cl- currents. The phorbol ester PDBu, which blocks ClC-3-mediated Cl- currents, had no effect on VRAC currents and up-regulated EAA release. In contrast, DCPIB, which selectively inhibits VRACs, potently suppressed both EAA release and VRAC currents. Two other relatively selective VRAC inhibitors, tamoxifen and phloretin, also blocked the VRAC currents and strongly reduced EAA release. Taken together, our data suggest that (i) astrocytic volume-dependent EAA release is largely mediated by the VRAC, and (ii) the ClC-2, ClC-3, ClC-4, ClC-5, VDAC, CaCC, MDR-1 and CFTR gene products do not contribute to EAA permeability.

Astrocyte dysfunction in neurological disorders: a molecular perspective

Recent work on glial cell physiology has revealed that glial cells, and astrocytes in particular, are much more actively involved in brain information processing than previously thought. This finding has stimulated the view that the active brain should no longer be regarded solely as a network of neuronal contacts, but instead as a circuit of integrated, interactive neurons and glial cells. Consequently, glial cells could also have as yet unexpected roles in the diseased brain. An improved understanding of astrocyte biology and heterogeneity and the involvement of these cells in pathogenesis offers the potential for developing novel strategies to treat neurological disorders.

Astrocytic and neuronal localization of the scaffold protein Na+/H+ exchanger regulatory factor 2 (NHERF-2) in mouse brain

The Na+/H+ exchanger regulatory factor 2 (NHERF-2) is a scaffold protein that regulates cellular signaling by forming protein complexes. Several proteins known to interact with NHERF-2 are abundantly expressed in the central nervous system, but little is known about NHERF-2 localization in the brain. By using immunohistochemistry combined with light and electron microscopy, we found that many populations of astrocytes, as well as some populations of neurons, were immunopositive for NHERF-2 throughout the mouse brain. Quantitative analysis of the subcellular distribution of NHERF-2 immunostaining in four brain structures, cerebral cortex, hippocampus, striatum, and cerebellar cortex, showed that NHERF-2 was expressed mainly in astrocytic processes but was also sometimes observed in both pre- and postsynaptic neuronal elements. NHERF-2 immunostaining was associated mainly with the plasma membrane of neurons and astrocytes. However, NHERF-2 immunoreactivity was also observed in association with synaptic vesicles in putative glutamatergic axon terminals. The subcellular localization of NHERF-2 in brain is consistent with a role for NHERF-2 in forming complexes between cell surface and cytosolic proteins, and the preferential expression of NHERF-2 in astrocytes suggests that this scaffold protein may play an important role in astrocytic physiology.