Developmental regulation of Na+ and K+ conductances in glial cells of mouse hippocampal brain slices

Dysfunction of NG2 glial cells affects neuronal plasticity and behavior

NG2 glia represents a distinct type of macroglial cells in the CNS and is unique among glia because they receive synaptic input from neurons. They are abundantly present in white and gray matter. While the majority of white matter NG2 glia differentiates into oligodendrocytes, the physiological impact of gray matter NG2 glia and their synaptic input are still ill defined. Here, we asked whether dysfunctional NG2 glia affect neuronal signaling and behavior. We generated mice with inducible deletion of the K⁺ channel Kir4.1 in NG2 glia and performed comparative electrophysiological, immunohistochemical, molecular and behavioral analyses. Kir4.1 was deleted at postnatal day 23-26 (recombination efficiency about 75%) and mice were investigated 3-8 weeks later. Notably, these mice with dysfunctional NG2 glia demonstrated improved spatial memory as revealed by testing new object location recognition while working and social memory remained unaffected. Focussing on the hippocampus, we found that loss of Kir4.1 potentiated synaptic depolarizations of NG2 glia and stimulated the expression of myelin basic protein while proliferation and differentiation of hippocampal NG2 glia remained largely unaffected. Mice with targeted deletion of the K⁺ channel in NG2 glia showed impaired long-term potentiation at CA3-CA1 synapses, which could be fully rescued by extracellular application of a TrkB receptor agonist. Our data demonstrate that proper NG2 glia function is important for normal brain function and behavior.

Therapeutic functions of astrocytes to treat α-synuclein pathology in Parkinson’s disease

Intraneuronal inclusions of misfolded α-synuclein (α-syn) and prion-like spread of the pathologic α-syn contribute to progressive neuronal death in Parkinson’s disease (PD). Despite the pathologic significance, no efficient therapeutic intervention targeting α-synucleinopathy has been developed. In this study, we provide evidence that astrocytes, especially those cultured from the ventral midbrain (VM), show therapeutic potential to alleviate α-syn pathology in multiple in vitro and in vivo α-synucleinopathic models. Regulation of neuronal α-syn proteostasis underlies the therapeutic function of astrocytes. Specifically, VM-derived astrocytes inhibited neuronal α-syn aggregation and transmission in a paracrine manner by correcting not only intraneuronal oxidative and mitochondrial stresses but also extracellular inflammatory environments, in which α-syn proteins are prone to pathologic misfolding. The astrocyte-derived paracrine factors also promoted disassembly of extracellular α-syn aggregates. In addition to the aggregated form of α-syn, VM astrocytes reduced total α-syn protein loads both by actively scavenging extracellular α-syn fibrils and by a paracrine stimulation of neuronal autophagic clearance of α-syn. Transplantation of VM astrocytes into the midbrain of PD model mice alleviated α-syn pathology and protected the midbrain dopamine neurons from neurodegeneration. We further showed that cografting of VM astrocytes could be exploited in stem cell–based therapy for PD, in which host-to-graft transmission of α-syn pathology remains a critical concern for long-term cell therapeutic effects.

Regional heterogeneity of astrocyte morphogenesis dictated by the formin protein, Daam2, modifies circuit function

Astrocytes display extraordinary morphological complexity that is essential to support brain circuit development and function. Formin proteins are key regulators of the cytoskeleton; however, their role in astrocyte morphogenesis across diverse brain regions and neural circuits is unknown. Here, we show that loss of the formin protein Daam2 in astrocytes increases morphological complexity in the cortex and olfactory bulb, but elicits opposing effects on astrocytic calcium dynamics. These differential physiological effects result in increased excitatory synaptic activity in the cortex and increased inhibitory synaptic activity in the olfactory bulb, leading to altered olfactory behaviors. Proteomic profiling and immunoprecipitation experiments identify Slc4a4 as a binding partner of Daam2 in the cortex, and combined deletion of Daam2 and Slc4a4 restores the morphological alterations seen in Daam2 mutants. Our results reveal new mechanisms regulating astrocyte morphology and show that congruent changes in astrocyte morphology can differentially influence circuit function.

Novel Toolboxes for the Investigation of Activity-Dependent Myelination in the Central Nervous System

Myelination is essential for signal processing within neural networks. Emerging data suggest that neuronal activity positively instructs myelin development and myelin adaptation during adulthood. However, the underlying mechanisms controlling activity-dependent myelination have not been fully elucidated. Myelination is a multi-step process that involves the proliferation and differentiation of oligodendrocyte precursor cells followed by the initial contact and ensheathment of axons by mature oligodendrocytes. Conventional end-point studies rarely capture the dynamic interaction between neurons and oligodendrocyte lineage cells spanning such a long temporal window. Given that such interactions and downstream signaling cascades are likely to occur within fine cellular processes of oligodendrocytes and their precursor cells, overcoming spatial resolution limitations represents another technical hurdle in the field. In this mini-review, we discuss how advanced genetic, cutting-edge imaging, and electrophysiological approaches enable us to investigate neuron-oligodendrocyte lineage cell interaction and myelination with both temporal and spatial precision.

Ion Channels and Electrophysiological Properties of Astrocytes: Implications for Emergent Stimulation Technologies

Astrocytes comprise a heterogeneous cell population characterized by distinct morphologies, protein expression and function. Unlike neurons, astrocytes do not generate action potentials, however, they are electrically dynamic cells with extensive electrophysiological heterogeneity and diversity. Astrocytes are hyperpolarized cells with low membrane resistance. They are heavily involved in the modulation of K⁺ and express an array of different voltage-dependent and voltage-independent channels to help with this ion regulation. In addition to these K⁺ channels, astrocytes also express several different types of Na⁺ channels; intracellular Na⁺ signaling in astrocytes has been linked to some of their functional properties. The physiological hallmark of astrocytes is their extensive intracellular Ca²⁺ signaling cascades, which vary at the regional, subregional, and cellular levels. In this review article, we highlight the physiological properties of astrocytes and the implications for their function and influence of network and synaptic activity. Furthermore, we discuss the implications of these differences in the context of optogenetic and DREADD experiments and consider whether these tools represent physiologically relevant techniques for the interrogation of astrocyte function.

Spontaneous Ultraslow Na+ Fluctuations in the Neonatal Mouse Brain

In the neonate forebrain, network formation is driven by the spontaneous synchronized activity of pyramidal cells and interneurons, consisting of bursts of electrical activity and intracellular Ca²⁺ oscillations. By employing ratiometric Na⁺ imaging in tissue slices obtained from animals at postnatal day 2-4 (P2-4), we found that 20% of pyramidal neurons and 44% of astrocytes in neonatal mouse hippocampus also exhibit transient fluctuations in intracellular Na⁺. These occurred at very low frequencies (~2/h), were exceptionally long (~8 min), and strongly declined after the first postnatal week. Similar Na⁺ fluctuations were also observed in the neonate neocortex. In the hippocampus, Na⁺ elevations in both cell types were diminished when blocking action potential generation with tetrodotoxin. Neuronal Na⁺ fluctuations were significantly reduced by bicuculline, suggesting the involvement of GABA_A-receptors in their generation. Astrocytic signals, by contrast, were neither blocked by inhibition of receptors and/or transporters for different transmitters including GABA and glutamate, nor of various Na⁺-dependent transporters or Na⁺-permeable channels. In summary, our results demonstrate for the first time that neonatal astrocytes and neurons display spontaneous ultraslow Na⁺ fluctuations. While neuronal Na⁺ signals apparently largely rely on suprathreshold GABAergic excitation, astrocytic Na⁺ signals, albeit being dependent on neuronal action potentials, appear to have a separate trigger and mechanism, the source of which remains unclear at present.

Heterogeneity of Activity-Induced Sodium Transients between Astrocytes of the Mouse Hippocampus and Neocortex: Mechanisms and Consequences

Activity-related sodium transients induced by glutamate uptake represent a special form of astrocyte excitability. Astrocytes of the neocortex, as opposed to the hippocampus proper, also express ionotropic glutamate receptors, which might provide additional sodium influx. We compared glutamate-related sodium transients in astrocytes and neurons in slices of the neocortex and hippocampus of juvenile mice of both sexes, using widefield and multiphoton imaging. Stimulation of glutamatergic afferents or glutamate application induced sodium transients that were twice as large in neocortical as in hippocampal astrocytes, despite similar neuronal responses. Astrocyte sodium transients were reduced by ∼50% upon blocking NMDA receptors in the neocortex, but not hippocampus. Neocortical, but not hippocampal, astrocytes exhibited marked sodium increases in response to NMDA. These key differences in sodium signaling were also observed in neonates and in adults. NMDA application evoked local calcium transients in processes of neocortical astrocytes, which were dampened upon blocking sodium/calcium exchange (NCX) with KB-R7943 or SEA0400. Mathematical computation based on our data predict that NMDA-induced sodium increases drive the NCX into reverse mode, resulting in calcium influx. Together, our study reveals a considerable regional heterogeneity in astrocyte sodium transients, which persists throughout postnatal development. Neocortical astrocytes respond with much larger sodium elevations to glutamatergic activity than hippocampal astrocytes. Moreover, neocortical astrocytes experience NMDA-receptor-mediated sodium influx, which hippocampal astrocytes lack, and which drives calcium import through reverse NCX. This pathway thereby links sodium to calcium signaling and represents a new mechanism for the generation of local calcium influx in neocortical astrocytes.SIGNIFICANCE STATEMENT Astrocyte calcium signals play a central role in neuron-glia interaction. Moreover, activity-related sodium transients may represent a new form of astrocyte excitability. Here we show that activation of NMDA receptors results in prominent sodium transients in neocortical, but not hippocampal, astrocytes in the mouse brain. NMDA receptor activation is accompanied by local calcium signaling in processes of neocortical astrocytes, which is augmented by sodium-driven reversal of the sodium/calcium exchanger. Our data demonstrate a significant regional heterogeneity in the magnitude and mechanisms of astrocyte sodium transients. They also suggest a close interrelation between NMDA-receptor-mediated sodium influx and calcium signaling through the reversal of sodium/calcium exchanger, thereby establishing a new pathway for the generation of local calcium signaling in astrocyte processes.

Methamphetamine decreases K+ channel function in human fetal astrocytes by activating the trace amine-associated receptor type-1

Methamphetamine (Meth) is a potent and commonly abused psychostimulant. Meth alters neuron and astrocyte activity; yet the underlying mechanism(s) is not fully understood. Here we assessed the impact of acute Meth on human fetal astrocytes (HFAs) using whole-cell patch-clamping. We found that HFAs displayed a large voltage-gated K⁺ efflux (I_Kv ) through K_v /K_v -like channels during membrane depolarization, and a smaller K⁺ influx (I_kir ) via inward-rectifying K_ir /K_ir -like channels during membrane hyperpolarization. Meth at a 'recreational' (20 μM) or toxic/fatal (100 μM) concentration depolarized resting membrane potential (RMP) and suppressed I_Kv/Kv-like . These changes were associated with a decreased time constant (Ƭ), and mimicked by blocking the two-pore domain K⁺ (K_2P )/K_2P -like and K_v /K_v -like channels, respectively. Meth also diminished I_Kir/Kir-like , but only at toxic/fatal levels. Given that Meth is a potent agonist for the trace amine-associated receptor type-1 (TAAR1), and TAAR1-coupled cAMP/cAMP-activated protein kinase (PKA) cascade, we further evaluated whether the Meth impact on K⁺ efflux was mediated by this pathway. We found that antagonizing TAAR1 with N-(3-Ethoxyphenyl)-4-(1-pyrrolidinyl)-3-(trifluoromethyl)benzamide (EPPTB) reversed Meth-induced suppression of I_Kv/Kv-like ; and inhibiting PKA activity by H89 abolished Meth effects on suppressing I_Kv/Kv-like . Antagonizing TAAR1 might also attenuate Meth-induced RMP depolarization. Voltage-gated Ca²⁺ currents were not detected in HFAs. These novel findings demonstrate that Meth suppresses I_Kv/Kv-like by facilitating the TAAR1/G_s /cAMP/PKA cascade and altering the kinetics of K_v /K_v -like channel gating, but reduces K_2P /K_2P -like channel activity through other pathway(s), in HFAs. Given that Meth-induced decrease in astrocytic K⁺ efflux through K_2P /K_2P -like and K_v /K_v -like channels reduces extracellular K⁺ levels, such reduction could consequently contribute to a decreased excitability of surrounding neurons. OPEN SCIENCE BADGES: This article has received a badge for *Open Materials* because it provided all relevant information to reproduce the study in the manuscript. The complete Open Science Disclosure form for this article can be found at the end of the article. More information about the Open Practices badges can be found at https://cos.io/our-services/open-science-badges/.

Cografting astrocytes improves cell therapeutic outcomes in a Parkinson's disease model

Transplantation of neural progenitor cells (NPCs) is a potential therapy for treating neurodegenerative disorders, but this approach has faced many challenges and limited success, primarily because of inhospitable host brain environments that interfere with enriched neuron engraftment and function. Astrocytes play neurotrophic roles in the developing and adult brain, making them potential candidates for helping with modification of hostile brain environments. In this study, we examined whether astrocytic function could be utilized to overcome the current limitations of cell-based therapies in a murine model of Parkinson's disease (PD) that is characterized by dopamine (DA) neuron degeneration in the midbrain. We show here that cografting astrocytes, especially those derived from the midbrain, remarkably enhanced NPC-based cell therapeutic outcomes along with robust DA neuron engraftment in PD rats for at least 6 months after transplantation. We further show that engineering of donor astrocytes with Nurr1 and Foxa2, transcription factors that were recently reported to polarize harmful immunogenic glia into the neuroprotective form, further promoted the neurotrophic actions of grafted astrocytes in the cell therapeutic approach. Collectively, these findings suggest that cografting astrocytes could be a potential strategy for successful cell therapeutic outcomes in neurodegenerative disorders.

Heterogeneity and function of hippocampal macroglia

The contribution of glial cells to normal and impaired hippocampal function is increasingly being recognized, although important questions as to the mechanisms that these cells use for their crosstalk with neurons and capillaries are still unanswered or lead to controversy. Astrocytes in the hippocampus are morphologically variable and a single cell contacts with its processes more than 100,000 synapses. They predominantly express inward rectifier K⁺ channels and transporters serving homeostatic function but may also release gliotransmitters to modify neuronal signaling and brain circulation. Intracellular Ca²⁺ transients are key events in the interaction of astrocytes with neurons and the vasculature. Hippocampal NG2 glia represent a population of cells with proliferative capacity throughout adulthood. Intriguingly, they receive direct synaptic input from pyramidal neurons and interneurons and express a multitude of ion channels and receptors. Despite in-depth knowledge about the features of these transmembrane proteins, the physiological impact of NG2 glial cells and their synaptic input remain nebulous. Because of the low abundance of oligodendrocytes in the hippocampus, limited information is available about their specific properties. Given the multitude of signaling molecules expressed by the various types of hippocampal glial cells (and because of space constraints), we focus, in this review, on those properties that are considered key for the interaction of the respective cell type with its neighborhood.

Modulatory Effects of Perineuronal Oligodendrocytes on Neuronal Activity in the Rat Hippocampus

Action potentials are fundamental to relaying information from region to region in the nervous system. Changes in action potential firing patterns in neural circuits influence how the brain processes information. In our previous study, we focused on interneuron/perineuronal astrocyte pairs in the hippocampal CA1 region and reported that direct depolarization of perineuronal astrocytes modulated the firing pattern of interneurons. In the current study, we investigated the morphological and electrophysiological properties of perineuronal oligodendrocytes, and examined their modulatory effects on interneuronal firing in the CA1 region. Perineuronal oligodendrocytes only had a few processes, which were crooked, intricately twisted, and twined around the soma and proximal region of the main processes of adjacent interneurons. Whole-cell current patterns of perineuronal oligodendrocytes were homogenous and the current-voltage relationship showed remarkable outward rectification. Although the K⁺ channel blockers, tetraethylammonium and 4-aminopyridine, clearly blocked outward currents, Ba²⁺ did not significantly alter whole-cell currents. Unlike perineuronal astrocytes, the depolarization of perineuronal oligodendrocytes had no effect on interneuronal firing; however, when the interneurons were firing at a higher frequency, the hyperpolarization of perineuronal oligodendrocytes suppressed their action potentials. The suppressive effects of perineuronal oligodendrocytes were inhibited in the presence of a low concentration of tetraethylammonium, which selectively blocked deep and fast afterhyperpolarization. These results suggest that perineuronal oligodendrocytes suppress interneuronal firing through their influence on K⁺ channels, which are responsible for deep and fast afterhyperpolarization.

Molecular and cellular physiology of sodium-dependent glutamate transporters

Glutamate is the major excitatory transmitter in the vertebrate brain. After its release from presynaptic nerve terminals, it is rapidly taken up by high-affinity sodium-dependent plasma membrane transporters. While both neurons and glial cells express these excitatory amino acid transporters (EAATs), the majority of glutamate uptake is accomplished by astrocytes, which convert synaptically-released glutamate to glutamine or feed it into their own metabolism. Glutamate uptake by astrocytes not only shapes synaptic transmission by regulating the availability of glutamate to postsynaptic neuronal receptors, but also protects neurons from hyper-excitability and subsequent excitotoxic damage. In the present review, we provide an overview of the molecular and cellular characteristics of sodium-dependent glutamate transporters and their associated anion permeation pathways, with a focus on astrocytic glutamate transport. We summarize their functional properties and roles within tripartite synapses under physiological and pathophysiological conditions, exemplifying the intricate interactions and interrelationships between neurons and glial cells in the brain.

Changes in the proliferative capacity of NG2 cell subpopulations during postnatal development of the mouse hippocampus

Besides astrocytes and oligodendrocytes, NG2 proteoglycan-expressing cells (NG2 glia) represent a third subtype of macroglia in the brain. Originally described as oligodendrocyte precursor cells, they feature several characteristics not expected from mere progenitor cells, including synaptic connections with neurons. There is accumulating evidence that the properties of NG2 glia differ between different brain regions and developmental stages. To further analyze this proposed heterogeneity, we studied electrophysiological properties, transcript and protein expression, distribution and proliferative capacity of NG2 glia during postnatal development, focusing on the hippocampus and corpus callosum. All NG2 glia displayed a 'complex' current pattern consisting of voltage- and time-dependent in- and outward currents. In juvenile mice, Kir current densities and rectification index were highly variable and on average significantly lower than in adult animals. Single cell RT-PCR analyses of electrophysiologically characterized cells demonstrated that different glial genes were expressed at variable extent, independent of developmental stage and genetic background. In the hippocampus proper and the corpus callosum, the density of NG2 glia was highest at postnatal days (P)10-12, decreased by ~50 % at P25-35 and then remained stable in adults (P80-90). Interestingly, co-expression of NG2 and S100β, a marker for mature astrocytes, increased from 7 % at P10-12 to 27 % at P25-35 in the hippocampus proper, and then dropped again in the stratum radiatum at P80-90. In the dentate gyrus and corpus callosum, co-expression of NG2 and S100β was very low (3 %) and constant throughout development. Age-related differences were also observed with Ki-67, a proliferation marker. In NG2 glia of the CA1 region, its expression decreased from 16 % at P10-12 to 9 % (P25-35) and then 3 % (P80-90). Triple-stainings revealed that Ki-67 was also expressed in 2-3 % of NG2/S100β-positive cells in the juvenile and mature stratum radiatum, indicating that the latter, in contrast to S100β-positive astrocytes, still host proliferative potential. Taken together, we found significant differences in transcript and protein expression, electrophysiological properties and proliferative capacity of NG2 glia in the mouse forebrain, suggesting the co-existence of several subpopulations of NG2 glia. Our data thus support the idea of a substantial regional and developmental heterogeneity in this subtype of macroglia.

Astrocytic glutamate transport regulates a Drosophila CNS synapse that lacks astrocyte ensheathment

Anatomical, molecular, and physiological interactions between astrocytes and neuronal synapses regulate information processing in the brain. The fruit fly Drosophila melanogaster has become a valuable experimental system for genetic manipulation of the nervous system and has enormous potential for elucidating mechanisms that mediate neuron-glia interactions. Here, we show the first electrophysiological recordings from Drosophila astrocytes and characterize their spatial and physiological relationship with particular synapses. Astrocyte intrinsic properties were found to be strongly analogous to those of vertebrate astrocytes, including a passive current-voltage relationship, low membrane resistance, high capacitance, and dye-coupling to local astrocytes. Responses to optogenetic stimulation of glutamatergic premotor neurons were correlated directly with anatomy using serial electron microscopy reconstructions of homologous identified neurons and surrounding astrocytic processes. Robust bidirectional communication was present: neuronal activation triggered astrocytic glutamate transport via excitatory amino acid transporter 1 (Eaat1), and blocking Eaat1 extended glutamatergic interneuron-evoked inhibitory postsynaptic currents in motor neurons. The neuronal synapses were always located within 1 μm of an astrocytic process, but none were ensheathed by those processes. Thus, fly astrocytes can modulate fast synaptic transmission via neurotransmitter transport within these anatomical parameters. J. Comp. Neurol. 524:1979-1998, 2016. © 2016 Wiley Periodicals, Inc.

A Novel Mechanism of pH Buffering in C. elegans Glia: Bicarbonate Transport via the Voltage-Gated ClC Cl- Channel CLH-1

An important function of glia is the maintenance of the ionic composition and pH of the synaptic microenvironment. In terms of pH regulation, HCO3 (-) buffering has been shown to be important in both glia and neurons. Here, we used in vivo fluorescent pH imaging and RNA sequencing of the amphid sheath glia of Caenorhabditis elegans to reveal a novel mechanism of cellular HCO3 (-) uptake. While the classical mechanism of HCO3 (-) uptake involves Na(+)/HCO3 (-) cotransporters, here we demonstrate that the C. elegans ClC Cl(-) channel CLH-1 is highly permeable to HCO3 (-) and mediates HCO3 (-) uptake into amphid sheath glia. CLH-1 has homology and electrophysiological properties similar to the mammalian ClC-2 Cl(-) channel. Our data suggest that, in addition to maintaining synaptic Cl(-) concentration, these channels may also be involved in maintenance of synaptic pH via HCO3 (-) flux. These findings provide an exciting new facet of study regarding how pH is regulated in the brain.

SIGNIFICANCE STATEMENT

Maintenance of pH is essential for the physiological function of the nervous system. HCO3 (-) is crucial for pH regulation and is transported into the cell via ion transporters, including ion channels, the molecular identity of which remains unclear. In this manuscript, we describe our discovery that the C. elegans amphid sheath glia regulate intracellular pH via HCO3 (-) flux through the voltage-gated ClC channel CLH-1. This represents a novel function for ClC channels, which has implications for their possible role in mammalian glial pH regulation. This discovery may also provide a novel therapeutic target for pathologic conditions, such as ischemic stroke where acidosis leads to widespread death of glia and subsequently neurons.

Heterogeneous astrocytes: Active players in CNS

Astrocytes, the predominant cell type that are broadly distributed in the brain and spinal cord, play key roles in maintaining homeostasis of the central nerve system (CNS) in physiological and pathological conditions. Increasing evidence indicates that astrocytes are a complex colony with heterogeneity on morphology, gene expression, function and many other aspects depending on their spatio-temporal distribution and activation level. In pathological conditions, astrocytes differentially respond to all kinds of insults, including injury and disease, and participate in the neuropathological process. Based on current studies, we here give an overview of the roles of heterogeneous astrocytes in CNS, especially in neuropathologies, which focuses on biological and functional diversity of astrocytes. We propose that a precise understanding of the heterogeneous astrocytes is critical to unlocking the secrets about pathogenesis and treatment of the mazy CNS.

Electrophysiological behavior of neonatal astrocytes in hippocampal stratum radiatum

BACKGROUND

Neonatal astrocytes are diverse in origin, and undergo dramatic change in gene expression, morphological differentiation and  syncytial networking throughout development. Neonatal astrocytes also play multifaceted roles in neuronal circuitry establishment. However, the extent to which neonatal astrocytes differ from their counterparts in the adult brain remains unknown.

RESULTS

Based on ALDH1L1-eGFP expression or sulforhodamine 101 staining, neonatal astrocytes at postnatal day 1-3 can be reliably identified in hippocampal stratum radiatum. They exhibit a more negative resting membrane potential (V M), -85 mV, than mature astrocytes, -80 mV and a variably rectifying whole-cell current profile due to complex expression of voltage-gated outward transient K(+) (IKa), delayed rectifying K(+) (IKd) and inward K(+) (IKin) conductances. Differing from NG2 glia, depolarization-induced inward Na(+) currents (INa) could not be detected in neonatal astrocytes. A quasi-physiological V M of -69 mV was retained when inwardly rectifying Kir4.1 was inhibited by 100 μM Ba(2+) in both wild type and TWIK-1/TREK-1 double gene knockout astrocytes, indicating expression of additional leak K(+) channels yet unknown. In dual patch recording, electrical coupling was detected in 74 % (14/19 pairs) of neonatal astrocytes with largely variable coupling coefficients. The increasing gap junction coupling progressively masked the rectifying K(+) conductances to account for an increasing number of linear voltage-to-current relationship passive astrocytes (PAs). Gap junction inhibition, by 100 μM meclofenamic acid, substantially reduced membrane conductance and converted all the neonatal PAs to variably rectifying astrocytes. The low density expression of leak K(+) conductance in neonatal astrocytes corresponded  to a ~50 % less K(+) uptake capacity compared to adult astrocytes.

CONCLUSIONS

Neonatal astrocytes predominantly express a variety of rectifying K(+) conductances, form discrete cell-to-cell gap junction coupling and are deficient in K(+) homeostatic capacity.

Sodium channels in astroglia and microglia

Voltage-gated sodium channels are required for electrogenesis in excitable cells. Their activation, triggered by membrane depolarization, generates transient sodium currents that initiate action potentials in neurons, cardiac, and skeletal muscle cells. Cells that have not traditionally been considered to be excitable (nonexcitable cells), including glial cells, also express sodium channels in physiological conditions as well as in pathological conditions. These channels contribute to multiple functional roles that are seemingly unrelated to the generation of action potentials. Here, we discuss the dynamics of sodium channel expression in astrocytes and microglia, and review evidence for noncanonical roles in effector functions of these cells including phagocytosis, migration, proliferation, ionic homeostasis, and secretion of chemokines/cytokines. We also examine possible mechanisms by which sodium channels contribute to the activity of glial cells, with an eye toward therapeutic implications for central nervous system disease. GLIA 2016;64:1628-1645.

Electrophysiological properties of NG2(+) cells: Matching physiological studies with gene expression profiles

NG2(+) glial cells are a dynamic population of non-neuronal cells that give rise to myelinating oligodendrocytes in the central nervous system. These cells express numerous ion channels and neurotransmitter receptors, which endow them with a complex electrophysiological profile that is unique among glial cells. Despite extensive analysis of the electrophysiological properties of these cells, relatively little was known about the molecular identity of the channels and receptors that they express. The generation of new RNA-Seq datasets for NG2(+) cells has provided the means to explore how distinct genes contribute to the physiological properties of these progenitors. In this review, we systematically compare the results obtained through RNA-Seq transcriptional analysis of purified NG2(+) cells to previous physiological and molecular studies of these cells to define the complement of ion channels and neurotransmitter receptors expressed by NG2(+) cells in the mammalian brain and discuss the potential significance of the unique physiological properties of these cells. This article is part of a Special Issue entitled SI:NG2-glia(Invited only).

Interneurons and oligodendrocyte progenitors form a structured synaptic network in the developing neocortex

NG2 cells, oligodendrocyte progenitors, receive a major synaptic input from interneurons in the developing neocortex. It is presumed that these precursors integrate cortical networks where they act as sensors of neuronal activity. We show that NG2 cells of the developing somatosensory cortex form a transient and structured synaptic network with interneurons that follows its own rules of connectivity. Fast-spiking interneurons, highly connected to NG2 cells, target proximal subcellular domains containing GABA_A receptors with γ2 subunits. Conversely, non-fast-spiking interneurons, poorly connected with these progenitors, target distal sites lacking this subunit. In the network, interneuron-NG2 cell connectivity maps exhibit a local spatial arrangement reflecting innervation only by the nearest interneurons. This microcircuit architecture shows a connectivity peak at PN10, coinciding with a switch to massive oligodendrocyte differentiation. Hence, GABAergic innervation of NG2 cells is temporally and spatially regulated from the subcellular to the network level in coordination with the onset of oligodendrogenesis.

DOI:http://dx.doi.org/10.7554/eLife.06953.001

Neurons are outnumbered in the brain by cells called glial cells. The brain contains various types of glial cells that perform a range of different jobs, including the supply of nutrients and the removal of dead neurons. The role of glial cells called oligodendrocytes is to produce a material called myelin: this is an electrical insulator that, when wrapped around a neuron, increases the speed at which electrical impulses can travel through the nervous system.

Neurons communicate with one another through specialized junctions called synapses, and at one time it was thought that only neurons could form synapses in the brain. However, this view had to be revised when researchers discovered synapses between neurons and glial cells called NG2 cells, which go on to become oligodendrocytes. These neuron-NG2 cell synapses have a lot in common with neuron–neuron synapses, but much less is known about them.

Orduz, Maldonado et al. have now examined these synapses in unprecedented detail by analyzing individual synapses between a type of neuron called an interneuron and an NG2 cell in mice aged only a few weeks. Interneurons can be divided into two major classes based on how quickly they fire, and Orduz, Maldonado et al. show that both types of interneuron form synapses with NG2 cells. However, these two types of interneuron establish synapses on different parts of the NG2 cell, and these synapses involve different receptor proteins.

Together, the synapses give rise to a local interneuron-NG2 cell network that reaches a peak of activity roughly two weeks after birth, after which the network is disassembled. This period of peak activity is accompanied by a sudden increase in the maturation of NG2 cells into oligodendrocytes. Further experiments are needed to test the possibility that activity in the interneuron-NG2 cell network acts as the trigger for the NG2 cells to turn into oligodendrocytes, which then supply myelin for the developing brain.

DOI:http://dx.doi.org/10.7554/eLife.06953.002

Voltage-dependent K+ currents contribute to heterogeneity of olfactory ensheathing cells

The olfactory nerve is permissive for axon growth throughout life. This has been attributed in part to the olfactory ensheathing glial cells that encompass the olfactory sensory neuron fascicles. Olfactory ensheathing cells (OECs) also promote axon growth in vitro and when transplanted in vivo to sites of injury. The mechanisms involved remain largely unidentified owing in part to the limited knowledge of the physiological properties of ensheathing cells. Glial cells rely for many functions on the properties of the potassium channels expressed; however, those expressed in ensheathing cells are unknown. Here we show that OECs express voltage-dependent potassium currents compatible with inward rectifier (Kir ) and delayed rectifier (KDR ) channels. Together with gap junction coupling, these contribute to the heterogeneity of membrane properties observed in OECs. The relevance of K(+) currents expressed by ensheathing cells is discussed in relation to plasticity of the olfactory nerve.

Direct conversion of fibroblasts into functional astrocytes by defined transcription factors

Direct cell reprogramming enables direct conversion of fibroblasts into functional neurons and oligodendrocytes using a minimal set of cell-lineage-specific transcription factors. This approach is rapid and simple, generating the cell types of interest in one step. However, it remains unknown whether this technology can be applied to convert fibroblasts into astrocytes, the third neural lineage. Astrocytes play crucial roles in neuronal homeostasis, and their dysfunctions contribute to the origin and progression of multiple human diseases. Herein, we carried out a screening using several transcription factors involved in defining the astroglial cell fate and identified NFIA, NFIB, and SOX9 to be sufficient to convert with high efficiency embryonic and postnatal mouse fibroblasts into astrocytes (iAstrocytes). We proved both by gene-expression profiling and functional tests that iAstrocytes are comparable to native brain astrocytes. This protocol can be then employed to generate functional iAstrocytes for a wide range of experimental applications.

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NFIA, NFIB, and SOX9 reprogram fibroblasts into induced astrocytes (iAstrocytes)

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iAstrocytes reprogramming induces a global change in gene-expression profiling

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iAstrocytes are functionally comparable to native astrocytes

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NFIA, NFIB, and SOX9 induce an astrocytic phenotype in human fibroblasts

Neuron-glia networks: integral gear of brain function

Astrocytes, the most abundant glial cell in the brain, play critical roles in metabolic and homeostatic functions of the Nervous System; however, their participation in coding information and cognitive processes has been largely ignored. The strategic position of astrocyte processes facing synapses and the astrocyte ability to uptake neurotransmitters and release neuroactive substances, so-called “gliotransmitters”, provide the scenario for prolific neuron-astrocyte signaling. From studies at single-cell level to animal behavior, recent advances in technology and genetics have revealed the impact of astrocyte activity in brain function from cellular and synaptic physiology, neuronal circuits to behavior. The present review critically discusses the consequences of astrocyte signaling on synapses and networks, as well as its impact on neuronal information processing, showing that some crucial brain functions arise from the coordinated activity of neuron-glia networks.

The NG2 Protein Is Not Required for Glutamatergic Neuron-NG2 Cell Synaptic Signaling

NG2 glial cells (as from now NG2 cells) are unique in receiving synaptic input from neurons. However, the components regulating formation and maintenance of these neuron-glia synapses remain elusive. The transmembrane protein NG2 has been considered a potential mediator of synapse formation and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) clustering, because it contains 2 extracellular Laminin G/Neurexin/Sex Hormone-Binding Globulin domains, which in neurons are crucial for formation of transsynaptic neuroligin-neurexin complexes. NG2 is connected via Glutamate Receptor-Interacting Protein with GluA2/3-containing AMPARs, thereby possibly mediating receptor clustering in glial postsynaptic density. To elucidate the role of NG2 in neuron-glia communication, we investigated glutamatergic synaptic transmission in juvenile and aged hippocampal NG2 cells of heterozygous and homozygous NG2 knockout mice. Neuron-NG2 cell synapses readily formed in the absence of NG2. Short-term plasticity, synaptic connectivity, postsynaptic AMPAR current kinetics, and density were not affected by NG2 deletion. During development, an NG2-independent acceleration of AMPAR current kinetics and decreased synaptic connectivity were observed. Our results indicate that the lack of NG2 does not interfere with genesis and basic properties of neuron-glia synapses. In addition, we demonstrate frequent expression of neuroligins 1-3 in juvenile and aged NG2 cells, suggesting a role of these molecules in synapse formation between NG2 glia and neurons.

Multiple Modes of Communication between Neurons and Oligodendrocyte Precursor Cells

The surprising discovery of bona fide synapses between neurons and oligodendrocytes precursor cells (OPCs) 15 years ago placed these progenitors as real partners of neurons in the CNS. The role of these synapses has not been established yet, but a main hypothesis is that neuron-OPC synaptic activity is a signaling pathway controlling OPC proliferation/differentiation, influencing the myelination process. However, new evidences describing non-synaptic mechanisms of communication between neurons and OPCs have revealed that neuron-OPC interactions are more complex than expected. The activation of extrasynaptic receptors by ambient neurotransmitter or local spillover and the ability of OPCs to sense neuronal activity through a potassium channel suggest that distinct modes of communication mediate different functions of OPCs in the CNS. This review discusses different mechanisms used by OPCs to interact with neurons and their potential roles during postnatal development and in brain disorders.

Synaptic integration by NG2 cells

NG2 expressing oligodendrocyte precursor cells stand out from other types of glial cells by receiving classical synaptic contacts from many neurons. This unconventional form of signaling between neurons and glial cells enables NG2 cells to receive information about the activity of presynaptic neurons with high temporal and spatial precision and has been postulated to be involved in activity-dependent myelination. While this still unproven concept is generally compelling, how NG2 cells may integrate synaptic input has hardly been addressed to date. Here we review the biophysical characteristics of synaptic currents and membrane properties of NG2 cells and discuss their capabilities to perform complex temporal and spatial signal integration and how this may be important for activity-dependent myelination.

White matter astrocytes in health and disease

Myelination by oligodendrocytes is a highly specialized process that relies on intimate interactions between the axon and the oligodendrocytes. Astrocytes have an important part in facilitating myelination in the CNS, however, comparatively less is known about how they affect myelination. This review therefore summarizes the literature and explores lingering questions surrounding differences between white matter and gray matter astrocytes, how astrocytes support myelination, how their dysfunction in pathological states contributes to myelin pathologies and how astrocytes may facilitate remyelination. We discuss how astrocytes in the white matter are specialized to promote myelination and myelin maintenance by clearance of extracellular ions and neurotransmitters and by secretion of pro-myelinating factors. Additionally, astrocyte-oligodendrocyte coupling via gap junctions is crucial for both myelin formation and maintenance, due to K(+) buffering and possibly metabolic support for oligodendrocytes via the panglial syncytium. Dysfunctional astrocytes aberrantly affect oligodendrocytes, as exemplified by a number of leukodystrophies in which astrocytic pathology is known as the direct cause of myelin pathology. Conversely, in primary demyelinating diseases, such as multiple sclerosis, astrocytes may facilitate remyelination. We suggest that specific manipulation of astrocytes could help prevent myelin pathologies and successfully restore myelin sheaths after demyelination.

Increase in neuroexcitability of unmyelinated C-type vagal ganglion neurons during initial postnatal development of visceral afferent reflex functions

BACKGROUND

Baroreflex gain increase up closely to adult level during initial postnatal weeks, and any interruption within this period will increase the risk of cardiovascular problems in later of life span. We hypothesize that this short period after birth might be critical for postnatal development of vagal ganglion neurons (VGNs).

METHODS

To evaluate neuroexcitability evidenced by discharge profiles and coordinate changes, ion currents were collected from identified A- and C-type VGNs at different developmental stages using whole-cell patch clamping.

RESULTS

C-type VGNs underwent significant age-dependent transition from single action potential (AP) to repetitive discharge. The coordinate changes between TTX-S and TTX-R Na(+) currents were also confirmed and well simulated by computer modeling. Although 4-AP or iberiotoxin age dependently increased firing frequency, AP duration was prolonged in an opposite fashion, which paralleled well with postnatal changes in 4-AP- and iberiotoxin-sensitive K(+) current activity, whereas less developmental changes were verified in A-types.

CONCLUSION

These data demonstrate for the first time that the neuroexcitability of C-type VGNs increases significantly compared with A-types within initial postnatal weeks evidenced by AP discharge profiles and coordinate ion channel changes, which explain, at least in part, that initial postnatal weeks may be crucial for ontogenesis in visceral afferent reflex function.

How do astrocytes shape synaptic transmission? Insights from electrophysiology

A major breakthrough in neuroscience has been the realization in the last decades that the dogmatic view of astroglial cells as being merely fostering and buffering elements of the nervous system is simplistic. A wealth of investigations now shows that astrocytes actually participate in the control of synaptic transmission in an active manner. This was first hinted by the intimate contacts glial processes make with neurons, particularly at the synaptic level, and evidenced using electrophysiological and calcium imaging techniques. Calcium imaging has provided critical evidence demonstrating that astrocytic regulation of synaptic efficacy is not a passive phenomenon. However, given that cellular activation is not only represented by calcium signaling, it is also crucial to assess concomitant mechanisms. We and others have used electrophysiological techniques to simultaneously record neuronal and astrocytic activity, thus enabling the study of multiple ionic currents and in depth investigation of neuro-glial dialogues. In the current review, we focus on the input such approach has provided in the understanding of astrocyte-neuron interactions underlying control of synaptic efficacy.

Oligodendrocyte precursor cells are accurate sensors of local K+ in mature gray matter

Oligodendrocyte precursor cells (OPCs) are the major source of myelinating oligodendrocytes during development. These progenitors are highly abundant at birth and persist in the adult where they are distributed throughout the brain. The large abundance of OPCs after completion of myelination challenges their unique role as progenitors in the healthy adult brain. Here we show that adult OPCs of the barrel cortex sense fine extracellular K(+) increases generated by neuronal activity, a property commonly assigned to differentiated astrocytes rather than to progenitors. Biophysical, pharmacological, and single-cell RT-PCR analyses demonstrate that this ability of OPCs establishes itself progressively through the postnatal upregulation of Kir4.1 K(+) channels. In animals with advanced cortical myelination, extracellular stimulation of layer V axons induces slow K(+) currents in OPCs, which amplitude correlates with presynaptic action potential rate. Moreover, using paired recordings, we demonstrate that the discharge of a single neuron can be detected by nearby adult OPCs, indicating that these cells are strategically located to detect local changes in extracellular K(+) concentration during physiological neuronal activity. These results identify a novel unitary neuron-OPC connection, which transmission does not rely on neurotransmitter release and appears late in development. Beyond their abundance in the mature brain, the postnatal emergence of a physiological response of OPCs to neuronal network activity supports the view that in the adult these cells are not progenitors only.

Albumin is taken up by hippocampal NG2 cells and astrocytes and decreases gap junction coupling

PURPOSE

  Dysfunction of the blood-brain barrier (BBB) and albumin extravasation have been suggested to play a role in the etiology of human epilepsy. In this context, dysfunction of glial cells attracts increasing attention. Our study was aimed to analyze in the hippocampus (1) which cell types internalize albumin injected into the lateral ventricle in vivo, (2) whether internalization into astrocytes impacts their coupling and expression of connexin 43 (Cx43), and (3) whether expression of Kir4.1, the predominating astrocytic K(+) channel subunit, is altered by albumin.

METHODS

  The patch-clamp method was combined with single cell tracer filling to investigate electrophysiologic properties and gap junction coupling (GJC). For cell identification, mice with cell type-specific expression of a fluorescent protein (NG2kiEYFP mice) and immunohistochemistry were employed. Semiquantitative real time polymerase chain reaction (RT-PCR) allowed analysis of Kir4.1 and Cx43 transcript levels.

KEY FINDINGS

  We show that fluorescently labeled albumin is taken up by astrocytes, NG2 cells, and neurons, with NG2 cells standing out in terms of the quantity of uptake. Within 5 days postinjection (dpi), intracellular albumin accumulation was largely reduced suggesting rapid degradation. Electrophysiologic analysis of astrocytes and NG2 cells revealed no changes in their membrane properties at either time point. However, astrocytic GJC was significantly decreased at 1 dpi but returned to control levels within 5 dpi. We found no changes in hippocampal Cx43 transcript expression, suggesting that other mechanisms account for the observed changes in coupling. Kir4.1 mRNA was regulated oppositely in the CA1 stratum radiatum, with a strong increase at 1 dpi followed by a decrease at 5 dpi.

SIGNIFICANCE

  The present study demonstrates that extravasal albumin in the hippocampus induces rapid changes of astrocyte function, which can be expected to impair ion and transmitter homeostasis and contribute to hyperactivity and epileptogenesis. Therefore, astrocytes may represent alternative targets for antiepileptic therapeutic approaches.

Age-dependent remodelling of inhibitory synapses onto hippocampal CA1 oriens-lacunosum moleculare interneurons

Stratum oriens-lacunosum moleculare interneurons (O-LM INs) represent the major element of the hippocampal feedback inhibitory circuit, which provides inhibition to the distal dendritic sites of CA1 pyramidal neurons. Although the intrinsic conductance profile and the properties of glutamatergic transmission to O-LM INs have become a subject of intense investigation, far less is known about the properties of the inhibitory synapses formed onto these cells. Here, we used whole-cell patch-clamp recordings in acute mouse hippocampal slices to study the properties and plasticity of GABAergic inhibitory synapses onto O-LM INs. Surprisingly, we found that the kinetics of inhibitory postsynaptic currents (IPSCs) were slower in mature synapses (P26-40) due to the synaptic incorporation of the α5 subunit of the GABA(A) receptor (a5-GABA(A)R). Moreover, this age-dependent synaptic expression of a5-GABA(A)Rs was directly associated with the emergence of long-term potentiation at IN inhibitory synapses. Finally, the slower time course of IPSCs observed in O-LM INs of mature animals had a profound effect on IN excitability by significantly delaying its spike firing. Our data suggest that GABAergic synapses onto O-LM INs undergo significant modifications during postnatal maturation. The developmental switch in IPSC properties and plasticity is controlled by the synaptic incorporation of the a5-GABA(A)R subunit and may represent a potential mechanism for the age-dependent modifications in the inhibitory control of the hippocampal feedback inhibitory circuit.

Pathologic and phenotypic alterations in a mouse expressing a connexin47 missense mutation that causes Pelizaeus-Merzbacher-like disease in humans

Gap junction channels are intercellular conduits that allow diffusional exchange of ions, second messengers, and metabolites. Human oligodendrocytes express the gap junction protein connexin47 (Cx47), which is encoded by the GJC2 gene. The autosomal recessive mutation hCx47M283T causes Pelizaeus-Merzbacher-like disease 1 (PMLD1), a progressive leukodystrophy characterized by hypomyelination, retarded motor development, nystagmus, and spasticity. We introduced the human missense mutation into the orthologous position of the mouse Gjc2 gene and inserted the mCx47M282T coding sequence into the mouse genome via homologous recombination in embryonic stem cells. Three-week-old homozygous Cx47M282T mice displayed impaired rotarod performance but unchanged open-field behavior. 10-15-day-old homozygous Cx47M282T and Cx47 null mice revealed a more than 80% reduction in the number of cells participating in glial networks after biocytin injections into oligodendrocytes in sections of corpus callosum. Homozygous expression of mCx47M282T resulted in reduced MBP expression and astrogliosis in the cerebellum of ten-day-old mice which could also be detected in Cx47 null mice of the same age. Three-month-old homozygous Cx47M282T mice exhibited neither altered open-field behavior nor impaired rotarod performance anymore. Adult mCx47M282T expressing mice did not show substantial myelin alterations, but homozygous Cx47M282T mice, additionally deprived of connexin32, which is also expressed in oligodendrocytes, died within six weeks after birth and displayed severe myelin defects accompanied by astrogliosis and activated microglia. These results strongly suggest that PMLD1 is caused by the loss of Cx47 channel function that results in impaired panglial coupling in white matter tissue.

Is neuronal communication with NG2 cells synaptic or extrasynaptic?

NG2-expressing glial cells (NG2 cells) represent a major pool of progenitors able to generate myelinating oligodendrocytes, and perhaps astrocytes and neurones, in the postnatal brain. In the last decade, it has been demonstrated that NG2 cells receive functional glutamatergic and GABAergic synapses mediating fast synaptic transmission in different brain regions. However, several controversies exist in this field. While two classes of NG2 cells have been defined by the presence or absence of Na(+) channels, action potential firing and neuronal input, other studies suggest that all NG2 cells possess Na(+) conductances and are the target of quantal neuronal release, but are unable to trigger action potential firing. Here we bring new evidence supporting the idea that the level of expression of Na(+) conductances is not a criterion to discriminate NG2 cell subpopulations in the somatosensory cortex. Surprisingly, recent reports demonstrated that NG2 cells detect quantal glutamate release from unmyelinated axons in white matter regions. Yet, it is difficult from these studies to establish whether axonal vesicular release in white matter occurs at genuine synaptic junctions or at ectopic release sites. In addition, we recently reported a new mode of extrasynaptic communication between neurones and NG2 cells that relies on pure GABA spillover and does not require GABAergic synaptic input. This review discusses the properties of quantal neuronal release onto NG2 cells and gives an extended overview of potential extrasynaptic modes of transmission, from ectopic to diffuse volume transmission, between neurones and NG2 cells in the brain.

Nitric oxide regulates astrocyte maturation in the hippocampus: involvement of NOS2

Neurons and astrocytes are generated sequentially from radial glia. Once neurogenesis is completed, radial glia starts to differentiate into immature astrocytes. Astrocytes then maturate and change their morphology and electrophysiological properties. Neurotrophic cytokines or bone morphogenetic proteins have been identified as inducers of the developmental switch from neurogenesis to astrogenesis. However, the factors and mechanisms regulating the late differentiation of radial glia and the subsequent astrocyte maturation are poorly described. We used two independent approaches to examine the role of nitric oxide (NO) in the process of astrogenesis and maturation of astrocytes. First using a pharmacological approach, we depleted NO from developing hippocampus by intraventricular injection of a specific scavenger. Then by a genetic approach, we analyzed a nitric oxide synthase-2 (NOS2) knockout mouse. In both models, we found that differentiation of RC2-positive radial glia into late GFAP-positive radial glia was impaired. The cell-fate analysis after incorporation of BrdU demonstrated that astrogenesis was not altered upon NOS2 deficiency. Maturation of astrocytes was assessed by electrophysiological recordings at P7 and functional analysis. In wild type, 20% of astrocytes were immature as shown by their non-linear I/V relationship and high membrane resistance, whereas in NOS2-/- hippocampus, 51% of the astrocytes displayed an immature profile. The reduced branching of astrocytes upon NOS2 deficiency and their low content in connexin-43 further confirmed their immature profile. Our results highlight a novel developmental role of NO and NOS2 in the differentiation of radial glia and the maturation of astrocytes.

Anti-inflammatory effects of the anticonvulsant drug levetiracetam on electrophysiological properties of astroglia are mediated via TGFβ1 regulation

BACKGROUND AND PURPOSE

The involvement of astrocytes as immune-competent players in inflammation and the pathogenesis of epilepsy and seizure-induced brain damage has recently been recognized. In clinical trials and practice, levetiracetam (LEV) has proven to be an effective antiepileptic drug (AED) in various forms of epileptic seizures, when applied as mono- or added therapy. Little is known about the mechanism(s) of action of LEV. Evidence so far suggests a mode of action different from that of classical AEDs. We have shown that LEV restored functional gap junction coupling and basic membrane properties in an astrocytic inflammatory model in vitro.

EXPERIMENTAL APPROACH

Here, we used neonatal rat astrocytes co-cultured with high proportions (30%) of activated microglia or treated with the pro-inflammatory cytokine interleukin-1β to provoke inflammatory responses. Effects of LEV (50 µg·mL⁻¹) on electrophysiological properties of astrocytes (by whole cell patch clamp) and on secretion of TGFβ1 (by (ELISA)) were studied in these co-cultures.

KEY RESULTS

LEV restored impaired astrocyte membrane resting potentials via modification of inward and outward rectifier currents, and promoted TGFβ1 expression in inflammatory and control co-cultures. Furthermore, LEV and TGFβ1 exhibited similar facilitating effects on the generation of astrocyte voltage-gated currents in inflammatory co-cultures and the effects of LEV were prevented by antibody to TGFβ1.

CONCLUSIONS AND IMPLICATIONS

Our data suggest that LEV is likely to reduce the harmful spread of excitation elicited by seizure events within the astro-glial functional syncytium, with stabilizing consequences for neuronal-glial interactions.

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.

Oligodendrocytes in mouse corpus callosum are coupled via gap junction channels formed by connexin47 and connexin32

According to previously published ultrastructural studies, oligodendrocytes in white matter exhibit gap junctions with astrocytes, but not among each other, while in vitro oligodendrocytes form functional gap junctions. We have studied functional coupling among oligodendrocytes in acute slices of postnatal mouse corpus callosum. By whole-cell patch clamp we dialyzed oligodendrocytes with biocytin, a gap junction-permeable tracer. On average 61 cells were positive for biocytin detected by labeling with streptavidin-Cy3. About 77% of the coupled cells stained positively for the oligodendrocyte marker protein CNPase, 9% for the astrocyte marker GFAP and 14% were negative for both CNPase and GFAP. In the latter population, the majority expressed Olig2 and some NG2, markers for oligodendrocyte precursors. Oligodendrocytes are known to express Cx47, Cx32 and Cx29, astrocytes Cx43 and Cx30. In Cx47-deficient mice, the number of coupled cells was reduced by 80%. Deletion of Cx32 or Cx29 alone did not significantly reduce the number of coupled cells, but coupling was absent in Cx32/Cx47-double-deficient mice. Cx47-ablation completely abolished coupling of oligodendrocytes to astrocytes. In Cx43-deficient animals, oligodendrocyte-astrocyte coupling was still present, but coupling to oligodendrocyte precursors was not observed. In Cx43/Cx30-double deficient mice, oligodendrocyte-to-astrocyte coupling was almost absent. Uncoupled oligodendrocytes showed a higher input resistance. We conclude that oligodendrocytes in white matter form a functional syncytium predominantly among each other dependent on Cx47 and Cx32 expression, while astrocytic connexins expression can promote the size of this network.

Neuron-glia synapses in the brain

The ability to investigate the electrophysiological properties of individual cells in acute brain tissue led to the discovery that many glial cells have the capacity to respond rapidly to neuronal activity. In particular, a distinct class of neuroglial cells known as NG2 cells, which exhibit many of the properties that have been described for glial subtypes such as complex cells, polydendrocytes, synantocytes and GluR cells, express ionotropic receptors for glutamate and GABA. In both gray and white matter, NG2 cells form direct synaptic junctions with axons, which enable transient activation of these receptors. Electrophysiological analyses have shown that these neuron-glia synapses exhibit all the hallmarks of 'classical' neuron-neuron synapses, including rapid activation, quantized responses, facilitation and depression, and presynaptic inhibition. Electron microscopy indicates that axons form morphologically distinct junctions at discrete sites along processes of NG2 cells, suggesting that NG2 cells are an overt target of axonal projections. AMPA receptors expressed by NG2 cells exhibit varying degrees of Ca(2+) permeability, depending on the brain region and stage of development, and in white matter NG2 cells have also been shown to express functional NMDA receptors. Ca(2+) influx through AMPA receptors following repetitive stimulation can trigger long term potentiation of synaptic currents in NG2 cells. The expression of receptors with significant Ca(2+) permeability may increase the susceptibility of NG2 cells to excitotoxic injury. Future studies using transgenic mice in which expression of receptors can be manipulated selectively in NG2 cells have to define the functions of this enigmatic neuron-glia signaling in the normal and diseased CNS.

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.

The endocannabinoid anandamide inhibits potassium conductance in rat cortical astrocytes

Endocannabinoids are a family of endogenous signaling molecules that modulate neuronal excitability in the central nervous system (CNS) by interacting with cannabinoid (CB) receptors. In spite of the evidence that astroglial cells also possess CB receptors, there is no information on the role of endocannabinoids in regulating CNS function through the modulation of ion channel-mediated homeostatic mechanisms in astroglial cells. We provide electrophysiological evidence that the two brain endocannabinoids anandamide (AEA) and 2-arachidonylglycerol (2-AG) markedly depress outward conductance mediated by delayed outward rectifier potassium current (IK(DR)) in primary cultured rat cortical astrocytes. Pharmacological experiments suggest that the effect of AEA does not result from the activation of known CB receptors. Moreover, neither the production of AEA metabolites nor variations in free cytosolic calcium are involved in the negative modulation of IK(DR). We show that the action of AEA is mediated by its interaction with the extracellular leaflet of the plasma membrane. Similar experiments performed in situ in cortical slices indicate that AEA downregulates IK(DR) in complex and passive astroglial cells. Moreover, IK(DR) is also inhibited by AEA in NG2 glia. Collectively, these results support the notion that endocannabinoids may exert their modulation of CNS function via the regulation of homeostatic function of the astroglial syncytium mediated by ion channel activity.

Role of Kir4.1 channels in growth control of glia

The inwardly rectifying potassium channel Kir4.1 is widely expressed by astrocytes throughout the brain. Kir4.1 channels are absent in immature, proliferating glial cells. The progressive expression of Kir4.1 correlates with astrocyte differentiation and is characterized by the establishment of a negative membrane potential (> -70 mV) and an exit from the cell cycle. Despite some correlative evidence, a mechanistic interdependence between Kir4.1 expression, membrane hyperpolarization, and control of cell proliferation has not been demonstrated. To address this question, we used astrocyte-derived tumors (glioma) that lack functional Kir4.1 channels, and generated two glioma cell lines that stably express either AcGFP-tagged Kir4.1 channels or AcGFP vectors only. Kir4.1 expression confers the same K+ conductance to glioma membranes and a similar responsiveness to changes in [K+]o that characterizes differentiated astrocytes. Kir4.1 expression was sufficient to move the resting potential of gliomas from -50 to -80 mV. Importantly, Kir4.1 expression impaired cell growth by shifting a significant number of cells from the G2/M phase into the quiescent G0/G1 stage of the cell cycle. Furthermore, these effects could be nullified entirely if Kir4.1 channels were either pharmacologically inhibited by 100 microM BaCl2 or if cells were chronically depolarized by 20 mM KCl to the membrane voltage of growth competent glioma cells. These studies therefore demonstrate directly that Kir4.1 causes a membrane hyperpolarization that is sufficient to account for the growth attenuation, which in turn induces cell maturation characterized by a shift of the cells from G2/M into G0/G1.

Developmental profile and properties of sulforhodamine 101--Labeled glial cells in acute brain slices of rat hippocampus

The reliable identification of astrocytes for physiological measurements was always time-consuming and difficult. Recently, the fluorescent dye sulforhodamine 101 (SR101) was reported to label cortical glial cells in vivo [Nimmerjahn A, Kirchhoff F, Kerr JN, Helmchen F. Sulforhodamine 101 as a specific marker of astroglia in the neocortex in vivo. Nat Methods 2004;1:31-7]. We adapted this technique for use in acute rat hippocampal slices at early postnatal stages (P3, 7, 15) and in young adults (P24-27) and describe a procedure for double-labeling of SR101 and ion-selective dyes. Using whole-cell patch-clamp, imaging, and immunohistochemistry, we characterized the properties of SR101-positive versus SR101-negative cells in the stratum radiatum. Our data show that SR101, in contrast to Fura-2 or SBFI, only stains a subset of glial cells. Throughout development, SR101-positive and SR101-negative cells differ in their basic membrane properties. Furthermore, SR101-positive cells undergo a developmental switch from variably rectifying to passive between P3 and P15 and lack voltage-gated Na+ currents. At P15, the majority of SR101-positive cells is positive for GFAP. Thus, our data demonstrate that SR101 selectively labels a subpopulation of glial cells in early juvenile hippocampi that shows the typical developmental changes and characteristics of classical astrocytes. Owing to its reliability and uncomplicated handling, we expect that this technique will be helpful in future investigations studying astrocytes in the developing brain.

Glutamate-induced exocytosis of glutamate from astrocytes

Recent studies indicate that astrocytes can play a much more active role in neuronal circuits than previously believed, by releasing neurotransmitters such as glutamate and ATP. Here we report that local application of glutamate or glutamine synthetase inhibitors induces astrocytic release of glutamate, which activates a slowly decaying transient inward current (SIC) in CA1 pyramidal neurons and a transient inward current in astrocytes in hippocampal slices. The occurrence of SICs was accompanied by an appearance of large vesicles around the puffing pipette. The frequency of SICs was positively correlated with [glutamate]o. EM imaging of anti-glial fibrillary acid protein-labeled astrocytes showed glutamate-induced large astrocytic vesicles. Imaging of FM 1-43 fluorescence using two-photon laser scanning microscopy detected glutamate-induced formation and fusion of large vesicles identified as FM 1-43-negative structures. Fusion of large vesicles, monitored by collapse of vesicles with a high intensity FM 1-43 stain in the vesicular membrane, coincided with SICs. Glutamate induced two types of large vesicles with high and low intravesicular [Ca2+]. The high [Ca2+] vesicle plays a major role in astrocytic release of glutamate. Vesicular fusion was blocked by infusing the Ca2+ chelator, 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, or the SNARE blocker, tetanus toxin, suggesting Ca2+- and SNARE-dependent fusion. Infusion of the vesicular glutamate transport inhibitor, Rose Bengal, reduced astrocytic glutamate release, suggesting the involvement of vesicular glutamate transports in vesicular transport of glutamate. Our results demonstrate that local [glutamate]o increases induce formation and exocytotic fusion of glutamate-containing large astrocytic vesicles. These large vesicles could play important roles in the feedback control of neuronal circuits and epileptic seizures.