Long-term potentiation depends on release of D-serine from astrocytes

D-serine regulates cerebellar LTD and motor coordination through the δ2 glutamate receptor

D-serine (D-Ser) is an endogenous co-agonist for NMDA receptors and regulates neurotransmission and synaptic plasticity in the forebrain. D-Ser is also found in the cerebellum during the early postnatal period. Although D-Ser binds to the δ2 glutamate receptor (GluD2, Grid2) in vitro, its physiological significance has remained unclear. Here we show that D-Ser serves as an endogenous ligand for GluD2 to regulate long-term depression (LTD) at synapses between parallel fibers and Purkinje cells in the immature cerebellum. D-Ser was released mainly from Bergmann glia after the burst stimulation of parallel fibers in immature, but not mature, cerebellum. D-Ser rapidly induced endocytosis of AMPA receptors and mutually occluded LTD in wild-type, but not Grid2-null, Purkinje cells. Moreover, mice expressing mutant GluD2 in which the binding site for D-Ser was disrupted showed impaired LTD and motor dyscoordination during development. These results indicate that glial D-Ser regulates synaptic plasticity and cerebellar functions by interacting with GluD2.

Imaging of Ca2+ and related signaling molecules and investigation of their functions in the brain

Intracellular Ca(2+) signaling, and related mechanisms involving inositol 1,4,5-trisphosphate (IP(3)), nitric oxide, and the excitatory neurotransmitter glutamate, play a major role in the regulation of cellular function in the brain. Due to the complex morphology of central neurons, the correct spatiotemporal distribution of signaling molecules is essential. Thus, imaging studies have been particularly useful in elucidating the functions of these signaling molecules. The advancement of imaging methods, together with the development of a new method for the specific inhibition of intracellular IP(3) signaling, have made it possible to identify pathways that are regulated by Ca(2+) signals in the brain, including Ca(2+)-dependent synaptic maintenance and glial cell-dependent neurite growth. Further investigation of Ca(2+)-related signaling is expected to increase our understanding of brain function in the future.

SNO-ing at the nociceptive synapse?

Nitric oxide is generally considered a pronociceptive retrograde transmitter that, by activation of soluble guanylyl cyclase-mediated cGMP production and activation of cGMP-dependent protein kinase, drives nociceptive hypersensitivity. The duality of its functions, however, is increasingly recognized. This review summarizes nitric-oxide-mediated direct S-nitrosylation of target proteins that may modify nociceptive signaling, including glutamate receptors and G-protein-coupled receptors, transient receptor potential channels, voltage-gated channels, proinflammatory enzymes, transcription factors, and redoxins. S-Nitrosylation events require close proximity of nitric oxide production and target proteins and a permissive redox state in the vicinity. Despite the diversity of potential targets and effects, three major schemes arise that may affect nociceptive signaling: 1) S-Nitrosylation-mediated changes of ion channel gating properties, 2) modulation of membrane fusion and fission, and thereby receptor and channel membrane insertion, and 3) modulation of ubiquitination, and thereby protein degradation or transcriptional activity. In addition, S-Nitrosylation may alter the production of nitric oxide itself.

A proposed mechanism for autism: an aberrant neuroimmune response manifested as a psychiatric disorder

Autism, an incurable neurodevelopmental brain disorder, is a complex psychopathology in which the affected individual cannot effectively self-regulate their sensory inputs toward coherent and focused motor outputs. There have been many hypotheses as to the etiology of autism - genetics, neurotransmitter imbalances, early childhood immunizations, xenobiotic and teratogenic agents, and maternal infection; the disorder can perhaps be studied best under the field of "Psychoneuroimmunology", which analyzes systemic and psychopathologies from an integrated approach through the combined effects of the nervous, immune, and endocrine systems. Using principles of psychoneuroimmunology along with previously established but yet un-linked scientific principles and observations, this paper proposes a neuroimmune-based mechanistic hypothesis for the etiology of autism that connects elevated levels of maternal pro-inflammatory cytokines to autistic symptoms in her offspring through a logical sequence of events. While both researchers and clinicians often note correlations between pro-inflammatory cytokine levels and autistic symptoms in affected individuals, no specific mechanism has been documented that logically and directly connects the two. I propose that pro-inflammatory cytokines arising from maternal inflammation, infection, and, possibly, autoimmunity, pass through the placenta; enter the fetal circulation; cross the fetal blood-brain barrier (BBB); and cause aberrant neuronal growth and plasticity within the fetal brain via a "cytokine-storm". Microglia and astrocyte stimulation lead to a positive-feedback loop that also facilitates the development of a chronic inflammatory environment within the fetus, pre-disposing it to lifelong comorbid psychiatric and systemic pathologies. Such a mechanism could account for many of the observed symptoms and behaviors of autistic individuals such as hyper-sensitivity to environmental stimuli, object fixation, echolalia, repetitive physical behaviors, chronic enterocolitis, autoimmune disease, and, at the extreme, savantism. The thiazolidinedione pioglitazone (and possibly rosiglitazone), a non-steroidal anti-inflammatory drug (NSAID), which is commonly used to lower blood glucose levels and associated inflammatory markers in patients with diabetes, and histamine receptor blockers, as well as monitoring and limiting sucrose-containing foods, might prove to be effective preventative therapies for the development of autism in the fetus for pregnant women displaying either a cytokine-induced depression or other elevated systemic inflammatory state conditions.

Glutamate released spontaneously from astrocytes sets the threshold for synaptic plasticity

Astrocytes exhibit spontaneous calcium oscillations that could induce the release of glutamate as gliotransmitter in rat hippocampal slices. However, it is unknown whether this spontaneous release of astrocytic glutamate may contribute to determining the basal neurotransmitter release probability in central synapses. Using whole-cell recordings and Ca(2+) imaging, we investigated the effects of the spontaneous astrocytic activity on neurotransmission and synaptic plasticity at CA3-CA1 hippocampal synapses. We show here that the metabolic gliotoxin fluorocitrate (FC) reduces the amplitude of evoked excitatory postsynaptic currents and increases the paired-pulse facilitation, mainly due to the reduction of the neurotransmitter release probability and the synaptic potency. FC also decreased intracellular Ca(2+) signalling and Ca(2+) -dependent glutamate release from astrocytes. The addition of glutamine rescued the effects of FC over the synaptic potency; however, the probability of neurotransmitter release remained diminished. The blockage of group I metabotropic glutamate receptors mimicked the effects of FC on the frequency of miniature synaptic responses. In the presence of FC, the Ca(2+) chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N ',N '-tetra-acetate or group I metabotropic glutamate receptor antagonists, the excitatory postsynaptic current potentiation induced by the spike-timing-dependent plasticity protocol was blocked, and it was rescued by delivering a stronger spike-timing-dependent plasticity protocol. Taken together, these results suggest that spontaneous glutamate release from astrocytes contributes to setting the basal probability of neurotransmitter release via metabotropic glutamate receptor activation, which could be operating as a gain control mechanism that regulates the threshold of long-term potentiation. Therefore, endogenous astrocyte activity provides a novel non-neuronal mechanism that could be critical for transferring information in the central nervous system.

Non-neuronal, slow GABA signalling in the ventrobasal thalamus targets δ-subunit-containing GABA(A) receptors

The rodent ventrobasal (VB) thalamus contains a relatively uniform population of thalamocortical (TC) neurons that receive glutamatergic input from the vibrissae and the somatosensory cortex, and inhibitory input from the nucleus reticularis thalami (nRT). In this study we describe γ-aminobutyric acid (GABA)(A) receptor-dependent slow outward currents (SOCs) in TC neurons that are distinct from fast inhibitory postsynaptic currents (IPSCs) and tonic currents. SOCs occurred spontaneously or could be evoked by hypo-osmotic stimulus, and were not blocked by tetrodotoxin, removal of extracellular Ca(2+) or bafilomycin A1, indicating a non-synaptic, non-vesicular GABA origin. SOCs were more common in TC neurons of the VB compared with the dorsal lateral geniculate nucleus, and were rarely observed in nRT neurons, whilst SOC frequency in the VB increased with age. Application of THIP, a selective agonist at δ-subunit-containing GABA(A) receptors, occluded SOCs, whereas the benzodiazepine site inverse agonist β-CCB had no effect, but did inhibit spontaneous and evoked IPSCs. In addition, the occurrence of SOCs was reduced in mice lacking the δ-subunit, and their kinetics were also altered. The anti-epileptic drug vigabatrin increased SOC frequency in a time-dependent manner, but this effect was not due to reversal of GABA transporters. Together, these data indicate that SOCs in TC neurons arise from astrocytic GABA release, and are mediated by δ-subunit-containing GABA(A) receptors. Furthermore, these findings suggest that the therapeutic action of vigabatrin may occur through the augmentation of this astrocyte-neuron interaction, and highlight the importance of glial cells in CNS (patho) physiology.

Glycine inhibitory dysfunction turns touch into pain through astrocyte-derived D-serine

Glycine inhibitory dysfunction provides a useful experimental model for studying the mechanism of dynamic mechanical allodynia, a widespread and intractable symptom of neuropathic pain. In this model, allodynia expression relies on N-methyl-d-aspartate receptors (NMDARs), and it has been shown that astrocytes can regulate their activation through the release of the NMDAR coagonist d-serine. Recent studies also suggest that astrocytes potentially contribute to neuropathic pain. However, the involvement of astrocytes in dynamic mechanical allodynia remains unknown. Here, we show that after blockade of glycine inhibition, orofacial tactile stimuli activated medullary dorsal horn (MDH) astrocytes, but not microglia. Accordingly, the glia inhibitor fluorocitrate, but not the microglia inhibitor minocycline, prevented allodynia. Fluorocitrate also impeded activation of astrocytes and blocked activation of the superficial MDH neural circuit underlying allodynia, as revealed by study of Fos expression. MDH astrocytes are thus required for allodynia. They may also produce d-serine because astrocytic processes were selectively immunolabeled for serine racemase, the d-serine synthesizing enzyme. Accordingly, selective degradation of d-serine with d-amino acid oxidase applied in vivo prevented allodynia and activation of the underlying neural circuit. Conversely, allodynia blockade by fluorocitrate was reversed by exogenous d-serine. These results suggest the following scenario: removal of glycine inhibition makes tactile stimuli able to activate astrocytes; activated astrocytes may provide d-serine to enable NMDAR activation and thus allodynia. Such a contribution of astrocytes to pathological pain fuels the emerging concept that astrocytes are critical players in pain signaling. Glycine disinhibition makes tactile stimuli able to activate astrocytes, which may provide d-serine to enable NMDA receptor activation and thus allodynia.

Astrocyte-neuron lactate transport is required for long-term memory formation

We report that, in the rat hippocampus, learning leads to a significant increase in extracellular lactate levels that derive from glycogen, an energy reserve selectively localized in astrocytes. Astrocytic glycogen breakdown and lactate release are essential for long-term but not short-term memory formation, and for the maintenance of long-term potentiation (LTP) of synaptic strength elicited in vivo. Disrupting the expression of the astrocytic lactate transporters monocarboxylate transporter 4 (MCT4) or MCT1 causes amnesia, which, like LTP impairment, is rescued by L-lactate but not equicaloric glucose. Disrupting the expression of the neuronal lactate transporter MCT2 also leads to amnesia that is unaffected by either L-lactate or glucose, suggesting that lactate import into neurons is necessary for long-term memory. Glycogenolysis and astrocytic lactate transporters are also critical for the induction of molecular changes required for memory formation, including the induction of phospho-CREB, Arc, and phospho-cofilin. We conclude that astrocyte-neuron lactate transport is required for long-term memory formation.

Learning an operant conditioning task differentially induces gliogenesis in the medial prefrontal cortex and neurogenesis in the hippocampus

Circuit modification associated with learning and memory involves multiple events, including the addition and remotion of newborn cells trough adulthood. Adult neurogenesis and gliogenesis were mainly described in models of voluntary exercise, enriched environments, spatial learning and memory task; nevertheless, it is unknown whether it is a common mechanism among different learning paradigms, like reward dependent tasks. Therefore, we evaluated cell proliferation, neurogenesis, astrogliogenesis, survival and neuronal maturation in the medial prefrontal cortex (mPFC) and the hippocampus (HIPP) during learning an operant conditioning task. This was performed by using endogenous markers of cell proliferation, and a bromodeoxiuridine (BrdU) injection schedule in two different phases of learning. Learning an operant conditioning is divided in two phases: a first phase when animals were considered incompletely trained (IT, animals that were learning the task) when they performed between 50% and 65% of the responses, and a second phase when animals were considered trained (Tr, animals that completely learned the task) when they reached 100% of the responses with a latency time lower than 5 seconds. We found that learning an operant conditioning task promoted cell proliferation in both phases of learning in the mPFC and HIPP. Additionally, the results presented showed that astrogliogenesis was induced in the medial prefrontal cortex (mPFC) in both phases, however, the first phase promoted survival of these new born astrocytes. On the other hand, an increased number of new born immature neurons was observed in the HIPP only in the first phase of learning, whereas, decreased values were observed in the second phase. Finally, we found that neuronal maturation was induced only during the first phase. This study shows for the first time that learning a reward-dependent task, like the operant conditioning, promotes neurogenesis, astrogliogenesis, survival and neuronal maturation depending on the learning phase in the mPFC-HIPP circuit.

Primary motor cortex in stroke: a functional MRI-guided proton MR spectroscopic study

BACKGROUND AND PURPOSE

Our goal was to investigate whether certain metabolites, specific to neurons, glial cells, or the neuronal-glial neurotransmission system, in primary motor cortices (M1), are altered and correlated with clinical motor severity in chronic stroke.

METHODS

Fourteen survivors of a single ischemic stroke located outside the M1 and 14 age-matched healthy control subjects were included. At >6 months after stroke, N-acetylaspartate, myo-inositol, and glutamate/glutamine were measured using proton magnetic resonance spectroscopic imaging (in-plane resolution=5×5 mm(2)) in radiologically normal-appearing gray matter of the hand representation area, identified by functional MRI, in each M1. Metabolite concentrations and analyses of metabolite correlations within M1 were determined. Relationships between metabolite concentrations and arm motor impairment were also evaluated.

RESULTS

The stroke survivors showed lower N-acetylaspartate and higher myo-inositol across ipsilesional and contralesional M1 compared with control subjects. Significant correlations between N-acetylaspartate and glutamate/glutamine were found in either M1. Ipsilesional N-acetylaspartate and glutamate/glutamine were positively correlated with arm motor impairment and contralesional N-acetylaspartate with time after stroke.

CONCLUSIONS

Our preliminary data demonstrated significant alterations of neuronal-glial interactions in spared M1 with the ipsilesional alterations related to stroke severity and contralesional alterations to stroke duration. Thus, MR spectroscopy might be a sensitive method to quantify relevant metabolite changes after stroke and consequently increase our knowledge of the factors leading from these changes in spared motor cortex to motor impairment after stroke.

Opposing actions of endothelin-1 on glutamatergic transmission onto vasopressin and oxytocin neurons in the supraoptic nucleus

Endothelin (ET-1) given centrally has many reported actions on hormonal and autonomic outputs from the CNS. However, it is unclear whether these effects are due to local ischemia via its vasoconstrictor properties or to a direct neuromodulatory action. ET-1 stimulates the release of oxytocin (OT) and vasopressin (VP) from supraoptic magnocellular (MNCs) neurons in vivo; therefore, we asked whether ET-1 modulates the excitatory inputs onto MNCs that are critical in sculpting the activity of these neurons. To investigate whether ET-1 modulates excitatory synaptic transmission, we obtained whole-cell recordings and analyzed quantal glutamate release onto MNCs in the supraoptic nucleus (SON). Neurons identified as VP-containing neurosecretory cells displayed a decrease in quantal frequency in response to ET-1 (10-100 pm). This decrease was mediated by ET(A) receptor activation and production of a retrograde messenger that targets presynaptic cannabinoid-1 receptors. In contrast, neurons identified as OT-containing MNCs displayed a transient increase in quantal glutamate release in response to ET-1 application via ET(B) receptor activation. Application of TTX to block action potential-dependent glutamate release inhibited the excitatory action of ET-1 in OT neurons. There were no changes in quantal amplitude in either MNC type, suggesting that the effects of ET-1 were via presynaptic mechanisms. A gliotransmitter does not appear to be involved as ET-1 failed to elevate astrocytic calcium in the SON. Our results demonstrate that ET-1 differentially modulates glutamate release onto VP- versus OT-containing MNCs, thus implicating it in the selective regulation of neuroendocrine output from the SON.

Contributions of the D-serine pathway to schizophrenia

The glutamate neurotransmitter system is one of the major candidate pathways for the pathophysiology of schizophrenia, and increased understanding of the pharmacology, molecular biology and biochemistry of this system may lead to novel treatments. Glutamatergic hypofunction, particularly at the NMDA receptor, has been hypothesized to underlie many of the symptoms of schizophrenia, including psychosis, negative symptoms and cognitive impairment. This review will focus on D-serine, a co-agonist at the NMDA receptor that in combination with glutamate, is required for full activation of this ion channel receptor. Evidence implicating D-serine, NMDA receptors and related molecules, such as D-amino acid oxidase (DAO), G72 and serine racemase (SRR), in the etiology or pathophysiology of schizophrenia is discussed, including knowledge gained from mouse models with altered D-serine pathway genes and from preliminary clinical trials with D-serine itself or compounds modulating the D-serine pathway. Abnormalities in D-serine availability may underlie glutamatergic dysfunction in schizophrenia, and the development of new treatments acting through the D-serine pathway may significantly improve outcomes for many schizophrenia patients.

Molecular signals of plasticity at the tetrapartite synapse

The emergence of astroglia as an important participant of the synaptic machinery has led to the 'tripartite synapse' hypothesis. Recent findings suggest that synaptic signaling also involves the surrounding extracellular matrix (ECM). The ECM can incorporate and store molecular traces of both neuronal and glial activities. It can also modulate function of local receptors or ion channels and send diffuse molecular signals using products of its use-dependent proteolytic cleavage. Recent experimental findings implicate the ECM in mechanisms of synaptic plasticity and glial remodeling, thus lending support to the 'tetrapartite synapse' concept. This inclusive view might help to understand better the mechanisms underlying signal integration and novel forms of long-term homeostatic regulation in the brain.

Astrocytes as brain interoceptors

Astrocytes form a vascular-neuronal interface and provide CNS neural networks with essential structural and metabolic support. They embrace all penetrating arterioles and capillaries, enwrap multiple neuronal somata, thousands of individual synapses, and upon activation release gliotransmitters (ATP, glutamate and D-serine) capable of modulating neuronal activity. The aim of this brief report is to review recent data implicating astrocytes in the brain mechanisms responsible for the detection of different sensory modalities and transmitting sensory information to the relevant neural networks controlling vital behaviours. The concept of astrocytes as brain interoceptors is strongly supported by our recent data obtained from studies of the central nervous mechanisms underlying the chemosensory control of breathing. At the level of the medulla oblongata, astrocytes indeed act as functional central respiratory chemoreceptors, sensing changes in the arterial blood and brain levels of /pH and then imparting these changes on the activity of the respiratory network to induce adaptive changes in lung ventilation.

The glycerophospho metabolome and its influence on amino acid homeostasis revealed by brain metabolomics of GDE1(-/-) mice

GDE1 is a mammalian glycerophosphodiesterase (GDE) implicated by in vitro studies in the regulation of glycerophophoinositol (GroPIns) and possibly other glycerophospho (GroP) metabolites. Here, we show using untargeted metabolomics that GroPIns is profoundly (>20-fold) elevated in brain tissue from GDE1(-/-) mice. Furthermore, two additional GroP metabolites not previously identified in eukaryotic cells, glycerophosphoserine (GroPSer) and glycerophosphoglycerate (GroPGate), were also highly elevated in GDE1(-/-) brains. Enzyme assays with synthetic GroP metabolites confirmed that GroPSer and GroPGate are direct substrates of GDE1. Interestingly, our metabolomic profiles also revealed that serine (both L-and D-) levels were significantly reduced in brains of GDE1(-/-) mice. These findings designate GroPSer as a previously unappreciated reservoir for free serine in the nervous system and suggest that GDE1, through recycling serine from GroPSer, may impact D-serine-dependent neural signaling processes in vivo.

Astrocytes control GABAergic inhibition of neurons in the mouse barrel cortex

Astrocytes in the barrel cortex respond with a transient Ca2+ increase to neuronal stimulation and this response is restricted to the stimulated barrel field. In the present study we suppressed the astrocyte response by dialysing these cells with the Ca2+ chelator BAPTA. Electrical stimulation triggered a depolarization in stellate or pyramidal ‘regular spiking' neurons from cortex layer 4 and 2/3 and this response was augmented in amplitude and duration after astrocytes were dialysed with BAPTA. Combined blockade of GABAA and GABAB receptors mimicked the effect of BAPTA dialysis, while glutamate receptor blockers had no effect. Moreover, the frequency of spontaneous postsynaptic currents was increased after BAPTA dialysis. Outside the range of BAPTA dialysis astrocytes responded with a Ca2+ increase, but in contrast to control, the response was no longer restricted to one barrel field. Our findings indicate that astrocytes control neuronal inhibition in the barrel cortex.

Serine racemase and the serine shuttle between neurons and astrocytes

d-Serine is a brain-enriched d-amino acid that works as a transmitter-like molecule by physiologically activating NMDA receptors. Synthesis of d-serine is carried out by serine racemase (SR), a pyridoxal 5'-phosphate-dependent enzyme. In addition to carry out racemization, SR α,β-eliminates water from l- or d-serine, generating pyruvate and NH(4)(+). Here I review the main mechanisms regulating SR activity and d-serine dynamics in the brain. I propose a role for SR in a novel form of astrocyte-neuron communication-the "serine shuttle", whereby astrocytes synthesize and export l-serine required for the synthesis of d-serine by the predominantly neuronal SR. d-Serine synthesized and released by neurons can be further taken up by astrocytes for storage and activity-dependent release. I discuss how SR α,β-elimination with d-serine itself may limit the achievable intracellular d-serine concentration, providing a mechanistic rationale on why neurons do not store as much d-serine as astrocytes. The higher content of d-serine in astrocytes appears to be related to increased d-serine stability, for their low SR expression will prevent substantial d-serine metabolism via α,β-elimination. SR and the serine shuttle pathway are therapeutic targets in neurodegenerative diseases in which NMDA receptor dysfunction plays pathological roles. This article is part of a Special Issue entitled: Pyridoxal Phospate Enzymology.

A new rapid protocol for eyeblink conditioning to assess cerebellar motor learning

Mice with spontaneous and induced mutations causing cerebellar phenotypes have provided key insights into how motor-related memories are stored in cerebellar circuits. Delayed eyeblink conditioning is a form of associative motor learning that depends on the cerebellum. However, neurochemical investigation of the underlying mechanisms has been hampered by the long training period (usually several days) required to establish conditioning. Here, we report a new rapid-training protocol that reliably induced delayed eyeblink conditioning within a single day. The associative memory formation depended on the expression of the δ2 glutamate receptor (GluD2) in cerebellar Purkinje cells. It lasted for several weeks, but could be erased by extinction sessions in a single day. In addition, using the rapid protocol, we found that eyeblink conditioning could be induced in juvenile mice at postnatal day 21, and that the Sindbis-virus-mediated expression of GluD2 could rescue the impaired eyeblink conditioning in GluD2-null mice in vivo.

Gliotransmission by prostaglandin e(2): a prerequisite for GnRH neuronal function?

Over the past four decades it has become clear that prostaglandin E(2) (PGE(2)), a phospholipid-derived signaling molecule, plays a fundamental role in modulating the gonadotropin-releasing hormone (GnRH) neuroendocrine system and in shaping the hypothalamus. In this review, after a brief historical overview, we highlight studies revealing that PGE(2) released by glial cells such as astrocytes and tanycytes is intimately involved in the active control of GnRH neuronal activity and neurosecretion. Recent evidence suggests that hypothalamic astrocytes surrounding GnRH neuronal cell bodies may respond to neuronal activity with an activation of the erbB receptor tyrosine kinase signaling, triggering the release of PGE(2) as a chemical transmitter from the glia themselves, and, in turn, leading to the feedback regulation of GnRH neuronal activity. At the GnRH neurohemal junction, in the median eminence of the hypothalamus, PGE(2) is released by tanycytes in response to cell-cell signaling initiated by glial cells and vascular endothelial cells. Upon its release, PGE(2) causes the retraction of the tanycyte end-feet enwrapping the GnRH nerve terminals, enabling them to approach the adjacent pericapillary space and thus likely facilitating neurohormone diffusion from these nerve terminals into the pituitary portal blood. In view of these new insights, we suggest that synaptically associated astrocytes and perijunctional tanycytes are integral modulatory elements of GnRH neuronal function at the cell soma/dendrite and nerve terminal levels, respectively.

EphrinBs regulate D-serine synthesis and release in astrocytes

There is growing evidence that astrocytes play critical roles in neuron-glial interactions at the synapse. Astrocytes are believed to regulate presynaptic and postsynaptic structures and functions, in part, by the release of gliotransmitters such as glutamate, ATP, and d-serine; however, little is known of how neurons and astrocytes communicate to regulate these processes. Here, we investigated a family of transmembrane proteins called ephrinBs and Eph receptors that are expressed in the synapse and are known to regulate synaptic transmission and plasticity. In addition to their presence on CA1 hippocampal neurons, we determined that ephrins and Eph receptors are also expressed on hippocampal astrocytes. Stimulation of hippocampal astrocytes with soluble ephrinB3, known to be expressed on CA1 postsynaptic dendrites, enhanced d-serine synthesis and release in culture. Conversely, ephrinB3 had no effect on d-serine release from astrocytes deficient in EphB3 and EphA4, which are the primary receptors for ephrinB3. Eph receptors mediate this response through interactions with PICK1 (protein interacting with C-kinase) and by dephosphorylating protein kinase C α to activate the conversion of l-serine to d-serine by serine racemase. These findings are supported in vivo, where reduced d-serine levels and synaptic transmissions are observed in the absence of EphB3 and EphA4. These data support a role for ephrins and Eph receptors in regulating astrocyte gliotransmitters, which may have important implications on synaptic transmission and plasticity.

Monitoring astrocyte calcium microdomains with improved membrane targeted GCaMP reporters

Astrocytes are involved in synaptic and cerebrovascular regulation in the brain. These functions are regulated by intracellular calcium signalling that is thought to reflect a form of astrocyte excitability. In a recent study, we reported modification of the genetically encoded calcium indicator (GECI) GCaMP2 with a membrane-tethering domain, Lck, to generate Lck-GCaMP2. This GECI allowed us to detect novel microdomain calcium signals. The microdomains were random and 'spotty' in nature. In order to detect such signals more reliably, in the present study we further modified Lck-GCaMP2 to carry three mutations in the GCaMP2 moiety (M153K, T203V within EGFP and N60D in the CaM domain) to generate Lck-GCaMP3. We directly compared Lck-GCaMP2 and Lck-GCaMP3 by assessing their ability to monitor several types of astrocyte calcium signals with a focus on spotty microdomains. Our data show that Lck-GCaMP3 is between two- and four-times better than Lck-GCaMP2 in terms of its basal fluorescence intensity, signal-to-noise and its ability to detect microdomains. The use of Lck-GCaMP3 thus represents a significantly improved way to monitor astrocyte calcium signals, including microdomains, and will facilitate detailed exploration of their molecular mechanisms and physiological roles.

Is astrocyte calcium signaling relevant for synaptic plasticity?

Astrocytes constitute a major group of glial cells which were long regarded as passive elements, fulfilling nutritive and structural functions for neurons. Calcium rise in astrocytes propagating to neurons was the first demonstration of direct interaction between the two cell types. Since then, calcium has been widely used, not only as an indicator of astrocytic activity but also as a stimulator switch to control astrocyte physiology. As a result, astrocytes have been elevated from auxiliaries to neurons, to cells involved in processing synaptic information. Curiously, while there is evidence that astrocytes play an important role in synaptic plasticity, the data relating to calcium's pivotal role are inconsistent. In this review, we will detail the various mechanisms of calcium flux in astrocytes, then briefly present the calcium-dependent mechanisms of gliotransmitter release. Finally, we will discuss the role of calcium in plasticity and present alternative explanations that could reconcile the conflicting results published recently.

The impact of glial activation in the aging brain

The past decade or so has witnessed a rekindling of interest in glia requiring a re-evaluation of the early descriptions of astrocytes as merely support cells, and microglia as adopting either a resting state or an activated state in a binary fashion. We now know that both cell types contribute to the optimal functioning of neurons in the healthy brain, and that altered function of either cell impacts on neuronal function and consequently cognitive function. The evidence indicates that both astrocytic and microglial phenotype change with age and that the shift from the resting state is associated with deterioration in synaptic function. In this review, we consider the rapidly-expanding array of functions attributed to these cells and focus on evaluating the changes in cell activation that accompany ageing.

Visualization of glutamate as a volume transmitter

Glutamate is the major excitatory neurotransmitter in the central nervous system. Although glutamate mediates synaptically confined point-to-point transmission, it has been suggested that under certain conditions glutamate may escape from the synaptic cleft (glutamate spillover), accumulate in the extrasynaptic space, and mediate volume transmission to regulate important brain functions. However, the inability to directly measure glutamate dynamics around active synapses has limited our understanding of glutamatergic volume transmission. The recent development of a family of fluorescent glutamate indicators has enabled the visualization of extrasynaptic glutamate dynamics in brain tissues. In this topical review, we examine glutamate as a volume transmitter based on novel results of glutamate imaging in the brain.

NOV/CCN3 upregulates CCL2 and CXCL1 expression in astrocytes through beta1 and beta5 integrins

Increasing evidence suggests that CCN matricellular proteins play important roles in inflammation. One of the major cell types that handle inflammation in the brain is the astrocyte, which, upon activation, dramatically increases its production of cytokines and chemokines. Here, we report that NOV/CCN3, added to primary cultured rat brain astrocytes, markedly increased the expression of CCL2 and CXCL1 chemokines, as indicated by ELISA and RT-qPCR assays. This effect was selective, as the production of thirteen other cytokines and chemokines was not affected by NOV. NOV expression by astrocytes was demonstrated by immunocytochemistry and Western blot analysis, and astrocyte transfection with NOV small interfering RNA (siRNA) markedly decreased CXCL1 and CCL2 production, indicating that endogenous NOV played a major role in the control of astrocytic chemokine synthesis. NOV was shown to mediate several of its actions through integrins. Here, we observed that siRNAs against integrins beta1 and beta5 decreased basal and abrogated NOV-stimulated astrocyte expression of CCL2 and CXCL1, respectively. Using a panel of kinase inhibitors, we demonstrated that NOV action on CCL2 and CXCL1 production involved a Rho/ROCK/JNK/NF-kappaB and a Rho/qROCK/p38/NF-kappaB pathway, respectively. Thus, distinct integrins and signaling mechanisms are involved in NOV-induced production of CCL2 and CXCL1 in astrocytes. Finally, astrocytic expression of NOV was detected in rat brain tissue sections, and NOV intracerebral injection increased CCL2 and CXCL1 brain levels in vivo. Altogether, our data shed light on the signaling pathways operated by NOV and strongly suggest that NOV mediates astrocyte activation and, therefore, might play a role in neuroinflammation.

Reactive astrocytes as therapeutic targets for CNS disorders

Reactive astrogliosis has long been recognized as a ubiquitous feature of CNS pathologies. Although its roles in CNS pathology are only beginning to be defined, genetic tools are enabling molecular dissection of the functions and mechanisms of reactive astrogliosis in vivo. It is now clear that reactive astrogliosis is not simply an all-or-nothing phenomenon but, rather, is a finely gradated continuum of molecular, cellular, and functional changes that range from subtle alterations in gene expression to scar formation. These changes can exert both beneficial and detrimental effects in a context-dependent manner determined by specific molecular signaling cascades. Dysfunction of either astrocytes or the process of reactive astrogliosis is emerging as an important potential source of mechanisms that might contribute to, or play primary roles in, a host of CNS disorders via loss of normal or gain of abnormal astrocyte activities. A rapidly growing understanding of the mechanisms underlying astrocyte signaling and reactive astrogliosis has the potential to open doors to identifying many molecules that might serve as novel therapeutic targets for a wide range of neurological disorders. This review considers general principles and examines selected examples regarding the potential of targeting specific molecular aspects of reactive astrogliosis for therapeutic manipulations, including regulation of glutamate, reactive oxygen species, and cytokines.

Functions of astrocytes and their potential as therapeutic targets

Astrocytes are often referred to, and historically have been regarded as, support cells of the mammalian CNS. Work over the last decade suggests otherwise-that astrocytes may in fact play a more active role in higher neural processing than previously recognized. Because astrocytes can potentially serve as novel therapeutic targets, it is critical to understand how astrocytes execute their diverse supportive tasks while maintaining neuronal health. To that end, this review focuses on the supportive roles of astrocytes, a line of study relevant to essentially all acute and chronic neurological diseases, and critically re-evaluates our concepts of the functional properties of astrocytes and relates these functions and properties to the intricate morphology of these cells.

Spinal glia and chronic pain

Therapeutic management of chronic pain has not been widely successful owing to a lack of understanding of factors that initiate and maintain the chronic pain condition. Efforts to delineate the mechanisms underlying pain long have focused on neuronal elements of pain pathways, and both opiate- and non-opiate-based therapeutics are thought largely to target neurons. Abnormal neuronal activity at the level of spinal cord "pain centers" in the dorsal horn leads to hypersensitivity or a hyperalgesic response subsequent to the initial painful stimulus. Only recently has the experimental literature implicated nonneuronal elements in pain because of the realization that glial-derived signaling molecules can contribute to and modulate pain signaling in the spinal cord. Most notably, glial proinflammatory mediators within the dorsal horn of the spinal cord appear to contribute to self-perpetuating pain. Chronic pain is modeled experimentally through a variety of manipulations of sensory nerves including cutting, crushing, resection, and ligation. The cellular and molecular responses in the spinal cord due to these manipulations often reveal activation of 2 types of glia: microglia and astrocytes. The activation states of both microglia and astrocytes are complex and may be driven by underlying chronic neuropathology and/or a chronically "primed" condition that accounts for their contribution to chronic pain. Recent evidence even suggests that opioid tolerance and withdrawal hyperalgesia may be initiated and maintained via actions of microglia and astroglia. Together, these recent findings suggest that glia will serve as novel therapeutic targets for the treatment of chronic pain. To fully exploit glia as novel therapeutic targets will require a greater understanding of glial biology, as well as the identification of agents able to control the glial reactions involved in chronic pain, without interfering with beneficial glial functions.

Perisynaptic glia discriminate patterns of motor nerve activity and influence plasticity at the neuromuscular junction

In the nervous system, the induction of plasticity is coded by patterns of synaptic activity. Glial cells are now recognized as dynamic partners in a wide variety of brain functions, including the induction and modulation of various forms of synaptic plasticity. However, it appears that glial cells are usually activated by stereotyped, sustained neuronal activity, and little attention has been given to more subtle changes in the patterns of synaptic activation. To this end, we used the mouse neuromuscular junction as a simple and useful model to study glial modulation of synaptic plasticity. We used two patterns of motor nerve stimulation that mimic endogenous motor-neuronal activity. A continuous stimulation induced a post-tetanic potentiation and a phasic Ca(2+) response in perisynaptic Schwann cells (PSCs), glial cells at this synapse. A bursting pattern of activity induced a post-tetanic depression and oscillatory Ca(2+) responses in PSCs. The different Ca(2+) responses in PSCs indicate that they decode the pattern of synaptic activity. Furthermore, the chelation of glial Ca(2+) impaired the production of the sustained plasticity events indicating that PSCs govern the outcome of synaptic plasticity. The mechanisms involved were studied using direct photo-activation of PSCs with caged Ca(2+) that mimicked endogenous plasticity. Using specific pharmacology and transgenic knock-out animals for adenosine receptors, we showed that the sustained depression was mediated by A1 receptors while the sustained potentiation is mediated by A(2A) receptors. These results demonstrate that glial cells decode the pattern of synaptic activity and subsequently provide bidirectional feedback to synapses.

Astroglial Wiring is Adding Complexity to Neuroglial Networking

Astrocytes are organized as networks of communicating cells due to their high expression level of connexins, the molecular constituents of gap junction channels. Based on their permeability properties for ions and small signaling molecules such astroglial wiring interferes with neuronal activity and survival. In this paper, I identify and discuss which future technical and conceptual progress or advances should be achieved in order to better understand how neuroglial networking contributes to brain functions and dysfunctions.

Integrated brain circuits: neuron-astrocyte interaction in sleep-related rhythmogenesis

Although astrocytes are increasingly recognized as important modulators of neuronal excitability and information transfer at the synapse, whether these cells regulate neuronal network activity has only recently started to be investigated. In this article, we highlight the role of astrocytes in the modulation of circuit function with particular focus on sleep-related rhythmogenesis. We discuss recent data showing that these glial cells regulate slow oscillations, a specific thalamocortical activity that characterizes non-REM sleep, and sleep-associated behaviors. Based on these findings, we predict that our understanding of the genesis and tuning of thalamocortical rhythms will necessarily go through an integrated view of brain circuits in which non-neuronal cells can play important neuromodulatory roles.

Synaptic plasticity and Ca2+ signalling in astrocytes

There is a growing body of evidence suggesting a functional relationship between Ca2+ signals generated in astroglia and the functioning of nearby excitatory synapses. Interference with endogenous Ca2+ homeostasis inside individual astrocytes has been shown to affect synaptic transmission and its use-dependent changes. However, establishing the causal link between source-specific, physiologically relevant intracellular Ca2+ signals, the astrocytic release machinery and the consequent effects on synaptic transmission has proved difficult. Improved methods of Ca2+ monitoring in situ will be essential for resolving the ambiguity in understanding the underlying Ca2+ signalling cascades.

Astrocytes control breathing through pH-dependent release of ATP

Astrocytes provide structural and metabolic support for neuronal networks, but direct evidence demonstrating their active role in complex behaviors is limited. Central respiratory chemosensitivity is an essential mechanism that, via regulation of breathing, maintains constant levels of blood and brain pH and partial pressure of CO2. We found that astrocytes of the brainstem chemoreceptor areas are highly chemosensitive. They responded to physiological decreases in pH with vigorous elevations in intracellular Ca2+ and release of adenosine triphosphate (ATP). ATP propagated astrocytic Ca2+ excitation, activated chemoreceptor neurons, and induced adaptive increases in breathing. Mimicking pH-evoked Ca2+ responses by means of optogenetic stimulation of astrocytes expressing channelrhodopsin-2 activated chemoreceptor neurons via an ATP-dependent mechanism and triggered robust respiratory responses in vivo. This demonstrates a potentially crucial role for brain glial cells in mediating a fundamental physiological reflex.

Astrocytes and human cognition: modeling information integration and modulation of neuronal activity

Recent research focusing on the participation of astrocytes in glutamatergic tripartite synapses has revealed mechanisms that support cognitive functions common to human and other mammalian species, such as learning, perception, conscious integration, memory formation/retrieval and the control of voluntary behavior. Astrocytes can modulate neuronal activity by means of release of glutamate, d-serine, adenosine triphosphate and other signaling molecules, contributing to sustain, reinforce or depress pre- and post-synaptic membranes. We review molecular mechanisms present in tripartite synapses and model the cognitive role of astrocytes. Single protoplasmic astrocytes operate as a "Local Hub", integrating information patterns from neuronal and glial populations. Two mechanisms, here modeled as the "domino" and "carousel" effects, contribute to the formation of intercellular calcium waves. As waves propagate through gap junctions and reach other types of astrocytes (interlaminar, polarized, fibrous and varicose projection), the active astroglial network functions as a "Master Hub" that integrates results of distributed processing from several brain areas and supports conscious states. Response of this network would define the effect exerted on neuronal plasticity (membrane potentiation or depression), behavior and psychosomatic processes. Theoretical results of our modeling can contribute to the development of new experimental research programs to test cognitive functions of astrocytes.

An active role for astrocytes in synaptic plasticity?

Recently, Henneberger and colleagues blocked hippocampal long-term synaptic potentiation (LTP) induction by "clamping" intracellular calcium concentration of individual CA1 astrocytes, suggesting calcium-dependent gliotransmitter release from astocytes plays a role in hippocampal LTP induction. However, using transgenic mice to manipulate astrocytic calcium, Agulhon and colleagues demonstrated no effect on LTP induction. Until the question of how intracellular calcium causes gliotransmitter release is answered, the role of astrocytes in synaptic plasticity will be incompletely understood.

Bone cancer induces a unique central sensitization through synaptic changes in a wide area of the spinal cord

BACKGROUND

Chronic bone cancer pain is thought to be partly due to central sensitization. Although murine models of bone cancer pain revealed significant neurochemical changes in the spinal cord, it is not known whether this produces functional alterations in spinal sensory synaptic transmission. In this study, we examined excitatory synaptic responses evoked in substantia gelatinosa (SG, lamina II) neurons in spinal cord slices of adult mice bearing bone cancer, using whole-cell voltage-clamp recording techniques.

RESULTS

Mice at 14 to 21 days after sarcoma implantation into the femur exhibited hyperalgesia to mechanical stimuli applied to the skin of the ipsilateral hind paw, as well as showing spontaneous and movement evoked pain-related behaviors. SG neurons exhibited spontaneous excitatory postsynaptic currents (EPSCs). The amplitudes of spontaneous EPSCs were significantly larger in cancer-bearing than control mice without any changes in passive membrane properties of SG neurons. In the presence of TTX, the amplitude of miniature EPSCs in SG neurons was increased in cancer-bearing mice and this was observed for cells sampled across a wide range of lumbar segmental levels. Alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor- and N-methyl-D-aspartate (NMDA) receptor-mediated EPSCs evoked by focal stimulation were also enhanced in cancer-bearing mice. Dorsal root stimulation elicited mono- and/or polysynaptic EPSCs that were caused by the activation of Adelta and/or C afferent fibers in SG neurons from both groups of animals. The number of cells receiving monosynaptic inputs from Adelta and C fibers was not different between the two groups. However, the amplitude of the monosynaptic C fiber-evoked EPSCs and the number of SG neurons receiving polysynaptic inputs from Adelta and C fibers were increased in cancer-bearing mice.

CONCLUSIONS

These results show that spinal synaptic transmission mediated through Adelta and C fibers is enhanced in the SG across a wide area of lumbar levels following sarcoma implantation in the femur. This widespread spinal sensitization may be one of the underlying mechanisms for the development of chronic bone cancer pain.

CXCR4-mediated glutamate exocytosis from astrocytes

The role of astrocytes as structural and metabolic support for neurons is known since the beginning of the last century. Because of their strategic localization between neurons and capillaries they can monitor and control the level of synaptic activity by providing energetic metabolites to neurons and remove excess of neurotransmitters. During the last two decades number of papers further established that the astrocytic plasma-membrane G-protein coupled receptors (GPCR) can sense external inputs (such as the spillover of neurotransmitters) and transduce them as intracellular calcium elevations and release of chemical transmitters such as glutamate. The chemokine CXCR4 receptor is a GPCR widely expressed on glial cells (especially astrocytes and microglia). Activation of the astrocytic CXCR4 by its natural ligand CXCL12 (or SDF1 alpha) results in a long chain of intracellular and extracellular events (including the release of the pro-inflammatory cytokine TNFalpha and prostanglandins) leading to glutamate release. The emerging role of CXCR4-CXCL12 signalling axis in brain physiology came from the recent observation that glutamate in astrocytes is released via a regulated exocytosis process and occurs with a relatively fast time-scale, in the order of few hundred milliseconds. Taking into account that astrocytes are electrically non-excitable and thus exocytosis rely only on a signalling pathway that involves the release Ca(2+) from the internal stores, these results suggested a close relationship between sites of Ca(2+) release and those of fusion events. Indeed, a recent observation describes structural sub-membrane microdomains where fast ER-dependent calcium elevations occur in spatial and temporal correlation with fusion events.

Spike-timing dependent plasticity beyond synapse - pre- and post-synaptic plasticity of intrinsic neuronal excitability

Long-lasting plasticity of synaptic transmission is classically thought to be the cellular substrate for information storage in the brain. Recent data indicate however that it is not the whole story and persistent changes in the intrinsic neuronal excitability have been shown to occur in parallel to the induction of long-term synaptic modifications. This form of plasticity depends on the regulation of voltage-gated ion channels. Here we review the experimental evidence for plasticity of neuronal excitability induced at pre- or postsynaptic sites when long-term plasticity of synaptic transmission is induced with Spike-Timing Dependent Plasticity (STDP) protocols. We describe the induction and expression mechanisms of the induced changes in excitability. Finally, the functional synergy between synaptic and non-synaptic plasticity and their spatial extent are discussed.

Presynaptic NMDA Receptors and Spike Timing-Dependent Depression at Cortical Synapses

It has recently been discovered that some forms of timing-dependent long-term depression (t-LTD) require presynaptic N-methyl-d-aspartate (NMDA) receptors. In this review, we discuss the evidence for the presence of presynaptic NMDA receptors at cortical synapses and their possible role in the induction of t-LTD. Two basic models emerge for the induction of t-LTD at cortical synapses. In one model, coincident activation of presynaptic NMDA receptors and CB1 receptors mediates t-LTD. In a second model, CB1 receptors are not necessary, and the activation of presynaptic NMDA receptors alone appears to be sufficient for the induction of t-LTD.

Blood-brain barrier breakdown as a therapeutic target in traumatic brain injury

Traumatic brain injury (TBI) is the leading cause of death in young adults and children. The treatment of TBI in the acute phase has improved substantially; however, the prevention and management of long-term complications remain a challenge. Blood-brain barrier (BBB) breakdown has often been documented in patients with TBI, but the role of such vascular pathology in neurological dysfunction has only recently been explored. Animal studies have demonstrated that BBB breakdown is involved in the initiation of transcriptional changes in the neurovascular network that ultimately lead to delayed neuronal dysfunction and degeneration. Brain imaging data have confirmed the high incidence of BBB breakdown in patients with TBI and suggest that such pathology could be used as a biomarker in the clinic and in drug trials. Here, we review the neurological consequences of TBI, focusing on the long-term complications of such injuries. We present the clinical evidence for involvement of BBB breakdown in TBI and examine the primary and secondary mechanisms that underlie such pathology. We go on to consider the consequences of BBB injury, before analyzing potential mechanisms linking vascular pathology to neuronal dysfunction and degeneration, and exploring possible targets for treatment. Finally, we highlight areas for future basic research and clinical studies into TBI.

Neuron-astrocyte communication and synaptic plasticity

By forming close contacts with synapses, astrocytes secrete neuroactive substances and remove neurotransmitters, thus influencing the processing of information by the nervous system. Here, we review recent work on astrocytes and their roles in regulating neuronal function and synaptic plasticity. Astrocytes are organized as networks and communicate with each other, thereby affecting larger neural circuits. They also provide a link between neurons and the vasculature, potentially changing the cerebral microcirculation. Recent work has provided insights into the relative contributions of specific astrocytic cues and transporters to synaptic transmission, plasticity, and animal behavior.

Functional 'glial' GLYT1 glycine transporters expressed in neurons

Glycine transporter 1 (GLYT1) and GLYT2 are the glycine transporters in CNS. While GLYT2 is largely expressed in glycinergic neurons, GLYT1 has long been considered to be exclusively present in glial cells. There is increasing evidence that significant amounts of the 'glial' transporter also exist on neurons, particularly on pre-synaptic nerve endings of glutamatergic neurons. The functions of 'neuronal GLYT1' may be manifold and are discussed in this review. Of major interest are the interactions between neuronal GLYT1 and glutamatergic receptors of the NMDA type the activity of which is modulated not only by astrocytic GLYT1 but also by neuronal GLYT1. Pathophysiological roles and therapeutic implications of neuronal GLYT1 are emerging from recent studies with genetically modified mice, particularly with animals lacking forebrain neuron-specific GLYT1 transporters. These mutant mice exhibit promnesic phenotypes reflecting enhancement of NMDA receptor function, as it occurs following administration of GLYT1 inhibitors. Inactivation of neuronal GLYT1 in the forebrain may represent an effective therapeutic intervention for the treatment of schizophrenia.