A mathematical model of the tripartite synapse: astrocyte-induced synaptic plasticity

In this paper, we present a biologically detailed mathematical model of tripartite synapses, where astrocytes modulate short-term synaptic plasticity. The model consists of a pre-synaptic bouton, a post-synaptic dendritic spine-head, a synaptic cleft and a peri-synaptic astrocyte controlling Ca^2 + dynamics inside the synaptic bouton. This in turn controls glutamate release dynamics in the cleft. As a consequence of this, glutamate concentration in the cleft has been modeled, in which glutamate reuptake by astrocytes has also been incorporated. Finally, dendritic spine-head dynamics has been modeled. As an application, this model clearly shows synaptic potentiation in the hippocampal region, i.e., astrocyte Ca^2 + mediates synaptic plasticity, which is in conformity with the majority of the recent findings (Perea and Araque (Science 317, 1083–1086, 2007); Henneberger et al. (Nature 463, 232–236, 2010); Navarrete et al. (PLoS Biol. 10, e1001259, 2012)).

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Astrocytes mediate in vivo cholinergic-induced synaptic plasticity

Long-term potentiation (LTP) of synaptic transmission represents the cellular basis of learning and memory. Astrocytes have been shown to regulate synaptic transmission and plasticity. However, their involvement in specific physiological processes that induce LTP in vivo remains unknown. Here we show that in vivo cholinergic activity evoked by sensory stimulation or electrical stimulation of the septal nucleus increases Ca²⁺ in hippocampal astrocytes and induces LTP of CA3-CA1 synapses, which requires cholinergic muscarinic (mAChR) and metabotropic glutamate receptor (mGluR) activation. Stimulation of cholinergic pathways in hippocampal slices evokes astrocyte Ca²⁺ elevations, postsynaptic depolarizations of CA1 pyramidal neurons, and LTP of transmitter release at single CA3-CA1 synapses. Like in vivo, these effects are mediated by mAChRs, and this cholinergic-induced LTP (c-LTP) also involves mGluR activation. Astrocyte Ca²⁺ elevations and LTP are absent in IP₃R2 knock-out mice. Downregulating astrocyte Ca²⁺ signal by loading astrocytes with BAPTA or GDPβS also prevents LTP, which is restored by simultaneous astrocyte Ca²⁺ uncaging and postsynaptic depolarization. Therefore, cholinergic-induced LTP requires astrocyte Ca²⁺ elevations, which stimulate astrocyte glutamate release that activates mGluRs. The cholinergic-induced LTP results from the temporal coincidence of the postsynaptic activity and the astrocyte Ca²⁺ signal simultaneously evoked by cholinergic activity. Therefore, the astrocyte Ca²⁺ signal is necessary for cholinergic-induced synaptic plasticity, indicating that astrocytes are directly involved in brain storage information.     

Long-term potentiation at hippocampal perforant path-dentate astrocyte synapses

Accumulated evidence indicates that astroglial cells actively participate in neuronal synaptic transmission and plasticity. However, it is still not clear whether astrocytes are able to undergo plasticity in response to synaptic inputs. Here we demonstrate that a long-term potentiation (LTP)-like response could be detected at perforant path-dentate astrocyte synapses following high-frequency stimulation (HFS) in hippocampal slices of GFAP-GFP transgenic mice. The potentiation was not dependent on the glutamate transporters nor the group I metabotropic glutamate receptors. However, the induction of LTP requires activation of the NMDA receptor (NMDAR). The presence of functional NMDAR was supported by isolating the NMDAR-gated current and by identifying mRNAs of NMDAR subunits in astrocytes. Our results suggest that astrocytes in the hippocampal dentate gyrus are able to undergo plasticity in response to presynaptic inputs.     

Role of CD23 in astrocytes inflammatory reaction during HIV-1 related encephalitis

Soluble factors released by intra-cerebral activated cells are implicated in neuronal alterations during central nervous system inflammatory diseases. In this study, the role of the CD23 pathway in astrocyte activation and its participation in human immunodeficiency virus-1 (HIV-1)-induced neuropathology were evaluated. In human primary astrocytes, CD23 protein membrane expression was dose-dependently upregulated by gp120. It was also upregulated by gamma-interferon (gamma-IFN) and modulated by interleukin-1-beta (IL-1beta) whereas microglial cells in these stimulation conditions did not express CD23. Cell surface stimulation of CD23 expressed by astrocytes induced production of nitric oxide (NO) and IL-1beta which was inhibited by a specific inducible NO-synthase (iNOS) inhibitor (aminoguanidine), indicating the implication of this receptor in the astrocyte inflammatory reaction. On brain tissues from five out of five patients with HIV-1-related encephalitis, CD23 was expressed by astrocytes and by some microglial cells, whereas it was not detectable on brain tissue from five of five HIV-1-infected patients without central nervous system (CNS) disease or from two of two control subjects. In addition, CD23 antigen was co-localized with iNOS and nitrotyrosine on brain tissue from patients with HIV1-related encephalitis, suggesting that CD23 participates in iNOS activation of astrocytes in vivo. In conclusion, CD23 ligation is an alternative pathway in the induction of inflammatory product synthesis by astrocytes and participates in CNS inflammation.     

Metabotropic glutamate receptors are required for the induction of long-term potentiation.

Recent observations have led to the suggestion that the metabotropic glutamate receptor may play a role in the induction or maintenance of long-term potentiation (LTP). However, experimental evidence supporting a role for this receptor in the induction of LTP is still inconclusive and controversial. Here we report that, in rat dorsolateral septal nucleus (DLSN) neurons, which have the highest density of metabotropic receptors and show functional responses, the induction of LTP is not blocked by the NMDA receptor antagonist 2-amino-5-phosphonovalerate, but is blocked by two putative metabotropic glutamate receptor antagonists, L-2-amino-3-phosphonopropionic acid and L-2-amino-4-phosphonobutyrate. Furthermore, superfusion of (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid, a selective metabotropic glutamate agonist, resulted in a long-lasting potentiation of synaptic transmission similar to that induced by tetanic stimuli. Our results demonstrated that activation of postsynaptic metabotropic receptors is both necessary and sufficient for the induction of LTP in the DLSN, and we suggest that such a mechanism may be important at other CNS synapses.     

Differences in kainate receptor involvement in hippocampal mossy fibre long-term potentiation depending on slice orientation

Long-term potentiation (LTP) is a well-established experimental model used to investigate the synaptic basis of learning and memory. LTP at mossy fibre - CA3 synapses in the hippocampus is unusual because it is normally N-methyl-d-aspartate (NMDA) receptor-independent. Instead it seems that the trigger for mossy fibre LTP involves kainate receptors (KARs). Although it is generally accepted that pre-synaptic KARs play an essential role in frequency facilitation and LTP, their subunit composition remains a matter of significant controversy. We have reported previously that both frequency facilitation and LTP can be blocked by selective antagonism of GluK1 (formerly GluR5/Glu(K5))-containing KARs, but other groups have failed to reproduce this effect. Moreover, data from receptor knockout and mRNA expression studies argue against a major role of GluK1, supporting a more central role for GluK2 (formerly GluR6/Glu(K6)). A potential reason underlying the controversy in the pharmacological experiments may reside in differences in the preparations used. Here we show differences in pharmacological sensitivity of synaptic plasticity at mossy fibre - CA3 synapses depend critically on slice orientation. In transverse slices, LTP of fEPSPs was invariably resistant to GluK1-selective antagonists whereas in parasagittal slices LTP was consistently blocked by GluK1-selective antagonists. In addition, there were pronounced differences in the magnitude of frequency facilitation and the sensitivity to the mGlu2/3 receptor agonist DCG-IV. Using anterograde labelling of granule cells we show that slices of both orientations possess intact mossy fibres and both large and small presynaptic boutons. Transverse slices have denser fibre tracts but a smaller proportion of giant mossy fibre boutons. These results further demonstrate a considerable heterogeneity in the functional properties of the mossy fibre projection.     

Glutamine in the mechanism of ammonia-induced astrocyte swelling

Brain edema and the subsequent increase in intracranial pressure are the major neurological complications in fulminant hepatic failure (FHF). Brain edema in FHF is predominantly "cytotoxic" due principally to astrocyte swelling. It is generally believed that ammonia plays a key role in this process, although the mechanism by which ammonia brings about such swelling is yet to be defined. It has been postulated that glutamine accumulation in astrocytes subsequent to ammonia detoxification results in increased osmotic forces leading to cell swelling. While the hypothesis is plausible and has gained support, it has never been critically tested. In this study, we examined whether a correlation exists between cellular glutamine levels and the degree of cell swelling in cultured astrocytes exposed to ammonia. Cultured astrocytes derived from rat brain cortices were exposed to ammonia (5 mM) for different time periods and cell swelling was measured. Cultures treated with ammonia for 1-3 days showed a progressive increase in astrocyte cell volume (59-127%). Parallel treatment of astrocyte cultures with ammonia showed a significant increase in cellular glutamine content (60-80%) only at 1-4 h, a time when swelling was absent, while glutamine levels were normal at 1-3 days, a time when peak cell swelling was observed. Thus no direct correlation between cell swelling and glutamine levels was detected. Additionally, acute increase in intracellular levels of glutamine by treatment with the glutaminase inhibitor 6-diazo-5-oxo-L-norleucine (DON) after ammonia exposure also did not result in swelling. On the contrary, DON treatment significantly blocked (66%) ammonia-induced astrocyte swelling at a later time point (24 h), suggesting that some process resulting from glutamine metabolism is responsible for astrocyte swelling. Additionally, ammonia-induced free radical production and induction of the mitochondrial permeability transition (MPT) were significantly blocked by treatment with DON, suggesting a key role of glutamine in the ammonia-induced free radical generation and the MPT. In summary, our findings indicate a lack of direct correlation between the extent of cell swelling and cellular levels of glutamine. While glutamine may not be acting as an osmolyte, we propose that glutamine-mediated oxidative stress and/or the MPT may be responsible for the astrocyte swelling by ammonia.     

Multiple forms of long-term potentiation and multiple regulatory sites of N-methyl-D-aspartate receptors: Role of the redox site

Long-term potentiation (LTP) is a form of synaptic plasticity thought to be involved in learning and memory. Althrough extensively studied, mainly in the CA1 region of the hippocampus, the mechanisms underlying the induction and expression of LTP are poorly elucidated. This is probably due to the fact that LTP is not a unique process and indeed recent studies have shown that several forms of LTP could be generated depending on the experimental conditions. Furthermore, LTP is generally associated with a long-lasting increase of the synaptic efficacy of AMPA receptors but an increasing number of data also suggested that NMDA receptors could be potentiated as well. NMDA receptor responses are modulated by a large number of extracellular and intracellular events, providing additional possibilities for the generation of LTP. The role of these different modulatory sites of the NMDA receptor and their relation with LTP are reviewed with a particular attention to the redox site which seems to be a selective target to distinguish between AMPA and NMDA-LTP. © 1995 John Wiley & Sons, Inc.     

Src and Pyk2 mediate angiotensin II effects in cultured rat astrocytes

Angiotensin II (Ang II)-induced proliferation of rat astrocytes is mediated by multiple signaling pathways. In the present study, we investigated the role of non-receptor tyrosine kinases on Ang II-signaling and proliferation of astrocytes cultured from neonatal rat pups. Ang II stimulated astrocyte growth, ERK1/2 phosphorylation and the phosphorylation of Src and proline-rich tyrosine kinase-2 (Pyk2), in astrocytes obtained from brainstem and cerebellum. Pretreatment with 10 microM PP2, a selective Src inhibitor, inhibited Ang II stimulated ERK1/2 phosphorylation by 59% to 91% both in brainstem and cerebellum astrocytes. PP2 also inhibited Ang II induction of brainstem (76% inhibition) and cerebellar (64% inhibition) astrocyte growth. Similarly, pretreatment with 25 microM dantrolene, the Pyk2 inhibitor, attenuated ERK1/2 activity in brainstem (62% inhibition) and in cerebellum astrocytes (44% inhibition). Interestingly, inhibition of Pyk2 inhibited Ang II-induced Src activation suggesting that these two non-receptor tyrosine kinases may be acting in concert to mediate Ang II effects in astrocytes. In summary, we found that Ang II stimulates the non-receptor tyrosine kinases Src and Pyk2 which mediate Ang II-induced ERK1/2 activation leading to stimulation of astrocyte growth. In addition, these two tyrosine kinases may be interacting to regulate effects of the peptide in these cells.     

Modeling synaptic transmission of the tripartite synapse

The tripartite synapse denotes the junction of a pre- and postsynaptic neuron modulated by a synaptic astrocyte. Enhanced transmission probability and frequency of the postsynaptic current-events are among the significant effects of the astrocyte on the synapse as experimentally characterized by several groups. In this paper we provide a mathematical framework for the relevant synaptic interactions between neurons and astrocytes that can account quantitatively for both the astrocytic effects on the synaptic transmission and the spontaneous postsynaptic events. Inferred from experiments, the model assumes that glutamate released by the astrocytes in response to synaptic activity regulates store-operated calcium in the presynaptic terminal. This source of calcium is distinct from voltage-gated calcium influx and accounts for the long timescale of facilitation at the synapse seen in correlation with calcium activity in the astrocytes. Our model predicts the inter-event interval distribution of spontaneous current activity mediated by a synaptic astrocyte and provides an additional insight into a novel mechanism for plasticity in which a low fidelity synapse gets transformed into a high fidelity synapse via astrocytic coupling.     

Glutamate-mediated overexpression of CD38 in astrocytes cultured with neurones

Recently, a new system of astrocyte-neurone glutamatergic signalling has been identified. It is started in astrocytes by ectocellular, CD38-catalysed conversion of NAD(+) to the calcium mobilizer cyclic ADP-ribose (cADPR). This is then pumped by CD38 itself into the cytosol where the resulting free intracellular Ca(2+) concentration [Ca(2+)](i) transients elicit an increased release of glutamate, which can induce an enhanced Ca(2+) response in neighbouring neurones. Here, we demonstrate that co-culture of either cortical or hippocampal astrocytes with neurones results in a significant overexpression of astrocyte CD38 both on the plasma membrane and intracellularly. The causal role of neurone-released glutamate in inducing overexpression of astrocyte CD38 is demonstrated by two observations: first, in the absence of neurones, induction of CD38 in pure astrocyte cultures can be obtained with glutamate and second, it can be prevented in co-cultures by glutamate receptor antagonists. The neuronal glutamate-mediated effect of neurones on astrocyte CD38 expression is paralleled by increased intracellular cADPR and [Ca(2+)](i) levels, both findings indicating functionality of overexpressed CD38. These results reveal a new neurone-to-astrocyte glutamatergic signalling based on the CD38/cADPR system, which affects the [Ca(2+)](i) in both cell types, adding further complexity to the bi-directional patterns of communication between astrocytes and neurones.     

Synaptic information processing by astrocytes.

Glial cells were classically considered as supportive cells that do not contribute to information processing in the nervous system. However, considerable amount of evidence obtained by several groups during the last few years has demonstrated the existence of a bidirectional communication between astrocytes and neurons, which prompted a re-examination of the role of glial cells in the physiology of the nervous system. This review will discuss recent advances in the neuron-to-astrocyte communication, focusing on the recently reported properties of the synaptically evoked astrocyte Ca2+ signal that indicate that astrocytes show integrative properties for synaptic information processing. Indeed, we have recently shown that hippocampal astrocytes discriminate between the activity of different synapses, and respond selectively to different axon pathways. Furthermore, the astrocyte Ca2+ signal is modulated by the simultaneous activity of different synaptic inputs. This Ca2+ signal modulation depends on cellular intrinsic properties of the astrocytes, is bidirectionally regulated by the level of synaptic activity, and controls the spatial extension of the intracellular Ca2+ signal. Consequently, we propose that astrocytes can be considered as cellular elements involved in information processing by the nervous system.     

Induction of Glutamine Synthetase in Rat Astrocytes by Co-Cultivation with Embryonic Chick Neurons

Co-cultivation of confluent rat astrocyte cultures with embryonic chick neurons resulted in induction of glutamine synthetase activity in the astrocytes. This induction of glutamine synthetase in astrocytes by neurons was independent of induction by hydrocortisone and forskolin, but was dependent on the length of co-cultivation and the number of neurons present in the co-culture. Cycloheximide and actinomycin D inhibited the induction of glutamine synthetase in astrocytes by neurons, whereas cytosine arabinoside had no apparent effect. Results suggest that this induction of glutamine synthetase in astrocytes is mediated by cell contact with neurons and may represent a specific neuronal and glial interaction.     

Modulation of astrocyte inducible nitric oxide synthase and cytokine expression by interferon beta is associated with induction and inhibition of interferon gamma-activated sequence binding activity

Although interferon (IFN)-beta is firmly established as a therapeutic agent for multiple sclerosis, information regarding its role in astrocyte cytokine production is limited. In primary cultures of human astrocytes, we determined the effects of IFN-beta on astrocyte cytokine [tumor necrosis factor-alpha (TNF-alpha) and interleukin (IL)-6] and inducible nitric oxide synthase (iNOS) expression by ribonuclease protection assay and ELISA. We found that IFN-beta inhibited astrocyte cytokine/iNOS induced by IL-1 plus IFN-gamma, but in the absence of IFN-gamma, IFN-beta enhanced IL-1-induced cytokine/iNOS expression. Electrophoretic mobility shift analysis (EMSA) demonstrated that IFN-gamma induced sustained IFN-gamma-activated sequence (GAS) binding, while IFN-beta induced transient GAS binding. When used together, IFN-beta inhibited IFN-gamma-induced GAS binding activity. Nuclear factor-kappa B (NF-kappaB) activation was not altered by either IFNs, whereas IFN stimulated response element (ISRE) was only activated by IFN-beta and not IFN-gamma. These results suggest that IFN-beta can both mimic and antagonize the effect of IFN-gamma by modulating induction of nuclear GAS binding activity. Our results demonstrating differential regulation of astrocyte cytokine/iNOS induction by IFN-beta are novel and have implications for inflammatory diseases of the human CNS.     

Biphasic regulation of tissue plasminogen activator activity in ischemic rat brain and in cultured neural cells: essential role of astrocyte-derived plasminogen activator inhibitor-1

In brain, the serine protease tissue plasminogen activator (tPA) and its endogenous inhibitor plasminogen activator inhibitor-1 (PAI-1) have been implicated in the regulation of various neurophysiological and pathological responses. In this study, we investigated the differential role of neurons and astrocytes in the regulation of tPA/PAI-1 activity in ischemic brain. The activity of tPA peaked transiently and then decreased in cortex and striatum along with delayed induction of PAI-1 in the inflammatory stage after MCAO/reperfusion injury. In cultured primary cells, glutamate stimulation increased tPA activity in neurons but not in other cells such as microglia and astrocytes. With LPS stimulation, a model of neuroinflammatory insults, robust PAI-1 induction was observed in astrocytes but not in neurons and microglia. The upregulation of PAI-1 by LPS in astrocytes was also verified by RT-PCR analysis as well as PAI-1 promoter reporter assay. Lastly, we checked the effects of hypoxia on tPA/PAI-1 activity. Hypoxia increased tPA release from neurons without effects on microglia, while the activity of tPA in astrocyte was decreased consistent with increased PAI-1 activity in astrocyte. Taken together, the results from the present study suggest that neurons are the major source of tPA and that the glutamate-induced stimulated release is mainly governed by neurons in the acute phase. In contrast, the massive up-regulation of PAI-1 in astrocytes during subchronic and chronic inflammatory conditions, leads to decreased tPA activity in the later stages of MCAO. Differential regulation of tPA and PAI-1 in neurons, astrocytes and microglia suggest more attention is required to understand the role of local tPA activity in the vicinity of individual cell types.     

Neuron-glia communication: metallothionein expression is specifically up-regulated by astrocytes in response to neuronal injury

Recent data suggests that metallothioneins (MTs) are major neuroprotective proteins within the CNS. In this regard, we have recently demonstrated that MT-IIA (the major human MT-I/-II isoform) promotes neural recovery following focal cortical brain injury. To further investigate the role of MTs in cortical brain injury, MT-I/-II expression was examined in several different experimental models of cortical neuron injury. While MT-I/-II immunoreactivity was not detectable in the uninjured rat neocortex, by 4 days, following a focal cortical brain injury, MT-I/-II was found in astrocytes aligned along the injury site. At latter time points, astrocytes, at a distance up to several hundred microns from the original injury tract, were MT-I/-II immunoreactive. Induced MT-I/-II was found both within the cell body and processes. Using a cortical neuron/astrocyte co-culture model, we observed a similar MT-I/-II response following in vitro injury. Intriguingly, scratch wound injury in pure astrocyte cultures resulted in no change in MT-I/-II expression. This suggests that MT induction was specifically elicited by neuronal injury. Based upon recent reports indicating that MT-I/-II are major neuroprotective proteins within the brain, our results provide further evidence that MT-I/-II plays an important role in the cellular response to neuronal injury.     

Neuronal survival depends on EGFR signaling in cortical but not midbrain astrocytes

Mice lacking epidermal growth factor receptor (EGFR) develop a neurodegeneration of unknown etiology affecting exclusively the frontal cortex and olfactory bulbs. Here, we show that EGFR signaling controls cortical degeneration by regulating cortical astrocyte apoptosis. Whereas EGFR(-/-) midbrain astrocytes are unaffected, mutant cortical astrocytes display increased apoptosis mediated by an Akt-caspase-dependent mechanism and are unable to support neuronal survival. The expression of many neurotrophic factors is unaltered in EGFR(-/-) cortical astrocytes suggesting that neuronal loss occurs as a consequence of increased astrocyte apoptosis rather than impaired secretion of trophic factors. Neuron-specific expression of activated Ras can compensate for the deficiency of EGFR(-/-) cortical astrocytes and prevent neuronal death. These results identify two functionally distinct astrocyte populations, which differentially depend on EGFR signaling for their survival and also for their ability to support neuronal survival. These spatial differences in astrocyte composition provide a mechanism for the region-specific neurodegeneration in EGFR(-/-) mice.     

Multiple astrocyte responses to lysophosphatidic acids

Lysophosphatidic acid (LPA) and LPA receptors are enriched in the brain. Moreover, the levels of these receptors and ligand are modulated during brain development and injury, respectively, suggesting multiple roles for LPA in the brain. In cultured astrocytes and glioma-derived cells, LPA increases intracellular calcium concentrations and causes morphological changes. LPA also induces glioma cell migration. In normal astrocytes, LPA stimulates reactive oxygen species synthesis, activation of multiple protein kinases and expression of c-fos and c-jun. It is noteworthy that LPA-induced astrocyte responses vary as a function of the specific brain region of origin of the astrocytes. This may be one factor in the finding of LPA-stimulated proliferation in some, but not all, astrocyte studies. The species and/or developmental stage also differed in many of the astrocyte proliferation analyses. Micromolar LPA is required to elicit some astrocyte responses, including the stimulation of cytokine expression and inhibition of glutamate uptake. These events could significantly impact on survival of injured neurons and micromolar LPA concentrations are likely in diverse brain pathologies. There are important aspects of astrocyte LPA responses still to be fully evaluated, including functions in development and activation, synergy between LPA and other biomediators, and astrocyte interactions with other cells.