Paola Bezzi

Prostaglandins stimulate calcium-dependent glutamate release in astrocytes

Astrocytes in the brain form an intimately associated network with neurons. They respond to neuronal activity and synaptically released glutamate by raising intracellular calcium concentration ([Ca2+]i), which could represent the start of back-signalling to neurons. Here we show that coactivation of the AMPA/kainate and metabotropic glutamate receptors (mGluRs) on astrocytes stimulates these cells to release glutamate through a Ca2+-dependent process mediated by prostaglandins. Pharmacological inhibition of prostaglandin synthesis prevents glutamate release, whereas application of prostaglandins (in particular PGE2) mimics and occludes the releasing action of GluR agonists. PGE2 promotes Ca2+-dependent glutamate release from cultured astrocytes and also from acute brain slices under conditions that suppress neuronal exocytotic release. When applied to the CA1 hippocampal region, PGE2 induces increases in [Ca2+]i both in astrocytes and in neurons. The [Ca2+]i increase in neurons is mediated by glutamate released from astrocytes, because it is abolished by GluR antagonists. Our results reveal a new pathway of regulated transmitter release from astrocytes and outline the existence of an integrated glutamatergic cross-talk between neurons and astrocytes in situ that may play critical roles in synaptic plasticity and in neurotoxicity.

CXCR4-activated astrocyte glutamate release via TNFalpha: amplification by microglia triggers neurotoxicity.

Astrocytes actively participate in synaptic integration by releasing transmitter (glutamate) via a calcium-regulated, exocytosis-like process. Here we show that this process follows activation of the receptor CXCR4 by the chemokine stromal cell-derived factor 1 (SDF-1). An extraordinary feature of the ensuing signaling cascade is the rapid extracellular release of tumor necrosis factor-α (TNFα). Autocrine/paracrine TNFα-dependent signaling leading to prostaglandin (PG) formation not only controls glutamate release and astrocyte communication, but also causes their derangement when activated microglia cooperate to dramatically enhance release of the cytokine in response to CXCR4 stimulation. We demonstrate that altered glial communication has direct neuropathological consequences and that agents interfering with CXCR4-dependent astrocyte–microglia signaling prevent neuronal apoptosis induced by the HIV-1 coat glycoprotein, gp120IIIB. Our results identify a new pathway for glia–glia and glia–neuron communication that is relevant to both normal brain function and neurodegenerative diseases.

Astrocytes contain a vesicular compartment that is competent for regulated exocytosis of glutamate

Astrocytes establish rapid cell-to-cell communication through the release of chemical transmitters. The underlying mechanisms and functional significance of this release are, however, not well understood. Here we identify an astrocytic vesicular compartment that is competent for glutamate exocytosis. Using postembedding immunogold labeling of the rat hippocampus, we show that vesicular glutamate transporters (VGLUT1/2) and the vesicular SNARE protein, cellubrevin, are both expressed in small vesicular organelles that resemble synaptic vesicles of glutamatergic terminals. Astrocytic vesicles, which are not as densely packed as their neuronal counterparts, can be observed in small groups at sites adjacent to neuronal structures bearing glutamate receptors. Fluorescently tagged VGLUT-containing vesicles were studied dynamically in living astrocytes by total internal reflection fluorescence (TIRF) microscopy. After activation of metabotropic glutamate receptors, astrocytic vesicles underwent rapid (milliseconds) Ca2+- and SNARE-dependent exocytic fusion that was accompanied by glutamate release. These data document the existence of a Ca2+-dependent quantal glutamate release activity in glia that was previously considered to be specific to synapses.

Glutamate exocytosis from astrocytes controls synaptic strength

The release of transmitters from glia influences synaptic functions. The modalities and physiological functions of glial release are poorly understood. Here we show that glutamate exocytosis from astrocytes of the rat hippocampal dentate molecular layer enhances synaptic strength at excitatory synapses between perforant path afferents and granule cells. The effect is mediated by ifenprodil-sensitive NMDA ionotropic glutamate receptors and involves an increase of transmitter release at the synapse. Correspondingly, we identify NMDA receptor 2B subunits on the extrasynaptic portion of excitatory nerve terminals. The receptor distribution is spatially related to glutamate-containing synaptic-like microvesicles in the apposed astrocytic processes. This glial regulatory pathway is endogenously activated by neuronal activity–dependent stimulation of purinergic P2Y1 receptors on the astrocytes. Thus, we provide the first combined functional and ultrastructural evidence for a physiological control of synaptic activity via exocytosis of glutamate from astrocytes.

A neuron–glia signalling network in the active brain

Glial cells are active partners of neurons in processing information and synaptic integration. They receive coded signals from synapses and elaborate modulatory responses. The active properties of glia, including long-range signalling and regulated transmitter release, are beginning to be elucidated. Recent insights suggest that the active brain should no longer be regarded as a circuitry of neuronal contacts, but as an integrated network of interactive neurons and glia.

TNFα Controls Glutamatergic Gliotransmission in the Hippocampal Dentate Gyrus

VIDEO ABSTRACT Glutamatergic gliotransmission provides a stimulatory input to excitatory synapses in the hippocampal dentate gyrus. Here, we show that tumor necrosis factor-alpha (TNFα) critically controls this process. With constitutive TNFα present, activation of astrocyte P2Y1 receptors induces localized [Ca(2+)](i) elevations followed by glutamate release and presynaptic NMDA receptor-dependent synaptic potentiation. In preparations lacking TNFα, astrocytes respond with identical [Ca(2+)](i) elevations but fail to induce neuromodulation. We find that TNFα specifically controls the glutamate release step of gliotransmission. In cultured astrocytes lacking TNFα glutamate exocytosis is dramatically slowed down due to altered vesicle docking. Addition of low picomolar TNFα promptly reconstitutes both normal exocytosis in culture and gliotransmission in situ. Alternatively, gliotransmission can be re-established without adding TNFα, by limiting glutamate uptake, which compensates slower release. These findings demonstrate that gliotransmission and its synaptic effects are controlled not only by astrocyte [Ca(2+)](i) elevations but also by permissive/homeostatic factors like TNFα.

P2Y1 Receptor-evoked Glutamate Exocytosis from Astrocytes CONTROL BY TUMOR NECROSIS FACTOR-α AND PROSTAGLANDINS

ATP, released by both neurons and glia, is an important mediator of brain intercellular communication. We find that selective activation of purinergic P2Y1 receptors (P2Y1R) in cultured astrocytes triggers glutamate release. By total internal fluorescence reflection imaging of fluorescence-labeled glutamatergic vesicles, we document that such release occurs by regulated exocytosis. The stimulus-secretion coupling mechanism involves Ca2+ release from internal stores and is controlled by additional transductive events mediated by tumor necrosis factor-α (TNFα) and prostaglandins (PG). P2Y1R activation induces release of both TNFα and PGE2 and blocking either one significantly reduces glutamate release. Accordingly, astrocytes from TNFα-deficient (TNF–/–) or TNF type 1 receptor-deficient (TNFR1–/–) mice display altered P2Y1R-dependent Ca2+ signaling and deficient glutamate release. In mixed hippocampal cultures, the P2Y1R-evoked process occurs in astrocytes but not in neurons or microglia. P2Y1R stimulation induces Ca2+-dependent glutamate release also from acute hippocampal slices. The process in situ displays characteristics resembling those in cultured astrocytes and is distinctly different from synaptic glutamate release evoked by high K+ stimulation as follows: (a) it is sensitive to cyclooxygenase inhibitors; (b) it is deficient in preparations from TNF–/– and TNFR1–/– mice; and (c) it is inhibited by the exocytosis blocker bafilomycin A1 with a different time course. No glutamate release is evoked by P2Y1R-dependent stimulation of hippocampal synaptosomes. Taken together, our data identify the coupling of purinergic P2Y1R to glutamate exocytosis and its peculiar TNFα- and PG-dependent control, and we strongly suggest that this cascade operates selectively in astrocytes. The identified pathway may play physiological roles in glial-glial and glial-neuronal communication.

Gliotransmission and the Tripartite Synapse

In the last years, the classical view of glial cells (in particular of astrocytes) as a simple supportive cell for neurons has been replaced by a new vision in which glial cells are active elements of the brain. Such a new vision is based on the existence of a bidirectional communication between astrocytes and neurons at synaptic level. Indeed, perisynaptic processes of astrocytes express active G-protein-coupled receptors that are able (1) to sense neurotransmitters released from the synapse during synaptic activity, (2) to increase cytosolic levels of calcium, and (3) to stimulate the release of gliotransmitters that in turn can interact with the synaptic elements. The mechanism(s) by which astrocytes can release gliotransmitter has been extensively studied during the last years. Many evidences have suggested that a fraction of astrocytes in situ release neuroactive substances both with calcium-dependent and calcium-independent mechanism(s); whether these mechanisms coexist and under what physiological or pathological conditions they occur, it remains unclear. However, the calcium-dependent exocytotic vesicular release has received considerable attention due to its potential to occur under physiological conditions via a finely regulated way. By releasing gliotransmitters in millisecond time scale with a specific vesicular apparatus, astrocytes can integrate and process synaptic information and control or modulate synaptic transmission and plasticity.

The Competitive Transport Inhibitor L‐trans‐pyrrolidine‐2,4‐dicarboxylate Triggers Excitotoxicity in Rat Cortical Neuron‐Astrocyte Co‐cultures via Glutamate Release rather than Uptake Inhibition

We studied the early and late effects of L‐trans‐pyrrolidine‐2,4‐dicarboxylate (PDC), a competitive inhibitor of glutamate uptake with low affinity for glutamate receptors, in co‐cultures of rat cortical neurons and glia expressing spontaneous excitatory amino acid (EAA) neurotransmission. At 100 or 200 μM, PDC induced different patterns of electrical changes: 100 μM prolonged tetrodotoxin‐sensitive excitation triggered by synaptic glutamate release; 200 μM produced sustained, tetrodotoxin‐insensitive and EAA‐mediated neuronal depolarization, overwhelming synaptic activity. At 200 μM, but not at 100 μM, PDC caused rapid elevation of the glutamate concentration ([Glu]0) in the culture medium, resulting in NMDA receptor‐mediated excitotoxic death of neurons 24 h later. The increase in [Glu]0 was largely insensitive to tetrodotoxin, independent of extracellular Ca2+, and present also in astrocyte‐pure cultures. By the use of glutamate transporters functionally reconstituted in liposomes, we showed directly that PDC activates carrier‐mediated release of glutamate via heteroexchange. Glutamate release and delayed neurotoxicity in our cultures were suppressed if PDC was applied in a Na+‐free medium containing Li+. However, replacement of Na+ with choline instead of Li+ did not result in an identical effect, suggesting that Li+ does not act simply as an external Na+ substitute. In conclusion, our data indicate that alteration of glutamate transport by PDC has excitotoxic consequences and that active release of glutamate rather than just uptake inhibition is responsible for the generation of neuronal injury.

Fast Subplasma Membrane Ca2+ Transients Control Exo-Endocytosis of Synaptic-Like Microvesicles in Astrocytes

Astrocytes are the most abundant glial cell type in the brain. Although not apposite for long-range rapid electrical communication, astrocytes share with neurons the capacity of chemical signaling via Ca2+-dependent transmitter exocytosis. Despite this recent finding, little is known about the specific properties of regulated secretion and vesicle recycling in astrocytes. Important differences may exist with the neuronal exocytosis, starting from the fact that stimulus-secretion coupling in astrocytes is voltage independent, mediated by G-protein-coupled receptors and the release of Ca2+ from internal stores. Elucidating the spatiotemporal properties of astrocytic exo-endocytosis is, therefore, of primary importance for understanding the mode of communication of these cells and their role in brain signaling. We here take advantage of fluorescent tools recently developed for studying recycling of glutamatergic vesicles at synapses (Voglmaier et al., 2006; Balaji and Ryan, 2007); we combine epifluorescence and total internal reflection fluorescence imaging to investigate with unprecedented temporal and spatial resolution, the stimulus-secretion coupling underlying exo-endocytosis of glutamatergic synaptic-like microvesicles (SLMVs) in astrocytes. Our main findings indicate that (1) exo-endocytosis in astrocytes proceeds with a time course on the millisecond time scale (τexocytosis = 0.24 ± 0.017 s; τendocytosis = 0.26 ± 0.03 s) and (2) exocytosis is controlled by local Ca2+ microdomains. We identified submicrometer cytosolic compartments delimited by endoplasmic reticulum tubuli reaching beneath the plasma membrane and containing SLMVs at which fast (time-to-peak, ∼50 ms) Ca2+ events occurred in precise spatial-temporal correlation with exocytic fusion events. Overall, the above characteristics of transmitter exocytosis from astrocytes support a role of this process in fast synaptic modulation.