Stimulation of excitatory amino acid release from adult mouse brain glia subcellular particles by high mobility group box 1 protein

Therapeutic potential of HMGB1-targeting agents in sepsis

Sepsis refers to a systemic inflammatory response syndrome resulting from a microbial infection. The inflammatory response is partly mediated by innate immune cells (such as macrophages, monocytes and neutrophils), which not only ingest and eliminate invading pathogens but also initiate an inflammatory response upon recognition of pathogen-associated molecular patterns (PAMPs). The prevailing theories of sepsis as a dysregulated inflammatory response, as manifested by excessive release of inflammatory mediators such as tumour necrosis factor and high-mobility group box 1 protein (HMGB1), are supported by extensive studies employing animal models of sepsis. Here we review emerging evidence that support extracellular HMGB1 as a late mediator of experimental sepsis, and discuss the therapeutic potential of several HMGB1-targeting agents (including neutralising antibodies and steroid-like tanshinones) in experimental sepsis.

Presynaptic mGlu1 and mGlu5 autoreceptors facilitate glutamate exocytosis from mouse cortical nerve endings

The effects of mGlu1 and mGlu5 receptor activation on the depolarization-evoked release of [3H]d-aspartate ([3H]D-ASP) from mouse cortical synaptosomes were investigated. The mGlu1/5 receptor agonist 3,5-DHPG (0.1-100microM) potentiated the K+(12mM)-evoked [3H]D-ASP overflow. The potentiation occurred in a concentration-dependent manner showing a biphasic pattern. The agonist potentiated [3H]D-ASP exocytosis when applied at 0.3microM; the efficacy of 3,5-DHPG then rapidly declined and reappeared at 30-100microM. The fall of efficacy of agonist at intermediate concentration may be consistent with 3,5-DHPG-induced receptor desensitization. Facilitation of [3H]D-ASP exocytosis caused by 0.3microM 3,5-DHPG was prevented by the selective mGlu5 receptor antagonist MPEP, but was insensitive to the selective mGlu1 receptor antagonist CPCCOEt. In contrast, CPCCOEt prevented the potentiation by 50microM 3,5-DHPG, while MPEP had minimal effect. Unexpectedly, LY 367385 antagonized both the 3,5-DHPG-induced effects. A total of 0.3microM 3,5-DHPG failed to facilitate the K+-evoked [3H]D-ASP overflow from mGlu5 receptor knockout (mGlu5-/-) cortical synaptosomes, but not from nerve terminals prepared from the cortex of animals lacking the mGlu1 receptors, the crv4/crv4 mice. On the contrary, 50microM 3,5-DHPG failed to affect the [3H]D-ASP exocytosis from cortical synaptosomes obtained from crv4/crv4 and mGlu5-/-mice. Western blot analyses in subsynaptic fractions support the existence of both mGlu1 and mGlu5 autoreceptors located presynaptically, while immunocytochemistry revealed their presence at glutamatergic terminals. We propose that mGlu1 and mGlu5 autoreceptors exist on mouse glutamatergic cortical terminals; mGlu5 receptors may represent the "high affinity" binding sites for 3,5-DHPG, while mGlu1 autoreceptors represent the "low affinity" binding sites.

Functional expression of release-regulating glycine transporters GLYT1 on GABAergic neurons and GLYT2 on astrocytes in mouse spinal cord

It is widely accepted that glycine transporters of the GLYT1 type are situated on astrocytes whereas GLYT2 are present on glycinergic neuronal terminals where they mediate glycine uptake. We here used purified preparations of mouse spinal cord nerve terminals (synaptosomes) and of astrocyte-derived subcellular particles (gliosomes) to characterize functionally and morphologically the glial versus neuronal distribution of GLYT1 and GLYT2. Both gliosomes and synaptosomes accumulated [3H]GABA through GAT1 transporters and, when exposed to glycine in superfusion conditions, they released the radioactive amino acid not in a receptor-dependent manner, but as a consequence of glycine penetration through selective transporters. The glycine-evoked release of [3H]GABA was exocytotic from synaptosomes but GAT1 carrier-mediated from gliosomes. Based on the sensitivity of the glycine effects to selective GLYT1 and GLYT2 blockers, the two transporters contributed equally to evoke [3H]GABA release from GABAergic synaptosomes; even more surprising, the 'neuronal' GLYT2 contributed more efficiently than the 'glial' GLYT1 to mediate the glycine effect in [3H]GABA releasing gliosomes. These functional results were largely confirmed by confocal microscopy analysis showing co-expression of GAT1 and GLYT2 in GFAP-positive gliosomes and of GAT1 and GLYT1 in MAP2-positive synaptosomes. To conclude, functional GLYT1 are present on neuronal axon terminals and functional GLYT2 are expressed on astrocytes, indicating not complete selectivity of glycine transporters in their glial versus neuronal localization in the spinal cord.

Adult astroglia is competent for Na+/Ca2+ exchanger-operated exocytotic glutamate release triggered by mild depolarization

Glutamate release induced by mild depolarization was studied in astroglial preparations from the adult rat cerebral cortex, that is acutely isolated glial sub-cellular particles (gliosomes), cultured adult or neonatal astrocytes, and neuron-conditioned astrocytes. K+ (15, 35 mmol/L), 4-aminopyridine (0.1, 1 mmol/L) or veratrine (1, 10 micromol/L) increased endogenous glutamate or [3H]D-aspartate release from gliosomes. Neurotransmitter release was partly dependent on external Ca2+, suggesting the involvement of exocytotic-like processes, and partly because of the reversal of glutamate transporters. K+ increased gliosomal membrane potential, cytosolic Ca2+ concentration [Ca2+]i, and vesicle fusion rate. Ca2+ entry into gliosomes and glutamate release were independent from voltage-sensitive Ca2+ channel opening; they were instead abolished by 2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]isothiurea (KB-R7943), suggesting a role for the Na+/Ca2+ exchanger working in reverse mode. K+ (15, 35 mmol/L) elicited increase of [Ca2+]i and Ca2+-dependent endogenous glutamate release in adult, not in neonatal, astrocytes in culture. Glutamate release was even more marked in in vitro neuron-conditioned adult astrocytes. As seen for gliosomes, K+-induced Ca2+ influx and glutamate release were abolished by KB-R7943 also in cultured adult astrocytes. To conclude, depolarization triggers in vitro glutamate exocytosis from in situ matured adult astrocytes; an aptitude grounding on Ca2+ influx driven by the Na+/Ca2+ exchanger working in the reverse mode.

The high-mobility group box 1 cytokine induces transporter-mediated release of glutamate from glial subcellular particles (gliosomes) prepared from in situ-matured astrocytes

The multifunctional protein high-mobility group box 1 (HMGB1) is expressed in restricted areas of adult brain where it can act as a proinflammatory cytokine. We report here that HMGB1 affects CNS transmission by inducing glutamatergic release from glial (gliosomes) but not neuronal (synaptosomes) resealed subcellular particles isolated from mouse cerebellum and hippocampus. Confocal microscopy showed that gliosomes are enriched with glia-specific proteins such as GFAP and S-100, but not with neuronal proteins such as PSD-95, MAP-2, and beta-tubulin III. Furthermore, gliosomes exhibit labeling neither for integrin-alphaM nor for myelin basic protein, specific for microglia and oligodendrocytes, respectively. The gliosomal fraction contains proteins of the exocytotic machinery coexisting with GFAP. Consistent with ultrastructural analysis, several approximately 30-nm nonclustered vesicles are present in the gliosome cytoplasm. Finally, gliosomes represent functional organelles that actively export glutamate when subjected to releasing stimuli, such as ionomycin or ATP, by mechanisms involving extracellular Ca(2+) and Ca(2+) release from intracellular stores. HMGB1-induced release of the stable glutamate analogue [(3)H]d-aspartate and endogenous glutamate form gliosomes, whereas nerve terminals were insensitive to the protein. The HMGB1-evoked release of glutamate was independent on modifications of cytosolic Ca(2+) concentration, but it was blocked by dl-threo-beta-benzyloxyaspartate, suggesting the involvement of transporter-mediated release mechanisms. Moreover, dihydrokainic acid, a selective inhibitor of glutamate transporter 1 does not block the HMGB1 effect, indicating a role for the glial glutamate-aspartate transporter (GLAST) subtype in this response. HMGB1 bind to gliosomes but not to synaptosomes and can physically interact with GLAST and receptor for advanced glycation end products (RAGE). Taken together, these results suggest that the HMGB1 cytokine could act as a modulator of glutamate homeostasis in adult mammalian brain.

Anti-high mobility group box 1 monoclonal antibody ameliorates brain infarction induced by transient ischemia in rats

The high mobility group box-1 (HMGB1), originally identified as an architectural nuclear protein, exhibits an inflammatory cytokine-like activity in the extracellular space. Here we show that treatment with neutralizing anti-HMGB1 monoclonal antibody (mAb; 200 microg, twice) remarkably ameliorated brain infarction induced by 2-h occlusion of the middle cerebral artery in rats, even when the mAb was administered after the start of reperfusion. Consistent with the 90% reduction in infarct size, the accompanying neurological deficits in locomotor function were significantly improved. Anti-HMGB1 mAb inhibited the increased permeability of the blood-brain barrier, the activation of microglia, the expression of TNF-alpha and iNOS, and suppressed the activity of MMP-9, whereas it had little effect on blood flow. Intracerebroventricular injection of HMGB1 increased the severity of infarction. Immunohistochemical study revealed that HMGB1 immunoreactivity in the cell nuclei decreased or disappeared in the affected areas, suggesting the release of HMGB1 into the extracellular space. These results indicate that HMGB1 plays a critical role in the development of brain infarction through the amplification of plural inflammatory responses in the ischemic region and could be an outstandingly suitable target for the treatment. Intravenous injection of neutralizing anti-HMGB1 mAb provides a novel therapeutic strategy for ischemic stroke.

Evidence that α7 nicotinic receptor modulates glutamate release from mouse neocortical gliosomes

The presence of nicotinic receptors on astrocytes in human and rat brain has been previously demonstrated however their possible functional role is still poorly understood. In this study we investigated on the presence of nicotinic receptors on gliosomes, purified from mouse cortex, and on their role in eliciting glutamate release. Epibatidine significantly increased basal release of [3H]D-aspartate and of endogenous glutamate from mouse gliosomes but not from synaptosomes. This effect was prevented by methyllycaconitine, alpha-bungarotoxin and mecamylamine but not by dihydro-beta-erythroidine. Epibatidine provoked also a significant increase of calcium concentration in gliosomes but not in synaptosomes; the increase in [Ca2+]i induced by epibatidine and KCl in gliosomes was very similar to each other. The present results indicate that alpha7 nicotinic receptors exist on mouse cortical glial particles and stimulate glutamate release.

High mobility group box 1 protein is released by neural cells upon different stresses and worsens ischemic neurodegeneration in vitro and in vivo

High mobility group proteins are chromatin binding factors with key roles in maintenance of nuclear homeostasis. The evidence indicates that extracellularly released high mobility group box 1 (HMGB1) protein behaves as a cytokine, promoting inflammation and participating to the pathogenesis of several disorders in peripheral organs. In this study, we have investigated the expression levels and relocation dynamics of HMGB1 in neural cells, as well as its neuropathological potential. We report that HMGB1 is released in the culture media of neurons and astrocytes challenged with necrotic but not apoptotic stimuli. Recombinant HMGB1 prompts induction of pro-inflammatory mediators such as inducible nitric oxide synthase (iNOS), cyclooxygenase-2, interleukin-1beta, and tumor necrosis factor alpha, and increases excitotoxic as well as ischemic neuronal death in vitro. Dexamethasone reduces HMGB1 dependent immune glia activation, having no effect on the protein's neurotoxic effects. HMGB1 is expressed in the nucleus of neurons and astrocytes of the mouse brain, and promptly (1 h) translocates into the cytoplasm of neurons within the ischemic brain. Brain microinjection of HMGB1 increases the transcript levels of pro-inflammatory mediators and sensitizes the tissue to the ischemic injury. Together, data underscore the neuropathological role of nuclear HMGB1, and point to the protein as a mediator of post-ischemic brain damage.

Relevance of high-mobility group protein box 1 to neurodegeneration

High-mobility group protein box 1 (HMGB1), also know as amphoterin, is a nonhistone nuclear protein with well-established functions in gene transcription and homeostasis regulation into the cell nucleus. Interestingly, the protein can be passively released in the extracellular space during necrosis, whereas retained into the nucleus by cells undergoing apoptosis. Also, immune cells actively release HMGB1 on stimulation. Emerging evidence undoubtedly demonstrates that extracellular HMGB1 engages membrane receptors on different cells signaling proliferation, differentiation, cytoprotection, as well as immune activation. Consistently, numerous reports point to HMGB1 as a novel player in disease pathogenesis in peripheral organs. This chapter provides an appraisal of the emerging roles of HMGB1 in neuropathology and the neuroinflammatory response, highlighting the relevance of HMGB1-blocking agents as innovative therapeutic tools to be harnessed for neuroprotection.