Glutamate Homeostasis In Brain Research Articles

Cyanidin-3-O-glucoside alleviates trimethyltin chloride-induced neurodegeneration by maintaining glutamate homeostasis through modulation of the gut microbiota

Trimethyltin chloride (TMT) is a potent neurotoxin to cause neurodegeneration, especially in hippocampus. This study aimed to identify dietary components that can effectively attenuate TMT-induced neurodegeneration in humans. The predominant anthocyanin in human diets, cyanidin-3-O-glucoside (C3G, 5 or 50 mg/kg), was given to mice for 16 days, and TMT (2.7 mg/kg) was injected intraperitoneally once on the eighth day. C3G (50 mg/kg) significantly alleviated TMT-induced seizures and subsequent cognitive impairment by ameliorating hippocampal neurodegeneration and synaptic dysfunction. Furthermore, C3G treatment restored glutamate homeostasis in brain and reversed glutamine synthetase (GS) inhibition in reactive astrogliosis and neuroinflammation, which are critical for C3G’s neuroprotective effects. Notably, C3G decreased the lipopolysaccharide, tumor necrosis factor-α, interleukin-6, and interleukin-1β levels in the mice, which potentially by modulating the relative abundance of Atopobiaceae and Lachnospiraceae in the gut. C3G may be a promising and practical dietary component for reducing TMT-induced neurodegeneration.

Astrocyte and glutamate involvement in the pathogenesis of epilepsy in Alzheimer's disease.

Alzheimer's disease (AD) can increase the risk of epilepsy by up to 10-fold compared to healthy age-matched controls. However, the pathological mechanisms that underlie this increased risk are poorly understood. Because disruption in brain glutamate homeostasis has been implicated in both AD and epilepsy, this might play a mechanistic role in the pathogenesis of epilepsy in AD. Prior to the formation of amyloid beta (Aβ) plaques, the brain can undergo pathological changes as a result of increased production of amyloid precursor protein (APP) and Aβ oligomers. Impairments in the glutamate uptake ability of astrocytes due to astrogliosis are hypothesized to be an early event occurring before Aβ plaque formation. Astrogliosis may increase the susceptibility to epileptogenesis of the brain via accumulation of extracellular glutamate and resulting excitotoxicity. Here we hypothesize that Aβ oligomers and proinflammatory cytokines can cause astrogliosis and accumulation of extracellular glutamate, which then contribute to the pathogenesis of epilepsy in AD. In this review article, we consider the evidence supporting a potential role of dysfunction of the glutamate-glutamine cycle and the astrocyte in the pathogenesis of epilepsy in AD.

Brain endothelial cells metabolize glutamate via glutamate dehydrogenase to replenish TCA-intermediates and produce ATP under hypoglycemic conditions.

The endothelial cells of the blood-brain barrier participate in the regulation of glutamate concentrations in the brain interstitial fluid by taking up brain glutamate. However, endothelial glutamate metabolism has not been characterized, nor is its role in brain glutamate homeostasis and endothelial energy production known. The aim of this study was to investigate endothelial glutamate dehydrogenase (GDH) expression and glutamate metabolism and probe its functional significance. The primary brain endothelial cells were isolated from bovine and mouse brains, and human brain endothelial cells were derived from induced pluripotent stem cells. GDH expression on the protein level and GDH function were investigated in the model systems using western blotting, confocal microscopy, 13 C-glutamate metabolism, and Seahorse assay. In this study, it was shown that GDH was expressed in murine and bovine brain capillaries and in cultured primary mouse and bovine brain endothelial cells as well as in human-induced pluripotent stem cell-derived endothelial cells. The endothelial GDH expression was confirmed in brain capillaries from mice carrying a central nervous system-specific GDH knockout. Endothelial cells from all tested species metabolized 13 C-glutamate to α-ketoglutarate, which subsequently entered the tricarboxylic acid (TCA)-cycle. Brain endothelial cells maintained mitochondrial oxygen consumption rates, when supplied with glutamate alone, whereas glutamate supplied in addition to glucose did not lead to additional oxygen consumption. In conclusion, brain endothelial cells directly take up and metabolize glutamate and utilize the resulting α-ketoglutarate in the tricarboxylic acid cycle to ultimately yield ATP if glucose is unavailable.

GLAST Activity is Modified by Acute Manganese Exposure in Bergmann Glial Cells.

Glutamate is the major excitatory amino acid neurotransmitter in the vertebrate brain. It exerts its actions through the activation of specific plasma membrane receptors expressed in neurons and glial cells. Overactivation of glutamate receptors results in neuronal death, known as excitotoxicity. A family of sodium-dependent glutamate transporters enriched in glial cells are responsible of the vast majority of the removal of this amino acid form the synaptic cleft. Therefore, a precise and exquisite regulation of these proteins is required not only for a proper glutamatergic transmission but also for the prevention of an excitotoxic insult. Manganese is a trace element essential as a cofactor for several enzymatic systems, although in high concentrations is involved in the disruption of brain glutamate homeostasis. The molecular mechanisms associated to manganese neurotoxicity have been focused on mitochondrial function, although energy depletion severely compromises the glutamate uptake process. In this context, in this contribution we analyze the effect of manganese exposure in glial glutamate transporters function. To this end, we used the well-established model of chick cerebellar Bergmann glia cultures. A time and dose dependent modulation of [3H]-D-aspartate uptake was found. An increase in the transporter catalytic efficiency, most probably linked to a discrete increase in the affinity of the transporter was detected upon manganese exposure. Interestingly, glucose uptake was reduced by this metal. These results favor the notion of a direct effect of manganese on glial cells, this in turn alters their coupling with neurons and might lead to changes in glutamatergic transmission.

AGO CLIP Reveals an Activated Network for Acute Regulation of Brain Glutamate Homeostasis in Ischemic Stroke

Post-transcriptional regulation by microRNAs (miRNAs) is essential for complex molecular responses to physiological insult and disease. Although many disease-associated miRNAs are known, their global targets and culminating network effects on pathophysiology remain poorly understood. We applied Argonaute (AGO) crosslinking immunoprecipitation (CLIP) to systematically elucidate altered miRNA-target interactions in brain following ischemia and reperfusion (I/R) injury. Among 1,190 interactions identified, the most prominent was the cumulative loss of target regulation by miR-29 family members. Integration of translational and time-course RNA profiles revealed a dynamic mode of miR-29 target de-regulation, led by acute translational activation and a later increase in RNA levels, allowing rapid proteomic changes to take effect. These functional regulatory events rely on canonical and non-canonical miR-29 binding and engage glutamate reuptake signals, such as glial glutamate transporter (GLT-1), to control local glutamate levels. These results uncover a miRNA target network that acts acutely to maintain brain homeostasis after ischemic stroke.

Targeting glutamatergic and cellular prion protein mechanisms of amyloid β-mediated persistent synaptic plasticity disruption: Longitudinal studies

Alzheimer's disease amyloid-β (Aβ) oligomers are synaptotoxic, inappropriately increasing extracellular glutamate concentration and glutamate receptor activation to thereby rapidly disrupt synaptic plasticity. Thus, acutely promoting brain glutamate homeostasis with a blood-based scavenging system, glutamate-oxaloacetate transaminase (GOT), and blocking metabotropic glutamate 5 (mGlu5) receptor or its co-receptor cellular prion protein (PrP), prevent the acute inhibition of long-term potentiation (LTP) by exogenous Aβ. Here, we evaluated the time course of the effects of such interventions in the persistent disruptive effects of Aβ oligomers, either exogenously injected in wild type rats or endogenously generated in transgenic rats that model Alzheimer's disease amyloidosis. We report that repeated, but not acute, systemic administration of recombinant GOT type 1, with or without the glutamate co-substrate oxaloacetate, reversed the persistent deleterious effect of exogenous Aβ on synaptic plasticity. Moreover, similar repetitive treatment reversibly abrogated the inhibition of LTP monitored longitudinally in freely behaving transgenic rats. Remarkably, brief repeated treatment with an mGlu5 receptor antagonist, basimglurant, or an antibody that prevents Aβ oligomer binding to PrP, ICSM35, also had similar reversible ameliorative effects in the transgenic rat model. Overall, the present findings support the ongoing development of therapeutics for early Alzheimer's disease based on these complementary approaches.

Expression of the human isoform of glutamate dehydrogenase, hGDH2, augments TCA cycle capacity and oxidative metabolism of glutamate during glucose deprivation in astrocytes.

A key enzyme in brain glutamate homeostasis is glutamate dehydrogenase (GDH) which links carbohydrate and amino acid metabolism mediating glutamate degradation to CO2 and expanding tricarboxylic acid (TCA) cycle capacity with intermediates, i.e. anaplerosis. Humans express two GDH isoforms, GDH1 and 2, whereas most other mammals express only GDH1. hGDH1 is widely expressed in human brain while hGDH2 is confined to astrocytes. The two isoforms display different enzymatic properties and the nature of these supports that hGDH2 expression in astrocytes potentially increases glutamate oxidation and supports the TCA cycle during energy-demanding processes such as high intensity glutamatergic signaling. However, little is known about how expression of hGDH2 affects the handling of glutamate and TCA cycle metabolism in astrocytes. Therefore, we cultured astrocytes from cerebral cortical tissue of hGDH2-expressing transgenic mice. We measured glutamate uptake and metabolism using [3 H]glutamate, while the effect on metabolic pathways of glutamate and glucose was evaluated by use of 13 C and 14 C substrates and analysis by mass spectrometry and determination of radioactively labeled metabolites including CO2 , respectively. We conclude that hGDH2 expression increases capacity for uptake and oxidative metabolism of glutamate, particularly during increased workload and aglycemia. Additionally, hGDH2 expression increased utilization of branched-chain amino acids (BCAA) during aglycemia and caused a general decrease in oxidative glucose metabolism. We speculate, that expression of hGDH2 allows astrocytes to spare glucose and utilize BCAAs during substrate shortages. These findings support the proposed role of hGDH2 in astrocytes as an important fail-safe during situations of intense glutamatergic activity. GLIA 2017;65:474-488.

Is glutamate dehydrogenase in astrocytes one of the keys to control brain glutamate homeostasis?

SummaryBrain glutamate concentration needs to be balanced to avoid excitoxicity. Following glutamatergic neurotransmission astrocytes are responsible for clearance of the synaptic cleft via glutamate transporters. In the astrocyte the conversion of glutamate to glutamine is an essential part of the glutamate‐glutamine cycle. But, a substantial amount of glutamate is oxidatively metabolized in the mitochondria, which to a large extent may be dependent on glutamate dehydrogenase (GDH). Thus, astrocytes are likely the main regulator of the brain glutamate concentration, but how do they do it? We have investigated the role of GDH in astrocytes with focus on energy and glutamate neurotransmitter homeostasis. We have used cultured astrocytes originating from CNS‐specific GDH1 knock‐out mice and cultures of astrocytes treated with siRNA against GDH. We find that an impaired GDH activity force glutamate to be only partially oxidized via the truncated TCA cycle and formation of another excitatory amino acid, aspartate. Astrocytes totally lacking GDH exhibit an increased glycolysis and impaired glucose oxidation, supporting that astrocytes are in a need for glutamate oxidation to sustain energy metabolism.

Peripheral Interventions Enhancing Brain Glutamate Homeostasis Relieve Amyloid β- and TNFα- Mediated Synaptic Plasticity Disruption in the Rat Hippocampus.

Dysregulation of glutamate homeostasis in the interstitial fluid of the brain is strongly implicated in causing synaptic dysfunction in many neurological and psychiatric illnesses. In the case of Alzheimer's disease (AD), amyloid β (Aβ)-mediated disruption of synaptic plasticity and memory can be alleviated by interventions that directly remove glutamate or block certain glutamate receptors. An alternative strategy is to facilitate the removal of excess glutamate from the nervous system by activating peripheral glutamate clearance systems. One such blood-based system, glutamate oxaloacetate transaminase (GOT), is activated by oxaloacetate, which acts as a co-substrate. We report here that synthetic and AD brain-derived Aβ-mediated inhibition of synaptic long-term potentiation in the hippocampus is alleviated by oxaloacetate. Moreover the effect of oxaloacetate was GOT-dependent. The disruptive effects of a general inhibitor of excitatory amino acid transport or TNFα, a pro-inflammatory mediator of Aβ action, were also reversed by oxaloacetate. Furthermore, another intervention that increases peripheral glutamate clearance, peritoneal dialysis, mimicked the beneficial effect of oxaloacetate. These findings lend support to the promotion of the peripheral clearance of glutamate as a means to alleviate synaptic dysfunction that is caused by impaired glutamate homeostasis in the brain.

N‐Acetylcysteine reduces early‐ and late‐stage cocaine seeking without affecting cocaine taking in rats

N-acetylcysteine (NAC) has been suggested to have therapeutic potential in the treatment of drug addiction through its effects on brain glutamate homeostasis. Here we show that NAC treatment resulted in dose-dependent reductions in cocaine seeking at both early and late stages of acquisition and maintenance of cocaine-seeking behavior, while confirming it had no effect on cocaine reinforcement. The results indicate that NAC is able to significantly diminish the propensity to seek cocaine early and late in the development of addiction and, taken together with previous work, indicates significant potential in relapse prevention.