Glutamate metabolism in the brain focusing on astrocytes

Endocannabinoid signaling in astrocytes

The study of the astrocytic contribution to brain functions has been growing in popularity in the neuroscience field. In the last years, and especially since the demonstration of the involvement of astrocytes in synaptic functions, the astrocyte field has revealed multiple functions of these cells that seemed inconceivable not long ago. In parallel, cannabinoid investigation has also identified different ways by which cannabinoids are able to interact with these cells, modify their functions, alter their communication with neurons and impact behavior. In this review, we will describe the expression of different endocannabinoid system members in astrocytes. Moreover, we will relate the latest findings regarding cannabinoid modulation of some of the most relevant astroglial functions, namely calcium (Ca²⁺ ) dynamics, gliotransmission, metabolism, and inflammation.

Glial Cells Response in Stroke

As the second-leading cause of death, stroke faces several challenges in terms of treatment because of the limited therapeutic interventions available. Previous studies primarily focused on metabolic and blood flow properties as a target for treating stroke, including recombinant tissue plasminogen activator and mechanical thrombectomy, which are the only USFDA approved therapies. These interventions have the limitation of a narrow therapeutic time window, the possibility of hemorrhagic complications, and the expertise required for performing these interventions. Thus, it is important to identify the contributing factors that exacerbate the ischemic outcome and to develop therapies targeting them for regulating cellular homeostasis, mainly neuronal survival and regeneration. Glial cells, primarily microglia, astrocytes, and oligodendrocytes, have been shown to have a crucial role in the prognosis of ischemic brain injury, contributing to inflammatory responses. They play a dual role in both the onset as well as resolution of the inflammatory responses. Understanding the different mechanisms driving these effects can aid in the development of therapeutic targets and further mitigate the damage caused. In this review, we summarize the functions of various glial cells and their contribution to stroke pathology. The review highlights the therapeutic options currently being explored and developed that primarily target glial cells and can be used as neuroprotective agents for the treatment of ischemic stroke.

Interrogating the Metabolomic Profile of Amyotrophic Lateral Sclerosis in the Post-Mortem Human Brain by Infrared Matrix-Assisted Laser Desorption Electrospray Ionization (IR-MALDESI) Mass Spectrometry Imaging (MSI)

Amyotrophic lateral sclerosis (ALS) is an idiopathic, fatal neurodegenerative disease characterized by progressive loss of motor function with an average survival time of 2-5 years after diagnosis. Due to the lack of signature biomarkers and heterogenous disease phenotypes, a definitive diagnosis of ALS can be challenging. Comprehensive investigation of this disease is imperative to discovering unique features to expedite the diagnostic process and improve diagnostic accuracy. Here, we present untargeted metabolomics by mass spectrometry imaging (MSI) for comparing sporadic ALS (sALS) and C9orf72 positive (C9Pos) post-mortem frontal cortex human brain tissues against a control cohort. The spatial distribution and relative abundance of metabolites were measured by infrared matrix-assisted laser desorption electrospray ionization (IR-MALDESI) MSI for association to biological pathways. Proteomic studies on the same patients were completed via LC-MS/MS in a previous study, and results were integrated with imaging metabolomics results to enhance the breadth of molecular coverage. Utilizing METASPACE annotation platform and MSiPeakfinder, nearly 300 metabolites were identified across the sixteen samples, where 25 were identified as dysregulated between disease cohorts. The dysregulated metabolites were further examined for their relevance to alanine, aspartate, and glutamate metabolism, glutathione metabolism, and arginine and proline metabolism. The dysregulated pathways discussed are consistent with reports from other ALS studies. To our knowledge, this work is the first of its kind, reporting on the investigation of ALS post-mortem human brain tissue analyzed by multiomic MSI.

Divergent Cellular Energetics, Glutamate Metabolism, and Mitochondrial Function Between Human and Mouse Cerebral Cortex

Disruptions of brain energy and neurotransmitter metabolism are associated with several pathological conditions including neurodegenerative diseases such as Alzheimer's disease. Transgenic rodent models, and in vitro preparations hereof, are often applied for studying pathological aspects of brain metabolism. However, despite the conserved cerebral development across mammalian species, distinct differences in cellular composition and structure may influence metabolism of the rodent and human brain. To address this, we investigated the metabolic function of acutely isolated brain slices and non-synaptic mitochondria obtained from the cerebral cortex of mice and neurosurgically resected neocortical tissue of humans. Utilizing dynamic isotope labeling with ¹³C-enriched metabolic substrates, we show that metabolism of glucose, acetate, β-hydroxybutyrate, and glutamine operates at lower rates in human cerebral cortical slices when compared to mouse slices. In contrast, human cerebral cortical slices display a higher capacity for converting exogenous glutamate into glutamine, which subsequently supports neuronal GABA synthesis, whereas mouse slices primarily convert glutamate into aspartate. In line with the reduced metabolic rate of the human brain slices, isolated non-synaptic mitochondria of the human cerebral cortex have a lower oxygen consumption rate when provided succinate as substrate. However, when provided pyruvate and malate, human mitochondria display a higher coupled respiration and lower proton leak, signifying a more efficient mitochondrial coupling compared to mouse mitochondria. This study reveals key differences between mouse and human brain metabolism concerning both neurons and astrocytes, which must be taken into account when applying in vitro rodent preparations as a model system of the human brain.

Alterations in Astrocytic Regulation of Excitation and Inhibition by Stress Exposure and in Severe Psychopathology

Dysregulation of excitatory and inhibitory signaling is commonly observed in major psychiatric disorders, including schizophrenia, depression, and bipolar disorder, and is often targeted by psychological and pharmacological treatment methods. The balance of excitation and inhibition is highly sensitive to severe psychological stress, one of the strongest risk factors for psychiatric disorders. The role of astrocytes in regulating excitatory and inhibitory signaling is now widely recognized; however, the specific involvement of astrocytes in the context of psychiatric disorders with a history of significant stress exposure remains unclear. In this review, we summarize how astrocytes regulate the balance of excitation and inhibition in the context of stress exposure and severe psychopathology, with a focus on the PFC, a brain area highly implicated in psychopathology. We first focus on preclinical models to demonstrate that the duration of stress (particularly acute vs chronic stress) is key to shaping astrocyte function and downstream behavior. We then provide a hypothesis for how astrocytes are involved in stress-associated cortical signaling imbalance, discuss how this directly contributes to phenotypes of psychopathologies, and provide suggestions for future research. We highlight that astrocytes are a key target to understand and treat the dysregulation of cortical signaling associated with stress-related psychiatric disorders.

Hormone-Glutamine Metabolism: A Critical Regulatory Axis in Endocrine-Related Cancers

The endocrine-related cancers and hormones are undoubtedly highly interconnected. How hormones support or repress tumor induction and progression has been extensively profiled. Furthermore, advances in understanding the role of glutamine metabolism in mediating tumorigenesis and development, coupled with these in-depth studies on hormone (e.g., estrogen, progesterone, androgen, prostaglandin, thyroid hormone, and insulin) regulation of glutamine metabolism, have led us to think about the relationship between these three factors, which remains to be elucidated. Accordingly, in this review, we present an updated overview of glutamine metabolism traits and its influence on endocrine oncology, as well as its upstream hormonal regulation. More importantly, this hormone/glutamine metabolism axis may help in the discovery of novel therapeutic strategies for endocrine-related cancer.

Phosphonate analog of 2-oxoglutarate regulates glutamate-glutamine homeostasis and counteracts amyloid beta induced learning and memory deficits in rats

BACKGROUND

Metabolic alteration is a mainstream concept underlying the cognitive decline in neurodegenerative disorders including Alzheimer's disease (AD). Mitochondrial enzyme α-ketoglutarate dehydrogenase complex (α-KGDHC) seems to play a dual-edged sword role in cytotoxic insult. Here, using succinyl phosphonate (SP), a specific α-KGDHC inhibitor, we aimed to examine its potential action on AD progression.

METHODS

Male Wistar rats were assigned to two separate experiments. First, they were bilaterally microinjected into the dorsal CA₁ area by amyloid-beta (Aβ)_25-35 for four consecutive days. Seven days after the last injection, they were trained to acquire Morris Water Maze (MWM) task for three successive days when they were treated with SP after each training session. In the second experiment, SP was administered 30 min after the first Aβ microinjection and behavioral tests were performed one week after the last Aβ administration. The activity of glutamate dehydrogenase (GDH), and glutamine synthetase (GS), as key enzymes involved in glutamate-glutamine homeostasis and histological assays were evaluated in the hippocampi.

RESULTS

Our behavioral results indicated that post-training SP treatment enhanced task acquisition but did not change memory performance in Aβ-treated rats. However, administration of SP at the time of Aβ injection precludes the deteriorative effect of Aβ and neuronal injury on both spatial learning and memory performances indicating its preventive action against Aβ pathology at its early stages. Measurement of enzymes activity shows that α-KGDHC activity was reduced in the Aβ treated group, and SP administration restored its activity; also, GDH and GS activities were increased and decreased respectively due to Aβ, and SP reversed the action of Aβ on these enzymes.

CONCLUSIONS

This study proposes that SP possibly a promising therapeutic approach to improve memory impairment in AD, especially in the early phases of this disease.

Loss of mitochondrial enzyme GPT2 causes early neurodegeneration in locus coeruleus

Locus coeruleus (LC) is among the first brain areas to degenerate in Alzheimer’s disease and Parkinson’s disease; however, the underlying causes for the vulnerability of LC neurons are not well defined. Here we report a novel mechanism of degeneration of LC neurons caused by loss of the mitochondrial enzyme glutamate pyruvate transaminase 2 (GPT2). GPT2 Deficiency is a newly-recognized childhood neurometabolic disorder. The GPT2 enzyme regulates cell growth through replenishment of tricarboxylic acid (TCA) cycle intermediates and modulation of amino acid metabolism. In Gpt2-null mice, we observe an early loss of tyrosine hydroxylase (TH)-positive neurons in LC and reduced soma size at postnatal day 18. Gpt2-null LC shows selective positive Fluoro-Jade C staining. Neuron loss is accompanied by selective, prominent microgliosis and astrogliosis in LC. We observe reduced noradrenergic projections to and norepinephrine levels in hippocampus and spinal cord. Whole cell recordings in Gpt2-null LC slices show reduced soma size and abnormal action potentials with altered firing kinetics. Strikingly, we observe early decreases in phosphorylated S6 in Gpt2-null LC, preceding prominent p62 aggregation, increased LC3B-II to LC3B-I ratio, and neuronal loss. These data are consistent with a possible mechanism involving deficiency in protein synthesis and cell growth, associated subsequently with abnormal autophagy and neurodegeneration. As compared to the few genetic animal models with LC degeneration, loss of LC neurons in Gpt2-null mice is developmentally the earliest. Early neuron loss in LC in a model of human neurometabolic disease provides important clues regarding the metabolic vulnerability of LC and may lead to new therapeutic targets.

Modulatory effects of vitamin B3 and its derivative on the levels of apoptotic and vascular regulators and cytoskeletal proteins in diabetic rat brain as signs of neuroprotection

BACKGROUND

Beneficial effects of nicotinamide (NAm) and its derivates have been earlier shown in animal models of diabetes mellitus (DM), but the mechanisms of their neuroprotective activities are still largely unknown. The aim of the present study was to investigate if NAm and conjugate of nicotinic acid with gamma-aminobutyric acid (N-GABA) are able to modulate expression levels of apoptosis regulators, angiogenesis-related molecules, and specific cytoskeletal proteins in diabetic rat brain.

METHODS

After six weeks of streptozotocin induced type 1 DM, rats were daily administered either by NAm (100 mg/kg) or N-GABA (55 mg/kg) intraperitoneally for two weeks. Protein levels were assessed by western blot and immunohistochemistry.

RESULTS

Both NAm and N-GABA down-regulated NF-κB and Bax levels in diabetic rat brain, suggesting their anti-apoptotic activities. Tested compounds normalized VEGF and nNOS contents improving pro-angiogenic signaling reduced by hyperglycemia. Western blot showed marked up-regulation of astroglial marker GFAP and lowering neurofilament protein levels in DM group, confirmed immunohistochemically, indicating the development of reactive astrogliosis as a major response to diabetes-induced neurodegeneration. NAm had no effects on GFAP and Nf-L levels in the diabetic brain, while N-GABA increased their expression. Inversely, NAm and N-GABA dramatically reduced enhanced levels of GFAP and Nf-L found in the blood serum of diabetic rats, providing for the first time strong evidence for preserving blood-brain barrier integrity by studied compounds.

CONCLUSION

Thus, NAm and N-GABA may exert neuroprotective effects by decreasing pro-apoptotic regulators levels and improving expression of angiogenic and cytoskeletal proteins impaired by hyperglycemia in rat brain.

Astrocyte energy and neurotransmitter metabolism in Alzheimer's disease: integration of the glutamate/GABA-glutamine cycle

Astrocytes contribute to the complex cellular pathology of Alzheimer's disease (AD). Neurons and astrocytes function in close collaboration through neurotransmitter recycling, collectively known as the glutamate/GABA-glutamine cycle, which is essential to sustain neurotransmission. Neurotransmitter recycling is intimately linked to astrocyte energy metabolism. In the course of AD, astrocytes undergo extensive metabolic remodeling, which may profoundly affect the glutamate/GABA-glutamine cycle. The consequences of altered astrocyte function and metabolism in relation to neurotransmitter recycling are yet to be comprehended. Metabolic alterations of astrocytes in AD deprive neurons of metabolic support, thereby contributing to synaptic dysfunction and neurodegeneration. In addition, several astrocyte-specific components of the glutamate/GABA-glutamine cycle, including glutamine synthesis and synaptic neurotransmitter uptake, are perturbed in AD. Integration of the complex astrocyte biology within the context of AD is essential for understanding the fundamental mechanisms of the disease, while restoring astrocyte metabolism may serve as an approach to arrest or even revert clinical progression of AD.

The Time Sequence of Gene Expression Changes after Spinal Cord Injury

Gene expression changes following spinal cord injury (SCI) are time-dependent, and an accurate understanding of these changes can be crucial in determining time-based treatment options in a clinical setting. We performed RNA sequencing of the contused spinal cord of rats at five different time points from the very acute to chronic stages (1 hour, 1 day, 1 week, 1 month, and 3 months) following SCI. We identified differentially expressed genes (DEGs) and Gene Ontology (GO) terms at each time point, and 14,257 genes were commonly expressed at all time points. The biological process of the inflammatory response was increased at 1 hour and 1 day, and the cellular component of the integral component of the synaptic membrane was increased at 1 day. DEGs associated with cell activation and the innate immune response were highly enriched at 1 week and 1 month, respectively. A total of 2841 DEGs were differentially expressed at any of the five time points, and 18 genes (17 upregulated and 1 downregulated) showed common expression differences at all time points. We found that interleukin signaling, neutrophil degranulation, eukaryotic translation, collagen degradation, LGI–ADAM interactions, GABA receptor, and L1CAM-ankyrin interactions were prominent after SCI depending on the time post injury. We also performed gene–drug network analysis and found several potential antagonists and agonists which can be used to treat SCI. We expect to discover effective treatments in the clinical field through further studies revealing the efficacy and safety of potential drugs.

Ceftriaxone as a Novel Therapeutic Agent for Hyperglutamatergic States: Bridging the Gap Between Preclinical Results and Clinical Translation

Dysregulation of glutamate homeostasis is a well-established core feature of neuropsychiatric disorders. Extracellular glutamate concentration is regulated by glutamate transporter 1 (GLT-1). The discovery of a beta-lactam antibiotic, ceftriaxone (CEF), as a safe compound with unique ability to upregulate GLT-1 sparked the interest in testing its efficacy as a novel therapeutic agent in animal models of neuropsychiatric disorders with hyperglutamatergic states. Indeed, more than 100 preclinical studies have shown the efficacy of CEF in attenuating the behavioral manifestations of various hyperglutamatergic brain disorders such as ischemic stroke, amyotrophic lateral sclerosis (ALS), seizure, Huntington’s disease, and various aspects of drug use disorders. However, despite rich and promising preclinical data, only one large-scale clinical trial testing the efficacy of CEF in patients with ALS is reported. Unfortunately, in that study, there was no significant difference in survival between placebo- and CEF-treated patients. In this review, we discussed the translational potential of preclinical efficacy of CEF based on four different parameters: (1) initiation of CEF treatment in relation to induction of the hyperglutamatergic state, (2) onset of response in preclinical models in relation to onset of GLT-1 upregulation, (3) mechanisms of action of CEF on GLT-1 expression and function, and (4) non-GLT-1-mediated mechanisms for CEF. Our detailed review of the literature brings new insights into underlying molecular mechanisms correlating the preclinical efficacy of CEF. We concluded here that CEF may be clinically effective in selected cases in acute and transient hyperglutamatergic states such as early drug withdrawal conditions.

A Non-Canonical Role for IRE1α Links ER and Mitochondria as Key Regulators of Astrocyte Dysfunction: Implications in Methamphetamine use and HIV-Associated Neurocognitive Disorders

Astrocytes are one of the most numerous glial cells in the central nervous system (CNS) and provide essential support to neurons to ensure CNS health and function. During a neuropathological challenge, such as during human immunodeficiency virus (HIV)-1 infection or (METH)amphetamine exposure, astrocytes shift their neuroprotective functions and can become neurotoxic. Identifying cellular and molecular mechanisms underlying astrocyte dysfunction are of heightened importance to optimize the coupling between astrocytes and neurons and ensure neuronal fitness against CNS pathology, including HIV-1-associated neurocognitive disorders (HAND) and METH use disorder. Mitochondria are essential organelles for regulating metabolic, antioxidant, and inflammatory profiles. Moreover, endoplasmic reticulum (ER)-associated signaling pathways, such as calcium and the unfolded protein response (UPR), are important messengers for cellular fate and function, including inflammation and mitochondrial homeostasis. Increasing evidence supports that the three arms of the UPR are involved in the direct contact and communication between ER and mitochondria through mitochondria-associated ER membranes (MAMs). The current study investigated the effects of HIV-1 infection and chronic METH exposure on astrocyte ER and mitochondrial homeostasis and then examined the three UPR messengers as potential regulators of astrocyte mitochondrial dysfunction. Using primary human astrocytes infected with pseudotyped HIV-1 or exposed to low doses of METH for 7 days, astrocytes had increased mitochondrial oxygen consumption rate (OCR), cytosolic calcium flux and protein expression of UPR mediators. Notably, inositol-requiring protein 1α (IRE1α) was most prominently upregulated following both HIV-1 infection and chronic METH exposure. Moreover, pharmacological inhibition of the three UPR arms highlighted IRE1α as a key regulator of astrocyte metabolic function. To further explore the regulatory role of astrocyte IRE1α, astrocytes were transfected with an IRE1α overexpression vector followed by activation with the proinflammatory cytokine interleukin 1β. Overall, our findings confirm IRE1α modulates astrocyte mitochondrial respiration, glycolytic function, morphological activation, inflammation, and glutamate uptake, highlighting a novel potential target for regulating astrocyte dysfunction. Finally, these findings suggest both canonical and non-canonical UPR mechanisms of astrocyte IRE1α. Thus, additional studies are needed to determine how to best balance astrocyte IRE1α functions to both promote astrocyte neuroprotective properties while preventing neurotoxic properties during CNS pathologies.

Dysregulation of Astrocytic Glutamine Transport in Acute Hyperammonemic Brain Edema

Acute liver failure (ALF) impairs ammonia clearance from blood, which gives rise to acute hyperammonemia and increased ammonia accumulation in the brain. Since in brain glutamine synthesis is the only route of ammonia detoxification, hyperammonemia is as a rule associated with increased brain glutamine content (glutaminosis) which correlates with and contributes along with ammonia itself to hyperammonemic brain edema-associated with ALF. This review focuses on the effects of hyperammonemia on the two glutamine carriers located in the astrocytic membrane: Slc38a3 (SN1, SNAT3) and Slc7a6 (y + LAT2). We emphasize the contribution of the dysfunction of either of the two carriers to glutaminosis- related aspects of brain edema: retention of osmotically obligated water (Slc38a3) and induction of oxidative/nitrosative stress (Slc7a6). The changes in glutamine transport link glutaminosis- evoked mitochondrial dysfunction to oxidative-nitrosative stress as formulated in the “Trojan Horse” hypothesis.

Glutamine Maintains Satellite Glial Cells Growth and Survival in Culture

Satellite glial cells (SGCs) tightly surround neurons and modulate sensory transmission in dorsal root ganglion (DRG). At present, the biological property of primary SGCs in culture deserves further investigation. To reveal the key factor for SGCs growth and survival, we examined the effects of different culture supplementations containing Dulbecco's Modified Eagle Medium (DMEM)/F12, DMEM high glucose (HG) or Neurobasal-A (NB). CCK-8 proliferation assay showed an increased proliferation of SGCs in DMEM/F12 and DMEM/HG, but not in NB medium. Bax, AnnexinV, and propidium iodide (PI) staining results showed that NB medium caused cell death and apoptosis. We showed that glutamine was over 2.5 mM in DMEM/F12 and DMEM/HG, whereas it was absence in NB medium. Interestingly, exogenous glutamine application significantly reversed the poor proliferation and cell death of SGCs in NB medium. These findings demonstrated that DMEM/F12 medium was optimal to get high-purity SGCs. Glutamine was the key molecule to maintain SGCs growth and survival in culture. Here, we provided a novel approach to get high-purity SGCs by changing the key component of culture medium. Our study shed a new light on understanding the biological property and modulation of glial cells of primary sensory ganglia.

N-Methyl-D-Aspartate (NMDA)-Type Glutamate Receptors and Demyelinating Disorders: A Neuroimmune Perspective

N-methyl-D-aspartate receptors (NMDARs) are ionotropic glutamate receptors, highly important in regulating substantial physiologic processes in the brain and the nervous system, and disturbance in their function could contribute to different pathologies. Overstimulation and hyperactivity of NMDARs, termed as glutamate toxicity, could promote cell death and apoptosis. Meanwhile, their blockade could lead to dysfunction of the brain and nervous system as well. A growing body of evidence has demonstrated the prominent role of NMDARs in demyelinating disorders and anti-NMDAR encephalitis. Herein, we provide an overview of the role of NMDARs' dysfunction in the physiopathology of demyelinating disorders such as multiple sclerosis and neuromyelitis optica spectrum disorders.

How expensive is the astrocyte?

The energy cost of information processing is thought to be chiefly neuronal, with a minor fraction attributed to glial cells. However, there is compelling evidence that astrocytes capture synaptic K⁺ using their Na⁺/K⁺ ATPase, and not solely through Kir4.1 channels as was once thought. When this active buffering is taken into account, the cost of astrocytes rises by >200%. Gram-per-gram, astrocytes turn out to be as expensive as neurons. This conclusion is supported by 3D reconstruction of the neuropil showing similar mitochondrial densities in neurons and astrocytes, by cell-specific transcriptomics and proteomics, and by the rates of the tricarboxylic acid cycle. Possible consequences for reactive astrogliosis and brain disease are discussed.

Glutamate Scavenging as a Neuroreparative Strategy in Ischemic Stroke

Stroke is the second highest reason of death in the world and the leading cause of disability. The ischemic stroke makes up the majority of stroke cases that occur due to the blockage of blood vessels. Therapeutic applications for ischemic stroke include thrombolytic treatments that are in limited usage and only applicable to less than 10% of the total stroke patients, but there are promising new approaches. The main cause of ischemic neuronal death is glutamate excitotoxicity. There have been multiple studies focusing on neuroprotection via reduction of glutamate both in ischemic stroke and other neurodegenerative diseases that ultimately failed due to the obstacles in delivery. At that point, systemic glutamate grabbing, or scavenging is an ingenious way of decreasing glutamate levels upon ischemic stroke. The main advantage of this new therapeutic method is the scavengers working in the circulating blood so that there is no interference with the natural brain neurophysiology. In this review, we explain the molecular mechanisms of ischemic stroke, provide brief information about existing drugs and approaches, and present novel systemic glutamate scavenging methods. This review hopefully will elucidate the potential usage of the introduced therapeutic approaches in stroke patients.

Ligand-Gated Ion Channels as Targets for Treatment and Management of Cancers

Ligand-gated ion channels are an ionotropic receptor subtype characterized by the binding of an extracellular ligand, followed by the transient passage of ions through a transmembrane pore. Ligand-gated ion channels are commonly subcategorized into three superfamilies: purinoreceptors, glutamate receptors, and Cys-loop receptors. This classification is based on the differing topographical morphology of the receptors, which in turn confers functional differences. Ligand-gated ion channels have a diverse spatial and temporal expression which implicate them in key cellular processes. Given that the transcellular electrochemical gradient is finely tuned in eukaryotic cells, any disruption in this homeostasis can contribute to aberrancies, including altering the activity of pro-tumorigenic molecular pathways, such as the MAPK/ERK, RAS, and mTOR pathways. Ligand-gated ion channels therefore serve as a potential targetable system for cancer therapeutics. In this review, we analyze the role that each of the three ligand-gated ion channel superfamilies has concerning tumor proliferation and as a target for the treatment of cancer symptomatology.

NPRL2 Inhibition of mTORC1 Controls Sodium Channel Expression and Brain Amino Acid Homeostasis

Genetic mutations in nitrogen permease regulator-like 2 (NPRL2) are associated with a wide spectrum of familial focal epilepsies, autism, and sudden unexpected death of epileptics (SUDEP), but the mechanisms by which NPRL2 contributes to these effects are not well known. NPRL2 is a requisite subunit of the GAP activity toward Rags 1 (GATOR1) complex, which functions as a negative regulator of mammalian target of rapamycin complex 1 (mTORC1) kinase when intracellular amino acids are low. Here, we show that loss of NPRL2 expression in mouse excitatory glutamatergic neurons causes seizures before death, consistent with SUDEP in humans with epilepsy. Additionally, the absence of NPRL2 expression increases mTORC1-dependent signal transduction and significantly alters amino acid homeostasis in the brain. Loss of NPRL2 reduces dendritic branching and increases the strength of electrically stimulated action potentials (APs) in neurons. The increased AP strength is consistent with elevated expression of epilepsy-linked, voltage-gated sodium channels in the NPRL2-deficient brain. Targeted deletion of NPRL2 in primary neurons increases the expression of sodium channel Scn1A, whereas treatment with the pharmacological mTORC1 inhibitor called rapamycin prevents Scn1A upregulation. These studies demonstrate a novel role of NPRL2 and mTORC1 signaling in the regulation of sodium channels, which can contribute to seizures and early lethality.

Caloric Restriction Mimetic 2-Deoxyglucose Reduces Inflammatory Signaling in Human Astrocytes: Implications for Therapeutic Strategies Targeting Neurodegenerative Diseases

Therapeutic interventions are greatly needed for age-related neurodegenerative diseases. Astrocytes regulate many aspects of neuronal function including bioenergetics and synaptic transmission. Reactive astrocytes are implicated in neurodegenerative diseases due to their pro-inflammatory phenotype close association with damaged neurons. Thus, strategies to reduce astrocyte reactivity may support brain health. Caloric restriction and a ketogenic diet limit energy production via glycolysis and promote oxidative phosphorylation, which has gained traction as a strategy to improve brain health. However, it is unknown how caloric restriction affects astrocyte reactivity in the context of neuroinflammation. We investigated how a caloric restriction mimetic and glycolysis inhibitor, 2-deoxyglucose (2-DG), affects interleukin 1β-induced inflammatory gene expression in human astrocytes. Human astrocyte cultures were exposed to 2-DG or vehicle for 24 h and then to recombinant IL-1β for 6 or 24 h to analyze mRNA and protein expression, respectively. Gene expression levels of proinflammatory genes (complement component 3, IL-1β, IL6, and TNFα) were analyzed by real-time PCR, immunoblot, and immunohistochemistry. As expected, IL-1β induced elevated levels of proinflammatory genes. 2-DG reversed this effect at the mRNA and protein levels without inducing cytotoxicity. Collectively, these data suggest that inhibiting glycolysis in human astrocytes reduces IL-1β-induced reactivity. This finding may lead to novel therapeutic strategies to limit inflammation and enhance bioenergetics toward the goal of preventing and treating neurodegenerative diseases.

Teriflunomide shifts the astrocytic bioenergetic profile from oxidative metabolism to glycolysis and attenuates TNFα-induced inflammatory responses

Astrocytes utilize both glycolytic and mitochondrial pathways to power cellular processes that are vital to maintaining normal CNS functions. These cells also mount inflammatory and acute phase reactive programs in response to diverse stimuli. While the metabolic functions of astrocytes under homeostatic conditions are well-studied, the role of cellular bioenergetics in astrocyte reactivity is poorly understood. Teriflunomide exerts immunomodulatory effects in diseases such as multiple sclerosis by metabolically reprogramming lymphocytes and myeloid cells. We hypothesized that teriflunomide would constrain astrocytic inflammatory responses. Purified murine astrocytes were grown under serum-free conditions to prevent acquisition of a spontaneous reactive state. Stimulation with TNFα activated NFκB and increased secretion of Lcn2. TNFα stimulation increased basal respiration, maximal respiration, and ATP production in astrocytes, as assessed by oxygen consumption rate. TNFα also increased glycolytic reserve and glycolytic capacity of astrocytes but did not change the basal glycolytic rate, as assessed by measuring the extracellular acidification rate. TNFα specifically increased mitochondrial ATP production and secretion of Lcn2 required ATP generated by oxidative phosphorylation. Inhibition of dihydroorotate dehydrogenase via teriflunomide transiently increased both oxidative phosphorylation and glycolysis in quiescent astrocytes, but only the increased glycolytic ATP production was sustained over time, resulting in a bias away from mitochondrial ATP production even at doses down to 1 μM. Preconditioning with teriflunomide prevented the TNFα-induced skew toward oxidative phosphorylation, reduced mitochondrial ATP production, and reduced astrocytic inflammatory responses, suggesting that this drug may limit neuroinflammation by acting as a metabolomodulator.

Research Progress on Neuroprotection of Insulin-like Growth Factor-1 towards Glutamate-Induced Neurotoxicity

Insulin-like growth factor-1 (IGF-1) and its binding proteins and receptors are widely expressed in the central nervous system (CNS), proposing IGF-1-induced neurotrophic actions in normal growth, development, and maintenance. However, while there is convincing evidence that the IGF-1 system has specific endocrine roles in the CNS, the concept is emerging that IGF-I might be also important in disorders such as ischemic stroke, brain trauma, Alzheimer’s disease, epilepsy, etc., by inducing neuroprotective effects towards glutamate-mediated excitotoxic signaling pathways. Research in rodent models has demonstrated rescue of pathophysiological and behavioral abnormalities when IGF-1 was administered by different routes, and several clinical studies have shown safety and promise of efficacy in neurological disorders of the CNS. Focusing on the relationship between IGF-1-induced neuroprotection and glutamate-induced excitatory neurotoxicity, this review addresses the research progress in the field, intending to provide a rationale for using IGF-I clinically to confer neuroprotective therapy towards neurological diseases with glutamate excitotoxicity as a common pathological pathway.

Metabolic Features of Brain Function with Relevance to Clinical Features of Alzheimer and Parkinson Diseases

Brain metabolism is comprised in Alzheimer’s disease (AD) and Parkinson’s disease (PD). Since the brain primarily relies on metabolism of glucose, ketone bodies, and amino acids, aspects of these metabolic processes in these disorders—and particularly how these altered metabolic processes are related to oxidative and/or nitrosative stress and the resulting damaged targets—are reviewed in this paper. Greater understanding of the decreased functions in brain metabolism in AD and PD is posited to lead to potentially important therapeutic strategies to address both of these disorders, which cause relatively long-lasting decreased quality of life in patients.

The Interconnected Mechanisms of Oxidative Stress and Neuroinflammation in Epilepsy

One of the most important characteristics of the brain compared to other organs is its elevated metabolic demand. Consequently, neurons consume high quantities of oxygen, generating significant amounts of reactive oxygen species (ROS) as a by-product. These potentially toxic molecules cause oxidative stress (OS) and are associated with many disorders of the nervous system, where pathological processes such as aberrant protein oxidation can ultimately lead to cellular dysfunction and death. Epilepsy, characterized by a long-term predisposition to epileptic seizures, is one of the most common of the neurological disorders associated with OS. Evidence shows that increased neuronal excitability—the hallmark of epilepsy—is accompanied by neuroinflammation and an excessive production of ROS; together, these factors are likely key features of seizure initiation and propagation. This review discusses the role of OS in epilepsy, its connection to neuroinflammation and the impact on synaptic function. Considering that the pharmacological treatment options for epilepsy are limited by the heterogeneity of these disorders, we also introduce the latest advances in anti-epileptic drugs (AEDs) and how they interact with OS. We conclude that OS is intertwined with numerous physiological and molecular mechanisms in epilepsy, although a causal relationship is yet to be established.

Roles of N-Methyl-D-Aspartate Receptors (NMDARs) in Epilepsy

Epilepsy is one of the most common neurological disorders characterized by recurrent seizures. The mechanism of epilepsy remains unclear and previous studies suggest that N-methyl-D-aspartate receptors (NMDARs) play an important role in abnormal discharges, nerve conduction, neuron injury and inflammation, thereby they may participate in epileptogenesis. NMDARs belong to a family of ionotropic glutamate receptors that play essential roles in excitatory neurotransmission and synaptic plasticity in the mammalian CNS. Despite numerous studies focusing on the role of NMDAR in epilepsy, the relationship appeared to be elusive. In this article, we reviewed the regulation of NMDAR and possible mechanisms of NMDAR in epilepsy and in respect of onset, development, and treatment, trying to provide more evidence for future studies.

Glutamatergic system components as potential biomarkers and therapeutic targets in cancer in non-neural organs

Glutamate is one of the most abundant amino acids in the blood. Besides its role as a neurotransmitter in the brain, it is a key substrate in several metabolic pathways and a primary messenger that acts through its receptors outside the central nervous system (CNS). The two main types of glutamate receptors, ionotropic and metabotropic, are well characterized in CNS and have been recently analyzed for their roles in non-neural organs. Glutamate receptor expression may be particularly important for tumor growth in organs with high concentrations of glutamate and might also influence the propensity of such tumors to set metastases in glutamate-rich organs, such as the liver. The study of glutamate transporters has also acquired relevance in the physiology and pathologies outside the CNS, especially in the field of cancer research. In this review, we address the recent findings about the expression of glutamatergic system components, such as receptors and transporters, their role in the physiology and pathology of cancer in non-neural organs, and their possible use as biomarkers and therapeutic targets.

Metabolic Enzyme Alterations and Astrocyte Dysfunction in a Murine Model of Alexander Disease With Severe Reactive Gliosis

Alexander disease (AxD) is a rare and fatal neurodegenerative disorder caused by mutations in the gene encoding glial fibrillary acidic protein (GFAP). In this report, a mouse model of AxD (GFAPTg;Gfap+/R236H) was analyzed that contains a heterozygous R236H point mutation in murine Gfap as well as a transgene with a GFAP promoter to overexpress human GFAP. Using label-free quantitative proteomic comparisons of brain tissue from GFAPTg;Gfap+/R236H versus wild-type mice confirmed upregulation of the glutathione metabolism pathway and indicated proteins were elevated in the peroxisome proliferator-activated receptor (PPAR) signaling pathway, which had not been reported previously in AxD. Relative protein-level differences were confirmed by a targeted proteomics assay, including proteins related to astrocytes and oligodendrocytes. Of particular interest was the decreased level of the oligodendrocyte protein, 2-hydroxyacylsphingosine 1-beta-galactosyltransferase (Ugt8), since Ugt8-deficient mice exhibit a phenotype similar to GFAPTg;Gfap+/R236H mice (e.g., tremors, ataxia, hind-limb paralysis). In addition, decreased levels of myelin-associated proteins were found in the GFAPTg;Gfap+/R236H mice, consistent with the role of Ugt8 in myelin synthesis. Fabp7 upregulation in GFAPTg;Gfap+/R236H mice was also selected for further investigation due to its uncharacterized association to AxD, critical function in astrocyte proliferation, and functional ability to inhibit the anti-inflammatory PPAR signaling pathway in models of amyotrophic lateral sclerosis (ALS). Within Gfap+ astrocytes, Fabp7 was markedly increased in the hippocampus, a brain region subjected to extensive pathology and chronic reactive gliosis in GFAPTg;Gfap+/R236H mice. Last, to determine whether the findings in GFAPTg;Gfap+/R236H mice are present in the human condition, AxD patient and control samples were analyzed by Western blot, which indicated that Type I AxD patients have a significant fourfold upregulation of FABP7. However, immunohistochemistry analysis showed that UGT8 accumulates in AxD patient subpial brain regions where abundant amounts of Rosenthal fibers are located, which was not observed in the GFAPTg;Gfap+/R236H mice.

Impact of monomeric and aggregated wild-type and A30P/A53T double-mutant α-synuclein on antioxidant mechanism and glutamate metabolic profile of cultured astrocytes

Serving as a source of glutathione and up-taking and metabolizing glutamate are the primary supportive role of astrocytes for the adjacent neurons. Despite the clear physical association between astrocytes and α-synuclein, the effect of extracellular α-synuclein on these astrocytic functions has not yet been elucidated. Hence, we aim to assess the effect of various forms of α-synuclein on antioxidant mechanism and glutamate metabolism. Wild-type and A53T/A30P double-mutant α-synuclein, both in monomeric and aggregated forms, were added extracellularly to media of midbrain rat astrocyte culture, with their survival, oxidative, and nitrative stress, glutathione and glutamate content, expression of enzymes associated with oxidative stress and glutamate metabolism, glutamate and glutathione transporters being assessed along with the association/engulfment of these peptides by astrocytes. A30P/A53T peptide associated more with astrocytes, and low-extracellular K⁺ concentration showed prominent reduction in the engulfment of the monomeric forms, suggesting that the association of the aggregated forms was greater with the membrane. The peptide-associated astrocytes showed lower survival and increased oxidative stress generation, owing to the decrease in nuclear localization of Nrf2 and increase in iNOS, and further aggravated by the decrease in glutathione content and related enzymes like glutathione synthetase, glutathione peroxidase, and glutathione reductase. Glutamate uptake increased in aggregate-treated cells due to the increase in GLAST1 expression, de novo synthesis of glutamate by pyruvate carboxylase, and/or glutamine synthase, bolstered by the differential glutamate dehydrogenase enzyme activity. We thus show for the first time that extracellular α-synuclein exposure leads to astrocytic dysfunction with respect to the antioxidant mechanism and glutamate metabolic profile. The impact was higher in the case of the aggregated and mutated peptide, with the highest dysfunction for the mutant aggregated α-synuclein treatment.

The trajectory of putative astroglial dysfunction in first episode schizophrenia: a longitudinal 7-Tesla MRS study

Myo-inositol is mainly found in astroglia and its levels has been shown to be reduced in the anterior cingulate cortex (ACC) of patients with schizophrenia. We investigate the status of astroglial integrity indexed by ACC myo-inositol at the onset and over the first 6 months of treatment of first episode schizophrenia. We employed 7 T magnetic resonance spectroscopy (1H-MRS) and quantified myo-inositol spectra at the dorsal ACC in 31 participants; 21 patients with schizophrenia with median lifetime antipsychotic exposure of less than 3 days, followed up after 6 months of treatment, and 10 healthy subjects scanned twice over the same period. We studied the time by group interaction for myo-inositol after adjusting for gender and age. We report significant reduction in myo-inositol concentration in the ACC in schizophrenia at an early, untreated state of acute illness that becomes insignificant over time, after instituting early intervention. This trajectory indicates that dynamic astroglial changes are likely to operate in the early stages of schizophrenia. MRS myo-inositol may be a critical marker of amelioration of active psychosis in early stages of schizophrenia.

Glia: A major player in glutamate-GABA dysregulation-mediated neurodegeneration

The imbalance between glutamate and γ-aminobutyric acid (GABA) results in the loss of synaptic strength leading to neurodegeneration. The dogma on the field considered neurons as the main players in this excitation-inhibition (E/I) balance. However, current strategies focusing only on neurons have failed to completely understand this condition, bringing up the importance of glia as an alternative modulator for neuroinflammation as glia alter the activity of neurons and is a source of both neurotrophic and neurotoxic factors. This review's primary goal is to illustrate the role of glia over E/I balance in the central nervous system and its interaction with neurons. Rather than focusing only on the neuronal targets, we take a deeper look at glial receptors and proteins that could also be explored as drug targets, as they are early responders to neurotoxic insults. This review summarizes the neuron-glia interaction concerning GABA and glutamate, possible targets, and its involvement in the E/I imbalance in neurodegenerative diseases like Alzheimer's disease, Parkinson's disease, Huntington's disease, and multiple sclerosis.

The tripartite glutamatergic synapse

Astroglial cells were long considered as structural and metabolic supporting cells are which do not directly participate in information processing in the brain. Discoveries of responsiveness of astrocytes to synaptically-released glutamate and their capability to release agonists of glutamate receptors awakened extensive studies of glia-neuron communications and led to the revolutionary changes in our understanding of brain cellular networks. Nowadays, astrocytes are widely acknowledged as inseparable element of glutamatergic synapses and role for glutamatergic astrocyte-neuron interactions in the brain computation is emerging. Astroglial glutamate receptors, in particular of NMDA, mGluR3 and mGluR5 types, can activate a variety of molecular cascades leading astroglial-driven modulation of extracellular levels of glutamate and activity of neuronal glutamate receptors. Their preferential location to the astroglial perisynaptic processes facilitates interaction of astrocytes with individual excitatory synapses. Bi-directional glutamatergic communication between astrocytes and neurons underpins a complex, spatially-distributed modulation of synaptic signalling thus contributing to the enrichment of information processing by the neuronal networks. Still, further research is needed to bridge the substantial gaps in our understanding of mechanisms and physiological relevance of astrocyte-neuron glutamatergic interactions, in particular ability of astrocytes directly activate neuronal glutamate receptors by releasing glutamate and, arguably, d-Serine. An emerging roles for aberrant changes in glutamatergic astroglial signalling, both neuroprotective and pathogenic, in neurological and neurodegenerative diseases also require further investigation. This article is part of the special Issue on 'Glutamate Receptors - The Glutamatergic Synapse'.

Glutamate and GABA in Microglia-Neuron Cross-Talk in Alzheimer's Disease

The physiological balance between excitation and inhibition in the brain is significantly affected in Alzheimer’s disease (AD). Several neuroactive compounds and their signaling pathways through various types of receptors are crucial in brain homeostasis, among them glutamate and γ-aminobutyric acid (GABA). Activation of microglial receptors regulates the immunological response of these cells, which in AD could be neuroprotective or neurotoxic. The novel research approaches revealed the complexity of microglial function, including the interplay with other cells during neuroinflammation and in the AD brain. The purpose of this review is to describe the role of several proteins and multiple receptors on microglia and neurons, and their involvement in a communication network between cells that could lead to different metabolic loops and cell death/survival. Our review is focused on the role of glutamatergic, GABAergic signaling in microglia–neuronal cross-talk in AD and neuroinflammation. Moreover, the significance of AD-related neurotoxic proteins in glutamate/GABA-mediated dialogue between microglia and neurons was analyzed in search of novel targets in neuroprotection, and advanced pharmacological approaches.

Glutamate Metabolism in Mitochondria is Closely Related to Alzheimer's Disease

Glutamate is the main excitatory neurotransmitter in the brain, and its excitatory neurotoxicity is closely related to the occurrence and development of Alzheimer's disease. However, increasing evidence shows that in the process of Alzheimer's disease, glutamate is not only limited to its excitotoxicity as a neurotransmitter but also related to the disorder of its metabolic balance. The balance of glutamate metabolism in the brain is an important determinant of central nervous system health, and the maintenance of this balance is closely related to glutamate uptake, glutamate circulation, intracellular mitochondrial transport, and mitochondrial metabolism. In this paper, we intend to elaborate the key role of mitochondrial glutamate metabolism in the pathogenesis of Alzheimer's disease and review glutamate metabolism in mitochondria as a potential target in the treatment of Alzheimer's disease.

NMDA and AMPA receptors dysregulation in Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative condition characterized by cognitive dysfunction and synaptic failure. The current therapeutic approaches are mainly focused on symptomatic treatment and possess limited effectiveness in addressing the pathophysiology of AD. It is known that neurodegeneration is negatively correlated with synaptic plasticity. This negative correlation highlights glutamatergic neurotransmission via N-methyl-D-aspartate (NMDA) and α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors and (AMPA) receptors as a critical mediator of synaptic plasticity. Despite this favorable role, extensive extracellular glutamate concentration induces excitotoxicity and neurodegeneration. NMDA receptors containing GluN2A subunits are located at synaptic sites, implicated in the protective pathways. In comparison, GluN2B containing receptors are located mainly at extrasynaptic sites and increase neuronal vulnerability. AMPA receptors are consistently endocytosed and recycled back to the membrane. An increase in the rate of endocytosis has been implicated as a part of AD pathophysiology through inducing long-term depression (LTD) and synaptic disintegration. In the present review, we focused on the mechanisms of glutamatergic system dysregulation in AD, particularly on its interaction with amyloid-beta. We concluded that assigning a specific role to an individual subtype of either NMDA receptors or AMPA receptors might be an oversimplification as they are not static receptors. Therefore, any imbalance between synaptic and extrasynaptic NMDA receptors and a reduced number of surface AMPA receptors will lead to synaptopathy.

Circadian Clock Regulates Inflammation and the Development of Neurodegeneration

The circadian clock regulates numerous key physiological processes and maintains cellular, tissue, and systemic homeostasis. Disruption of circadian clock machinery influences key activities involved in immune response and brain function. Moreover, Immune activation has been closely linked to neurodegeneration. Here, we review the molecular clock machinery and the diurnal variation of immune activity. We summarize the circadian control of immunity in both central and peripheral immune cells, as well as the circadian regulation of brain cells that are implicated in neurodegeneration. We explore the important role of systemic inflammation on neurodegeneration. The circadian clock modulates cellular metabolism, which could be a mechanism underlying circadian control. We also discuss the circadian interventions implicated in inflammation and neurodegeneration. Targeting circadian clocks could be a potential strategy for the prevention and treatment of inflammation and neurodegenerative diseases.

Microbially-derived short-chain fatty acids impact astrocyte gene expression in a sex-specific manner

Recent investigations in neuroscience implicate the role of microbial-derived metabolites, such as short-chain fatty acids (SCFAs) in brain health and disease. The SCFAs acetate, propionate and butyrate have pleiotropic effects within the nervous system. They are crucial for the maturation of the brain's innate immune cells, the microglia, and modulate other glial cells through the aryl-hydrocarbon receptor. Investigations in preclinical and clinical models find that SCFAs exert neuroprotective and antidepressant affects, while also modulating the stress response and satiety. However, many investigations thus far have not assessed the impact of sex on SCFA activity. Our novel investigation tested the impact of physiologically relevant doses of SCFAs on male and female primary cortical astrocytes. We find that butyrate (0–25 ​μM) correlates with increased Bdnf and Pgc1-α expression, implicating histone-deacetylase inhibitor pathways. Intriguingly, this effect is only seen in females. We also find that acetate (0–1500 ​μM) correlates with increased Ahr and Gfap expression in males only, suggesting immune modulatory pathways. In males, propionate (0–35 ​μM) correlates with increased Il-22 expression, further suggesting immunomodulatory actions. These findings show a novel sex-dependent impact of acetate and butyrate, but not propionate on astrocyte gene expression.

Mitochondrial enzyme GPT2 regulates metabolic mechanisms required for neuron growth and motor function in vivo

The metabolic needs for postnatal growth of the human nervous system are vast. Recessive loss-of-function mutations in the mitochondrial enzyme glutamate pyruvate transaminase 2 (GPT2) in humans cause postnatal undergrowth of brain, and cognitive and motor disability. We demonstrate that GPT2 governs critical metabolic mechanisms in neurons required for neuronal growth and survival. These metabolic processes include neuronal alanine synthesis and anaplerosis, the replenishment of tricarboxylic acid (TCA) cycle intermediates. We performed metabolomics across postnatal development in Gpt2-null mouse brain to identify the trajectory of dysregulated metabolic pathways: alterations in alanine occur earliest; followed by reduced TCA cycle intermediates and reduced pyruvate; followed by elevations in glycolytic intermediates and amino acids. Neuron-specific deletion of GPT2 in mice is sufficient to cause motor abnormalities and death pre-weaning, a phenotype identical to the germline Gpt2-null mouse. Alanine biosynthesis is profoundly impeded in Gpt2-null neurons. Exogenous alanine is necessary for Gpt2-null neuronal survival in vitro, but is not needed for Gpt2-null astrocytes. Dietary alanine supplementation in Gpt2-null mice enhances animal survival, and improves the metabolic profile of Gpt2-null brain, but does not alone appear to correct motor function. In surviving Gpt2-null animals, we observe smaller upper and lower motor neurons in vivo. We also observe selective death of lower motor neurons in vivo with worsening motor behavior with age. In conclusion, these studies of the pathophysiology of GPT2 Deficiency have identified metabolic mechanisms required for neuronal growth and that potentially underlie selective neuronal vulnerabilities in motor neurons.

Metabolic Regulation of Glia and Their Neuroinflammatory Role in Alzheimer's Disease

Alzheimer's disease (AD) is an aging-related neurodegenerative disorder. It is characterized clinically by progressive memory loss and impaired cognitive function. Its progression occurs from neuronal synapse loss to amyloid pathology and Tau deposit which eventually leads to the compromised neuronal function. Neurons in central nervous tissue work in a composite and intricate network with the glia and vascular cells. Microglia and astrocytes are becoming the prime focus due to their involvement in various aspects of neurophysiology, such as trophic support to neurons, synaptic modulation, and brain surveillance. AD is also often considered as the sequela of prolonged metabolic dyshomeostasis. The neuron and glia have different metabolic profiles as cytosolic glycolysis and mitochondrial-dependent oxidative phosphorylation (OXPHOS), especially under dyshomeostasis or with aging pertaining to their unique genetic built-up. Various efforts are being put in to decipher the role of mitochondrial dynamics regarding their trafficking, fission/fusion imbalance, and mitophagy spanning over both neurons and glia to improve aging-related brain health. The mitochondrial dysfunction may lead to activation in various signaling mechanisms causing metabolic reprogramming in glia cells, further accelerating AD-related pathogenic events. The glycolytic-dominant astrocytes switch to the neurotoxic phenotype, i.e., disease-associated astrocyte under metabolic stress. The microglia also transform from resting to reactive phenotype, i.e., disease-associated microglia. It may also exist in otherwise a misconception an M1, glycolytic, or M2, an OXPHOS-dependent phenotype. Further, glial transformation plays a vital role in regulating hallmarks of AD pathologies like synapse maintenance, amyloid, and Tau clearance. In this updated review, we have tried to emphasize the metabolic regulation of glial reactivity, mitochondrial quality control mechanisms, and their neuroinflammatory response in Alzheimer's progression.

Naked mole-rat brain mitochondria tolerate in vitro ischaemia

Naked mole-rats (NMRs; Heterocephalus glaber) are among the most hypoxia-tolerant mammals. There is evidence that the NMR brain tolerates in vitro hypoxia and NMR brain mitochondria exhibit functional plasticity following in vivo hypoxia; however, if and how these organelles tolerate ischaemia and how ischaemic stress impacts mitochondrial energetics and redox regulation is entirely unknown. We hypothesized that mitochondria fundamentally contribute to in vitro ischaemia resistance in the NMR brain. To test this, we treated NMR and CD-1 mouse cortical brain sheets with an in vitro ischaemic mimic and evaluated mitochondrial respiration capacity and redox regulation following 15 or 30 min of ischaemia or ischaemia/reperfusion (I/R). We found that, relative to mice, the NMR brain largely retains mitochondrial function and redox balance post-ischaemia and I/R. Specifically: (i) ischaemia reduced complex I and II-linked respiration ∼50-70% in mice, vs. ∼20-40% in NMR brain, (ii) NMR but not mouse brain maintained relatively steady respiration control ratios and robust mitochondrial membrane integrity, (iii) electron leakage post-ischaemia was lesser in NMR than mouse brain and NMR brain retained higher coupling efficiency, and (iv) free radical generation during and following ischaemia and I/R was lower from NMR brains than mice. Taken together, our results indicate that NMR brain mitochondria are more tolerant of ischaemia and I/R than mice and retain respiratory capacity while avoiding redox derangements. Overall, these findings support the hypothesis that hypoxia-tolerant NMR brain is also ischaemia-tolerant and suggest that NMRs may be a natural model of ischaemia tolerance in which to investigate evolutionarily derived solutions to ischaemic pathology. KEY POINTS: Ischaemia is highly deleterious to the mammalian brain and this damage is largely mediated by mitochondrial dysfunction. Naked mole-rats are among the most hypoxia-tolerant mammals and their brain tolerates ischaemia ex vivo, but the impact of ischaemia on mitochondrial function is unknown. Naked mole-rat but not mouse brain mitochondria retain respiratory capacity and membrane integrity following ischaemia or ischaemia/reperfusion. Differences in free radical management and respiratory pathway control between species may mediate this tolerance. These results help us understand how natural models of hypoxia tolerance also tolerate ischaemia in the brain.

Derivation and Molecular Characterization of a Morphological Subpopulation of Human iPSC Astrocytes Reveal a Potential Role in Schizophrenia and Clozapine Response

Astrocytes are the most abundant cell type in the human brain and are important regulators of several critical cellular functions, including synaptic transmission. Although astrocytes are known to play a central role in the etiology and pathophysiology of schizophrenia, little is known about their potential involvement in clinical response to the antipsychotic clozapine. Moreover, astrocytes display a remarkable degree of morphological diversity, but the potential contribution of astrocytic subtypes to disease biology and drug response has received little attention. Here, we used state-of-the-art human induced pluripotent stem cell (hiPSC) technology to derive a morphological subtype of astrocytes from healthy individuals and individuals with schizophrenia, including responders and nonresponders to clozapine. Using functional assays and transcriptional profiling, we identified a distinct gene expression signature highly specific to schizophrenia as shown by disease association analysis of more than 10 000 diseases. We further found reduced levels of both glutamate and the NMDA receptor coagonist d-serine in subtype astrocytes derived from schizophrenia patients, and that exposure to clozapine only rescued this deficiency in cells from clozapine responders, providing further evidence that d-serine in particular, and NMDA receptor-mediated glutamatergic neurotransmission in general, could play an important role in disease pathophysiology and clozapine action. Our study represents a first attempt to explore the potential contribution of astrocyte diversity to schizophrenia pathophysiology using a human cellular model. Our findings suggest that specialized subtypes of astrocytes could be important modulators of disease pathophysiology and clinical drug response, and warrant further investigations.

Glutamate and depression: Reflecting a deepening knowledge of the gut and brain effects of a ubiquitous molecule

The versatility of glutamate as the brain’s foremost excitatory neurotransmitter and modulator of neurotransmission and function is considered common knowledge. Years of research have continued to uncover glutamate’s effects and roles in several neurological and neuropsychiatric disorders, including depression. It had been considered that a deeper understanding of the roles of glutamate in depression might open a new door to understanding the pathological basis of the disorder, improve the approach to patient management, and lead to the development of newer drugs that may benefit more patients. This review examines our current understanding of the roles of endogenous and exogenous sources of glutamate and the glutamatergic system in the aetiology, progression and management of depression. It also examines the relationships that link the gut-brain axis, glutamate and depression; as it emphasizes how the gut-brain axis could impact depression pathogenesis and management via changes in glutamate homeostasis. Finally, we consider what the likely future of glutamate-based therapies and glutamate-based therapeutic manipulations in depression are, and if with them, we are now on the final chapter of understanding the neurochemical milieu of depressive disorders.

Glutamate metabolism and recycling at the excitatory synapse in health and neurodegeneration

Glutamate is the primary excitatory neurotransmitter of the brain. Cellular homeostasis of glutamate is of paramount importance for normal brain function and relies on an intricate metabolic collaboration between neurons and astrocytes. Glutamate is extensively recycled between neurons and astrocytes in a process known as the glutamate-glutamine cycle. The recycling of glutamate is closely linked to brain energy metabolism and is essential to sustain glutamatergic neurotransmission. However, a considerable amount of glutamate is also metabolized and serves as a metabolic hub connecting glucose and amino acid metabolism in both neurons and astrocytes. Disruptions in glutamate clearance, leading to neuronal overstimulation and excitotoxicity, have been implicated in several neurodegenerative diseases. Furthermore, the link between brain energy homeostasis and glutamate metabolism is gaining attention in several neurological conditions. In this review, we provide an overview of the dynamics of synaptic glutamate homeostasis and the underlying metabolic processes with a cellular focus on neurons and astrocytes. In particular, we review the recently discovered role of neuronal glutamate uptake in synaptic glutamate homeostasis and discuss current advances in cellular glutamate metabolism in the context of Alzheimer's disease and Huntington's disease. Understanding the intricate regulation of glutamate-dependent metabolic processes at the synapse will not only increase our insight into the metabolic mechanisms of glutamate homeostasis, but may reveal new metabolic targets to ameliorate neurodegeneration.

Deficiency in MT5-MMP Supports Branching of Human iPSCs-Derived Neurons and Reduces Expression of GLAST/S100 in iPSCs-Derived Astrocytes

For some time, it has been accepted that the β-site APP cleaving enzyme 1 (BACE1) and the γ-secretase are two main players in the amyloidogenic processing of the β-amyloid precursor protein (APP). Recently, the membrane-type 5 matrix metalloproteinase (MT5-MMP/MMP-24), mainly expressed in the nervous system, has been highlighted as a new key player in APP-processing, able to stimulate amyloidogenesis and also to generate a neurotoxic APP derivative. In addition, the loss of MT5-MMP has been demonstrated to abrogate pathological hallmarks in a mouse model of Alzheimer’s disease (AD), thus shedding light on MT5-MMP as an attractive new therapeutic target. However, a more comprehensive analysis of the role of MT5-MMP is necessary to evaluate how its targeting affects neurons and glia in pathological and physiological situations. In this study, leveraging on CRISPR-Cas9 genome editing strategy, we established cultures of human-induced pluripotent stem cells (hiPSC)-derived neurons and astrocytes to investigate the impact of MT5-MMP deficiency on their phenotypes. We found that MT5-MMP-deficient neurons exhibited an increased number of primary and secondary neurites, as compared to isogenic hiPSC-derived neurons. Moreover, MT5-MMP-deficient astrocytes displayed higher surface area and volume compared to control astrocytes. The MT5-MMP-deficient astrocytes also exhibited decreased GLAST and S100β expression. These findings provide novel insights into the physiological role of MT5-MMP in human neurons and astrocytes, suggesting that therapeutic strategies targeting MT5-MMP should be controlled for potential side effects on astrocytic physiology and neuronal morphology.

Metabolic Drivers of Invasion in Glioblastoma

Glioblastoma is a primary malignant brain tumor with a median survival under 2 years. The poor prognosis glioblastoma caries is largely due to cellular invasion, which enables escape from resection, and drives inevitable recurrence. While most studies to date have focused on pathways that enhance the invasiveness of tumor cells in the brain microenvironment as the primary driving forces behind GBM’s ability to invade adjacent tissues, more recent studies have identified a role for adaptations in cellular metabolism in GBM invasion. Metabolic reprogramming allows invasive cells to generate the energy necessary for colonizing surrounding brain tissue and adapt to new microenvironments with unique nutrient and oxygen availability. Historically, enhanced glycolysis, even in the presence of oxygen (the Warburg effect) has dominated glioblastoma research with respect to tumor metabolism. More recent global profiling experiments, however, have identified roles for lipid, amino acid, and nucleotide metabolism in tumor growth and invasion. A thorough understanding of the metabolic traits that define invasive GBM cells may provide novel therapeutic targets for this devastating disease. In this review, we focus on metabolic alterations that have been characterized in glioblastoma, the dynamic nature of tumor metabolism and how it is shaped by interaction with the brain microenvironment, and how metabolic reprogramming generates vulnerabilities that may be ripe for exploitation.

Dysregulation of Astrocyte Ion Homeostasis and Its Relevance for Stroke-Induced Brain Damage

Ischemic stroke is a leading cause of mortality and chronic disability. Either recovery or progression towards irreversible failure of neurons and astrocytes occurs within minutes to days, depending on remaining perfusion levels. Initial damage arises from energy depletion resulting in a failure to maintain homeostasis and ion gradients between extra- and intracellular spaces. Astrocytes play a key role in these processes and are thus central players in the dynamics towards recovery or progression of stroke-induced brain damage. Here, we present a synopsis of the pivotal functions of astrocytes at the tripartite synapse, which form the basis of physiological brain functioning. We summarize the evidence of astrocytic failure and its consequences under ischemic conditions. Special emphasis is put on the homeostasis and stroke-induced dysregulation of the major monovalent ions, namely Na+, K+, H+, and Cl-, and their involvement in maintenance of cellular volume and generation of cerebral edema.

Not Just a Bystander: The Emerging Role of Astrocytes and Research Tools in Studying Cognitive Dysfunctions in Schizophrenia

Cognitive dysfunction is one of the core symptoms in schizophrenia, and it is predictive of functional outcomes and therefore useful for treatment targets. Rather than improving cognitive deficits, currently available antipsychotics mainly focus on positive symptoms, targeting dopaminergic/serotoninergic neurons and receptors in the brain. Apart from investigating the neural mechanisms underlying schizophrenia, emerging evidence indicates the importance of glial cells in brain structure development and their involvement in cognitive functions. Although the etiopathology of astrocytes in schizophrenia remains unclear, accumulated evidence reveals that alterations in gene expression and astrocyte products have been reported in schizophrenic patients. To further investigate the role of astrocytes in schizophrenia, we highlighted recent progress in the investigation of the effect of astrocytes on abnormalities in glutamate transmission and impairments in the blood–brain barrier. Recent advances in animal models and behavioral methods were introduced to examine schizophrenia-related cognitive deficits and negative symptoms. We also highlighted several experimental tools that further elucidate the role of astrocytes. Instead of focusing on schizophrenia as a neuron-specific disorder, an additional astrocytic perspective provides novel and promising insight into its causal mechanisms and treatment. The involvement of astrocytes in the pathogenesis of schizophrenia and other brain disorders is worth further investigation.