Mitochondrial transport proteins of the brain

The mitochondrial aspartate/glutamate carrier does not transport GABA

ɣ-aminobutyric acid (GABA) is a four‑carbon amino acid acting as the main inhibitory transmitter in the invertebrate and vertebrate nervous systems. The metabolism of GABA is well compartmentalized in the cell and the uptake of cytosolic GABA into the mitochondrial matrix is required for its degradation. A previous study carried out in the fruit fly Drosophila melanogaster indicated that the mitochondrial aspartate/glutamate carrier (AGC) is responsible for mitochondrial GABA accumulation. Here, we investigated the transport of GABA catalysed by the human and D. melanogaster AGC proteins through a well-established method for the study of the substrate specificity and the kinetic parameters of the mitochondrial carriers. In this experimental system, the D. melanogaster spliced AGC isoforms (Aralar1-PA and Aralar1-PE) and the human AGC isoforms (AGC1/aralar1 and AGC2/citrin) are unable to transport GABA both in homo- and in hetero-exchange with either glutamate or aspartate, i.e. the canonical substrates of AGC. Moreover, GABA has no inhibitory effect on the exchange activities catalysed by the investigated AGCs. Our data demonstrate that AGC does not transport GABA and the molecular identity of the GABA transporter in human and D. melanogaster mitochondria remains unknown.

Hippocampal transcriptome-wide association study and pathway analysis of mitochondrial solute carriers in Alzheimer's disease

The etiopathogenesis of late-onset Alzheimer's disease (AD) is increasingly recognized as the result of the combination of the aging process, toxic proteins, brain dysmetabolism, and genetic risks. Although the role of mitochondrial dysfunction in the pathogenesis of AD has been well-appreciated, the interaction between mitochondrial function and genetic variability in promoting dementia is still poorly understood. In this study, by tissue-specific transcriptome-wide association study (TWAS) and further meta-analysis, we examined the genetic association between mitochondrial solute carrier family (SLC25) genes and AD in three independent cohorts and identified three AD-susceptibility genes, including SLC25A10, SLC25A17, and SLC25A22. Integrative analysis using neuroimaging data and hippocampal TWAS-predicted gene expression of the three susceptibility genes showed an inverse correlation of SLC25A22 with hippocampal atrophy rate in AD patients, which outweighed the impacts of sex, age, and apolipoprotein E4 (ApoE4). Furthermore, SLC25A22 downregulation demonstrated an association with AD onset, as compared with the other two transcriptome-wide significant genes. Pathway and network analysis related hippocampal SLC25A22 downregulation to defects in neuronal function and development, echoing the enrichment of SLC25A22 expression in human glutamatergic neurons. The most parsimonious interpretation of the results is that we have identified AD-susceptibility genes in the SLC25 family through the prediction of hippocampal gene expression. Moreover, our findings mechanistically yield insight into the mitochondrial cascade hypothesis of AD and pave the way for the future development of diagnostic tools for the early prevention of AD from a perspective of precision medicine by targeting the mitochondria-related genes.

Mechanistic stoichiometric relationship between the rates of neurotransmission and neuronal glucose oxidation: Reevaluation of and alternatives to the pseudo-malate-aspartate shuttle model

The ~1:1 stoichiometry between the rates of neuronal glucose oxidation (CMRglc-ox-N) and glutamate (Glu)/γ-aminobutyric acid (GABA)-glutamine (Gln) neurotransmitter (NT) cycling between neurons and astrocytes (VNTcycle) has been firmly established. However, the mechanistic basis for this relationship is not fully understood, and this knowledge is critical for the interpretation of metabolic and brain imaging studies in normal and diseased brain. The pseudo-malate-aspartate shuttle (pseudo-MAS) model established the requirement for glycolytic metabolism in cultured glutamatergic neurons to produce NADH that is shuttled into mitochondria to support conversion of extracellular Gln (i.e., astrocyte-derived Gln in vivo) into vesicular neurotransmitter Glu. The evaluation of this model revealed that it could explain half of the 1:1 stoichiometry and it has limitations. Modifications of the pseudo-MAS model were, therefore, devised to address major knowledge gaps, that is, submitochondrial glutaminase location, identities of mitochondrial carriers for Gln and other model components, alternative mechanisms to transaminate α-ketoglutarate to form Glu and shuttle glutamine-derived ammonia while maintaining mass balance. All modified models had a similar 0.5 to 1.0 predicted mechanistic stoichiometry between VNTcycle and the rate of glucose oxidation. Based on studies of brain β-hydroxybutyrate oxidation, about half of CMRglc-ox-N may be linked to glutamatergic neurotransmission and localized in pre-synaptic structures that use pseudo-MAS type mechanisms for Glu-Gln cycling. In contrast, neuronal compartments that do not participate in transmitter cycling may use the MAS to sustain glucose oxidation. The evaluation of subcellular compartmentation of neuronal glucose metabolism in vivo is a critically important topic for future studies to understand glutamatergic and GABAergic neurotransmission.

Integrated transcriptomic and proteomic analyses reveal the mechanism of easy acceptance of artificial pelleted diets during food habit domestication in Largemouth bass (Micropterus salmoides)

Acceptance of artificial pelleted diets contributes to increasing the cultured areas and output of carnivorous fish. However, the mechanism of acceptance of artificial pelleted diets remains largely unknown. In this study, the easy acceptance of artificial pelleted diets (EAD) group and the not easy acceptance of artificial pelleted diets (NAD) group of Largemouth bass (Micropterus salmoides) were divided based on the ratios of stomach weight/body weight (SB) after 0.5 h feeding, which was bigger than 18% in the EAD group and ranged from 8 to 12% in the NAD group. Through transcriptome and proteome sequencing, a total of 2463 differentially expressed genes (DEGs) and 230 differentially expressed proteins (DEPs) were identified, respectively. Integrated analyses of transcriptome and proteome data revealed that 152 DEPs were matched with the corresponding DEGs (named co-DEGs-DEPs), and 54 co-DEGs-DEPs were enriched in 16 KEGG pathways, including the metabolic pathways, steroid biosynthesis, fatty acid biosynthesis, etc. Furthermore, 3 terpenoid backbone biosynthesis-related genes (Hmgcr, Hmgcs, and Fdps) in metabolic pathways, 10 steroid biosynthesis-related genes (Fdft1, Sqle, Lss, Cyp51a1, Tm7sf2, Nsdhl, Hsd17b7, Dhcr24, Sc5d, and Dhcr7), and 3 fatty acid biosynthesis-related genes (Acaca, Fasn, and Ascl) were all up-regulated in the EAD group, suggesting that the lipid metabolism pathway and steroid biosynthesis pathway play important roles in early food habit domestication in Largemouth bass. In addition, the detection results of randomly selected 15 DEGs and 15 DEPs indicated that both transcriptome and proteome results in the study were reliable. Our study provides useful information for further research on the mechanisms of food habit domestication in fish.

Metabolomic Footprint of Disrupted Energetics and Amino Acid Metabolism in Neurodegenerative Diseases: Perspectives for Early Diagnosis and Monitoring of Therapy

The prevalence of neurodegenerative diseases (NDs) is increasing due to the aging population and improved longevity. They are characterized by a range of pathological hallmarks, including protein aggregation, mitochondrial dysfunction, and oxidative stress. The aim of this review is to summarize the alterations in brain energy and amino acid metabolism in Alzheimer’s disease (AD), Parkinson’s disease (PD), and Huntington’s disease (HD). Based on our findings, we proposed a group of selected metabolites related to disturbed energy or mitochondrial metabolism as potential indicators or predictors of disease. We also discussed the hidden challenges of metabolomics studies in NDs and proposed future directions in this field. We concluded that biochemical parameters of brain energy metabolism disruption (obtained with metabolomics) may have potential application as a diagnostic tool for the diagnosis, prediction, and monitoring of the effectiveness of therapies for NDs. However, more studies are needed to determine the sensitivity of the proposed candidates. We suggested that the most valuable biomarkers for NDs studies could be groups of metabolites combined with other neuroimaging or molecular techniques. To attain clinically applicable results, the integration of metabolomics with other “omic” techniques might be required.

Comparative phylogenomic analyses and co-expression gene network reveal insights in flowering time and aborted meiosis in woody bamboo, Bambusa oldhamii 'Xia Zao' ZSX

Woody bamboos have peculiar flowering characteristics with intervals ranging from several years to more than 100 years. Elucidating flowering time and reproductive development in bamboo could be beneficial for both humans and wildlife. To identity the mechanisms responsible for flowering time and embryo abortion in Bambusa oldhamii 'Xia Zao' ZSX, a transcriptome sequencing project was initiated to characterize the genes involved in developing flowers in this bamboo species. Morphological studies showed that pollen abortion in this bamboo species was mainly caused by a delay in tapetum degradation and abnormal meiotic process. Differential expression (DE) and optimized hierarchical clustering analyses identified three of nine gene expression clusters with decreasing expression at the meiosis of flowering stages. Together with enriched Gene Ontology Biological Process terms for meiosis, this suggests that their expression pattern may be associated with aborted meiosis in B. oldhamii 'Xia Zao'. Moreover, our large-scale phylogenomic analyses comparing meiosis-related transcripts of B. oldhamii 'Xia Zao' with well annotated genes in 22 representative angiosperms and sequence evolution analyses reveal two core meiotic genes NO EXINE FORMATION 1 (NFE1) and PMS1 with nonsense mutations in their coding regions, likely providing another line of evidence supporting embryo abortion in B. oldhamii 'Xia Zao'. Similar analyses, however, reveal conserved sequence evolution in flowering pathways such as LEAFY (LFY) and FLOWERING LOCUS T (FT). Seventeen orthogroups associated with flowering were identified by DE analyses between nonflowering and flowering culm buds. Six regulators found primarily in several connected network nodes of the photoperiod pathway were confirmed by mapping to the flowering time network in rice, such as Heading date (Hd3a) and Rice FT-like 1 (RFT1) which integrate upstream signaling into the downstream effectors. This suggests the existence of an intact photoperiod pathway is likely the key regulators that switch on/off flowering in B. oldhamii 'Xia Zao'.

Aging astrocytes metabolically support aging axon function by proficiently regulating astrocyte-neuron lactate shuttle

The astrocyte-neuron lactate shuttle (ANLS) is an essential metabolic support system that uptakes glucose, stores it as glycogen in astrocytes, and provides glycogen-derived lactate for axonal function. Aging intrinsically increases the vulnerability of white matter (WM) to injury. Therefore, we investigated the regulation of this shuttle to understand vascular-glial metabolic coupling to support axonal function during aging in two different WM tracts. Aging astrocytes displayed larger cell bodies and thicker horizontal processes in contrast to thinner vertically oriented processes of young astrocytes. Aging axons recovered less following aglycemia in mouse optic nerves (MONs) compared to young axons, although providing lactate during aglycemia equally supported young and aging axonal function. Incubating MONs in high glucose to upregulate glycogen stores in astrocytes delayed loss of function during aglycemia and improved recovery in both young and aging axons. Providing lactate during recovery from aglycemia unmasked a metabolic switch from glucose to lactate in aging axons. Young and aging corpus callosum consisting of a mixture of myelinated and unmyelinated axons sustained their function fully when lactate was available during aglycemia and surprisingly showed a greater resilience to aglycemia compared to fully myelinated axons of optic nerve. We conclude that lactate is a universal substrate for axons independent of their myelination content and age.

AGC1 Deficiency: Pathology and Molecular and Cellular Mechanisms of the Disease

AGC1/Aralar/Slc25a12 is the mitochondrial carrier of aspartate-glutamate, the regulatory component of the NADH malate-aspartate shuttle (MAS) that transfers cytosolic redox power to neuronal mitochondria. The deficiency in AGC1/Aralar leads to the human rare disease named "early infantile epileptic encephalopathy 39" (EIEE 39, OMIM # 612949) characterized by epilepsy, hypotonia, arrested psychomotor neurodevelopment, hypo myelination and a drastic drop in brain aspartate (Asp) and N-acetylaspartate (NAA). Current evidence suggest that neurons are the main brain cell type expressing Aralar. However, paradoxically, glial functions such as myelin and Glutamine (Gln) synthesis are markedly impaired in AGC1 deficiency. Herein, we discuss the role of the AGC1/Aralar-MAS pathway in neuronal functions such as Asp and NAA synthesis, lactate use, respiration on glucose, glutamate (Glu) oxidation and other neurometabolic aspects. The possible mechanism triggering the pathophysiological findings in AGC1 deficiency, such as epilepsy and postnatal hypomyelination observed in humans and mice, are also included. Many of these mechanisms arise from findings in the aralar-KO mice model that extensively recapitulate the human disease including the astroglial failure to synthesize Gln and the dopamine (DA) mishandling in the nigrostriatal system. Epilepsy and DA mishandling are a direct consequence of the metabolic defect in neurons due to AGC1/Aralar deficiency. However, the deficits in myelin and Gln synthesis may be a consequence of neuronal affectation or a direct effect of AGC1/Aralar deficiency in glial cells. Further research is needed to clarify this question and delineate the transcellular metabolic fluxes that control brain functions. Finally, we discuss therapeutic approaches successfully used in AGC1-deficient patients and mice.

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.

βOHB Protective Pathways in Aralar-KO Neurons and Brain: An Alternative to Ketogenic Diet

Aralar/AGC1/Slc25a12, the mitochondrial aspartate-glutamate carrier expressed in neurons, is the regulatory component of the NADH malate-aspartate shuttle. AGC1 deficiency is a neuropediatric rare disease characterized by hypomyelination, hypotonia, developmental arrest, and epilepsy. We have investigated whether β-hydroxybutyrate (βOHB), the main ketone body (KB) produced in ketogenic diet (KD), is neuroprotective in aralar-knock-out (KO) neurons and mice. We report that βOHB efficiently recovers aralar-KO neurons from deficits in basal-stimulated and glutamate-stimulated respiration, effects requiring βOHB entry into the neuron, and protects from glutamate excitotoxicity. Aralar-deficient mice were fed a KD to investigate its therapeutic potential early in development, but this approach was unfeasible. Therefore, aralar-KO pups were treated without distinction of gender with daily intraperitoneal injections of βOHB during 5 d. This treatment resulted in a recovery of striatal markers of the dopaminergic system including dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC)/DA ratio, and vesicular monoamine transporter 2 (VMAT2) protein. Regarding postnatal myelination, myelin basic protein (MBP) and myelin-associated glycoprotein (MAG) myelin proteins were markedly increased in the cortices of βOHB-treated aralar-KO mice. Although brain Asp and NAA levels did not change by βOHB administration, a 4-d βOHB treatment to aralar-KO, but not to control, neurons led to a substantial increase in Asp (3-fold) and NAA (4-fold) levels. These results suggest that the lack of increase in brain Asp and NAA is possibly because of its active utilization by the aralar-KO brain and the likely involvement of neuronal NAA in postnatal myelination in these mice. The effectiveness of βOHB as a therapeutic treatment in AGC1 deficiency deserves further investigation.SIGNIFICANCE STATEMENT Aralar deficiency induces a fatal phenotype in humans and mice and is associated with impaired neurodevelopment, epilepsy, and hypomyelination. In neurons, highly expressing aralar, its deficiency causes a metabolic blockade hampering mitochondrial energetics and respiration. Here, we find that βOHB, the main metabolic product in KD, recovers defective mitochondrial respiration bypassing the metabolic failure in aralar-deficient neurons. βOHB oxidation in mitochondria boosts the synthesis of cytosolic aspartate (Asp) and NAA, which is impeded by aralar deficiency, presumably through citrate-malate shuttle. In aralar-knock-out (KO) mice, βOHB recovers from the drastic drop in specific dopaminergic and myelin markers. The βOHB-induced myelin synthesis occurring together with the marked increment in neuronal NAA synthesis supports the role of NAA as a lipid precursor during postnatal myelination.

Glutamate Carrier Involvement in Mitochondrial Dysfunctioning in the Brain White Matter

Glutamate homeostasis is an important determinant of health of the central nervous system (CNS). Mitochondria play crucial roles in glutamate metabolism, especially in processes with a high energy demand such as action potential generation. Mitochondrial glutamate carriers (GCs) and aspartate-GCs (AGCs) regulate the transport of glutamate from the cytoplasm across the mitochondrial membrane, which is needed to control energy demand, lipid metabolism, and metabolic activity including oxidative phosphorylation and glycolysis. Dysfunction in these carriers are associated with seizures, spasticity, and/or myelin deficits, all of which are associated with inherited metabolic disorders. Since solute carrier functioning and associated processes are cell type- and context-specific, selective vulnerability to glutamate excitotoxicity and mitochondrial dysfunctioning is expected. Understanding this could offer important insights into the pathomechanisms of associated disorders. This perspective aims to explore the link between functions of both AGCs and GCs and their role in metabolic disorders, with a focus on a subclass of lysosomal storage disorders called leukodystrophies (LDs).

Comparison of defense responses of transgenic potato lines expressing three different Rpi genes to specific Phytophthora infestans races based on transcriptome profiling

Potato late blight, one of the most devastating diseases in potato, is caused by the oomycete Phytophthora infestans. Over 20 resistance genes have been cloned including R1, R3a, and R3b. The distinctions between defense response mechanisms mediated by different resistance genes are still unclear. Here we performed transcriptome profiling in three transgenic lines, R1, R3a, and R3b, and wild-type Desiree under inoculation with two P. infestans isolates, 89148 (race 0) and CN152 (super race), using RNA-seq. Compared with wild type, specific differentially expressed genes (DEGs) were identified in the three transgenic lines. The highest number of DEGs occurred in transgenic R3b, with 779 DEGs in response to isolate 89148 and 864 DEGs in response to infection by CN152, followed by transgenic R1 lines with 408 DEGs for isolate 89148 and 267 DEGs for CN152. Based on gene ontology, the most common GO terms (15 for 89148 and 20 for CN152) were enriched in transgenic R3a and R3b lines. This indicates that the defense pathways mediated by R3a and R3b are more similar than those mediated by R1. Further separate GO analysis of up- or down-regulated DEGs showed that the down-regulated DEGs mainly functioned in mediating the resistance of potato to P. infestans 89148 by response to stress biological process and to CN152 by oxidation reduction biological process. KEGG pathways of DNA replication, plant-pathogen interaction and pentose and glucuronate interconversions are unique for transgenic R1, R3a, and R3b lines in incompatible interactions. Quantitative real-time PCR experimental validation confirmed the induced expression of DEGs in the late blight resistance signaling pathway. Our results will lay a solid foundation for further understanding the mechanisms of plant-pathogen interactions, and provide a theoretical reference for durable resistance in potato.

Report on three additional patients and genotype-phenotype correlation in SLC25A22-related disorders group

Early infantile epileptic encephalopathy (EIEE) is a heterogeneous group of severe forms of age-related developmental and epileptic encephalopathies with onset during the first weeks or months of life. The interictal electroencephalogram (EEG) shows a "suppression burst" (SB) pattern. The prognosis is usually poor and most children die within the first two years or survive with very severe intellectual disabilities. EIEE type 3 is caused by variants affecting function, in SLC25A22, which is also responsible for epilepsy of infancy with migrating focal seizures (EIMFS). We report a family with a less severe phenotype of EIEE type 3. We performed exome sequencing and identified two unreported variants in SLC25A22 in the compound heterozygous state: NM_024698.4: c.[813_814delTG];[818 G>A] (p.[Ala272Glnfs*144];[Arg273Lys]). Functional studies in cultured skin fibroblasts from a patient showed that glutamate oxidation was strongly defective, based on a literature review. We clustered the 18 published patients (including those from this family) into three groups according to the severity of the SLC25A22-related disorders. In an attempt to identify genotype-phenotype correlations, we compared the variants according to the location depending on the protein domains. We observed that patients with two variants located in helical transmembrane domains presented a severe phenotype, whereas patients with at least one variant outside helical transmembrane domains presented a milder phenotype. These data are suggestive of a continuum of disorders related to SLC25A22 that could be called SLC25A22-related disorders. This might be a first clue to enable geneticists to outline a prognosis based on genetic molecular data regarding the SLC25A22 gene.

Neurointegrity and neurophysiology: astrocyte, glutamate, and carbon monoxide interactions

Astrocyte contributions to brain function and prevention of neuropathologies are as extensive as that of neurons. Astroglial regulation of glutamate, a primary neurotransmitter, is through uptake, release through vesicular and non-vesicular pathways, and catabolism to intermediates. Homeostasis by astrocytes is considered to be of primary importance in determining normal central nervous system health and central nervous system physiology - glutamate is central to dynamic physiologic changes and central nervous system stability. Gasotransmitters may affect diverse glutamate interactions positively or negatively. The effect of carbon monoxide, an intrinsic central nervous system gasotransmitter, in the complex astrocyte homeostasis of glutamate may offer insights to normal brain development, protection, and its use as a neuromodulator and neurotherapeutic. In this article, we will review the effects of carbon monoxide on astrocyte homeostasis of glutamate.

Molecular basis for increased lactate formation in the Müller glial cells of retina

Müller glial cells are highly metabolic active cells that compensate for the high energy demand of retinal neurons. It has been believed that glucose provides the energy needs by the complete oxidation within Müller cells. However, numerous studies indicated that glial cells convert the majority of glucose to lactate, which may serve as an energy source for neurons. It is still not well understood why within glia, glucose is not completely oxidized under aerobic glycolysis conditions. The aspartate glutamate carrier (AGC) is a major component of the malate-aspartate shuttle (MAS) responsible for transporting the reducing equivalent of glycolysis to the mitochondria for the complete oxidation of glucose. Here, we report the absence of AGC within Müller glial cells which impairs the ability to oxidize glucose. We investigated the expression and localization of AGC and its isoforms (aralar and citrin) in the intact rat retina. We also analyzed the expression and regulation of AGC and its metabolic activity within cultured Müller cells (TR-MUL). The results suggest that AGC and its isoforms seem to be neuronal, with no or low expression within Müller cells of the intact retina. The study of cultured Müller cells suggests a very low expression of AGC and a decreased metabolic activity of the carrier especially under cell differentiation conditions due to low serum and hydrocortisone treatments. Thus, these data give a molecular explanation of increased levels of lactate formation due to a lack of AGC in the retina by Müller glial cells.

Towards an Understanding of Energy Impairment in Huntington's Disease Brain

This review systematically examines the evidence for shifts in flux through energy generating biochemical pathways in Huntington’s disease (HD) brains from humans and model systems. Compromise of the electron transport chain (ETC) appears not to be the primary or earliest metabolic change in HD pathogenesis. Rather, compromise of glucose uptake facilitates glucose flux through glycolysis and may possibly decrease flux through the pentose phosphate pathway (PPP), limiting subsequent NADPH and GSH production needed for antioxidant protection. As a result, oxidative damage to key glycolytic and tricarboxylic acid (TCA) cycle enzymes further restricts energy production so that while basal needs may be met through oxidative phosphorylation, those of excessive stimulation cannot. Energy production may also be compromised by deficits in mitochondrial biogenesis, dynamics or trafficking. Restrictions on energy production may be compensated for by glutamate oxidation and/or stimulation of fatty acid oxidation. Transcriptional dysregulation generated by mutant huntingtin also contributes to energetic disruption at specific enzymatic steps. Many of the alterations in metabolic substrates and enzymes may derive from normal regulatory feedback mechanisms and appear oscillatory. Fine temporal sequencing of the shifts in metabolic flux and transcriptional and expression changes associated with mutant huntingtin expression remain largely unexplored and may be model dependent. Differences in disease progression among HD model systems at the time of experimentation and their varying states of metabolic compensation may explain conflicting reports in the literature. Progressive shifts in metabolic flux represent homeostatic compensatory mechanisms that maintain the model organism through presymptomatic and symptomatic stages.

Cognate Ligand Chaperoning: a Novel Mechanism for the Post-translational Regulation of Neurotransmitter Receptor Biogenesis

The functional unit for inter-neuronal communication in the central nervous system is the neuronal synapse. The number of postsynaptic neurotransmitter receptors at the cell surface is an important determinant of synaptic efficacy and plasticity. A diverse array of post-translational processes regulate postsynaptic receptor number, including receptor exocytosis, lateral diffusion, surface stabilization, endocytosis, and recycling, thus highlighting the importance of mechanisms that control postsynaptic receptor levels. Another putative post-translational mechanism for regulating receptor surface expression is cognate ligand chaperoning. It has been proposed that neurotransmitters function as cognate ligand chaperones by binding, within the endoplasmic reticulum (ER) lumen, to their nascent neurotransmitter receptors and facilitating receptor biogenesis. Here we discuss proof-of-concept evidence that small molecules can selectively facilitate the biogenesis of their targets and examine the specific evidence in support of cognate ligand chaperoning of neurotransmitter receptor biogenesis.

Inhibition of the Mitochondrial Glutamate Carrier SLC25A22 in Astrocytes Leads to Intracellular Glutamate Accumulation

The solute carrier family 25 (SLC25) drives the import of a large diversity of metabolites into mitochondria, a key cellular structure involved in many metabolic functions. Mutations of the mitochondrial glutamate carrier SLC25A22 (also named GC1) have been identified in early epileptic encephalopathy (EEE) and migrating partial seizures in infancy (MPSI) but the pathophysiological mechanism of GC1 deficiency is still unknown, hampered by the absence of an in vivo model. This carrier is mainly expressed in astrocytes and is the principal gate for glutamate entry into mitochondria. A sufficient supply of energy is essential for the proper function of the brain and mitochondria have a pivotal role in maintaining energy homeostasis. In this work, we wanted to study the consequences of GC1 absence in an in vitro model in order to understand if glutamate catabolism and/or mitochondrial function could be affected. First, short hairpin RNA (shRNA) designed to specifically silence GC1 were validated in rat C6 glioma cells. Silencing GC1 in C6 resulted in a reduction of the GC1 mRNA combined with a decrease of the mitochondrial glutamate carrier activity. Then, primary astrocyte cultures were prepared and transfected with shRNA-GC1 or mismatch-RNA (mmRNA) constructs using the Neon® Transfection System in order to target a high number of primary astrocytes, more than 64%. Silencing GC1 in primary astrocytes resulted in a reduced nicotinamide adenine dinucleotide (Phosphate) (NAD(P)H) formation upon glutamate stimulation. We also observed that the mitochondrial respiratory chain (MRC) was functional after glucose stimulation but not activated by glutamate, resulting in a lower level of cellular adenosine triphosphate (ATP) in silenced astrocytes compared to control cells. Moreover, GC1 inactivation resulted in an intracellular glutamate accumulation. Our results show that mitochondrial glutamate transport via GC1 is important in sustaining glutamate homeostasis in astrocytes.

Main Points:

The mitochondrial respiratory chain is functional in absence of GC1

Lack of glutamate oxidation results in a lower global ATP level

Lack of mitochondrial glutamate transport results in intracellular glutamate accumulation

The Role of Astrocytes in Neuroprotection after Brain Stroke: Potential in Cell Therapy

Astrocytes are commonly involved in negative responses through their hyperreactivity and glial scar formation in excitotoxic and/or mechanical injuries. But, astrocytes are also specialized glial cells of the nervous system that perform multiple homeostatic functions for the survival and maintenance of the neurovascular unit. Astrocytes have neuroprotective, angiogenic, immunomodulatory, neurogenic, and antioxidant properties and modulate synaptic function. This makes them excellent candidates as a source of neuroprotection and neurorestoration in tissues affected by ischemia/reperfusion, when some of their deregulated genes can be controlled. Therefore, this review analyzes pro-survival responses of astrocytes that would allow their use in cell therapy strategies.

Glutamine-Glutamate Cycle Flux Is Similar in Cultured Astrocytes and Brain and Both Glutamate Production and Oxidation Are Mainly Catalyzed by Aspartate Aminotransferase

The glutamine-glutamate cycle provides neurons with astrocyte-generated glutamate/γ-aminobutyric acid (GABA) and oxidizes glutamate in astrocytes, and it returns released transmitter glutamate/GABA to neurons after astrocytic uptake. This review deals primarily with the glutamate/GABA generation/oxidation, although it also shows similarity between metabolic rates in cultured astrocytes and intact brain. A key point is identification of the enzyme(s) converting astrocytic α-ketoglutarate to glutamate and vice versa. Most experiments in cultured astrocytes, including those by one of us, suggest that glutamate formation is catalyzed by aspartate aminotransferase (AAT) and its degradation by glutamate dehydrogenase (GDH). Strongly supported by results shown in Table 1 we now propose that both reactions are primarily catalyzed by AAT. This is possible because the formation occurs in the cytosol and the degradation in mitochondria and they are temporally separate. High glutamate/glutamine concentrations abolish the need for glutamate production from α-ketoglutarate and due to metabolic coupling between glutamate synthesis and oxidation these high concentrations render AAT-mediated glutamate oxidation impossible. This necessitates the use of GDH under these conditions, shown by insensitivity of the oxidation to the transamination inhibitor aminooxyacetic acid (AOAA). Experiments using lower glutamate/glutamine concentration show inhibition of glutamate oxidation by AOAA, consistent with the coupled transamination reactions described here.

Glutamate oxidation in astrocytes: Roles of glutamate dehydrogenase and aminotransferases

The cellular distribution of transporters and enzymes related to glutamate metabolism led to the concept of the glutamate-glutamine cycle. Glutamate is released as a neurotransmitter and taken up primarily by astrocytes ensheathing the synapses. The glutamate carbon skeleton is transferred back to the presynaptic neurons as the nonexcitatory amino acid glutamine. The cycle was initially thought to function with a 1:1 ratio between glutamate released and glutamine taken up by neurons. However, studies of glutamate metabolism in astrocytes have shown that a considerable proportion of glutamate undergoes oxidative degradation; thus, quantitative formation of glutamine from the glutamate taken up is not possible. Oxidation of glutamate is initiated by transamination catalyzed by an aminotransferase, or oxidative deamination catalyzed by glutamate dehydrogenase (GDH). We discuss methods available to elucidate the enzymes that mediate this conversion. Methods include pharmacological tools such as the transaminase inhibitor aminooxyacetic acid, studies using GDH knockout mice, and siRNA-mediated knockdown of GDH in astrocytes. Studies in brain slices incubated with [¹⁵ N]glutamate demonstrated activity of GDH in astrocytes in situ. These results, in conjunction with reports in the literature, support the conclusion that GDH is active in astrocytes both in culture and in vivo and that this enzyme plays a significant role in glutamate oxidation. Oxidative metabolism of glutamate, primarily mediated by GDH, but also by transamination by aspartate aminotransferase, provides considerably more energy than is required to maintain the activity of the high-affinity glutamate transporters needed for efficient removal of glutamate from the synaptic cleft. © 2016 Wiley Periodicals, Inc.

Metabolic and Inflammatory Adaptation of Reactive Astrocytes: Role of PPARs

Astrocyte-mediated inflammation is associated with degenerative pathologies such as Alzheimer's and Parkinson's diseases and multiple sclerosis. The acute inflammation and morphological and metabolic changes that astrocytes develop after the insult are known as reactive astroglia or astrogliosis that is an important response to protect and repair the lesion. Astrocytes optimize their metabolism to produce lactate, glutamate, and ketone bodies in order to provide energy to the neurons that are deprived of nutrients upon insult. Firstly, we review the basis of inflammation and morphological changes of the different cell population implicated in reactive gliosis. Next, we discuss the more active metabolic pathways in healthy astrocytes and explain the metabolic response of astrocytes to the insult in different pathologies and which metabolic alterations generate complications in these diseases. We emphasize the role of peroxisome proliferator-activated receptors isotypes in the inflammatory and metabolic adaptation of astrogliosis developed in ischemia or neurodegenerative diseases. Based on results reported in astrocytes and other cells, we resume and hypothesize the effect of peroxisome proliferator-activated receptor (PPAR) activation with ligands on different metabolic pathways in order to supply energy to the neurons. The activation of selective PPAR isotype activity may serve as an input to better understand the role played by these receptors on the metabolic and inflammatory compensation of astrogliosis and might represent an opportunity to develop new therapeutic strategies against traumatic brain injuries and neurodegenerative diseases.

Diagnostic Approach to Genetic Causes of Early-Onset Epileptic Encephalopathy

Epileptic encephalopathies are characterized by recurrent clinical seizures and prominent interictal epileptiform discharges seen during the early infantile period. Although epileptic encephalopathies are mostly associated with structural brain defects and inherited metabolic disorders, pathogenic gene mutations may also be involved in the development of epileptic encephalopathies even when no clear genetic inheritance patterns or consanguinity exist. The most common epileptic encephalopathies are Ohtahara syndrome, early myoclonic encephalopathy, epilepsy of infancy with migrating focal seizures, West syndrome and Dravet syndrome, which are usually unresponsive to traditional antiepileptic medication. Many of the diagnoses describe the phenotype of these electroclinical syndromes, but not the underlying causes. To date, approximately 265 genes have been defined in epilepsy and several genes including STXBP1, ARX, SLC25A22, KCNQ2, CDKL5, SCN1A, and PCDH19 have been found to be associated with early-onset epileptic encephalopathies. In this review, we aimed to present a diagnostic approach to primary genetic causes of early-onset epileptic encephalopathies.

Astrocyte Cultures Mimicking Brain Astrocytes in Gene Expression, Signaling, Metabolism and K+ Uptake and Showing Astrocytic Gene Expression Overlooked by Immunohistochemistry and In Situ Hybridization

Based on differences in gene expression between cultured astrocytes and freshly isolated brain astrocytes it has been claimed that cultured astrocytes poorly reflect the characteristics of their in vivo counterparts. This paper shows that this is not the case with the cultures of mouse astrocytes we have used since 1978. The culture is prepared following guidelines provided by Drs. Monique Sensenbrenner and John Booher, with the difference that dibutyryl cyclic AMP is added to the culture medium from the beginning of the third week. This addition has only minor effects on glucose and glutamate metabolism, but it is crucial for effects by elevated K⁺ concentrations and for Ca²⁺ homeostasis, important aspects of astrocyte function. Work by Liang Peng and her colleagues has shown identity between not only gene expression but also drug-induced gene upregulations and editings in astrocytes cultured by this method and astrocytes freshly isolated from brains of drug-treated animals. Dr. Norenberg's laboratory has demonstrated identical upregulation of the cotransporter NKCC1 in ammonia-exposed astrocytes and rats with liver failure. Similarity between cultured and freshly isolated astrocytes has also been shown in metabolism, K⁺ uptake and several aspects of signaling. However, others have shown that the gene for the glutamate transporter GLT1 is not expressed, and rat cultures show some abnormalities in K⁺ effects. Nevertheless, the overall reliability of the cultured cells is important because immunohistochemistry and in situ hybridization poorly demonstrate many astrocytic genes, e.g., those of nucleoside transporters, and even microarray analysis of isolated cells can be misleading.

Glutaminases

Mammalian glutaminases catalyze the stoichiometric conversion of L-glutamine to L-glutamate and ammonium ions. In brain, glutaminase is considered the prevailing pathway for synthesis of the neurotransmitter pool of glutamate. Besides neurotransmission, the products of glutaminase reaction also fulfill crucial roles in energy and metabolic homeostasis in mammalian brain. In the last years, new functional roles for brain glutaminases are being uncovered by using functional genomic and proteomic approaches. Glutaminases may act as multifunctional proteins able to perform different tasks: the discovery of multiple transcript variants in neurons and glial cells, novel extramitochondrial localizations, and isoform-specific proteininteracting partners strongly support possible moonlighting functions for these proteins. In this chapter, we present a critical account of essential works on brain glutaminase 80 years after its discovery. We will highlight the impact of recent findings and thoughts in the context of the glutamate/glutamine brain homeostasis.

Neuronal gamma-aminobutyric acid (GABA) type A receptors undergo cognate ligand chaperoning in the endoplasmic reticulum by endogenous GABA

GABA_A receptors mediate fast inhibitory neurotransmission in the brain. Dysfunction of these receptors is associated with various psychiatric/neurological disorders and drugs targeting this receptor are widely used therapeutic agents. Both the efficacy and plasticity of GABA_A receptor-mediated neurotransmission depends on the number of surface GABA_A receptors. An understudied aspect of receptor cell surface expression is the post-translational regulation of receptor biogenesis within the endoplasmic reticulum (ER). We have previously shown that exogenous GABA can act as a ligand chaperone of recombinant GABA_A receptors in the early secretory pathway leading us to now investigate whether endogenous GABA facilitates the biogenesis of GABA_A receptors in primary cerebral cortical cultures. In immunofluorescence labeling experiments, we have determined that neurons expressing surface GABA_A receptors contain both GABA and its degradative enzyme GABA transaminase (GABA-T). Treatment of neurons with GABA-T inhibitors, a treatment known to increase intracellular GABA levels, decreases the interaction of the receptor with the ER quality control protein calnexin, concomittantly increasing receptor forward-trafficking and plasma membrane insertion. The effect of GABA-T inhibition on the receptor/calnexin interaction is not due to the activation of surface GABA_A or GABA_B receptors. Consistent with our hypothesis that GABA acts as a cognate ligand chaperone in the ER, immunogold-labeling of rodent brain slices reveals the presence of GABA within the rough ER. The density of this labeling is similar to that present in mitochondria, the organelle in which GABA is degraded. Lastly, the effect of GABA-T inhibition on the receptor/calnexin interaction was prevented by pretreatment with a GABA transporter inhibitor. Together, these data indicate that endogenous GABA acts in the rough ER as a cognate ligand chaperone to facilitate the biogenesis of neuronal GABA_A receptors.

Two siblings with early infantile myoclonic encephalopathy due to mutation in the gene encoding mitochondrial glutamate/H+ symporter SLC25A22

AIM

To characterize a new subset of early myoclonic encephalopathy usually associated with metabolic etiologies with a new genetic entity.

METHODS

We describe two siblings with early myoclonic encephalopathy born to consanguineous parents of Arab Muslim origin from Israel. We used homozygosity mapping and candidate gene sequencing to reveal the genetic basis of the myoclonic syndrome.

RESULTS

We found a rare missense mutation in the gene encoding one of the two mitochondrial glutamate/H symporters, SLC25A22. The phenotype of early myoclonic encephalopathy was first linked to the same mutation in 2005 in patients of the same ethnicity as our family.

CONCLUSIONS

Owing to the devastating nature of this encephalopathy, we focus attention on its clinical history, epileptic semiology, distinct electroencephalography features, and genetic basis. We provide the evidence that an integrated diagnostic strategy combining homozygosity mapping with candidate gene sequencing is efficient in consanguineous families with highly heterogeneous autosomal recessive diseases.

Expression of Gls and Gls2 glutaminase isoforms in astrocytes

The expression of glutaminase in glial cells has been a controversial issue and matter of debate for many years. Actually, glutaminase is essentially considered as a neuronal marker in brain. Astrocytes are endowed with efficient and high capacity transport systems to recapture synaptic glutamate which seems to be consistent with the absence of glutaminase in these glial cells. In this work, a comprehensive study was devised to elucidate expression of glutaminase in neuroglia and, more concretely, in astrocytes. Immunocytochemistry in rat and human brain tissues employing isoform-specific antibodies revealed expression of both Gls and Gls2 glutaminase isozymes in glutamatergic and GABAergic neuronal populations as well as in astrocytes. Nevertheless, there was a different subcellular distribution: Gls isoform was always present in mitochondria while Gls2 appeared in two different locations, mitochondria and nucleus. Confocal microscopy and double immunofluorescence labeling in cultured astrocytes confirmed the same pattern previously seen in brain tissue samples. Astrocytic glutaminase expression was also assessed at the mRNA level, real-time quantitative RT-PCR detected transcripts of four glutaminase isozymes but with marked differences on their absolute copy number: the predominance of Gls isoforms over Gls2 transcripts was remarkable (ratio of 144:1). Finally, we proved that astrocytic glutaminase proteins possess enzymatic activity by in situ activity staining: concrete populations of astrocytes were labeled in the cortex, cerebellum and hippocampus of rat brain demonstrating functional catalytic activity. These results are relevant for the stoichiometry of the Glu/Gln cycle at the tripartite synapse and suggest novel functions for these classical metabolic enzymes.

Localization of excitatory amino acid transporters EAAT1 and EAAT2 in human postmortem cortex: a light and electron microscopic study

The process of glutamate release, activity, and reuptake involves the astrocyte, the presynaptic and postsynaptic neurons. Glutamate is released into the synapse and may occupy and activate receptors on both neurons and astrocytes. Glutamate is rapidly removed from the synapse by a family of plasma membrane excitatory amino acid transporters (EAATs), also localized to neurons and astrocytes. The purpose of the present study was to examine EAAT labeling in the postmortem human cortex at the light and electron microscopic (EM) levels. The postmortem prefrontal cortex was processed for EAAT1 and EAAT2 immunohistochemistry. At the light microscopic level, EAAT1 and EAAT2 labeling was found in both gray and white matter. Most cellular labeling was in small cells which were morphologically similar to glia. In addition, EAAT1-labeled neurons were scattered throughout, some of which were pyramidal in shape. At the EM level, EAAT1 and EAAT2 labeling was found in astrocytic soma and processes surrounding capillaries. EAAT labeling was also found in small astrocytic processes adjacent to axon terminals forming asymmetric (glutamatergic) synapses. While EAAT2 labeling was most prevalent in astrocytic processes, EAAT1 labeling was also present in neuronal processes including the soma, axons, and dendritic spines. Expression of EAAT1 protein on neurons may be due to the hypoxia associated with the postmortem interval, and requires further confirmation. The localization of EAATs on the astrocytic plasma membrane and adjacent to excitatory synapses is consistent with the function of facilitating glutamate reuptake and limiting glutamate spillover. Establishment that EAAT1 and EAAT2 can be measured at the EM level in human postmortem tissues will permit testing of hypotheses related to these molecules in diseases lacking analogous animal models.

Fatty acids in energy metabolism of the central nervous system

In this review, we analyze the current hypotheses regarding energy metabolism in the neurons and astroglia. Recently, it was shown that up to 20% of the total brain's energy is provided by mitochondrial oxidation of fatty acids. However, the existing hypotheses consider glucose, or its derivative lactate, as the only main energy substrate for the brain. Astroglia metabolically supports the neurons by providing lactate as a substrate for neuronal mitochondria. In addition, a significant amount of neuromediators, glutamate and GABA, is transported into neurons and also serves as substrates for mitochondria. Thus, neuronal mitochondria may simultaneously oxidize several substrates. Astrocytes have to replenish the pool of neuromediators by synthesis de novo, which requires large amounts of energy. In this review, we made an attempt to reconcile β-oxidation of fatty acids by astrocytic mitochondria with the existing hypothesis on regulation of aerobic glycolysis. We suggest that, under condition of neuronal excitation, both metabolic pathways may exist simultaneously. We provide experimental evidence that isolated neuronal mitochondria may oxidize palmitoyl carnitine in the presence of other mitochondrial substrates. We also suggest that variations in the brain mitochondrial metabolic phenotype may be associated with different mtDNA haplogroups.

Nobiletin restores impaired hippocampal mitochondrial bioenergetics in hypothyroidism through activation of matrix substrate-level phosphorylation

OBJECTIVE

Evaluation of the effect of citrus flavonoid - nobiletin on the bioenergetics of synaptic and non-synaptic mitochondria in the hippocampus of hypothyroid rats.

METHODS

Male Wistar rats were divided into hypothyroid (methimazole-treated), nobiletin supplemented hypothyroid, thyroxine-treated hypothyroid, and euthyroid (control) groups. Synaptic and non-synaptic (cell) mitochondria were isolated from hippocampus. Oligomycin-sensitive, oligomycin-insensitive, α-ketoglutarate dehydrogenase-dependent synthesis of adenosine triphosphate (ATP), succinate dehydrogenase, and hexokinase activities were determined luminometrically and spectrophotometrically, respectively.

RESULTS

Decreased synthesis of oligomycin-sensitive and oligomycin-insensitive ATP in hypothyroid rat hippocampus was observed in synaptic and non-synaptic mitochondria. Supplementation of hypothyroid rats with nobiletin increases oligomycin-insensitive and α-ketoglutarate-dependent production of ATP in both types of mitochondria. The activity of succinate dehydrogenase in non-synaptic mitochondria and the activities of hexokinase in both types of mitochondria were normalized in nobiletin-treated hypothyroid rats.

DISCUSSION

Nobiletin restores reduced mitochondrial metabolism in hypothyroid rat hippocampus through acceleration of matrix substrate-level phosphorylation that may be important for the prevention of hypometabolic complications in neurological disorders.

6-Hydroxydopamine impairs mitochondrial function in the rat model of Parkinson's disease: respirometric, histological, and behavioral analyses

Mitochondrial defects have been shown to be associated with the pathogenesis of Parkinson's disease (PD). Yet, experience in PD research linking mitochondrial dysfunction, e.g., deregulation of oxidative phosphorylation, with neuronal degeneration and behavioral changes is rather limited. Using the 6-hydroxydopamine (6-OHDA) rat model of PD, we have investigated the potential role of mitochondria in dopaminergic neuronal cell death in the substantia nigra pars compacta by high-resolution respirometry. Mitochondrial function was correlated with the time course of disease-related motor behavior asymmetry and dopaminergic neuronal cell loss, respectively. Unilateral 6-OHDA injections (>2.5 μg/2 μl) into the median forebrain bundle induced an impairment of oxidative phosphorylation due to a decrease in complex I activity. This was indicated by increased flux control coefficient. During the period of days 2-21, a progressive decrease in respiratory control ratio of up to -58 % was observed in the lesioned compared to the non-lesioned substantia nigra of the same animals. This decrease was associated with a marked uncoupling of oxidative phosphorylation. Mitochondrial dysfunction, motor behavior asymmetry, and dopaminergic neuronal cell loss correlated with dosage (1.25-5 μg/2 μl). We conclude that high-resolution respirometry may allow the detection of distinct mitochondrial dysfunction as a suitable surrogate marker for the preclinical assessment of potential neuroprotective strategies in the 6-OHDA model of PD.

Methodological limitations in determining astrocytic gene expression

Traditionally, astrocytic mRNA and protein expression are studied by in situ hybridization (ISH) and immunohistochemically. This led to the concept that astrocytes lack aralar, a component of the malate-aspartate-shuttle. At least similar aralar mRNA and protein expression in astrocytes and neurons isolated by fluorescence-assisted cell sorting (FACS) reversed this opinion. Demonstration of expression of other astrocytic genes may also be erroneous. Literature data based on morphological methods were therefore compared with mRNA expression in cells obtained by recently developed methods for determination of cell-specific gene expression. All Na,K-ATPase-α subunits were demonstrated by immunohistochemistry (IHC), but there are problems with the cotransporter NKCC1. Glutamate and GABA transporter gene expression was well determined immunohistochemically. The same applies to expression of many genes of glucose metabolism, whereas a single study based on findings in bacterial artificial chromosome (BAC) transgenic animals showed very low astrocytic expression of hexokinase. Gene expression of the equilibrative nucleoside transporters ENT1 and ENT2 was recognized by ISH, but ENT3 was not. The same applies to the concentrative transporters CNT2 and CNT3. All were clearly expressed in FACS-isolated cells, followed by biochemical analysis. ENT3 was enriched in astrocytes. Expression of many nucleoside transporter genes were shown by microarray analysis, whereas other important genes were not. Results in cultured astrocytes resembled those obtained by FACS. These findings call for reappraisal of cellular nucleoside transporter expression. FACS cell yield is small. Further development of cell separation methods to render methods more easily available and less animal and cost consuming and parallel studies of astrocytic mRNA and protein expression by ISH/IHC and other methods are necessary, but new methods also need to be thoroughly checked.

Metabolic Flux and Compartmentation Analysis in the Brain In vivo

Through significant developments and progresses in the last two decades, in vivo localized nuclear magnetic resonance spectroscopy (MRS) became a method of choice to probe brain metabolic pathways in a non-invasive way. Beside the measurement of the total concentration of more than 20 metabolites, (1)H MRS can be used to quantify the dynamics of substrate transport across the blood-brain barrier by varying the plasma substrate level. On the other hand, (13)C MRS with the infusion of (13)C-enriched substrates enables the characterization of brain oxidative metabolism and neurotransmission by incorporation of (13)C in the different carbon positions of amino acid neurotransmitters. The quantitative determination of the biochemical reactions involved in these processes requires the use of appropriate metabolic models, whose level of details is strongly related to the amount of data accessible with in vivo MRS. In the present work, we present the different steps involved in the elaboration of a mathematical model of a given brain metabolic process and its application to the experimental data in order to extract quantitative brain metabolic rates. We review the recent advances in the localized measurement of brain glucose transport and compartmentalized brain energy metabolism, and how these reveal mechanistic details on glial support to glutamatergic and GABAergic neurons.

Unraveling the complex metabolic nature of astrocytes

Since the initial description of astrocytes by neuroanatomists of the nineteenth century, a critical metabolic role for these cells has been suggested in the central nervous system. Nonetheless, it took several technological and conceptual advances over many years before we could start to understand how they fulfill such a role. One of the important and early recognized metabolic function of astrocytes concerns the reuptake and recycling of the neurotransmitter glutamate. But the description of this initial property will be followed by several others including an implication in the supply of energetic substrates to neurons. Indeed, despite the fact that like most eukaryotic non-proliferative cells, astrocytes rely on oxidative metabolism for energy production, they exhibit a prominent aerobic glycolysis capacity. Moreover, this unusual metabolic feature was found to be modulated by glutamatergic activity constituting the initial step of the neurometabolic coupling mechanism. Several approaches, including biochemical measurements in cultured cells, genetic screening, dynamic cell imaging, nuclear magnetic resonance spectroscopy and mathematical modeling, have provided further insights into the intrinsic characteristics giving rise to these key features of astrocytes. This review will provide an account of the different results obtained over several decades that contributed to unravel the complex metabolic nature of astrocytes that make this cell type unique.

Quantitative clinical proteomic study of autopsied human infarcted brain specimens to elucidate the deregulated pathways in ischemic stroke pathology

Ischemic stroke, still lacking an effective neuroprotective therapy is the third leading cause of global mortality and morbidity. Here, we have applied an 8-plex iTRAQ-based 2D-LC-MS/MS strategy to study the commonly regulated infarct proteome from three different brain regions (putamen, thalamus and the parietal lobe) of female Japanese patients. Infarcts were compared with age-, post-mortem interval- and location-matched control specimens. The iTRAQ experiment confidently identified 1520 proteins with 0.1% false discovery rate. Bioinformatics data mining and immunochemical validation of pivotal perturbed proteins revealed a global failure of the cellular energy metabolism in the infarcted tissues as seen by the parallel down-regulation of proteins related to glycolysis, pyruvate dehydrogenase complex, TCA cycle and oxidative phosphorylation. The concomitant down-regulation of all participating proteins (SLC25A11, SLC25A12, GOT2 and MDH2) of malate-aspartate shuttle might be responsible for the metabolic in-coordination between the cytosol and mitochondria resulting in the failure of energy metabolism. The levels of proteins related to reactive gliosis (VIM, GFAP) and anti-inflammatory response (ANXA1, ANXA2) showed an increasing trend. The elevation of ferritin (FTL, FTH1) may indicate an iron-mediated oxidative imbalance aggravating the mitochondrial failure and neurotoxicity. The deregulated proteins could be useful as potential therapeutic targets or biomarkers for ischemic stroke.

BIOLOGICAL SIGNIFICANCE

Clinical proteomics of stroke has been lagging behind other areas of clinical proteomics like Alzheimer's disease or schizophrenia. Our study is the first quantitative clinical proteomics study where iTRAQ-2D-LC-MS/MS has been utilized in the area of ischemic stroke to obtain a comparative profile of human ischemic infarcts and age-, sex-, location- and post-mortem interval-matched control brain specimens. Different pathological attributes of ischemic stroke well-known through basic and pre-clinical research such as failure of cellular energy metabolism, reactive gliosis, activation of anti-inflammatory response and aberrant iron metabolism have been observed at the bedside. Our dataset could act as a reference for similar studies done in the future using ischemic brain samples from various brain banks across the world. A meta-analysis of these studies could help to map the pathological proteome specific to ischemic stroke that will guide the scientific community to better evaluate the pros and cons of the pre-clinical models for efficacy and mechanistic studies. Infarct being the core of injury should have the most intense regulation for several key proteins involved in the pathophysiology of ischemic stroke. Hence, a part of the up-regulated proteome could leak into the general circulation that may offer candidates of interest as potential biomarkers. In support of our proposed hypothesis, we report ferritin in the current study as one of the most elevated proteins in the infarct, which has been documented as a biomarker in the context of ischemic stroke by an independent study. Overall, our approach has the potential to identify probable therapeutic targets and biomarkers in the area of ischemic stroke.

Formaldehyde in brain: an overlooked player in neurodegeneration?

Formaldehyde is an environmental pollutant that is also generated in substantial amounts in the human body during normal metabolism. This aldehyde is a well-established neurotoxin that affects memory, learning, and behavior. In addition, in several pathological conditions, including Alzheimer's disease, an increase in the expression of formaldehyde-generating enzymes and elevated levels of formaldehyde in brain have been reported. This article gives an overview on the current knowledge on the generation and metabolism of formaldehyde in brain cells as well as on formaldehyde-induced alterations in metabolic processes. Brain cells have the potential to generate and to dispose formaldehyde. In culture, both astrocytes and neurons efficiently oxidize formaldehyde to formate which can be exported or further oxidized. Although moderate concentrations of formaldehyde are not acutely toxic for brain cells, exposure to formaldehyde severely affects their metabolism as demonstrated by the formaldehyde-induced acceleration of glycolytic flux and by the rapid multidrug resistance protein 1-mediated export of glutathione from both astrocytes and neurons. These formaldehyde-induced alterations in the metabolism of brain cells may contribute to the impaired cognitive performance observed after formaldehyde exposure and to the neurodegeneration in diseases that are associated with increased formaldehyde levels in brain.

Gene expression analysis of novel genes in the prefrontal cortex of major depressive disorder subjects

Dysregulation of the glutamatergic system has been implicated not only in the treatment of major depressive disorder (MDD), but also in the excitotoxic effects of stress and anxiety on the prefrontal cortex, which may precede the onset of a depressive episode. Our previous studies demonstrate marked deficits in prominent postsynaptic proteins involved in glutamate neurotransmission in the prefrontal cortex (PFC), Brodmann's area 10 (BA 10) from subjects diagnosed with major depressive disorder (MDD). In the same group of subjects we have identified deficits in expression and phosphorylation level of key components of the mammalian target of rapamycin (mTOR) signaling pathway, known to regulate translation initiation. Based on our previous findings, we have postulated that glutamate-dependent dysregulation of mTOR-initiated protein synthesis in the PFC may underlie the pathology of MDD. The aim of this study was to use the NanoString nCounter System to perform analysis of genes coding for glutamate transporters, glutamate metabolizing enzymes, neurotrophic factors and other intracellular signaling markers involved in glutamate signaling that were not previously investigated by our group in the PFC BA 10 from subjects with MDD. We have analyzed a total of 200 genes from 16 subjects with MDD and 16 healthy controls. These are part of the same cohort used in our previous studies. Setting our cutoff p-value≤0.01, marked upregulation of genes coding for mitochondrial glutamate carrier (GC1; p=0.0015), neuropilin 1 (NRP-1; p=0.0019), glutamate receptor ionotropic N-methyl-d-aspartate-associated protein 1 (GRINA; p=0.0060), and fibroblast growth factor receptor 1 (FGFR-1; p=0.010) was identified. No significant differences in expression of the remaining 196 genes were observed between MDD subjects and controls. While upregulation of FGFR-1 has been previously shown in MDD; abnormalities in GC-1, GRINA, and NRP-1 have not been reported. Therefore, this postmortem study identifies GC1, GRINA, and NRP-1 as novel factors associated with MDD; however, future studies will be needed to address the significance of these genes in the pathophysiology of depression and antidepressant activity.

The Glutamate-Glutamine (GABA) Cycle: Importance of Late Postnatal Development and Potential Reciprocal Interactions between Biosynthesis and Degradation

The gold standard for studies of glutamate-glutamine (GABA) cycling and its connections to brain biosynthesis from glucose of glutamate and GABA and their subsequent metabolism are the elegant in vivo studies by (13)C magnetic resonance spectroscopy (NMR), showing the large fluxes in the cycle. However, simpler experiments in intact brain tissue (e.g., immunohistochemistry), brain slices, cultured brain cells, and mitochondria have also made important contributions to the understanding of details, mechanisms, and functional consequences of glutamate/GABA biosynthesis and degradation. The purpose of this review is to attempt to integrate evidence from different sources regarding (i) the enzyme(s) responsible for the initial conversion of α-ketoglutarate to glutamate; (ii) the possibility that especially glutamate oxidation is essentially confined to astrocytes; and (iii) the ontogenetically very late onset and maturation of glutamine-glutamate (GABA) cycle function. Pathway models based on the functional importance of aspartate for glutamate synthesis suggest the possibility of interacting pathways for biosynthesis and degradation of glutamate and GABA and the use of transamination as the default mechanism for initiation of glutamate oxidation. The late development and maturation are related to the late cortical gliogenesis and convert brain cortical function from being purely neuronal to becoming neuronal-astrocytic. This conversion is associated with huge increases in energy demand and production, and the character of potentially incurred gains of function are discussed. These may include alterations in learning mechanisms, in mice indicated by lack of pairing of odor learning with aversive stimuli in newborn animals but the development of such an association 10-12 days later. The possibility is suggested that analogous maturational changes may contribute to differences in the way learning is accomplished in the newborn human brain and during later development.

The mitochondrial oxoglutarate carrier: from identification to mechanism

The 2-oxoglutarate carrier (OGC) belongs to the mitochondrial carrier protein family whose members are responsible for the exchange of metabolites, cofactors and nucleotides between the cytoplasm and mitochondrial matrix. Initially, OGC was characterized by determining substrate specificity, kinetic parameters of transport, inhibitors and molecular probes that form covalent bonds with specific residues. It was shown that OGC specifically transports oxoglutarate and certain carboxylic acids. The substrate specificity combination of OGC is unique, although many of its substrates are also transported by other mitochondrial carriers. The abundant recombinant expression of bovine OGC in Escherichia coli and its ability to functionally reconstitute into proteoliposomes made it possible to deduce the individual contribution of each and every residue of OGC to the transport activity by a complete set of cys-scanning mutants. These studies give experimental support for a substrate binding site constituted by three major contact points on the even-numbered α-helices and identifies other residues as important for transport function through their crucial positions in the structure for conserved interactions and the conformational changes of the carrier during the transport cycle. The results of these investigations have led to utilize OGC as a model protein for understanding the transport mechanism of mitochondrial carriers.

Multifunctional role of astrocytes as gatekeepers of neuronal energy supply

Dynamic adjustments to neuronal energy supply in response to synaptic activity are critical for neuronal function. Glial cells known as astrocytes have processes that ensheath most central synapses and express G-protein-coupled neurotransmitter receptors and transporters that respond to neuronal activity. Astrocytes also release substrates for neuronal oxidative phosphorylation and have processes that terminate on the surface of brain arterioles and can influence vascular smooth muscle tone and local blood flow. Membrane receptor or transporter-mediated effects of glutamate represent a convergence point of astrocyte influence on neuronal bioenergetics. Astrocytic glutamate uptake drives glycolysis and subsequent shuttling of lactate from astrocytes to neurons for oxidative metabolism. Astrocytes also convert synaptically reclaimed glutamate to glutamine, which is returned to neurons for glutamate salvage or oxidation. Finally, astrocytes store brain energy currency in the form of glycogen, which can be mobilized to produce lactate for neuronal oxidative phosphorylation in response to glutamatergic neurotransmission. These mechanisms couple synaptically driven astrocytic responses to glutamate with release of energy substrates back to neurons to match demand with supply. In addition, astrocytes directly influence the tone of penetrating brain arterioles in response to glutamatergic neurotransmission, coordinating dynamic regulation of local blood flow. We will describe the role of astrocytes in neurometabolic and neurovascular coupling in detail and discuss, in turn, how astrocyte dysfunction may contribute to neuronal bioenergetic deficit and neurodegeneration. Understanding the role of astrocytes as a hub for neurometabolic and neurovascular coupling mechanisms is a critical underpinning for therapeutic development in a broad range of neurodegenerative disorders characterized by chronic generalized brain ischemia and brain microvascular dysfunction.

AGC1-malate aspartate shuttle activity is critical for dopamine handling in the nigrostriatal pathway

The mitochondrial transporter of aspartate-glutamate Aralar/AGC1 is a regulatory component of the malate-aspartate shuttle. Aralar deficiency in mouse and human causes a shutdown of brain shuttle activity and global cerebral hypomyelination. A lack of neurofilament-labeled processes is detected in the cerebral cortex, but whether different types of neurons are differentially affected by Aralar deficiency is still unknown. We have now found that Aralar-knockout (Aralar-KO) post-natal mice show hyperactivity, anxiety-like behavior, and hyperreactivity with a decrease of dopamine (DA) in terminal-rich regions. The striatum is the brain region most affected in terms of size, amino acid and monoamine content. We find a decline in vesicular monoamine transporter-2 (VMAT2) levels associated with increased DA metabolism through MAO activity (DOPAC/DA ratio) in Aralar-KO striatum. However, no decrease in DA or in the number of nigral tyrosine hydroxylase-positive cells was detected in Aralar-KO brainstem. Adult Aralar-hemizygous mice presented also increased DOPAC/DA ratio in striatum and enhanced sensitivity to amphetamine. Our results suggest that Aralar deficiency causes a fall in GSH/GSSG ratio and VMAT2 in striatum that might be related to a failure to produce mitochondrial NADH and to an increase of reactive oxygen species (ROS) in the cytosol. The results indicate that the nigrostriatal dopaminergic system is a target of Aralar deficiency.

Formaldehyde metabolism and formaldehyde-induced stimulation of lactate production and glutathione export in cultured neurons

Formaldehyde is endogenously produced in the human body and brain levels of this compound are elevated in neurodegenerative conditions. Although the toxic potential of an excess of formaldehyde has been studied, little is known on the molecular mechanisms underlying its neurotoxicity as well as on the ability of neurons to metabolize formaldehyde. To address these topics, we have used cerebellar granule neuron cultures as model system. These cultures express mRNAs of various enzymes that are involved in formaldehyde metabolism and were remarkably resistant toward acute formaldehyde toxicity. Cerebellar granule neurons metabolized formaldehyde with a rate of around 200 nmol/(h × mg) which was accompanied by significant increases in the cellular and extracellular concentrations of formate. In addition, formaldehyde application significantly increased glucose consumption, almost doubled the rate of lactate release from viable neurons and strongly accelerated the export of the antioxidant glutathione. The latter process was completely prevented by inhibition of the known glutathione exporter multidrug resistance protein 1. These data indicate that cerebellar granule neurons are capable of metabolizing formaldehyde and that the neuronal glycolysis and glutathione export are severely affected by the presence of formaldehyde.

Quantitative in silico Analysis of Neurotransmitter Pathways Under Steady State Conditions

The modeling of glutamate/GABA-glutamine cycling in the brain tissue involving astrocytes, glutamatergic and GABAergic neurons leads to a complex compartmentalized metabolic network that comprises neurotransmitter synthesis, shuttling, and degradation. Without advanced computational tools, it is difficult to quantitatively track possible scenarios and identify viable ones. In this article, we follow a sampling-based computational paradigm to analyze the biochemical network in a multi-compartment system modeling astrocytes, glutamatergic, and GABAergic neurons, and address some questions about the details of transmitter cycling, with particular emphasis on the ammonia shuttling between astrocytes and neurons, and the synthesis of transmitter GABA. More specifically, we consider the joint action of the alanine-lactate shuttle, the branched chain amino acid shuttle, and the glutamine-glutamate cycle, as well as the role of glutamate dehydrogenase (GDH) activity. When imposing a minimal amount of bound constraints on reaction and transport fluxes, a preferred stoichiometric steady state equilibrium requires an unrealistically high reductive GDH activity in neurons, indicating the need for additional bound constants which were included in subsequent computer simulations. The statistical flux balance analysis also suggests a stoichiometrically viable role for leucine transport as an alternative to glutamine for replenishing the glutamate pool in neurons.

Feasibility of direct mapping of cerebral fluorodeoxy-D-glucose metabolism in situ at subcellular resolution using soft X-ray fluorescence

Glucose metabolism is difficult to image with cellular resolution in mammalian brain tissue, particularly with (18) fluorodeoxy-D-glucose (FDG) positron emission tomography (PET). To this end, we explored the potential of synchrotron-based low-energy X-ray fluorescence (LEXRF) to image the stable isotope of fluorine (F) in phosphorylated FDG (DG-6P) at 1 μm(2) spatial resolution in 3-μm-thick brain slices. The excitation-dependent fluorescence F signal at 676 eV varied linearly with FDG concentration between 0.5 and 10 mM, whereas the endogenous background F signal was undetectable in brain. To validate LEXRF mapping of fluorine, FDG was administered in vitro and in vivo, and the fluorine LEXRF signal from intracellular trapped FDG-6P over selected brain areas rich in radial glia was spectrally quantitated at 1 μm(2) resolution. The subsequent generation of spatial LEXRF maps of F reproduced the expected localization and gradients of glucose metabolism in retinal Müller glia. In addition, FDG uptake was localized to periventricular hypothalamic tanycytes, whose morphological features were imaged simultaneously by X-ray absorption. We conclude that the high specificity of photon emission from F and its spatial mapping at ≤1 μm resolution demonstrates the ability to identify glucose uptake at subcellular resolution and holds remarkable potential for imaging glucose metabolism in biological tissue.