Role of specific aminotransferases in de novo glutamate synthesis and redox shuttling in the retina

Retinal metabolism displays evidence for uncoupling of glycolysis and oxidative phosphorylation via Cori-, Cahill-, and mini-Krebs-cycle

The retina consumes massive amounts of energy, yet its metabolism and substrate exploitation remain poorly understood. Here, we used a murine explant model to manipulate retinal energy metabolism under entirely controlled conditions and utilised 1H-NMR spectroscopy-based metabolomics, in situ enzyme detection, and cell viability readouts to uncover the pathways of retinal energy production. Our experimental manipulations resulted in varying degrees of photoreceptor degeneration, while the inner retina and retinal pigment epithelium were essentially unaffected. This selective vulnerability of photoreceptors suggested very specific adaptations in their energy metabolism. Rod photoreceptors were found to rely strongly on oxidative phosphorylation, but only mildly on glycolysis. Conversely, cone photoreceptors were dependent on glycolysis but insensitive to electron transport chain decoupling. Importantly, photoreceptors appeared to uncouple glycolytic and Krebs-cycle metabolism via three different pathways: (1) the mini-Krebs-cycle, fuelled by glutamine and branched chain amino acids, generating N-acetylaspartate; (2) the alanine-generating Cahill-cycle; (3) the lactate-releasing Cori-cycle. Moreover, the metabolomics data indicated a shuttling of taurine and hypotaurine between the retinal pigment epithelium and photoreceptors, likely resulting in an additional net transfer of reducing power to photoreceptors. These findings expand our understanding of retinal physiology and pathology and shed new light on neuronal energy homeostasis and the pathogenesis of neurodegenerative diseases.

Inaugurating a novel adjuvant therapy in urological cancers: Ferroptosis

Ferroptosis, a distinctive form of programmed cell death, is involved in numerous diseases with specific characteristics, including certain cell morphology, functions, biochemistry, and genetics, that differ from other forms of programmed cell death, such as apoptosis. Many studies have explored ferroptosis and its associated mechanisms, drugs, and clinical applications in diseases such as kidney injury, stroke, ischemia-reperfusion injury, and prostate cancer. In this review, we summarize the regulatory mechanisms of some ferroptosis inducers, such as enzalutamide and erastin. These are current research focuses and have already been studied extensively. In summary, this review focuses on the use of ferroptosis induction as a therapeutic strategy for treating tumors of the urinary system.

Implications of Diabetes-Induced Altered Metabolites on Retinal Neurodegeneration

Diabetic retinopathy (DR) is one of the major complications of diabetic eye diseases, causing vision loss and blindness worldwide. The concept of diabetic retinopathy has evolved from microvascular disease into more complex neurovascular disorders. Early in the disease progression of diabetes, the neuronal and glial cells are compromised before any microvascular abnormalities clinically detected by the ophthalmoscopic examination. This implies understanding the pathophysiological mechanisms at the early stage of disease progression especially due to diabetes-induced metabolic alterations to damage the neural retina so that early intervention and treatments options can be identified to prevent and inhibit the progression of DR. Hyperglycemia has been widely considered the major contributor to the progression of the retinal damage, even though tight control of glucose does not seem to have a bigger effect on the incidence or progression of retinal damage that leads to DR. Emerging evidence suggests that besides diabetes-induced hyperglycemia, dyslipidemia and amino acid defects might be a major contributor to the progression of early neurovascular retinal damage. In this review, we have discussed recent advances in the alterations of key metabolites of carbohydrate, lipid, and amino acids and their implications for neurovascular damage in DR.

An overview of branched-chain amino acid aminotransferases: functional differences between mitochondrial and cytosolic isozymes in yeast and human

Branched-chain amino acid aminotransferase (BCAT) catalyzes bidirectional transamination in the cell between branched-chain amino acids (BCAAs; valine, leucine, and isoleucine) and branched-chain α-keto acids (BCKAs; α-ketoisovalerate, α-ketoisocaproate, and α-keto-β-methylvalerate). Eukaryotic cells contain two types of paralogous BCATs: mitochondrial BCAT (BCATm) and cytosolic BCAT (BCATc). Both isozymes have identical enzymatic functions, so they have long been considered to perform similar physiological functions in the cells. However, many studies have gradually revealed the differences in physiological functions and regulatory mechanisms between them. In this article, we present overviews of BCATm and BCATc in both yeast and human. We also introduce BCAT variants found natively or constructed artificially, which could have significant implications for research into the relationship between the primary structures and protein functions of BCATs. KEY POINTS: • BCAT catalyzes bidirectional transamination in the cell between BCAAs and BCKAs. • BCATm and BCATc are different in the metabolic roles and regulatory mechanisms. • BCAT variants offer insight into a relationship between the structure and function.

Emerging Moonlighting Functions of the Branched-Chain Aminotransferase Proteins

Significance: Unique to the branched-chain aminotransferase (BCAT) proteins is their redox-active CXXC motif. Subjected to post-translational modification by reactive oxygen species and reactive nitrogen species, these proteins have the potential to adopt numerous cellular roles, which may be fundamental to their role in oncogenesis and neurodegenerative diseases. An understanding of the interplay of the redox regulation of BCAT with important cell signaling mechanisms will identify new targets for future therapeutics. Recent Advances: The BCAT proteins have been assigned novel thiol oxidoreductase activity that can accelerate the refolding of proteins, in particular when S-glutathionylated, supporting a chaperone role for BCAT in protein folding. Other metabolic proteins were also shown to have peroxide-mediated redox associations with BCAT, indicating that the cellular function of BCAT is more diverse. Critical Issues: While the role of branched-chain amino acid metabolism and its metabolites has dominated aspects of cancer research, less is known about the role of BCAT. The importance of the CXXC motif in regulating the BCAT activity under hypoxic conditions, a characteristic of tumors, has not been addressed. Understanding how these proteins operate under various cellular redox conditions will become important, in particular with respect to their moonlighting roles. Future Directions: Advances in the quantification of thiols, their measurement, and the manipulation of metabolons that rely on redox-based interactions should accelerate the investigation of the cellular role of moonlighting proteins such as BCAT. Given the importance of cross talk between signaling pathways, research should focus more on these "housekeeping" proteins paying attention to their wider application. Antioxid. Redox Signal. 34, 1048-1067.

Branched-chain amino acid aminotransferase 2 regulates ferroptotic cell death in cancer cells

Ferroptosis, a form of iron-dependent cell death driven by cellular metabolism and iron-dependent lipid peroxidation, has been implicated as a tumor-suppressor function for cancer therapy. Recent advance revealed that the sensitivity to ferroptosis is tightly linked to numerous biological processes, including metabolism of amino acid and the biosynthesis of glutathione. Here, by using a high-throughput CRISPR/Cas9-based genetic screen in HepG2 hepatocellular carcinoma cells to search for metabolic proteins inhibiting ferroptosis, we identified a branched-chain amino acid aminotransferase 2 (BCAT2) as a novel suppressor of ferroptosis. Mechanistically, ferroptosis inducers (erastin, sorafenib, and sulfasalazine) activated AMPK/SREBP1 signaling pathway through iron-dependent ferritinophagy, which in turn inhibited BCAT2 transcription. We further confirmed that BCAT2 as the key enzyme mediating the metabolism of sulfur amino acid, regulated intracellular glutamate level, whose activation by ectopic expression specifically antagonize system Xc^– inhibition and protected liver and pancreatic cancer cells from ferroptosis in vitro and in vivo. On the contrary, direct inhibition of BCAT2 by RNA interference, or indirect inhibition by blocking system Xc^– activity, triggers ferroptosis. Finally, our results demonstrate the synergistic effect of sorafenib and sulfasalazine in downregulating BCAT2 expression and dictating ferroptotic death, where BCAT2 can also be used to predict the responsiveness of cancer cells to ferroptosis-inducing therapies. Collectively, these findings identify a novel role of BCAT2 in ferroptosis, suggesting a potential therapeutic strategy for overcoming sorafenib resistance.

The Role of Amino Acids in Neurotransmission and Fluorescent Tools for Their Detection

Neurotransmission between neurons, which can occur over the span of a few milliseconds, relies on the controlled release of small molecule neurotransmitters, many of which are amino acids. Fluorescence imaging provides the necessary speed to follow these events and has emerged as a powerful technique for investigating neurotransmission. In this review, we highlight some of the roles of the 20 canonical amino acids, GABA and β-alanine in neurotransmission. We also discuss available fluorescence-based probes for amino acids that have been shown to be compatible for live cell imaging, namely those based on synthetic dyes, nanostructures (quantum dots and nanotubes), and genetically encoded components. We aim to provide tool developers with information that may guide future engineering efforts and tool users with information regarding existing indicators to facilitate studies of amino acid dynamics.

BCAT-induced autophagy regulates Aβ load through an interdependence of redox state and PKC phosphorylation-implications in Alzheimer's disease

Leucine, nutrient signal and substrate for the branched chain aminotransferase (BCAT) activates the mechanistic target of rapamycin (mTORC1) and regulates autophagic flux, mechanisms implicated in the pathogenesis of neurodegenerative conditions such as Alzheimer's disease (AD). BCAT is upregulated in AD, where a moonlighting role, imparted through its redox-active CXXC motif, has been suggested. Here we demonstrate that the redox state of BCAT signals differential phosphorylation by protein kinase C (PKC) regulating the trafficking of cellular pools of BCAT. We show inter-dependence of BCAT expression and proteins associated with the P13K/Akt/mTORC1 and autophagy signalling pathways. In response to insulin or an increase in ROS, BCATc is trafficked to the membrane and docks via palmitoylation, which is associated with BCATc-induced autophagy through PKC phosphorylation. In response to increased levels of BCATc, as observed in AD, amyloid β (Aβ) levels accumulate due to a shift in autophagic flux. This effect was diminished when incubated with leucine, indicating that dietary levels of amino acids show promise in regulating Aβ load. Together these findings show that increased BCATc expression, reported in human AD brain, will affect autophagy and Aβ load through the interdependence of its redox-regulated phosphorylation offering a novel target to address AD pathology.

Alzheimer's disease: targeting the glutamatergic system

Alzheimer’s disease (AD) is a debilitating neurodegenerative disease that causes a progressive decline in memory, language and problem solving. For decades mechanism-based therapies have primarily focused on amyloid β (Aβ) processing and pathways that govern neurofibrillary tangle generation. With the potential exception to Aducanumab, a monotherapy to target Aβ, clinical trials in these areas have been challenging and have failed to demonstrate efficacy. Currently, the prescribed therapies for AD are those that target the cholinesterase and glutamatergic systems that can moderately reduce cognitive decline, dependent on the individual. In the brain, over 40% of neuronal synapses are glutamatergic, where the glutamate level is tightly regulated through metabolite exchange in neuronal, astrocytic and endothelial cells. In AD brain, Aβ can interrupt effective glutamate uptake by astrocytes, which evokes a cascade of events that leads to neuronal swelling, destruction of membrane integrity and ultimately cell death. Much work has focussed on the post-synaptic response with little insight into how glutamate is regulated more broadly in the brain and the influence of anaplerotic pathways that finely tune these mechanisms. The role of blood branched chain amino acids (BCAA) in regulating neurotransmitter profiles under disease conditions also warrant discussion. Here, we review the importance of the branched chain aminotransferase proteins in regulating brain glutamate and the potential consequence of dysregulated metabolism in the context of BCAA or glutamate accumulation. We explore how the reported benefits of BCAA supplementation or restriction in improving cognitive function in other neurological diseases may have potential application in AD. Given that memantine, the glutamate receptor agonist, shows clinical relevance it is now timely to research related pathways, an understanding of which could identify novel approaches to treatment of AD.

The retina and retinal pigment epithelium differ in nitrogen metabolism and are metabolically connected

Defects in energy metabolism in either the retina or the immediately adjacent retinal pigment epithelium (RPE) underlie retinal degeneration, but the metabolic dependence between retina and RPE remains unclear. Nitrogen-containing metabolites such as amino acids are essential for energy metabolism. Here, we found that ¹⁵N-labeled ammonium is predominantly assimilated into glutamine in both the retina and RPE/choroid ex vivo [¹⁵N]Ammonium tracing in vivo show that, like the brain, the retina can synthesize asparagine from ammonium, but RPE/choroid and the liver cannot. However, unless present at toxic concentrations, ammonium cannot be recycled into glutamate in the retina and RPE/choroid. Tracing with ¹⁵N-labeled amino acids show that the retina predominantly uses aspartate transaminase for de novo synthesis of glutamate, glutamine, and aspartate, whereas RPE uses multiple transaminases to utilize and synthesize amino acids. Retina consumes more leucine than RPE, but little leucine is catabolized. The synthesis of serine and glycine is active in RPE but limited in the retina. RPE, but not the retina, uses alanine as mitochondrial substrates through mitochondrial pyruvate carrier. However, when the mitochondrial pyruvate carrier is inhibited, alanine may directly enter the retinal mitochondria but not those of RPE. In conclusion, our results demonstrate that the retina and RPE differ in nitrogen metabolism and highlight that the RPE supports retinal metabolism through active amino acid metabolism.

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.

Role of l-Type Amino Acid Transporter 1 at the Inner Blood-Retinal Barrier in the Blood-to-Retina Transport of Gabapentin

Gabapentin is an antiseizure drug that is known to also have beneficial effects on the retinal cells. To use gabapentin in retinal pharmacotherapy, it is critical to understand gabapentin distribution in the retina. The purpose of this study was to clarify the kinetics of gabapentin influx transport across the inner and outer blood-retinal barrier (BRB), which regulates the exchange of compounds/drugs between the circulating blood and the retina. In vivo blood-to-retina gabapentin transfer was evaluated by the rat carotid artery injection technique. In addition, gabapentin transport was examined using in vitro models of the inner (TR-iBRB2 cells) and outer BRB (RPE-J cells). The in vivo [³H]gabapentin transfer to the rat retina across the BRB was significantly reduced in the presence of unlabeled gabapentin, suggesting transporter-mediated blood-to-retina distribution of gabapentin. Substrates of the Na⁺-independent l-type amino acid transporter 1 (LAT1), such as 2-aminobicyclo[2.2.1]heptane-2-carboxylic acid (BCH), also significantly inhibited the in vivo [³H]gabapentin transfer. [³H]Gabapentin uptake in TR-iBRB2 and RPE-J cells exhibited Na⁺-independent and saturable kinetics with a K_m of 735 and 507 μM, respectively. Regarding the effect of various transporter substrates/inhibitors on gabapentin transport in these cells, LAT1 substrates significantly inhibited [³H]gabapentin uptake in TR-iBRB2 and RPE-J cells. In addition, preloaded [³H]gabapentin release from TR-iBRB2 and RPE-J cells was trans-stimulated by LAT1 substrates through the obligatory exchange mechanism as LAT1. Immunoblot analysis indicates the protein expression of LAT1 in TR-iBRB2 and RPE-J cells. These results imply that LAT1 at the inner and outer BRB takes part in gabapentin transport between the circulating blood and retina. Moreover, treatment of LAT1-targeted small interfering RNA to TR-iBRB2 cells significantly reduced both the level of LAT1 protein expression and [³H]gabapentin uptake activities in TR-iBRB2 cells. In conclusion, data from the present study indicate that LAT1 at the inner BRB is involved in retinal gabapentin transfer, and also suggest that LAT1 mediates gabapentin transport in the RPE cells.

Altered Expression of Human Mitochondrial Branched Chain Aminotransferase in Dementia with Lewy Bodies and Vascular Dementia

Cytosolic and mitochondrial human branched chain aminotransferase (hBCATc and hBCATm, respectively) play an integral role in brain glutamate metabolism. Regional increased levels of hBCATc in the CA1 and CA4 region of Alzheimer’s disease (AD) brain together with increased levels of hBCATm in frontal and temporal cortex of AD brains, suggest a role for these proteins in glutamate excitotoxicity. Glutamate toxicity is a key pathogenic feature of several neurological disorders including epilepsy associated dementia, AD, vascular dementia (VaD) and dementia with Lewy bodies (DLB). To further understand if these increases are specific to AD, the expression profiles of hBCATc and hBCATm were examined in other forms of dementia including DLB and VaD. Similar to AD, levels of hBCATm were significantly increased in the frontal and temporal cortex of VaD cases and in frontal cortex of DLB cases compared to controls, however there were no observed differences in hBCATc between groups in these areas. Moreover, multiple forms of hBCATm were observed that were particular to the disease state relative to matched controls. Real-time PCR revealed similar expression of hBCATm mRNA in frontal and temporal cortex for all cohort comparisons, whereas hBCATc mRNA expression was significantly increased in VaD cases compared to controls. Collectively our results suggest that hBCATm protein expression is significantly increased in the brains of DLB and VaD cases, similar to those reported in AD brain. These findings indicate a more global response to altered glutamate metabolism and suggest common metabolic responses that might reflect shared neurodegenerative mechanisms across several forms of dementia.

Effective disposal of nitrogen waste in blood-fed Aedes aegypti mosquitoes requires alanine aminotransferase

To better understand the mechanisms responsible for the success of female mosquitoes in their disposal of excess nitrogen, we investigated the role of alanine aminotransferase (ALAT) in blood-fed Aedes aegypti. Transcript and protein levels from the 2 ALAT genes were analyzed in sucrose- and blood-fed A. aegypti tissues. ALAT1 and ALAT2 exhibit distinct expression patterns in tissues during the first gonotrophic cycle. Injection of female mosquitoes with either double-stranded RNA (dsRNA)-ALAT1 or dsRNA ALAT2 significantly decreased mRNA and protein levels of ALAT1 or ALAT2 in fat body, thorax, and Malpighian tubules compared with dsRNA firefly luciferase-injected control mosquitoes. The silencing of either A. aegypti ALAT1 or ALAT2 caused unexpected phenotypes such as a delay in blood digestion, a massive accumulation of uric acid in the midgut posterior region, and a significant decrease of nitrogen waste excretion during the first 48 h after blood feeding. Concurrently, the expression of genes encoding xanthine dehydrogenase and ammonia transporter (Rhesus 50 glycoprotein) were significantly increased in tissues of both ALAT1- and ALAT2-deficient females. Moreover, perturbation of ALAT1 and ALAT2 in the female mosquitoes delayed oviposition and reduced egg production. These novel findings underscore the efficient mechanisms that blood-fed mosquitoes use to avoid ammonia toxicity and free radical damage.-Mazzalupo, S., Isoe, J., Belloni, V., Scaraffia, P. Y. Effective disposal of nitrogen waste in blood-fed Aedes aegypti mosquitoes requires alanine aminotransferase.

New insights into the role of the branched-chain aminotransferase proteins in the human brain

The human cytosolic branched-chain aminotransferase (hBCATc) enzyme is strategically located in glutamatergic neurons, where it is thought to provide approximately 30% of de novo nitrogen for brain glutamate synthesis. In health, glutamate plays a dominant role in facilitating learning and memory. However, in patients with Alzheimer's disease (AD), synaptic levels of glutamate become toxic, resulting in a direct increase in postsynaptic neuronal calcium, causing a cascade of events that contributes to the destruction of neuronal integrity and cell death, pathological features of AD. Our group is the first to map the hBCAT proteins to the human brain, where cell-specific compartmentation indicates key roles for these proteins in regulating glutamate homeostasis. Moreover, increased expression of hBCAT was observed in the brains of patients with AD relative to matched controls. We reflect on the importance of the redox-active CXXC motif, which confers novel roles for the hBCAT proteins, particularly with respect to substrate channeling and protein folding. This implies that, in addition to their role in glutamate metabolism, these proteins have additional functional roles that might impact redox cell signaling. This review discusses how these proteins behave as potential neuroprotectors during periods of oxidative stress. These findings are particularly important because an increase in misfolded proteins, linked to increased oxidative stress, occurs in several neurodegenerative conditions. Together, these studies give an overview of the diverse role that these proteins play in brain metabolism, in which a dysregulation of their expression may contribute to neurodegenerative conditions such as AD.

Exposure to L-cycloserine incurs survival costs and behavioral alterations in Aedes aegypti females

Background

It was previously demonstrated that alanine aminotransferase (ALAT, EC 2.6.1.2) participates in maintaining the alanine-proline cycle between flight muscles and fat body during Aedes aegypti flight. ALAT is also actively involved in the metabolism of ammonia in A. aegypti. Here, we investigated the survival and behavioral costs of ALAT inhibition in A. aegypti females to better understand the role of ALAT in blood-fed mosquitoes.

Methods

We analyzed how A. aegypti female mosquitoes respond to blood meals supplemented with 0, 2.5, 5 and 10 mM L-cycloserine, a well-known inhibitor of ALAT in animals. Mosquitoes were also exposed to blood meals supplemented with L-cycloserine and different concentrations of glucose (0, 10 and 100 mM). Additionally, the effects of ALAT inhibitor and glucose in mosquitoes starved for 24 or 48 h were investigated. Survival and behavioral phenotypes were analyzed during a time course (1, 2, 4, 6, 12, 24, 48 and 72 h after feeding).

Results

L-cycloserine at 10 mM resulted in high mortality relative to control, with an acute effect during the first 6 h after treatment. A significant decrease in the number of active mosquitoes coinciding with an increase in futile wing fanning during the first 24 h was observed at all inhibitor concentrations. A high occurrence of knockdown phenotype was also recorded at this time for both 5 and 10 mM L-cycloserine. The supplementation of glucose in the blood meal amplified the effects of the ALAT inhibitor. In particular, we observed a higher mortality rate concomitant with an increase in the knockdown phenotype. Starvation prior to blood feeding also increased the effects of L-cycloserine with a rapid increase in mortality.

Conclusions

Our results provide evidence that exposure of high doses of L-cycloserine during A. aegypti blood feeding affects mosquito survival and motor activity, suggesting an interference with carbohydrate and ammonia metabolism in a time-dependent manner.

Electronic supplementary material

The online version of this article (doi:10.1186/1756-3305-7-373) contains supplementary material, which is available to authorized users.

Inhibition of mitochondrial pyruvate transport by zaprinast causes massive accumulation of aspartate at the expense of glutamate in the retina

Transport of pyruvate into mitochondria by the mitochondrial pyruvate carrier is crucial for complete oxidation of glucose and for biosynthesis of amino acids and lipids. Zaprinast is a well known phosphodiesterase inhibitor and lead compound for sildenafil. We found Zaprinast alters the metabolomic profile of mitochondrial intermediates and amino acids in retina and brain. This metabolic effect of Zaprinast does not depend on inhibition of phosphodiesterase activity. By providing (13)C-labeled glucose and glutamine as fuels, we found that the metabolic profile of the Zaprinast effect is nearly identical to that of inhibitors of the mitochondrial pyruvate carrier. Both stimulate oxidation of glutamate and massive accumulation of aspartate. Moreover, Zaprinast inhibits pyruvate-driven O2 consumption in brain mitochondria and blocks mitochondrial pyruvate carrier in liver mitochondria. Inactivation of the aspartate glutamate carrier in retina does not attenuate the metabolic effect of Zaprinast. Our results show that Zaprinast is a potent inhibitor of mitochondrial pyruvate carrier activity, and this action causes aspartate to accumulate at the expense of glutamate. Our findings show that Zaprinast is a specific mitochondrial pyruvate carrier (MPC) inhibitor and may help to elucidate the roles of MPC in amino acid metabolism and hypoglycemia.

GABA and Glutamate Uptake and Metabolism in Retinal Glial (Müller) Cells

Müller cells, the principal glial cells of the retina, support the synaptic activity by the uptake and metabolization of extracellular neurotransmitters. Müller cells express uptake and exchange systems for various neurotransmitters including glutamate and γ-aminobutyric acid (GABA). Müller cells remove the bulk of extracellular glutamate in the inner retina and contribute to the glutamate clearance around photoreceptor terminals. By the uptake of glutamate, Müller cells are involved in the shaping and termination of the synaptic activity, particularly in the inner retina. Reactive Müller cells are neuroprotective, e.g., by the clearance of excess extracellular glutamate, but may also contribute to neuronal degeneration by a malfunctioning or even reversal of glial glutamate transporters, or by a downregulation of the key enzyme, glutamine synthetase. This review summarizes the present knowledge about the role of Müller cells in the clearance and metabolization of extracellular glutamate and GABA. Some major pathways of GABA and glutamate metabolism in Müller cells are described; these pathways are involved in the glutamate-glutamine cycle of the retina, in the defense against oxidative stress via the production of glutathione, and in the production of substrates for the neuronal energy metabolism.

Distribution of the branched chain aminotransferase proteins in the human brain and their role in glutamate regulation

The branched chain aminotransferase enzymes (BCAT) serve as nitrogen donors for the production of 30% of de novo glutamate synthesis in rat brain. Despite the importance of this major metabolite and excitatory neurotransmitter, the distribution of BCAT proteins in the human brain (hBCAT) remains unreported. We have studied this and report, for the first time, that the mitochondrial isoform, hBCATm is largely confined to vascular endothelial cells, whereas the cytosolic hBCATc is restricted to neurons. The majority of hBCATc-labelled neurons were either GABA-ergic or glutamatergic showing both cell body and axonal staining indicating a role for hBCATc in both glutamate production and glutamate release during excitation. Strong staining in hormone secreting cells suggests a further role for the transaminases in hormone regulation potentially similar to that proposed for insulin secretion. Expression of hBCATm in the endothelial cells of the vasculature demonstrates for the first time that glutamate could be metabolized by aminotranferases in these cells. This has important implications given that the dysregulation of glutamate metabolism, leading to glutamate excitotoxicity, is an important contributor to the pathogenesis of several neurodegenerative conditions, where the role of hBCATm in metabolizing excess glutamate may factor more prominently.

Is there in vivo evidence for amino acid shuttles carrying ammonia from neurons to astrocytes?

The high in vivo flux of the glutamate/glutamine cycle puts a strong demand on the return of ammonia released by phosphate activated glutaminase from the neurons to the astrocytes in order to maintain nitrogen balance. In this paper we review several amino acid shuttles that have been proposed for balancing the nitrogen flows between neurons and astrocytes in the glutamate/glutamine cycle. All of these cycles depend on the directionality of glutamate dehydrogenase, catalyzing reductive glutamate synthesis (forward reaction) in the neuron in order to capture the ammonia released by phosphate activated glutaminase, while catalyzing oxidative deamination of glutamate (reverse reaction) in the astrocytes to release ammonia for glutamine synthesis. Reanalysis of results from in vivo experiments using (13)N and (15)N labeled ammonia and (15)N leucine in rats suggests that the maximum flux of the alanine/lactate or branched chain amino acid/branched chain amino acid transaminase shuttles between neurons and astrocytes are approximately 3-5 times lower than would be required to account for the ammonia transfer from neurons to astrocytes needed for glutamine synthesis (amide nitrogen) to sustain the glutamate/glutamine cycle. However, in the rat brain both the total ammonia fixation rate by glutamate dehydrogenase and the total branched chain amino acid transaminase activity are sufficient to support a branched chain amino acid/branched chain keto acid shuttle, as proposed by Hutson and coworkers, which would support the de novo synthesis of glutamine in the astrocyte to replace the ~20 % of neurotransmitter glutamate that is oxidized. A higher fraction of the nitrogen needs of total glutamate neurotransmitter cycling could be supported by hybrid cycles in which glutamate and tricarboxylic acid cycle intermediates act as a nitrogen shuttle. A limitation of all in vivo studies in animals conducted to date is that none have shown transfer of nitrogen for glutamine amide synthesis, either as free ammonia or via an amino acid from the neurons to the astrocytes. Future work will be needed, perhaps using methods for selectively labeling nitrogen in neurons, to conclusively establish the rate of amino acid nitrogen shuttles in vivo and their coupling to the glutamate/glutamine cycle.

Expression of mitochondrial branched-chain aminotransferase and α-keto-acid dehydrogenase in rat brain: implications for neurotransmitter metabolism

In the brain, metabolism of the essential branched chain amino acids (BCAAs) leucine, isoleucine, and valine, is regulated in part by protein synthesis requirements. Excess BCAAs are catabolized or excreted. The first step in BCAA catabolism is catalyzed by the branched chain aminotransferase (BCAT) isozymes, mitochondrial BCATm and cytosolic BCATc. A product of this reaction, glutamate, is the major excitatory neurotransmitter and precursor of the major inhibitory neurotransmitter γ-aminobutyric acid (GABA). The BCATs are thought to participate in a α-keto-acid nitrogen shuttle that provides nitrogen for synthesis of glutamate from α-ketoglutarate. The branched-chain α-keto acid dehydrogenase enzyme complex (BCKDC) catalyzes the second, irreversible step in BCAA metabolism, which is oxidative decarboxylation of the branched-chain α-keto acid (BCKA) products of the BCAT reaction. Maple Syrup Urine Disease (MSUD) results from genetic defects in BCKDC, which leads to accumulation of toxic levels of BCAAs and BCKAs that result in brain swelling. Immunolocalization of BCATm and BCKDC in rats revealed that BCATm is present in astrocytes in white matter and in neuropil, while BCKDC is expressed only in neurons. BCATm appears uniformly distributed in astrocyte cell bodies throughout the brain. The segregation of BCATm to astrocytes and BCKDC to neurons provides further support for the existence of a BCAA-dependent glial-neuronal nitrogen shuttle since the data show that BCKAs produced by glial BCATm must be exported to neurons. Additionally, the neuronal localization of BCKDC suggests that MSUD is a neuronal defect involving insufficient oxidation of BCKAs, with secondary effects extending beyond the neuron.

Regulation of glutamate metabolism by hydrocortisone and branched chain keto acids in cultured rat retinal Müller cells (TR-MUL)

Glutamate released from retinal neurons during neurotransmission is taken up by retinal Müller cells, where much of the amino acid is subsequently amidated to glutamine or transaminated to α-ketoglutarate for oxidation. Müller cell glutamate levels may have to be carefully maintained at fairly low concentrations to avoid excesses of glutamate in extracellular spaces of the retina that would otherwise cause excitotoxicity. We employed a cultured rat retinal Müller cell line in order to study the metabolism and the role of Müller cell specific enzymes on the glutamate disposal pathways. We found that the TR-MUL cells express the glial specific enzymes, glutamine synthetase, the mitochondrial isoform of branched chain aminotransferase (BCATm) and pyruvate carboxylase, all of which are involved in glutamate metabolism and homeostasis in the retina. Hydrocortisone treatment of TR-MUL cells increased glutamine synthetase expression and the rate of glutamate amidation to glutamine. Addition of branched chain keto acids (BCKAs) increased lactate and aspartate formation from glutamate and also oxidation of glutamate to CO(2) and H(2)O. The two glutamate disposal pathways (amidation and oxidation) did not influence each other. When glutamate levels were independently depleted within TR-MUL cells, the uptake of glutamate from the extracellular fluid increased compared to uptake from control (undepleted) cells suggesting that the level of intracellular glutamate may influence clearing of extracellular glutamate.

The significance of vascular and neural apoptosis to the pathology of diabetic retinopathy

The most striking features of diabetic retinopathy are the vascular abnormalities that are apparent by fundus examination. There is also strong evidence that diabetes causes apoptosis of neural and vascular cells in the retina. Thus, there is good reason to define diabetic retinopathy as a form of chronic neurovascular degeneration. In keeping with the gradual onset of retinopathy in humans, the rate of cell loss in the animal models is insidious, even in uncontrolled diabetes. This is not surprising given that a sustained high rate of cell loss without regeneration would soon lead to catastrophic tissue destruction. The consequences of ongoing cell death are difficult to detect, and even the quantification of cumulative cell loss requires painstaking histology and microscopy. This subtle cell loss raises the issue of the relevance of the phenomenon to the progression of diabetic retinopathy and the ultimate loss of vision. Neuronal function may be compromised in advance of apoptosis, contributing to an early deterioration of vision. Here we review some of the evidence supporting apoptotic cell death as a contributing mechanism of diabetic retinopathy, explore some of the potential causes, and discuss the potential links between apoptosis and loss of visual function in diabetic retinopathy.

Metabolic fingerprint of ischaemic cardioprotection: importance of the malate-aspartate shuttle

The convergence of cardioprotective intracellular signalling pathways to modulate mitochondrial function as an end-target of cytoprotective stimuli is well described. However, our understanding of whether the complementary changes in mitochondrial energy metabolism are secondary responses or inherent mechanisms of ischaemic cardioprotection remains incomplete. In the heart, the malate-aspartate shuttle (MAS) constitutes the primary metabolic pathway for transfer of reducing equivalents from the cytosol into the mitochondria for oxidation. The flux of MAS is tightly linked to the flux of the tricarboxylic acid cycle and the electron transport chain, partly by the amino acid l-glutamate. In addition, emerging evidence suggests the MAS is an important regulator of cytosolic and mitochondrial calcium homeostasis. In the isolated rat heart, inhibition of MAS during ischaemia and early reperfusion by the aminotransferase inhibitor aminooxyacetate induces infarct limitation, improves haemodynamic responses, and modulates glucose metabolism, analogous to effects observed in classical ischaemic preconditioning. On the basis of these findings, the mechanisms through which MAS preserves mitochondrial function and cell survival are reviewed. We conclude that the available evidence is supportive of a down-regulation of mitochondrial respiration during lethal ischaemia with a gradual 'wake-up' during reperfusion as a pivotal feature of ischaemic cardioprotection. Finally, comments on modulating myocardial energy metabolism by the cardioprotective amino acids glutamate and glutamine are given.

Transamination is required for {alpha}-ketoisocaproate but not leucine to stimulate insulin secretion

It remains unclear how α-ketoisocaproate (KIC) and leucine are metabolized to stimulate insulin secretion. Mitochondrial BCATm (branched-chain aminotransferase) catalyzes reversible transamination of leucine and α-ketoglutarate to KIC and glutamate, the first step of leucine catabolism. We investigated the biochemical mechanisms of KIC and leucine-stimulated insulin secretion (KICSIS and LSIS, respectively) using BCATm(-/-) mice. In static incubation, BCATm disruption abolished insulin secretion by KIC, D,L-α-keto-β-methylvalerate, and α-ketocaproate without altering stimulation by glucose, leucine, or α-ketoglutarate. Similarly, during pancreas perfusions in BCATm(-/-) mice, glucose and arginine stimulated insulin release, whereas KICSIS was largely abolished. During islet perifusions, KIC and 2 mM glutamine caused robust dose-dependent insulin secretion in BCATm(+/+) not BCATm(-/-) islets, whereas LSIS was unaffected. Consistently, in contrast to BCATm(+/+) islets, the increases of the ATP concentration and NADPH/NADP(+) ratio in response to KIC were largely blunted in BCATm(-/-) islets. Compared with nontreated islets, the combination of KIC/glutamine (10/2 mM) did not influence α-ketoglutarate concentrations but caused 120 and 33% increases in malate in BCATm(+/+) and BCATm(-/-) islets, respectively. Although leucine oxidation and KIC transamination were blocked in BCATm(-/-) islets, KIC oxidation was unaltered. These data indicate that KICSIS requires transamination of KIC and glutamate to leucine and α-ketoglutarate, respectively. LSIS does not require leucine catabolism and may be through leucine activation of glutamate dehydrogenase. Thus, KICSIS and LSIS occur by enhancing the metabolism of glutamine/glutamate to α-ketoglutarate, which, in turn, is metabolized to produce the intracellular signals such as ATP and NADPH for insulin secretion.

Involvement of LAT1 and LAT2 in the high- and low-affinity transport of L-leucine in human retinal pigment epithelial cells (ARPE-19 cells)

System L, which is encoded by LAT1 and LAT2, is an amino acid transport system that transports neutral amino acids, including several essential amino acids in an Na+-independent manner. Due to its broad substrate selectivity, system L has been proposed to mediate the transport of amino-acid-related drugs across the blood-tissue barriers. We characterized L-leucine transport and its corresponding transporter in a human retinal pigment epithelial cell line (ARPE-19 cells) as an in vitro model of the outer blood-retinal barrier. [3H]L-leucine uptake by ARPE-19 cells took place in an Na+-, Cl(-)-independent and saturable manner with K(m) values of 8.71 and 220 microM. This process was more potently cis-inhibited by substrates of LAT1 than those of LAT2. [3H]L-leucine efflux from ARPE-19 cells was trans-stimulated by substrates of LAT1 and LAT2 through the obligatory exchange mechanism of system L. Although RT-PCR analysis demonstrated that LAT1 and LAT2 mRNA are expressed in ARPE-19 cells, the LAT1 mRNA concentration is 42-fold higher than that of LAT2. Moreover, immunoblot analysis demonstrated that LAT1 is expressed in ARPE-19 cells. In conclusion, although the transport function of LAT1 is greater than that of LAT2, LAT1 and LAT2 are involved in L-leucine transport in ARPE-19 cells.

Inner blood-retinal barrier transporters: role of retinal drug delivery

The inner blood-retinal barrier (inner BRB) forms complex tight junctions of retinal capillary endothelial cells to prevent the free diffusion of substances between the circulating blood and the neural retina. Thus, understanding of the inner BRB transport mechanisms could provide a basis for the development of strategies for drug delivery to the retina. Recent progress in inner BRB research has revealed that retinal endothelial cells express a variety of unique transporters which play a role in the influx transport of essential molecules and the efflux transport of xenobiotics. In this review we focus on the transport mechanism at the inner BRB in relation to its importance in influencing the inner BRB permeability of drugs.

Role of aralar, the mitochondrial transporter of aspartate-glutamate, in brain N-acetylaspartate formation and Ca(2+) signaling in neuronal mitochondria

Aralar, the Ca(2+)-dependent mitochondrial aspartate-glutamate carrier expressed in brain and skeletal muscle, is a member of the malate-aspartate NADH shuttle. Disrupting the gene for aralar, SLC25a12, in mice has enabled the discovery of two new roles of this carrier. On the one hand, it is required for synthesis of brain aspartate and N-acetylaspartate, a neuron-born metabolite that supplies acetate for myelin lipid synthesis; and on the other, it is essential for the transmission of small Ca(2+) signals to mitochondria via an increase in mitochondrial NADH.

Mitochondrial transport proteins of the brain

In this study, cellular distribution and activity of glutamate and gamma-aminobutyric acid (GABA) transport as well as oxoglutarate transport across brain mitochondrial membranes were investigated. A goal was to establish cell-type-specific expression of key transporters and enzymes involved in neurotransmitter metabolism in order to estimate neurotransmitter and metabolite traffic between neurons and astrocytes. Two methods were used to isolate brain mitochondria. One method excludes synaptosomes and the organelles may therefore be enriched in astrocytic mitochondria. The other method isolates mitochondria derived from all regions of the brain. Immunological and enzymatic methods were used to measure enzymes and carriers in the different preparations, in addition to studying transport kinetics. Immunohistochemistry was also employed using brain slices to confirm cell type specificity of enzymes and carriers. The data suggest that the aspartate/glutamate carriers (AGC) are expressed predominantly in neurons, not astrocytes, and that one of two glutamate/hydroxyl carriers is expressed predominantly in astrocytes. The GABA carrier and the oxoglutarate carrier appear to be equally distributed in astrocytes and neurons. As expected, pyruvate carboxylase and branched-chain aminotransferase were predominantly astrocytic. Insofar as the aspartate/glutamate exchange carriers are required for the malate/aspartate shuttle and for reoxidation of cytosolic NADH, the data suggest a compartmentation of glucose metabolism in which astrocytes catalyze glycolytic conversion of glucose to lactate, whereas neurons are capable of oxidizing both lactate and glucose to CO(2) + H(2)O.

Functional activity of a large neutral amino acid transporter (LAT) in rabbit retina: A study involving the in vivo retinal uptake and vitreal pharmacokinetics of l-phenyl alanine

PURPOSE

The purpose of this study is to elucidate the functional activity of large neutral amino acid transporter (LAT) in rabbit retina and to delineate its role in the retinal uptake and intravitreal pharmacokinetics of L-phenylalanine (L-Phe).

METHODS

In vivo retinal uptake of L-Phe and L-alanine (L-Ala) was determined in the presence and absence of specific transport inhibitors following intravitreal administration. L and D isomers of amino acids were employed as inhibitors to determine the stereo-selectivity of LAT. Reverse transcription-polymerase chain reaction (RT-PCR) was carried out for LAT isoforms (LAT1 and LAT2). Vitreal disposition of L-Phe following administration in rabbit vitreous was studied in the presence of an other competing LAT substrate D-methionine using microdialysis.

RESULTS

Retinal uptake of L-Phe was significantly inhibited in presence of a specific LAT inhibitor, 2-aminobicyclo-[2,2,1]-heptane-2-carboxylic acid (BCH), L isomers of large neutral amino acids and LAT1 specific but not by LAT2 specific and charged amino acids. No significant inhibition of L-Ala retinal uptake was observed with LAT substrates. LAT isoforms (LAT1 and LAT2) were identified by RT-PCR in rabbit retina. The mean residence time (MRT) and area under curve (AUC) values of L-Phe following intravitreal administration were significantly increased in the presence of D-methionine, a LAT substrate.

CONCLUSIONS

This study demonstrates the functional activity and molecular expression of large neutral amino acid transporter in the rabbit retina. Furthermore, based on these studies it can be concluded that LAT is involved in the retinal uptake and intravitreal elimination of L-Phe.

Energy sources for glutamate neurotransmission in the retina: absence of the aspartate/glutamate carrier produces reliance on glycolysis in glia

The mitochondrial transporter, the aspartate/glutamate carrier (AGC), is a necessary component of the malate/aspartate cycle, which promotes the transfer into mitochondria of reducing equivalents generated in the cytosol during glycolysis. Without transfer of cytosolic reducing equivalents into mitochondria, neither glucose nor lactate can be completely oxidized. In the present study, immunohistochemistry was used to demonstrate the absence of AGC from retinal glia (Müller cells), but its presence in neurons and photoreceptor cells. To determine the influence of the absence of AGC on sources of ATP for glutamate neurotransmission, neurotransmission was estimated in both light- and dark-adapted retinas by measuring flux through the glutamate/glutamine cycle and the effect of light on ATP-generating reactions. Neurotransmission was 80% faster in the dark as expected, because photoreceptors become depolarized in the dark and this depolarization induces release of excitatory glutamate neurotransmitter. Oxidation of [U-14C]glucose, [1-14C]lactate, and [1-14C]pyruvate in light- and dark-adapted excised retinas was estimated by collecting 14CO2. Neither glucose nor lactate oxidation that require participation of the malate/aspartate shuttle increased in the dark, but pyruvate oxidation that does not require the malate/aspartate shuttle increased to 36% in the dark. Aerobic glycolysis was estimated by measuring the rate of lactate appearance. Glycolysis was 37% faster in the dark. It appears that in the retina, ATP consumed during glutamatergic neurotransmission is replenished by ATP generated glycolytically within the retinal Müller cells and that oxidation of glucose within the Müller cells does not occur or occurs only slowly.

Alanine metabolism, transport, and cycling in the brain

Brain glutamate/glutamine cycling is incomplete without return of ammonia to glial cells. Previous studies suggest that alanine is an important carrier for ammonia transfer. In this study, we investigated alanine transport and metabolism in Guinea pig brain cortical tissue slices and prisms, in primary cultures of neurons and astrocytes, and in synaptosomes. Alanine uptake into astrocytes was largely mediated by system L isoform LAT2, whereas alanine uptake into neurons was mediated by Na(+)-dependent transporters with properties similar to system B(0) isoform B(0)AT2. To investigate the role of alanine transport in metabolism, its uptake was inhibited in cortical tissue slices under depolarizing conditions using the system L transport inhibitors 2-aminobicyclo[2.2.1]heptane-2-carboxylic acid and cycloleucine (1-aminocyclopentanecarboxylic acid; cLeu). The results indicated that alanine cycling occurs subsequent to glutamate/glutamine cycling and that a significant proportion of cycling occurs via amino acid transport system L. Our results show that system L isoform LAT2 is critical for alanine uptake into astrocytes. However, alanine does not provide any significant carbon for energy or neurotransmitter metabolism under the conditions studied.

Widespread neuronal expression of branched-chain aminotransferase in the CNS: implications for leucine/glutamate metabolism and for signaling by amino acids

Transamination of the branched-chain amino acids produces glutamate and branched-chain alpha-ketoacids. The reaction is catalyzed by branched-chain aminotransferase (BCAT), of which there are cytosolic and mitochondrial isoforms (BCATc and BCATm). BCATc accounts for 70% of brain BCAT activity, and contributes at least 30% of the nitrogen required for glutamate synthesis. In previous work, we showed that BCATc is present in the processes of glutamatergic neurons and in cell bodies of GABAergic neurons in hippocampus and cerebellum. Here we show that this metabolic enzyme is expressed throughout the brain and spinal cord, with distinct differences in regional and intracellular patterns of expression. In the cerebral cortex, BCATc is present in GABAergic interneurons and in pyramidal cell axons and proximal dendrites. Axonal labeling for BCATc continues into the corpus callosum and internal capsule. BCATc is expressed by GABAergic neurons in the basal ganglia and by glutamatergic neurons in the hypothalamus, midbrain, brainstem, and dorsal root ganglia. BCATc is also expressed in hypothalamic peptidergic neurons, brainstem serotoninergic neurons, and spinal cord motor neurons. The results indicate that BCATc accumulates in neuronal cell bodies in some regions, while elsewhere it is exported to axons and nerve terminals. The enzyme is in a position to influence pools of glutamate in a variety of neuronal types. BCATc may also provide neurons with sensitivity to nutrient-derived BCAAs, which may be important in regions that control feeding behavior, such as the arcuate nucleus of the hypothalamus, where neurons express high levels of BCATc.

Expression of cytosolic branched chain aminotransferase (BCATc) mRNA in the developing mouse brain

Branched-chain aminotransferase (BCAT) catalyzes the transamination of essential branched-chain amino acids (BCAAs: leucine, isoleucine and valine) with alpha-ketoglutarate. Through this reaction, BCAAs provide nitrogen for the synthesis of glutamate, the predominant excitatory neurotransmitter. Two BCAT isoforms have been identified: one cytosolic (BCATc) and one mitochondrial (BCATm). In adult rodents, BCATc is expressed in a wide variety of structures of the central nervous system (CNS), in neurons. So far, no data were available about the expression of BCATc in the developing CNS. Here, we analyse the expression profile of BCATc mRNA in the mouse brain from embryonic day 12.5 to adult age. BCATc mRNA gradually appears in different brain regions starting from early stages of neural development, and is maintained until adulthood. BCATc mRNA is predominantly present in the cerebral cortex, hippocampus, thalamus, ventral midbrain, raphe, cerebellum and precerebellar system. This study represents the first detailed analysis of BCATc mRNA expression in the developing mouse brain.

Structural Determinants for Branched-chain Aminotransferase Isozyme-specific Inhibition by the Anticonvulsant Drug Gabapentin

This study presents the first three-dimensional structures of human cytosolic branched-chain aminotransferase (hBCATc) isozyme complexed with the neuroactive drug gabapentin, the hBCATc Michaelis complex with the substrate analog, 4-methylvalerate, and the mitochondrial isozyme (hBCATm) complexed with gabapentin. The branched-chain aminotransferases (BCAT) reversibly catalyze transamination of the essential branched-chain amino acids (leucine, isoleucine, valine) to alpha-ketoglutarate to form the respective branched-chain alpha-keto acids and glutamate. The cytosolic isozyme is the predominant BCAT found in the nervous system, and only hBCATc is inhibited by gabapentin. Pre-steady state kinetics show that 1.3 mm gabapentin can completely inhibit the binding of leucine to reduced hBCATc, whereas 65.4 mm gabapentin is required to inhibit leucine binding to hBCATm. Structural analysis shows that the bulky gabapentin is enclosed in the active-site cavity by the shift of a flexible loop that enlarges the active-site cavity. The specificity of gabapentin for the cytosolic isozyme is ascribed at least in part to the location of the interdomain loop and the relative orientation between the small and large domain which is different from these relationships in the mitochondrial isozyme. Both isozymes contain a CXXC center and form a disulfide bond under oxidizing conditions. The structure of reduced hBCATc was obtained by soaking the oxidized hBCATc crystals with dithiothreitol. The close similarity in active-site structures between cytosolic enzyme complexes in the oxidized and reduced states is consistent with the small effect of oxidation on pre-steady state kinetics of the hBCATc first half-reaction. However, these kinetic data do not explain the inactivation of hBCATm by oxidation of the CXXC center. The structural data suggest that there is a larger effect of oxidation on the interdomain loop and residues surrounding the CXXC center in hBCATm than in hBCATc.

Branched-chain [corrected] amino acid metabolism: implications for establishing safe intakes

There are several features of the metabolism of the indispensable BCAAs that set them apart from other indispensable amino acids. BCAA catabolism involves 2 initial enzymatic steps that are common to all 3 BCAAs; therefore, the dietary intake of an individual BCAA impacts on the catabolism of all 3. The first step is reversible transamination followed by irreversible oxidative decarboxylation of the branched-chain alpha-keto acid transamination products, the branched chain alpha-keto acids (BCKAs). The BCAA catabolic enzymes are distributed widely in body tissues and, with the exception of the nervous system, all reactions occur in the mitochondria of the cell. Transamination provides a mechanism for dispersing BCAA nitrogen according to the tissue's requirements for glutamate and other dispensable amino acids. The intracellular compartmentalization of the branched-chain aminotransferase isozymes (mitochondrial branched-chain aminotransferase, cytosolic branched-chain aminotransferase) impacts on intra- and interorgan exchange of BCAA metabolites, nitrogen cycling, and net nitrogen transfer. BCAAs play an important role in brain neurotransmitter synthesis. Moreover, a dysregulation of the BCAA catabolic pathways that leads to excess BCAAs and their derivatives (e.g., BCKAs) results in neural dysfunction. The relatively low activity of catabolic enzymes in primates relative to the rat may make the human more susceptible to excess BCAA intake. It is hypothesized that the symptoms of excess intake would mimic the neurological symptoms of hereditary diseases of BCAA metabolism.

Brain amino acid requirements and toxicity: the example of leucine

Glutamic acid is an important excitatory neurotransmitter of the brain. Two key goals of brain amino acid handling are to maintain a very low intrasynaptic concentration of glutamic acid and also to provide the system with precursors from which to synthesize glutamate. The intrasynaptic glutamate level must be kept low to maximize the signal-to-noise ratio upon the release of glutamate from nerve terminals and to minimize the risk of excitotoxicity consequent to excessive glutamatergic stimulation of susceptible neurons. The brain must also provide neurons with a constant supply of glutamate, which both neurons and glia robustly oxidize. The branched-chain amino acids (BCAAs), particularly leucine, play an important role in this regard. Leucine enters the brain from the blood more rapidly than any other amino acid. Astrocytes, which are in close approximation to brain capillaries, probably are the initial site of metabolism of leucine. A mitochondrial branched-chain aminotransferase is very active in these cells. Indeed, from 30 to 50% of all alpha-amino groups of brain glutamate and glutamine are derived from leucine alone. Astrocytes release the cognate ketoacid [alpha-ketoisocaproate (KIC)] to neurons, which have a cytosolic branched-chain aminotransferase that reaminates the KIC to leucine, in the process consuming glutamate and providing a mechanism for the "buffering" of glutamate if concentrations become excessive. In maple syrup urine disease, or a congenital deficiency of branched-chain ketoacid dehydrogenase, the brain concentration of KIC and other branched-chain ketoacids can increase 10- to 20-fold. This leads to a depletion of glutamate and a consequent reduction in the concentration of brain glutamine, aspartate, alanine, and other amino acids. The result is a compromise of energy metabolism because of a failure of the malate-aspartate shuttle and a diminished rate of protein synthesis.

Branched-chain amino acids and neurotransmitter metabolism: expression of cytosolic branched-chain aminotransferase (BCATc) in the cerebellum and hippocampus

In the brain, catabolism of the branched-chain amino acids (BCAAs) provides nitrogen for the synthesis of glutamate and glutamine. Glutamate is formed through transfer of an amino group from BCAA to alpha-ketoglutarate in reaction catalyzed by branched-chain aminotransferases (BCAT). There are two isozymes of BCAT: cytosolic BCATc, which is found in the nervous system, ovary, and placenta, and mitochondrial BCATm, which is found in all organs except rat liver. In cell culture systems, BCATc is found only in neurons and developing oligodendrocytes, whereas BCATm is the isoform in astroglia. In this study, we used immunohistochemistry to examine the distribution of BCATc in the rat brain, focusing on the well-known neural architecture of the cerebellum and hippocampus. We show that BCATc is expressed only in neurons in the adult rat brain. In glutamatergic neurons such as granule cells of the cerebellar cortex and of the dentate gyrus, BCATc is localized to axons and nerve terminals. In contrast, in GABAergic neurons such as cerebellar Purkinje cells and hippocampal pyramidal basket cells, BCATc is concentrated in cell bodies. A common function for BCATc in these neurotransmitter systems may be to modulate amounts of glutamate available either for release as neurotransmitter or for use as precursor for synthesis of GABA. Particularly striking in our findings is the strong expression of BCATc in the mossy fiber pathway of the hippocampal formation. This result is discussed in light of the effectiveness of the anticonvulsant drug gabapentin, which is a specific inhibitor of BCATc.

Regulation of glutamate-synthesizing enzymes by NMDA and potassium in cerebellar granule cells

The presence of 25 mm potassium (KCl) or N-methyl-d-aspartate (NMDA) in cultured cerebellar granule neurons (CGN) induces a trophic effect, including a specific regulation of the enzymes involved in the glutamate neurotransmitter synthesis. In this study we explored the effect of these conditions on the cytosolic and mitochondrial isoenzymes of aspartate aminotransferase (AAT), and phosphate-activated glutaminase (PAG) in CGN. We found that NMDA and KCl increased the AAT total activity by 40% and 70%, respectively. This effect was mediated by an augmentation in the protein levels (68% by NMDA, 58% by KCl). NMDA raised the Vmax and KCl raised both the maximol velocity (Vmax) and Michaelis constant (Km) of AAT. NMDA increased cytosolic AAT activity by 30% and mitochondrial activity by 70%; KCl increased cytosolic and mitochondrial AAT activity by 60% and 100%, respectively. This activation was also related to an increase in the protein levels. The effect of both conditions on the activity and protein levels were more pronounced in mitochondrial than cytosolic AAT and the increment elicited by KCl was higher in both isoforms than that produced by NMDA. The PAG and AAT mRNA levels were also regulated by incubation with NMDA and KCl similarly to the observed changes in the protein levels. These results suggest that NMDA receptor stimulation during CGN development differentially regulates the two AAT isoenzymes involved in the maturation of CGN and that the regulation of both AAT and PAG occurs also at the mRNA expression level, suggesting the involvement of a mechanism of gene expression regulation.