Function of leucine in excitatory neurotransmitter metabolism in the central nervous system

Intraocular delivery of BDNF following visual cortex lesion upregulates cytosolic branched chain aminotransferase (BCATc) in the rat dorsal lateral geniculate nucleus

Visual cortex ablation in newborn rats determines the almost complete degeneration of neurons in the dorsal lateral geniculate nucleus (dLGN), as a consequence of the axotomy of the geniculo-cortical fibres. Death of dLGN neurons is massive and rapid, and occurs by apoptosis. We recently showed that exogenous administration of the neurotrophin brain-derived neurotrophic factor (BDNF) in the eye prevents the degeneration of dLGN neurons occurring after visual cortex lesion in newborn rats. To elucidate the molecular mechanisms of BDNF-mediated neuroprotection, we sought to identify novel genes regulated by BDNF in the rat dLGN after visual cortex lesion. By using mRNA fingerprinting, we isolated a cDNA fragment upregulated in the dLGN of lesioned rats treated with BDNF. This cDNA fragment shared 100% homology with the rat cytosolic branched chain aminotransferase (BCATc), a key enzyme of glutamate metabolism. Quantitative reverse transcription-polymerase chain reaction and in situ hybridization confirmed that BCATc mRNA is markedly overexpressed by exogenous supply of BDNF to axotomized dLGNs. Immunohistochemical analysis showed that upregulation of BCATc in the dLGN of lesioned rats treated with BDNF takes place in astrocytes. These results suggest that modulation of glutamate metabolism by astrocytes might play an important role in BDNF-mediated survival of axotomized dLGN neurons.

Mechanisms responsible for regulation of branched-chain amino acid catabolism

The branched-chain amino acids (BCAAs) are essential amino acids and therefore must be continuously available for protein synthesis. However, BCAAs are toxic at high concentrations as evidenced by maple syrup urine disease (MSUD), which explains why animals have such an efficient oxidative mechanism for their disposal. Nevertheless, it is clear that leucine is special among the BCAAs. Leucine promotes global protein synthesis by signaling an increase in translation, promotes insulin release, and inhibits autophagic protein degradation. However, leucine's effects are self-limiting because leucine promotes its own disposal by an oxidative pathway, thereby terminating its positive effects on body protein accretion. A strong case can therefore be made that the proper leucine concentration in the various compartments of the body is critically important for maintaining body protein levels beyond simply the need of this essential amino acid for protein synthesis. The goal of the work of this laboratory is to establish the importance of regulation of the branched chain alpha-ketoacid dehydrogenase complex (BCKDC) to growth and maintenance of body protein. We hypothesize that proper regulation of the activity state of BCKDC by way of its kinase (BDK) and its phosphatase (BDP) is critically important for body growth, tissue repair, and maintenance of body protein. We believe that growth and protection of body protein during illness and stress will be improved by therapeutic control of BCKDC activity. We also believe that it is possible that the negative effects of some drugs (PPAR alpha ligands) and dietary supplements (medium chain fatty acids) on growth and body protein maintenance can be countered by therapeutic control of BCDKC activity.

Leucine-nitrogen metabolism in the brain of conscious rats: its role as a nitrogen carrier in glutamate synthesis in glial and neuronal metabolic compartments

The source of nitrogen (N) for the de novo synthesis of brain glutamate, glutamine and GABA remains controversial. Because leucine is readily transported into the brain and the brain contains high activities of branched-chain aminotransferase (BCAT), we hypothesized that leucine is the predominant N-precursor for brain glutamate synthesis. Conscious and unstressed rats administered with [U-13C] and/or [15N]leucine as additions to the diet were killed at 0-9 h of continuous feeding. Plasma and brain leucine equilibrated rapidly and the brain leucine-N turnover was more than 100%/min. The isotopic dilution of [U-13C]leucine (brain/plasma ratio 0.61 +/- 0.06) and [15N]leucine (0.23 +/- 0.06) differed markedly, suggesting that 15% of cerebral leucine-N turnover derived from proteolysis and 62% from leucine synthesis via reverse transamination. The rate of glutamate synthesis from leucine was 5 micro mol/g/h and at least 50% of glutamate-N originally derived from leucine. The enrichment of [5-15N]glutamine was higher than [15N]ammonia in the brain, indicating glial ammonia generation from leucine via glutamate. The enrichment of [15N]GABA, [15N]aspartate, [15N]glutamate greater than [2-15N]glutamine suggests direct incorporation of leucine-N into both glial and neuronal glutamate. These findings provide a new insight for the role of leucine as N-carrier from the plasma pool and within the cerebral compartments.

Branched-chain amino acid catabolism: unique segregation of pathway enzymes in organ systems and peripheral nerves

We have examined the localization of the first two enzymes in the branched-chain amino acid (BCAA) catabolic pathway: the branched-chain aminotransferase (BCAT) isozymes (mitochondrial BCATm and cytosolic BCATc) and the branched-chain alpha-keto acid dehydrogenase (BCKD) enzyme complex. Antibodies specific for BCATm or BCATc were used to immunolocalize the respective isozymes in cryosections of rat tissues. BCATm was expressed in secretory epithelia throughout the digestive tract, with the most intense expression in the stomach. BCATm was also strongly expressed in secretory cells of the exocrine pancreas, uterus, and testis, as well as in the transporting epithelium of convoluted tubules in kidney. In muscle, BCATm was located in myofibrils. Liver, as predicted, was not immunoreactive for BCATm. Unexpectedly, BCATc was localized in elements of the autonomic innervation of the digestive tract, as well as in axons in the sciatic nerve. The distributions of BCATc and BCATm did not overlap. BCATm-expressing cells also expressed the second enzyme of the BCAA catabolic pathway, BCKD. In selected monkey and human tissues examined by immunoblot and/or immunohistochemistry, BCATm and BCATc were distributed in patterns very similar to those found in the rat. The results show that BCATm is in a position to regulate BCAA availability as protein precursors and anabolic signals in secretory portions of the digestive and other organ systems. The unique expression of BCATc in neurons of the peripheral nervous system, without coexpression of BCKD, raises new questions about the physiological function of this BCAT isozyme.

Activity of the lactate-alanine shuttle is independent of glutamate-glutamine cycle activity in cerebellar neuronal-astrocytic cultures

The glutamate-glutamine cycle describes the neuronal release of glutamate into the synaptic cleft, astrocytic uptake, and conversion into glutamine, followed by release for use as a neuronal glutamate precursor. This only explains the fate of the carbon atoms, however, and not that of the ammonia. Recently, a role for alanine has been proposed in transfer of ammonia between glutamatergic neurons and astrocytes, denoted the lactate-alanine shuttle (Waagepetersen et al. [ 2000] J. Neurochem. 75:471-479). The role of alanine in this context has been studied further using cerebellar neuronal cultures and corresponding neuronal-astrocytic cocultures. A superfusion paradigm was used to induce repetitively vesicular glutamate release by N-methyl-D-aspartate (NMDA) in the neurons, allowing the relative activity dependency of the lactate-alanine shuttle to be assessed. [(15)N]Alanine (0.2 mM), [2-(15)N]/[5-(15)N]glutamine (0.25 mM), and [(15)N]ammonia (0.3 mM) were used as precursors and cell extracts were analyzed by mass spectrometry. Labeling from [(15)N]alanine in glutamine, aspartate, and glutamate in cerebellar cocultures was independent of depolarization of the neurons. Employing glutamine with the amino group labeled ([2-(15)N]glutamine) as the precursor, an activity-dependent increase in the labeling of both glutamate and aspartate (but not alanine) was observed in the cerebellar neurons. When the amide group of glutamine was labeled ([5-(15)N]glutamine), no labeling could be detected in the analyzed metabolites. Altogether, the results of this study support the existence of the lactate-alanine shuttle and the associated glutamate-glutamine cycle. No direct coupling of the two shuttles was observed, however, and only the glutamate-glutamine cycle seemed activity dependent.

Acute effects of gabapentin and pregabalin on rat forebrain cellular GABA, glutamate, and glutamine concentrations

The effects of antiepileptic drugs, gabapentin, pregabalin and vigabatrin, on brain gamma-aminobutyric acid (GABA), glutamate and glutamine concentrations were studied in Long Evans rats using proton magnetic resonance spectroscopy (MRS) of perchloric acid extracts. Cellular glutamate concentrations significantly decreased by 7% (P<0.05) 2 hours after intraperitoneal injection of 100mg/kg gabapentin and 4% (P<0.05) with 1000 mg/kg. No differences were observed in cellular GABA and cellular glutamine concentrations in rats treated with gabapentin. Pregabalin, an analogue of gabapentin, significantly decreased cellular glutamate concentrations by 4% (P<0.05), while no effect was observed on cellular GABA or glutamine concentrations in the healthy rat forebrain. Vigabatrin, used as a positive control to increase GABA levels, produced a 50% increase in cellular GABA compared to saline treated rats (P<0.003). Although, gabapentin and pregabalin are anticonvulsants designed to mimic GABA, these drugs do not raise cellular GABA levels acutely but modestly decreased cellular glutamate levels in our healthy rat forebrain model.

A new view of diabetic retinopathy: a neurodegenerative disease of the eye

Diabetic retinopathy (DR) is a common complication of diabetes and a leading cause of legal blindness in working-age adults. The clinical hallmarks of DR include increased vascular permeability, leading to edema, and endothelial cell proliferation. Much of the research effort has been focused on vascular changes, but it is becoming apparent that other degenerative changes occur beyond the vascular cells of the retina. These include increased apoptosis, glial cell reactivity, microglial activation, and altered glutamate metabolism. When occurring together, these changes may be considered as neurodegenerative and could explain some of the functional deficits in vision that begin soon after the onset of diabetes. This review will present the current evidence that neurodegeneration of the retina is a critical component of DR. There are two basic hypotheses that account for loss of cells in the neural retina. First, the loss of blood-retinal barrier integrity, which initially manifests as an increase in vascular permeability, causes a failure to control the composition of the extracellular fluid in the retina, which in turn leads to edema and neuronal cell loss. Alternatively, diabetes has a direct effect on metabolism within the neural retina, leading to an increase in apoptosis, which in turn causes breakdown of the blood-retinal barrier. It is not clear which hypothesis will be found to be correct, and, in fact, it is likely that vascular permeability and neuronal apoptosis are closely linked components of DR. However, the gradual loss of neurons suggests that progress of the disease is ultimately irreversible, since these cells cannot usually be replaced. In light of this possibility, new treatments for DR should be preventive in nature, being implemented before overt clinical symptoms develop. While vascular permeability is the target that is primarily considered for new treatments of DR, evidence presented here suggests that apoptosis of neurons is also an essential target for pharmacological studies. The vision of people with diabetes will be protected only when we have discovered a means to prevent the gradual but constant loss of neurons within the inner retina.

Astrocytic amino acid metabolism under control conditions and during oxygen and/or glucose deprivation

Amino acid contents were measured in 1- and 3-week-old primary cultures of astrocytes and in their incubation media, an amino acid-free salt solution with or without glucose, during 3-h incubation under normoxic or anoxic conditions. Most essential amino acids were rapidly released to the medium during the beginning of the incubation. A subsequent slow medium increase reflected proteolysis. Glutamate and aspartate were absent from the media during all conditions, indicating fueling of their uptake by either glycolytically or oxidatively derived energy. The total content of glutamine increased, except during incubation in glucose-deprived media, when it declined or remained constant. Changes in aspartate were negligible, suggesting oxidative degradation of aspartate-derived oxaloacetate during normoxia and its reduction to succinate during anoxia, driving regeneration of NAD+ from NADH. An increase of alanine was reduced in glucose-free media, whereas serine showed especially large increase during isolated glucose deprivation, suggesting its production from glutamine via 3-phosphoglycerate.

Metabolism of brain amino acids following pentylenetetrazole treatment

We studied the effects of pentylenetetrazole (PTZ) on brain amino acid metabolism in mice. Administration of this convulsant did not change forebrain concentrations of amino acids, but when treated animals also received an injection of [15N]leucine, which served as a tracer of brain nitrogen metabolism, total (14N+15N) forebrain [leucine] exceeded control and [glutamate] and [aspartate] were less than control, as were forebrain concentrations of [15N]glutamate and [2-15N]glutamine. These data suggest greater uptake of [15N]leucine but diminished transamination of leucine to glutamate in experimental mice. In contrast to the [15N]leucine studies, which were associated with increased brain [leucine], the administration of [15N]alanine did not alter levels of alanine, glutamate or glutamine. However, label appeared in [2-15N]glutamine much more readily with [15N]alanine than with [15N]leucine as precursor and the ratio of enrichment in [2-15N]glutamine/[15N]alanine was much higher than that in [2-15N]glutamine/[15N]leucine, a finding that is compatible with preferential metabolism of alanine in astrocytes, which are the primary site of brain glutamine synthetase. We conclude that PTZ treatment favors the uptake of selected amino acids such as leucine but also diminishes transamination of leucine to yield glutamate via branched-chain amino acid transaminase. PTZ treatment may favor the "reverse" transamination of 2-keto-isocaproate (KIC), the ketoacid of leucine, to form leucine and to consume glutamate. A net result of these processes may be to enable the brain more readily to dispose of the glutamate that is released from neurons during convulsive activity.

Human mitochondrial and cytosolic branched-chain aminotransferases are cysteine S-conjugate beta-lyases, but turnover leads to inactivation

The mitochondrial and cytosolic branched-chain aminotransferases (BCAT(m) and BCAT(c)) are homodimers in the fold type IV class of pyridoxal 5'-phosphate-containing enzymes that also contains D-amino acid aminotransferase and 4-amino-4-deoxychorismate lyase (a beta-lyase). Recombinant human BCAT(m) and BCAT(c) were shown to have beta-lyase activity toward three toxic cysteine S-conjugates [S-(1,1,2,2-tetrafluoroethyl)-L-cysteine, S-(1,2-dichlorovinyl)-L-cysteine, and S-(2-chloro-1,1,2-trifluoroethyl)-L-cysteine] and toward beta-chloro-L-alanine. Human BCAT(m) is a much more effective beta-chloro-L-alanine beta-lyase than two aminotransferases (cytosolic and mitochondrial isozymes of aspartate aminotransferase) previously shown to possess this activity. BCAT(m), but not BCAT(c), also exhibits measurable beta-lyase activity toward a relatively bulky cysteine S-conjugate [benzothiazolyl-L-cysteine]. Benzothiazolyl-L-cysteine, however, inhibits the L-leucine-alpha-ketoglutarate transamination reaction catalyzed by both enzymes. Inhibition was more pronounced with BCAT(m). In the presence of beta-lyase substrates and alpha-ketoisocaproate (the alpha-keto acid analogue of leucine), no transamination could be detected. Therefore, with an amino acid containing a good leaving group in the beta position, beta-elimination is greatly preferred over transamination. Both BCAT isozymes are rapidly inactivated by the beta-lyase substrates. The ratio of turnover to inactivation per monomer in the presence of toxic halogenated cysteine S-conjugates is approximately 170-280 for BCAT(m) and approximately 40-50 for BCAT(c). Mitochondrial enzymes of energy metabolism are especially vulnerable to thioacylation and inactivation by the reactive fragment released from toxic, halogenated cysteine S-conjugates such as S-(1,1,2,2-tetrafluoroethyl)-L-cysteine. The present results suggest that BCAT isozymes may contribute to the mitochondrial toxicity of these compounds by providing thioacylating fragments, but inactivation of the BCAT isozymes might also block essential metabolic pathways.

A continuous 96-well plate spectrophotometric assay for branched-chain amino acid aminotransferases

A new, continuous 96-well plate spectrophotometric assay for the branched-chain amino acid aminotransferases is described. Transamination of L-leucine with alpha-ketoglutarate results in formation of alpha-ketoisocaproate, which is reductively aminated back to L-leucine by leucine dehydrogenase in the presence of ammonia and NADH. The disappearance of absorbance at 340 nm due to NADH oxidation is measured continuously. The specific activities obtained by this procedure for the highly purified human mitochondrial and cytosolic isoforms of BCAT compare favorably with those obtained by a commonly used radiochemical procedure, which measures transamination between alpha-ketoiso[1-14C]valerate and L-isoleucine. Due to the presence of glutamate dehydrogenase substrates (alpha-ketoglutarate, ammonia, and NADH) and L-leucine (an activator of glutamate dehydrogenase) in the standard assay mixture, interference with the measurement of BCAT activity in tissue homogenates by glutamate dehydrogenase is observed. However, by limiting the amount of ammonia and including the inhibitor GTP in the assay mixture, the interference from the glutamate dehydrogenase reaction is minimized. By comparing the rate of loss of absorbance at 340 nm in the modified spectrophotometric assay mixture containing leucine dehydrogenase to that obtained in the modified spectrophotometric assay mixture lacking leucine dehydrogenase, it is possible to measure BCAT activity in microliter amounts of rat tissue homogenates. The specific activities of BCAT in homogenates of selected rat tissues obtained by this method are comparable to those obtained previously by the radiochemical procedure.

Toxic, halogenated cysteine S-conjugates and targeting of mitochondrial enzymes of energy metabolism

Several haloalkenes are metabolized in part to nephrotoxic cysteine S-conjugates; for example, trichloroethylene and tetrafluoroethylene are converted to S-(1,2-dichlorovinyl)-L-cysteine (DCVC) and S-(1,1,2,2-tetrafluoroethyl)-L-cysteine (TFEC), respectively. Although DCVC-induced toxicity has been investigated since the 1950s, the toxicity of TFEC and other haloalkene-derived cysteine S-conjugates has been studied more recently. Some segments of the US population are exposed to haloalkenes either through drinking water or in the workplace. Therefore, it is important to define the toxicological consequences of such exposures. Most halogenated cysteine S-conjugates are metabolized by cysteine S-conjugate beta-lyases to pyruvate, ammonia, and an alpha-chloroenethiolate (with DCVC) or an alpha-difluoroalkylthiolate (with TFEC) that may eliminate halide to give a thioacyl halide, which reacts with epsilon-amino groups of lysine residues in proteins. Nine mammalian pyridoxal 5'-phosphate (PLP)-containing enzymes catalyze cysteine S-conjugate beta-lyase reactions, including mitochondrial aspartate aminotransferase (mitAspAT), and mitochondrial branched-chain amino acid aminotransferase (BCAT(m)). Most of the cysteine S-conjugate beta-lyases are syncatalytically inactivated. TFEC-induced toxicity is associated with covalent modification of several mitochondrial enzymes of energy metabolism. Interestingly, the alpha-ketoglutarate- and branched-chain alpha-keto acid dehydrogenase complexes (KGDHC and BCDHC), but not the pyruvate dehydrogenase complex (PDHC), are susceptible to inactivation. mitAspAT and BCAT(m) may form metabolons with KGDHC and BCDHC, respectively, but no PLP enzyme is known to associate with PDHC. Consequently, we hypothesize that not only do these metabolons facilitate substrate channeling, but they also facilitate toxicant channeling, thereby promoting the inactivation of proximate mitochondrial enzymes and the induction of mitochondrial dysfunction.

Structure and function of branched chain aminotransferases

Branched chain aminotransferases (BCATs) catalyze transamination of the branched chain amino acids (BCAAs) leucine, isoleucine, and valine. Except for the Escherichia coli and Salmonella proteins, which are homohexamers arranged as a double trimer, the BCATs are homodimers. Structurally, the BCATs belong to the fold type IV class of pyridoxal phosphate (PLP) enzymes. Other members are D-alanine aminotransferase and 4-amino-4-deoxychorismate lyase. Catalysis is on the re face of the PLP cofactor, whereas in other classes, catalysis occurs from the si face of PLP. Crystal structures of the fold type IV proteins show that they are distinct from the fold type I aspartate aminotransferase family and represent a new protein fold. Because the fold type IV enzymes catalyze diverse reactions, it is not surprising that the greatest structural similarities involve residues that participate in PLP binding rather than residues involved in substrate binding. The BCATs are widely distributed in the bacterial kingdom, where they are involved in the synthesis/degradation of the BCAAs. Bacteria contain a single BCAT. In eukaryotes there are two isozymes, one is mitochondrial (BCATm) and the other is cytosolic (BCATc). In mammals, BCATm is in most tissues, and BCATm is thought to be important in body nitrogen metabolism. BCATc is largely restricted to the central nervous system (CNS). Recently, BCATc has been recognized as a target of the neuroactive drug gabapentin. BCATc is involved in excitatory neurotransmitter glutamate synthesis in the CNS. Ongoing structural studies of the BCATs may facilitate the design of therapeutic compounds to treat neurodegenerative disorders involving disturbances of the glutamatergic system.

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

In this study aminotransferase inhibitors were used to determine the relative importance of different aminotransferases in providing nitrogen for de novo glutamate synthesis in the retina. Aminooxyacetate, which inhibits all aminotransferases, blocked de novo glutamate synthesis from H(14)CO(3)(-) by more than 60%. Inhibition of neuronal cytosolic branched chain amino acid transamination by gabapentin or branched chain amino acid transport by the L-system substrate analog, 2-amino-bicyclo-(2,2,1)-heptane-2-carboxylic acid, lowered total de novo synthesis of glutamate by 30%, suggesting that branched chain amino acids may account for half of the glutamate nitrogen contributed by transamination reactions. L-cycloserine, an inhibitor of alanine aminotransferase, inhibited glutamate synthesis less than 15% when added in the presence of 5 mM pyruvate but 47% in the presence of 0.2 mM pyruvate. Although high levels of pyruvate blunted the inhibitory effectiveness of L-cycloserine, the results indicate that, under physiological conditions, alanine as well as branched chain amino acids are probably the predominant sources of glutamate nitrogen in ex vivo retinas. The L-cycloserine results were also used to evaluate activity of the malate/aspartate shuttle. In this shuttle, cytosolic aspartate (synthesized in mitochondria) generates cytosolic oxaloacetate that oxidizes cytosolic NADH via malate dehydrogenase. Because L-cycloserine inhibits cytosolic but not mitochondrial aspartate aminotransferase, L-cycloserine should prevent the utilization of aspartate but not its generation, thereby increasing levels of (14)C-aspartate. Instead, L-cycloserine caused a significant decline in (14)C-aspartate. The results suggest the possibility that shuttle activity is low in retinal Müller cells. Low malate/aspartate shuttle activity may be the molecular basis for the high rate of aerobic glycolysis in retinal Müller cells.

Ketogenic diet, amino acid metabolism, and seizure control

The ketogenic diet has been utilized for many years as an adjunctive therapy in the management of epilepsy, especially in those children for whom antiepileptic drugs have not permitted complete relief. The biochemical basis of the dietary effect is unclear. One possibility is that the diet leads to alterations in the metabolism of brain amino acids, most importantly glutamic acid, the major excitatory neurotransmitter. In this review, we explore the theme. We present evidence that ketosis can lead to the following: 1) a diminution in the rate of glutamate transamination to aspartate that occurs because of reduced availability of oxaloacetate, the ketoacid precursor to aspartate; 2) enhanced conversion of glutamate to GABA; and 3) increased uptake of neutral amino acids into the brain. Transport of these compounds involves an uptake system that exchanges the neutral amino acid for glutamine. The result is increased release from the brain of glutamate, particularly glutamate that had been resident in the synaptic space, in the form of glutamine. These putative adaptations of amino acid metabolism occur as the system evolves from a glucose-based fuel economy to one that utilizes ketone bodies as metabolic substrates. We consider mechanisms by which such changes might lead to the antiepileptic effect.