Metabolism related to the tricarboxylic acid cycle in rat brain slices. Observations on CO 2 fixation and metabolic compartmentation

Which way does the citric acid cycle turn during hypoxia? The critical role of α-ketoglutarate dehydrogenase complex

The citric acid cycle forms a major metabolic hub and as such it is involved in many disease states involving energetic imbalance. In spite of the fact that it is being branded as a "cycle", during hypoxia, when the electron transport chain does not oxidize reducing equivalents, segments of this metabolic pathway remain operational but exhibit opposing directionalities. This serves the purpose of harnessing high-energy phosphates through matrix substrate-level phosphorylation in the absence of oxidative phosphorylation. In this Mini-Review, these segments are appraised, pointing to the critical importance of the α-ketoglutarate dehydrogenase complex dictating their directionalities.

Pyruvate carboxylation in neurons

Carboxylation of pyruvate in the brain was for many years thought to occur only in glia, an assumption that formed much of the basis for the concept of the glutamine cycle. It was shown recently, however, that carboxylation of pyruvate to malate occurs in neurons and that it supports formation of transmitter glutamate. The role of pyruvate carboxylation in neurons is to ensure tricarboxylic acid cycle activity by compensating for losses of alpha-ketoglutarate that occur through release of transmitter glutamate and GABA; these amino acids are alpha-ketoglutarate derivatives. Available data suggest that neuronal pyruvate carboxylation is quantitatively important. But because there is no net CO(2) fixation in the brain, pyruvate carboxylation must be balanced by decarboxylation of malate or oxaloacetate. Such decarboxylation occurs in both neurons and astrocytes. Several in vitro studies have shown a neuroprotective effect of pyruvate supplementation. Pyruvate carboxylation may be one mechanism through which such treatment is effective, because pyruvate carboxylation through malic enzyme is active during energy deficiency and leads to an increase in the level of dicarboxylates that can be metabolized through the tricarboxylic acid cycle for ATP production.

Carboxylation and anaplerosis in neurons and glia

Anaplerosis, or de novo formation of intermediates of the tricarboxylic acid (TCA) cycle, compensates for losses of TCA cycle intermediates, especially alpha-ketoglutarate, from brain cells. Loss of alpha-ketoglutarate occurs through release of glutamate and GABA from neurons and through export of glutamine from glia, because these amino acids are alpha-ketoglutarate derivatives. Anaplerosis in the brain may involve four different carboxylating enzymes: malic enzyme, phosphoenopyruvate carboxykinase (PEPCK), propionyl-CoA carboxylase, and pyruvate carboxylase. Anaplerotic carboxylation was for many years thought to occur only in glia through pyruvate carboxylase; therefore, loss of transmitter glutamate and GABA from neurons was thought to be compensated by uptake of glutamine from glia. Recently, however, anaplerotic pyruvate carboxylation was demonstrated in glutamatergic neurons, meaning that these neurons to some extent can maintain transmitter synthesis independently of glutamine. Malic enzyme, which may carboxylate pyruvate, was recently detected in neurons. The available data suggest that neuronal and glial pyruvate carboxylation could operate at as much as 30% and 40-60% of the TCA cycle rate, respectively. Cerebral carboxylation reactions are probably balanced by decarboxylation reactions,, because cerebral CO2 formation equals O2 consumption. The finding of pyruvate carboxylation in neurons entails a major revision of the concept of the glutamine cycle.

Cerebral metabolism of lactate in vivo: evidence for neuronal pyruvate carboxylation

The cerebral metabolism of lactate was investigated. Awake mice received [3-13C]lactate or [1-13C]glucose intravenously, and brain and blood extracts were analyzed by 13C nuclear magnetic resonance spectroscopy. The cerebral uptake and metabolism of [3-13C]lactate was 50% that of [1-13C]glucose. [3-13C]Lactate was almost exclusively metabolized by neurons and hardly at all by glia, as revealed by the 13C labeling of glutamate, gamma-aminobutyric acid and glutamine. Injection of [3-13C]lactate led to extensive formation of [2-13C]lactate, which was not seen with [1-13C]glucose, nor has it been seen in previous studies with [2-13C]acetate. This formation probably reflected reversible carboxylation of [3-13C]pyruvate to malate and equilibration with fumarate, because inhibition of succinate dehydrogenase with nitropropionic acid did not block it. Of the [3-13C]lactate that reached the brain, 20% underwent this reaction, which probably involved neuronal mitochondrial malic enzyme. The activities of mitochondrial malic enzyme, fumarase, and lactate dehydrogenase were high enough to account for the formation of [2-13C]lactate in neurons. Neuronal pyruvate carboxylation was confirmed by the higher specific activity of glutamate than of glutamine after intrastriatal injection of [1-14C]pyruvate into anesthetized mice. This procedure also demonstrated equilibration of malate, formed through pyruvate carboxylation, with fumarate. The demonstration of neuronal pyruvate carboxylation demands reconsideration of the metabolic interrelationship between neurons and glia.

The toxin kainic acid: a study of avian nerve and glial cell response utilizing tritiated kainic acid and electron microscopic autoradiography

Three questions are asked regarding the toxin kainic acid (KA). Does it destroy specific glial cells as well as neurons? Does KA gain access to the cytoplasm in intact cells and to which organelles does it bind? Intracerebral injections of tritiated KA into the pigeon (Columba livia) paleostriatal complex (basal ganglia) coupled with electron microscopic autoradiography revealed the following major points. Kainic acid destroyes oligodendrocytes, with pathophysiology apparent by 30 min after challenge with KA leading to cell destruction by 4 h. The response of astrocytes at the longest observation period (4 h) involves swelling of perivascular endfeet and processes in the neuropil. Reactive microglial-like cells show an accumulation of label in their cytoplasm, but no apparent morphological changes. The label appears in the cytoplasm of intact cells, both glia and neurons early after challenge with the toxin. Label is associated (bound) with mitochondria at an incidence significantly above chance at 30 min, 2 and 4 h after challenge with KA. Two hours after exposure to KA is the critical period where metabolic, physiological and morphological changes occur that lead to cell death. Cell destruction may be a consequence of KA-induced energy depletion. Kainate may interfere with adequate energy production by uncoupling glycolysis and the Krebs cycle in the mitochondria.

Control of aerobic glycolysis in the brain in vitro

Protoveratrine-(5 microM) stimulated aerobic glycolysis of incubated rat brain cortex slices that accompanies the enhanced neuronal influx of Na+ is blocked by tetrodotoxin (3 microM) and the local anesthetics, cocaine (0.1 mM) and lidocaine (0.5 mM). On the other hand, high [K+]-stimulated aerobic glycolysis that accompanies the acetylcholine-sensitive enhanced glial uptakes of Na+ and water is unaffected by acetylcholine (2 mM). Experiments done under a variety of metabolic conditions show that there exists a better correlation between diminished ATP content of the tissue and enhanced aerobic glycolysis than between tissue ATP and the ATP-dependent synthesis of glutamine. Whereas malonate (2 mM) and amino oxyacetate (5 mM) suppress ATP content and O2 uptake, stimulate lactate formation, but have little effect on glutamine levels, fluoroacetate (3 mM) suppresses glutamine synthesis in glia, presumably by suppressing the operation of the citric acid cycle, with little effect on ATP content, O2 uptake, and lactate formation. Exogenous citrate (5 mM), which may be transported and metabolized in glia but not in neurons, inhibits lactate formation by cell free acetone-dried powder extracts of brain cortex but not by brain cortex slices. These results suggest that the neuron is the major site of stimulated aerobic glycolysis in the brain, and that under our experimental conditions glycolysis in glia is under lesser stringent metabolic control than that in the neuron. Stimulation of aerobic glycolysis by protoveratrine occurs due to diminution of the energy charge of the neuron as a result of stimulation of the sodium pump following tetrodotoxin-sensitive influx of Na+; stimulation by high [K+], NH4+, or Ca2+ deprivation occurs partly by direct stimulation of key enzymes of glycolysis and partly by a fall in the tissue ATP concentration.

A comparative analysis of compartmentation of metabolism in the dorsal root ganglion and ventral spinal cord gray using [U-14C]glucose, [2-14C]glucose, [6-14C]glucose, [3,4-14C]glucose, NaH14CO3, and [2-14C]pyruvate

A detailed temporal comparison of glucose metabolism, in the production of glutamate and glutamine as well as aspartate and alamine, was conducted in order to further define the uniqueness of the dorsal root ganglion compared to the ventral spinal cord gray. Experiments with injected labeled NaHCO3 and pyruvate were used in an attempt to clarify certain aspects of the above results with different [14C]glucose precursors. The glutamine/glutamate relative specific activity ratio (RSA) was consistently lower in the ganglion than in the ventral spinal cord gray, as was also true for glutamate specific activity from the same amount of injected [14C]glucose. The ganglion is characterized by a high level of alanine production from glucose and pyruvate. The NaH14CO3 experiments suggest that CO2 fixation from [3,4-14C]-glucose in the dorsal rool ganglion resul .ts in a higher glutamine/glutamate RSA when compared to results using either [6-14C] or [2-14C]glucose.

Compartmentation of amino acid metabolism in the rat dorsal root ganglion; a metabolic and autoradiographic study

The incorporation of radioactivity into glutamate, glutamine, GABA and other amino acids was followed after incubation of desheathed rat dorsal root ganglia in media containing [14C]glucose or [14C]acetate. The results indicated that [14C]glucose was incorporated into a large pool of glutamate, but that this glutamate pool did not synthesize glutamine or GABA to any great extent. [14C]Acetate, on the other hand, was incorporated into a small glutamate pool which was readily converted to glutamine, and which synthesized GABA to a greater extent than the large pool. Light microscopic autoradiography of ganglia incubated with [14C]glucose or [14C]acetate confirmed that the small pool labelled by acetate was probably associated with satellite glial cells, while the large pool was located within the ganglion neurons. The results are discussed within the context of previous work on compartmentation of glutamate metabolism in the central nervous system.

Glutamate and aspartate transport in rat brain mitochondria

1. Rat brain mitochondria did not swell in iso-osmotic solutions of ammonium or potassium (plus valinomycin) glutamate or aspartate, with or without addition of uncouplers. 2. Glutamate was able to reduce intramitochondrial NAD(P)(+); aspartate was able to cause partial re-oxidation. 3. These effects were inhibited by threo-hydroxy-aspartate in whole but not in lysed mitochondria. 4. The existence of a ;malate-aspartate shuttle' for the oxidation of extramitochondrial NADH was demonstrated. This shuttle requires the net exchange of glutamate for aspartate across the mitochondrial membrane. 5. Extramitochondrial glutamate did not inhibit intramitochondrial glutaminase under conditions in which the inhibition in lysed mitochondria was virtually complete. 6. The glutaminase activity of these mitochondria was not energy-dependent. 7. We conclude that these mitochondria do not possess a glutamate-hydroxyl antiporter similar to that of liver mitochondria nor a glutamate-glutamine antiporter similar to that of pig kidney mitochondria, but that they do possess a glutamate-aspartate antiporter.