Glutamate-glutamine cycle

In biochemistry, the glutamate-glutamine cycle is a sequence of events by which an adequate supply of the neurotransmitter glutamate is maintained in the central nervous system.^[1] Neurons are not able to perform new synthesis of the neurotransmitter glutamate and γ-aminobutyric acid (GABA) from glucose. Discoveries of glutamine and glutamate pools within intercellular compartments led to suggestions of the glutamate-glutamine cycle working between neurons and astrocytes. The glutamate/GABA-glutamine cycle is a metabolic pathway that describes the release of glutamate or GABA from neurons which are then taken up into astrocytes (star shaped glial cells). In return, astrocytes release glutamine to be taken up into neurons for use as a precursor to the synthesis of glutamate or GABA.^[2]

Production

Glutamate

Initially, in a glutamatergic synapse, the neurotransmitter glutamate is released from the neurons and is taken up into the synaptic cleft. Glutamate resident in the synapse must then be removed rapidly. There are three possibilities, as follows:

1. Uptake into the postsynaptic compartment.
2. Re-uptake into the presynaptic compartment.
3. Uptake into a third, nonneuronal compartment.

Postsynaptic neurons remove little glutamate from the synapse. There is active reuptake into presynaptic neurons, but this mechanism appears to be less important than astrocytic transport. Astrocytes could dispose of transported glutamate in two ways. They could export it to blood capillaries, which about the astrocyte foot processes. However, this strategy would result in a net loss of carbon and nitrogen from the system. An alternate approach would be to convert glutamate into another compound, preferably a non-neuroactive species. The advantage of this approach is that neuronal glutamate could be restored without the risk of trafficking the transmitter through extracellular fluid, where glutamate would cause neuronal depolarization. The latter approach appears to have been favored by evolutionary processes. Astrocytes readily convert glutamate to glutamine via the glutamine synthetase pathway and released into the extracellular space.^[3] The glutamine is taken into the presynaptic terminals and metabolized into glutamate by the phosphate-activated glutaminase (a mitochondrial enzyme). The glutamate that is synthesized in the presynaptic terminal is packaged into synaptic vesicles by the transporter VGLUT, also known as glutamate transporters. Once the vesicle is released, glutamate is removed from the synaptic cleft by excitatory amino-acid transporters (EAATs). This allows synaptic terminals and glial cells to work together in order to maintain a proper supply of glutamate.Glutamate can also be produced by transamination of 2-oxoglutarate, an intermediate in the Citric acid cycle.<ref name="Purves" ==/> Recent electrophysiological evidence suggests that active synapses require presynaptically localized glutamine glutamate cycle to maintain excitatory neurotransmission in specific circumstances.^[4] In other systems, it has been suggested that neurons have alternate mechanisms to cope with compromised glutamate-glutamine cycling.^[5]

GABA

At GABAergic synapses, the cycle is called the GABA-glutamine cycle. Here the glutamine taken up by neurons is converted to glutamate, which is then metabolized into GABA by glutamate decarboxylase. Upon release, GABA is taken up into astrocytes via GABA transporters and then catabolized into succinate by the joint actions of GABA transaminase and succinate-semialdehyde dehydrogenase. Glutamine is synthesized from succinate via the TCA cycle including a condensation reaction of oxaloacetate and acetyl-CoA forming citrate. After that, the synthesis of α-ketoglutarate and glutamate occurs. Glutamate is converted by glutamate decarboxylase (GAD) to GABA. The supply of glutamine to GABAergic neurons is less significant, as these neurons exhibit a larger proportion of reuptake of the released neurotransmitter compared to their glutamatergic counterparts ^[6]

Ammonia Homeostasis

One of the problems of both the Glutamate-Glutamine cycle and the GABA-Glutamine cycle is ammonia homeostasis. When one molecule of glutamate or GABA is converted to glutamine in the astrocytes, one molecule of ammonia is absorbed. Also, for each molecule of glutamate or GABA cycled into the astrocytes from the synapse, one molecule of ammonia will be produced in the neurons. This ammonia will obviously have to be transported out of the neurons and back into the astrocytes for detoxification, as an elevated ammonia concentration has detrimental effects on a number of cellular functions and can cause a spectrum of neuropsychiatric and neurological symptoms (impaired memory, shortened attention span, sleep-wake inversions, brain edema, intracranial hypertension, seizures, ataxia and coma). ^[7]

Transportation & Detoxification

This can basically happen in two different ways; ammonia itself might simply diffuse (as NH3) or be transported (as NH4+) across the cell membranes in and out of the extracellular space or a shuttle system involving carrier molecules (amino acids) might be employed. Certainly, ammonia can diffuse across lipid membranes and it has been shown that ammonia can be transported by K+/CI– co-transporters.^[8]

Amino Acid Shuttles and Transporting Ammonia

As diffusion and transport of free ammonia across the cell membrane will affect the pH level of the cell, the more attractive and regulated way of transporting ammonia between the neuronal and the astrocytic compartment is via an amino acid shuttle, of which there are two: leucine and alanine. The amino acid moves in the opposite direction of glutamine. In the opposite direction of the amino acid a corresponding molecule transported; for alanine this molecule is lactate, and for leucine the molecule is α-ketoisocaproate.

Leucine

The ammonia fixed in the glutamate dehydrogenase reaction in the neurons is transaminated into α-ketoisocaproate to form leucine, which is exported to the astrocytes. In the astrocytes, the process is reversed. α-Ketoisocaproate is transported in the other direction.

Alanine

The ammonia produced in neurons is fixed into α-ketoglutarate by the glutamate dehydrogenase reaction to form glutamate. Then transaminated by alanine aminotransferase into lactate-derived pyruvate to form alanine which is exported to astrocytes. In the astrocytes this process is then reversed, and lactate is transported in the other direction.

Disorders and Conditions

Numerous reports have been published indicating that the glutamate/GABA-glutamine cycle is affected in a variety of neurological disorders and conditions. Biopsies of sclerotic hippocampus tissue from human subjects suffering from epilepsy have shown decreased glutamate-glutamine cycling. Another pathology in which the glutamate/GABA-glutamine cycle might be compromised is Alzheimer's disease, NMR spectroscopy showed decreased glutamate neurotransmission activity and TCA cycling rate in patients suffering from Alzheimer's disease. Hyperammonemia in the brain, typically occurring as a secondary complication of primary liver disease and known as hepatic encephalopathy, is a condition that has an impact on glutamate/GABA-glutamine cycling in the brain.^[9]

Potential Drug Targets

In the treatment of epilepsy, drugs targeting both GABA transporters and the GABA metabolizing enzyme GABA-transaminase (vigabatrin) have been marketed, providing proof of principle for the neurotransmitter cycling systems as pharmacological targets. However, with regard to glutamate transport and metabolism no such drugs have been developed This is because glutamatergic synapses are abundant and the neurotransmitter glutamate is an important metabolite in metabolism, making interference capable of adverse effects. So far, most of the drug development directed at the glutamatergic system seems to have been focused on ionotropic glutamate receptors as pharmacological targets, although G-protein coupled receptors have been attracting increased attention over the years.^[10]

References