Glutamate transporters are family of neurotransmitter transporters that contains two subclasses: the excitatory amino acid transporters (EAATs) and the vesicular glutamate transporters (VGLUTs). Glutamate transporters are proteins that move glutamate – the principal excitatory neurotransmitter – across a membrane; in the brain, they normally serve to remove glutamate from the synaptic cleft and extrasynaptic sites via reuptake into neuroglia and neurons as well as move glutamate from the cytoplasm into synaptic vesicles. Glutamate transporters also transport aspartate and are present in virtually all peripheral tissues including bone, heart, liver, and testes. They exhibit stereoselectivity for L-glutamate but transport both L- and D-aspartate.
The EAATs are membrane-bound secondary transporters that superficially resemble ion channels. These transporters play the important role of regulating concentrations of glutamate in the extracellular space by transporting it along with other ions across cellular membranes. After glutamate is released as the result of an action potential, glutamate transporters quickly remove it from the extracellular space to keep its levels low, thereby terminating the synaptic transmission.
Without the activity of glutamate transporters, glutamate would build up and kill cells in a process called excitotoxicity, in which excessive amounts of glutamate acts as a toxin to neurons by triggering a number of biochemical cascades. The activity of glutamate transporters also allows glutamate to be recycled for repeated release.
|EAAT2||SLC1A2||astroglial cells; low levels in some neurons|
|EAAT3||SLC1A1||all neurons - dendrites and axon-terminals|
There are two general classes of glutamate transporters, those that are dependent on an electrochemical gradient of sodium ions (the EAATs) and those that are not (VGLUTs and xCT). The cystine-glutamate antiporter (xCT) is localised to the plasma membrane of cells whilst vesicular glutamate transporters (VGLUTs) are found in the membrane of glutamate-containing synaptic vesicles. Na+-dependent EAATs are also dependent on transmembrane K+ and H+concentration gradients, and so are also known as 'sodium and potassium coupled glutamate transporters'. Na+-dependent transporters have also been called 'high-affinity glutamate transporters', though their glutamate affinity actually varies widely.
In humans (as well as in rodents), five subtypes have been identified and named EAAT1-5 (SLC1A3, SLC1A2, SLC1A1, SLC1A6, SLC1A7). Subtypes EAAT1-2 are found in membranes of glial cells (astrocytes, microglia, and oligodendrocytes). However, low levels of EAAT2 are also found in the axon-terminals of hippocampal CA3 pyramidal cells. EAAT2 is responsible for over 90% of glutamate reuptake within the brain. The EAAT3-4 subtypes are exclusively neuronal, and are expressed in axon terminals, cell bodies, and dendrites. Finally, EAAT5 is only found in the retina where it is principally localized to photoreceptors and bipolar neurons in the retina.
When glutamate is taken up into glial cells by the EAATs, it is converted to glutamine and subsequently transported back into the presynaptic neuron, converted back into glutamate, and taken up into synaptic vesicles by action of the VGLUTs. This process is named the glutamate-glutamine cycle.
Three types of vesicular glutamate transporters are known, VGLUTs 1–3 (SLC17A7, SLC17A6, and SLC17A8 respectively) and the novel glutamate/aspartate transporter sialin. These transporters pack the neurotransmitter into synaptic vesicles so that they can be released into the synapse. VGLUTs are dependent on the proton gradient that exists in the secretory system (vesicles being more acidic than the cytosol). VGLUTs have only between one hundredth and one thousandth the affinity for glutamate that EAATs have. Also unlike EAATs, they do not appear to transport aspartate.
During injury processes such as ischemia and traumatic brain injury, the action of glutamate transporters may fail, leading to toxic buildup of glutamate. In fact, their activity may also actually be reversed due to inadequate amounts of adenosine triphosphate to power ATPase pumps, resulting in the loss of the electrochemical ion gradient. Since the direction of glutamate transport depends on the ion gradient, these transporters release glutamate instead of removing it, which results in neurotoxicity due to overactivation of glutamate receptors.
Loss of the Na+-dependent glutamate transporter EAAT2 is suspected to be associated with neurodegenerative diseases such as Alzheimer's disease, Huntington's disease, and ALS–parkinsonism dementia complex. Also, degeneration of motor neurons in the disease amyotrophic lateral sclerosis has been linked to loss of EAAT2 from patients' brains and spinal cords.
- Dopamine transporters
- Norepinephrine transporters
- Serotonin transporters
- NMDA receptors
- AMPA receptors
- Kainate receptors
- Metabotropic glutamate receptors
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TAAR1 overexpression significantly decreased EAAT-2 levels and glutamate clearance ... METH treatment activated TAAR1 leading to intracellular cAMP in human astrocytes and modulated glutamate clearance abilities. Furthermore, molecular alterations in astrocyte TAAR1 levels correspond to changes in astrocyte EAAT-2 levels and function.
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Since then, a family of five high-affinity glutamate transporters has been characterized that is responsible for the precise regulation of glutamate levels at both synaptic and extrasynaptic sites, although the glutamate transporter 1 (GLT1) is responsible for more than 90% of glutamate uptake in the brain.3 The importance of GLT1 is further highlighted by the large number of neuropsychiatric disorders associated with glutamate-induced neurotoxicity.
Clarification of nomenclature
The major glial glutamate transporter is referred to as GLT1 in the rodent literature and excitatory amino acid transporter 2 (EAAT2) in the human literature.
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