Глутаматные транспортеры: функции, подтипы и роль в нейротрансмиссии.

Family of neurotransmitter transporter proteins
Glutamate transporters are a family of neurotransmitter transporter proteins that move glutamate – the principal excitatory neurotransmitter – across a membrane. The family of glutamate transporters is composed of two primary subclasses: the excitatory amino acid transporter (EAAT) family and vesicular glutamate transporter (VGLUT) family. In the brain, EAATs remove glutamate from the synaptic cleft and extrasynaptic sites via glutamate reuptake into glial cells and neurons, while VGLUTs move glutamate from the cell cytoplasm into synaptic vesicles. Glutamate transporters also transport aspartate and are present in virtually all peripheral tissues, including the heart, liver, testes, and bone. They exhibit stereoselectivity for L glutamate but transport both L aspartate 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.

Three types of vesicular glutamate transporters are known, VGLUTs 1–3 (SLC17A7, SLC17A6, and SLC17A8 respectively) 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. The role of this unconventional transporter (VGluT3) still remains unknown but, at the moment, has been demonstrated that, in auditory system, the VGluT3 is involved in fast excitatory glutamatergic transmission very similar to the other two vesicular glutamate transporters, VGluT1 and VGluT2. There are behavioral and physiological consequences of VGluT3 ablation because it modulates a wide range of neuronal and physiological processes like anxiety, mood regulation, impulsivity, aggressive behavior, pain perception, sleep–wake cycle, appetite, body temperature and sexual behavior. Certainly, no significant change was found in aggression and depression like behaviors, but in contrast, the loss of VGluT3 resulted in a specific anxiety related phenotype. The sensory nerve fibers have different ways to detect the pain hypersensivity throughout their sensory modalities and conduction velocities, but at the moment is still unknown which types of sensory is related to the different forms of inflammatory and neuropathic pain hypersensivity. In this case, Vesicular glutamate transporter 3 (VGluT3), have been implicated in mechanical hypersensitivity after inflammation, but their role in neuropathic pain still remains under debate. VGluT3 has extensive somatic throughout development, which could be involved in non synaptic modulation by glutamate in developing retina, and could influence trophic and extra synaptic neuronal signaling by glutamate in the inner retina.

Like all glutamate transporters, EAATs are trimers, with each protomer consisting of two domains : the central scaffold domain (Figure 1A, wheat) and the peripheral transport domain (Figure 1A, blue). The transport conformational path is as follows. First, the outward facing conformation occurs (OF, open) which allows the glutamate to bind. Then the HP2 region closes after uptake (OF, closed) and the elevator like movement carries the substrate to the intracellular side of the membrane. It worth nothing that this elevator motion consists of several yet to be categorized/identified conformational changes. After the elevator motion brings the substrate to the IC side of the membrane, EAAT adopts the inward facing (IF, closed) state in which the transport domain is lowered, but the HP2 gate is still closed with the glutamate still bound to the transporter. Lastly, the HP2 gate opens and the glutamate diffuses into the cytoplasm of the cell.

Overactivity of glutamate transporters may result in inadequate synaptic glutamate and may be involved in schizophrenia and other mental illnesses. 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.