Silvia Catagini Elena D'Ambrosio
GLUTAMATE AS NEUROTRANSMITTER
Glutamic acid (abbreviated as Glu or E) is one of the 20-22 proteinogenic amino acids, and its codons are GAA and GAG. It is a non-essential amino acid with a side chain carboxylic acid functional group. The carboxylate anions and salts of glutamic acid are known as glutamates. Glutamate is the most abundant excitatory neurotransmitter in the nervous system and it plays a key role in long-term potentiation. It is involved in most aspects of normal brain functions, including cognition, memory and learning.
Outside the community of biomedical scientists, glutamate is probably best known as "monosodium glutamate" which is used as a flavor or taste enhancer in food.
Significant amounts of free glutamic acid are present in a wide variety of foods, including cheese and soy sauce, and it is responsible for umami, one of the five basic tastes of the human sense of taste.
BIOSYNTHESIS
Glutamate is biosynthesized in the mitochondrion from the tricarboxylic acid (TCA) cycle intermediate α-ketoglutarate by transaminase.
Glutamate does not pass easily the blood brain barrier but it is instead transported by a high-affinity transport system. It can also be converted into glutamine. Glutamine can cross the blood brain barrier, and then be converted into glutamate by the action of phosphate-activated Glutaminase. This is the full reaction: Glutamine + H2O → Glutamate + NH3
It appears that glutamate derived from glutamine via this route is produced intramitochondrially and may subsequently undergo a transamination catalyzed by the mitochondrial isoform of aspartate aminotransferase. The α-ketoglutarate thus formed is translocated out of the mitochondria by the dicarboxylate carrier and transaminated in the cytoplasm by the cytoplasmic isoform of aspartate aminotransferase. Alternatively, glutamate may be formed from α-ketoglutarate and alanine catalyzed by alanine amino- transferase. This cytoplasmic glutamate is transported into vesicles by vesicular glutamate transporters.
The neurotransmitter GABA is formed from glutamate by the action of glutamate decarboxylase. It appears that glutamine serves as the precursor for glutamate, making phosphate-activated glutaminase, an important enzyme for GABA synthesis.
STORAGE, EXOCYTOSIS AND RECEPTORS
In presynaptic terminals, glutamate is stored in vesicles in the axon, and it is released by an increased concentration of intracellular Ca2+, due to the activation of voltage gated channels for Calcium. The synaptic release of glutamate is controlled by a wide range of presynaptic receptor. These include not only group II and III of metabotropic receptors, but also colinergic receptors, GABAB, and neuropeptide Y receptors.
The glutamate released in the synaptic cleft binds to its receptor on the postsynaptic terminals, and it produces an excitatory postsynaptic potential (EPSP).
The glutamate receptors can be divided into two groups according to the mechanism by which their activation gives rise to a postsynaptic current. Ionotropic glutamate receptors (iGluRs) form the ion channel pore that activates when glutamate binds to the receptor. Metabotropic glutamate receptors (mGluRs) indirectly activate ion channels on the plasma membrane through a signaling cascade that involves G proteins.
Ionotropic receptors tend to be quicker in relaying information, but metabotropic ones are associated with a more prolonged stimulus. This is due to the usage of many different messengers to carry out the signal, but since there is a cascade, just one activation of a G-protein can lead to multiple activations. Glutamate receptors are usually not specifically geared towards exclusively glutamate as the ligand, and sometimes another agonist is required.
Ionotropic receptors
There are three families of ionotropic receptors, divided into two groups: NMDA receptors and non-NMDA receptors. The non-NMDA receptors are divided into AMPA receptors and Kainate receptors.
All glutamate ionotropic receptors are ligand-gated nonselective cation channels which allow the flow of K+, Na+ and sometimes Ca2+ in response to glutamate binding. These receptors are pentameric.
Upon binding, the agonist will stimulate direct action of the central pore of the receptor, an ion channel, allowing ion flow and causing EPSC (excitatory postsynaptic current). This current is depolarizing and, if enough glutamate receptors are activated, may trigger an action potential in the postsynaptic neuron. All produce excitatory postsynaptic current, but the speed and duration of the current is different for each type. NMDA receptors have an internal binding site for an Mg2+ ion, creating a voltage-dependent block which is removed by outward flow of positive current. Since the block must be removed by outward current flow, NMDA receptors rely on the EPSC produced by AMPA receptors to open. NMDA receptors are permeable to Ca2+. The flow of Ca2+ through NMDA receptors is thought to cause both LTP and LTD by transducing signaling cascades and regulating gene expression.
Metabotropic receptors
These receptors share a common molecular morphology with other G protein–linked metabotropic receptors. They are presumed to have seven trans-membrane domains with an extracellular N-terminal and intracellular COOH terminal. They have little sequence homology with other metabotropic receptors, except for a modest resemblance to GABAB receptors.
Group I receptors activate phospholipase C, producing diacylglycerol and inositol triphosphate as second messengers. Groups II and III are negatively coupled to adenylyl cyclase.
Studies using oocyte or human embryonic kidney cells expressing specific mGluR show marked variation in the sensitivity of the receptors to glutamate, with mGluR7 being remarkably insensitive. The sensitivity to glutamate has to be considered in relation to the location of the receptor on the cell membrane relative to the synaptic cleft. Immunochemistry at the electron microscopy (EM) level reveals a highly selective expression of mGluR), with some occurring presynaptically in close relationship to the presynaptic density (mGluR7, mGluR8) and some occurring on the presynaptic axon, relatively distant from the synaptic cleft (mGluR2, mGluR3)
Glutamate transporters
The mechanisms which can maintain low extracellular concentrations of glutamate are essential for brain functioning. The only (significant) mechanism for removal of glutamate from the extracellular fluid is cellular uptake of glutamate, the so called “glutamate uptake”. This uptake is mediated by a family of special transporter proteins which act as pumps. These proteins bind glutamate, one molecule at the time, and transfer them into the cells. In agreement with the abundance of glutamate and the ubiquity of glutamate receptors, brain tissue displays a very high glutamate uptake activity.
Glutamate is taken up into both glial cells and nerve terminals. The former is believed to be the more important from a quantitative point of view. Glutamate taken up by astroglial cells is converted to glutamine. Glutamine is inactive in the sense that it cannot activate glutamate receptors, and it is released from the glial cells into to extracellular fluid. Nerve terminals take up glutamine and convert glutamine back to glutamate. This process is referred to as the glutamate-glutamine, and it is important because it allows glutamate to be inactivated by glial cells and transported back to neurons in an inactive (non-toxic) form.
The glutamate transporters (Excitatory Amino Acid Transporters; EAATs) are indicated in red. These are found in the plasma membranes. The glutamate-cystine exchanger (xCT) uses the transmembrane gradient of glutamate to import cystine needed for glutathione synthesis. The vesicular glutamate transporters (VGLUTs) pack glutamate into synaptic vesicles.
GLUTAMATE TOXICITY
Glutamate is toxic, not in spite of its importance, but because of it
. Glutamate mediate a lot of information, included information that regulates brain development and information which determines cellular survival, differentiation and elimination as well as formation and elimination of nerve contacts (synapses). From this ,it follows that glutamate has to be present in the right concentrations in the right places for the right time. Both too much and too little glutamate is harmful. This implies that glutamate is both essential and highly toxic at the same time.
Transporters found in neuronal and glial membranes rapidly remove glutamate from extracellular space. In brain injury or disease, they can work in reverse, and excess glutamate can accumulate outside cells. This process causes calcium ions to enter cells via NMDA receptor channels, leading to neuronal damage and eventual cell death, and is called excitotoxicity. The mechanisms of cell death include damage to mitochondria from excessively high intracellular Ca2+ and Glu/Ca2+-mediated promotion of transcription factors for pro-apoptotic genes, or downregulation of transcription factors for anti-apoptotic genes.
Excitotoxicity due to glutamate occurs as part of the ischemic cascade and is associated with stroke and diseases like amyotrophic lateral sclerosis, lathyrism, autism, some forms of mental retardation, and Alzheimer's disease.
Glutamate is of particular interest to neurologists because of its possible involvement in acute or chronic neurodegenerative processes. It is useful to consider three distinct possible mechanisms. One is the possibility that exogenous glutamate, or related compounds acting on glutamate receptors, can be consumed in the diet and damage the brain. Second, there is the possibility that endogenous glutamate released from neurons can contribute to acute neurodegeneration occurring in relation to cerebral ischemia or traumatic brain injury. Third, there is the possibility that activation of glutamate receptors contributes to the process of cell death in chronic neurodegenerative disorders, such as motor neuron disease (MND) or amyotrophic lateral sclerosis (ALS), Huntington’s disease, Parkinson’s disease and Alzheimer’s disease.
Glutamic acid is also implicated in epileptic seizures. Microinjection of glutamic acid into neurons produces spontaneous depolarizations around one second apart, and this firing pattern is similar to what is known as paroxysmal depolarizing shift in epileptic attacks. This change in the resting membrane potential at seizure foci could cause spontaneous opening of voltage-activated calcium channels, leading to glutamic acid release and further depolarization.
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