Glutamate Synthesis, Release and Inactivation
Glutamate is the term we use for the ionized (i.e., electrically charged) form of the amino acid glutamic acid. Since most of the glutamic acid in our bodies is in this ionized state, we will refer to it as glutamate throughout the text. Like other common amino acids, glutamate is used by all of our cells to help make new proteins. But glutamate also has numerous other biochemical functions (for example, in energy metabolism), which is reflected in the fact that it is the most abundant amino acid in the brain. Glutamate and aspartate (the name for the ionized form of aspartic acid) are the two principal members of a small family of excitatory amino acid neurotransmitters. These transmitters are so named because they cause a powerful excitatory response when applied to most neurons in the brain or spinal cord. We will focus on glutamate, which seems to be the more widely used excitatory amino acid transmitter and which has been more intensively studied than aspartate.
Neurons generate glutamate from the precursor glutamine
When a nerve cell synthesizes a molecule of norepinephrine (NE), acetylcholine (ACh), or serotonin (5-HT), it is almost always for the purpose of neurotransmission. Moreover, in the brain these substances are localized specifically within the cells using them as transmitters. However, we must recognize that the situation is different for glutamate due to its roles in protein synthesis and general cellular metabolism. First, all neurons and glial cells contain significant amounts of glutamate, although neurons that use glutamate as a transmitter (called glutamatergic neurons) possess even greater concentrations than other cells in the brain. Second, glutamatergic neurons are thought to segregate the pool of glutamate they use for transmission from the pool of glutamate used for other cellular functions. These facts complicate both our ability to determine which nerve cells actually are glutamatergic and our understanding of how these cells synthesize and dispose of the transmitter-related glutamate. Nevetheless, researchers have accumulated considerable information, which we summarize in this section.
Glutamate can be synthesized by several different chemical reactions. Most molecules of glutamate are derived ultimately from the normal metabolic breakdown of the sugar glucose. The more immediate precursor for much of the transmitter-related glutamate is a related substance known as glutamine. Neurons can transform glutamine into glutamate using an enzyme called glutaminase. We will see in the next section that the role of glutamine in glutamate synthesis involves a fascinating metabolic partnership between glutamatergic neurons and neighboring glial cells, specifically astrocytes.
Glutamate is released from vesicles and removed from the synaptic cleft by both neuronal and glial transport systems
For a long time, no one knew how glutamate got into synaptic vesicles for the purpose of storage and release. Then between the years 2000 and 2002, researchers discovered three distinct proteins that package glutamate into vesicles: VGLUT1, VGLUT2, and VGLUT3 (VGLUT standing for vesicular glutamate transporter). These proteins provide good markers for glutamatergic neurons, because unlike glutamate itself, they are found only in cells that use glutamate as a neurotransmitter. Glutamatergic neurons generally possess either VGLUT1 or VGLUT2 (but not both), with VGLUT3 being less abundant than the other two transporters. mRNAs for the VGLUT1 and VGLUT2 genes show very little overlap across different brain regions, confirming that the glutamatergic neurons in most regions manufacture only one VGLUT. What difference does it make which vesicular glutamate transporter is expressed by a particular nerve cell? This question is being investigated, but we don’t yet have a clear answer.
After glutamate molecules are released into the synaptic cleft, they are rapidly removed by other glutamate transporters located on cell membranes. Always keep in mind that the plasma membrane transporters that remove neurotransmitters from the synaptic cleft are distinct from the transporters on the vesicle membranes that are responsible for loading the vesicles in preparation for transmitter release. In the case of glutamate, five different plasma membrane transporters have already been identified. Because these transporters take up aspartate as well as glutamate, they are called EAAT1-EAAT5 (EAAT standing for excitatory amino acid transporter). Two of these transporters, EAAT1 and EAAT2, are located mainly on astrocytes instead of neurons. Of the neuronal transporters, EAAT3 is the most widely distributed in the brain. As we will see later, prolonged high levels of glutamate in the extracellular fluid are very dangerous, produc-ing excessive neuronal excitation and even cell death. With this in mind, it is interesting to discover that uptake by astrocytes seems to be particularly important in controlling the amount of extracellular glutamate.
For example, there is evidence that more than half of patients with amyotrophic lateral sclerosis (ALS; also known as Lou Gehrig’s disease), a neurological disorder involving degeneration of motor neurons in the spinal cord and cortex, have abnormalities in EAAT2 in the affected areas of their nervous systems (Lin et al., 1998). In rats, inhibition of EAAT1 or EAAT2 synthesis led to large increases in extracellular glutamate levels in the striatum, indicating that these transporters are the most important ones for normal glutamate uptake in this brain area (Rothstein et al., 1996). Furthermore, there were signs of neural degeneration in the striatum in the treated animals, and all of the animals exhibited progressive motor deficits. In contrast, inhibition of the neuronal glutamate transporter EAAT3 was much less effective in producing either neural degeneration or behavioral symptoms.
Besides playing a key role in removing excess glutamate from the extracellular space, the astrocyte transporters are also intimately involved in the metabolic partnership between neurons and astrocytes. After astrocytes have taken up glutamate by means of EAAT1 or EAAT2, they convert a major portion of it to glutamine by means of an enzyme called glutamine synthetase. The glutamine is then transported out of the astrocytes and picked up by neurons, where it can be converted back into glutamate by glutaminase, as described earlier. This interplay between glutamatergic neurons and neighboring astrocytes. It is reasonable to wonder why such a complex system has evolved; why don’t the neurons themselves have the primary responsibility for glutamate reuptake, as we have seen previously for the catecholamines neurotransmitters and for serotonin? Although we aren’t certain about the answer to this question, it’s worth noting that glutamine does not produce neuronal excitation and therefore is not potentially dangerous like glutamate. Hence, glial cell production of glutamine may be the brain’s way of storing glutamate in a form that is “safe” but still available for use once the glutamine has been transferred to the neurons and reconverted to glutamate.
Glutamate and aspartate are amino acid neurotransmitters that have potent excitatory effects on neurons throughout the brain and spinal cord. Although glutamate is contained within all cells due to its multiple biochemical functions, glutamatergic neurons are thought to possess higher glutamate concentrations than other cells and to segregate their neurotransmitter pool of this amino acid. Many of the glutamate molecules that are released synaptically are synthesized from glutamine in a chemical reaction catalyzed by the enzyme glutaminase.
Glutamate is packaged into vesicles by the vesicular transporters VGLUT1, VGLUT2, and VGLUT3. After being released, glutamate molecules are removed from the extracellular space by several different excitatory amino acid transporters, designated EAAT1-EAAT5. EAAT1 and EAAT2 mediate glutamate uptake into astrocytes, after which some of the glutamate is converted into glutamine. This glutamine can subsequently be transported from the astrocytes to the glutamatergic neurons, where it is transformed back into glutamate and reutilized. This constitutes an important metabolic interplay between glutamatergic nerve cells and their neighboring glial cells. The importance of EAAT2, in particular, is exemplified in recent findings that many patients suffering from the neurological disorder ALS seem to have abnormalities in this transporter.