NMDA receptor


The N-methyl-D-aspartate 'receptor', is a glutamate receptor and predominantly Ca2+ ion channel found in neurons. The NMDA receptor is one of three types of ionotropic glutamate receptors, the other two being AMPA and kainate receptors. Depending on its subunit composition, its ligands are glutamate and glycine. However, the binding of the ligands is typically not sufficient to open the channel as it may be blocked by Mg2+ ions which are only removed when the neuron is sufficiently depolarized. Thus, the channel acts as a "coincidence detector" and only once both of these conditions are met, the channel opens and it allows positively charged ions to flow through the cell membrane. The NMDA receptor is thought to be very important for controlling synaptic plasticity and mediating learning and memory functions.
The NMDA receptor is ionotropic, meaning it is a protein which allows the passage of ions through the cell membrane. The NMDA receptor is so named because the agonist molecule N-methyl-D-aspartate binds selectively to it, and not to other glutamate receptors. Activation of NMDA receptors results in the opening of the ion channel that is nonselective to cations, with a combined reversal potential near 0 mV. While the opening and closing of the ion channel is primarily gated by ligand binding, the current flow through the ion channel is voltage-dependent. Specifically located on the receptor, extracellular magnesium and zinc ions can bind and prevent other cations from flowing through the open ion channel. A voltage-dependent flow of predominantly calcium, sodium, and potassium ions into and out of the cell is made possible by the depolarization of the cell, which displaces and repels the Mg2+ and Zn2+ ions from the pore. Ca2+ flux through NMDA receptors in particular is thought to be critical in synaptic plasticity, a cellular mechanism for learning and memory, due to proteins which bind to and are activated by Ca2+ ions.
Activity of the NMDA receptor is blocked by many psychoactive drugs such as phencyclidine, alcohol and dextromethorphan. The anaesthetic and analgesic effects of the drugs ketamine and nitrous oxide are also partially due to their effects at blocking NMDA receptor activity. In contrast, overactivation of NMDAR by NMDA agonists increases the cytosolic concentrations of calcium and zinc, which significantly contributes to neural death, an effect known to be prevented by cannabinoids, mediated by activation of the CB1 receptor, which leads HINT1 protein to counteract the toxic effects of NMDAR-mediated NO production and zinc release. As well as preventing methamphetamine-induced neurotoxicity via inhibition of nitric oxide synthase expression and astrocyte activation, it is seen to reduce methamphetamine-induced brain damage through CB1-dependent and independent mechanisms, respectively, and inhibition of methamphetamine-induced astrogliosis is likely to occur through a CB2 receptor dependent mechanism for THC. Since 1989, memantine has been recognized to be an uncompetitive antagonist of the NMDA receptor, entering the channel of the receptor after it has been activated and thereby blocking the flow of ions.
Overactivation of the receptor, causing excessive influx of Ca2+ can lead to excitotoxicity which is implied to be involved in some neurodegenerative disorders. Blocking of NMDA receptors could therefore, in theory, be useful in treating such diseases. However, hypofunction of NMDA receptors may be involved in impairment of synaptic plasticity and could have other negative repercussions. The main problem with the utilization of NMDA receptor antagonists for neuroprotection is that the physiological actions of the NMDA receptor are essential for normal neuronal function. To be clinically useful NMDA antagonists need to block excessive activation without interfering with normal functions. Memantine has this property.

History

The discovery of NMDA receptors was followed by the synthesis and study of N-methyl-D-aspartic acid in the 1960s by Jeff Watkins and colleagues. In the early 1980s, NMDA receptors were shown to be involved in several central synaptic pathways. Receptor subunit selectivity was discovered in the early 1990s, which led to recognition of a new class of compounds that selectively inhibit the NR2B subunit. These findings led to vigorous campaign in the pharmaceutical industry. From this it was considered that NMDA receptors were associated with a variety of neurological disorders such as epilepsy, Parkinson's, Alzheimer's, Huntington's and other CNS disorders.
In 2002, it was discovered by Hilmar Bading and co-workers that the cellular consequences of NMDA receptor stimulation depend on the receptor's location on the neuronal cell surface. Synaptic NMDA receptors promote gene expression, plasticity-related events, and acquired neuroprotection. Extrasynaptic NMDA receptors promote death signaling; they cause transcriptional shut-off, mitochondrial dysfunction, and structural disintegration. This pathological triad of extrasynaptic NMDA receptor signaling represents a common conversion point in the etiology of several acute and chronic neurodegenerative conditions. The molecular basis for toxic extrasynaptic NMDA receptor signaling was uncovered by Hilmar Bading and co-workers in 2020. Extrasynaptic NMDA receptors form a death signaling complex with TRPM4. NMDAR/TRPM4 interaction interface inhibitors disrupt the NMDAR/TRPM4 complex and detoxify extrasynaptic NMDA receptors.
A fortuitous finding was made in 1968 when a woman was taking amantadine as flu medicine and experienced remarkable remission of her Parkinson's symptoms. This finding, reported by Scawab et al., was the beginning of medicinal chemistry of adamantane derivatives in the context of diseases affecting the CNS. Before this finding, memantine, another adamantane derivative, had been synthesized by Eli Lilly and Company in 1963. The purpose was to develop a hypoglycemic drug, but it showed no such efficacy. It was not until 1972 that a possible therapeutic importance of memantine for treating neurodegenerative disorders was discovered. From 1989 memantine has been recognized to be an uncompetitive antagonist of the NMDA receptor.

Structure

Functional NMDA receptors are heterotetramers comprising different combinations of the GluN1, GluN2, and GluN3 subunits derived from distinct gene families. All NMDARs contain two of the obligatory GluN1 subunits, which when assembled with GluN2 subunits of the same type, give rise to canonical diheteromeric NMDARs. Triheteromeric NMDARs, by contrast, contain three different types of subunits, and include receptors that are composed of one or more subunits from each of the three gene families, designated t-NMDARs. There is one GluN1, four GluN2, and two GluN3 subunit encoding genes, and each gene may produce more than one splice variant.
The NMDA receptor is a glutamate and ion channel protein receptor that is activated when glycine and glutamate bind to it. The receptor is a highly complex and dynamic heteromeric protein that interacts with a multitude of intracellular proteins via three distinct subunits, namely GluN1, GluN2, and GluN3. The GluN1 subunit, which is encoded by the GRIN1 gene, exhibits eight distinct isoforms owing to alternative splicing. On the other hand, the GluN2 subunit, of which there are four different types, as well as the GluN3 subunit, of which there are two types, are each encoded by six separate genes. This intricate molecular structure and genetic diversity enable the receptor to carry out a wide range of physiological functions within the nervous system. All the subunits share a common membrane topology that is dominated by a large extracellular N-terminus, a membrane region comprising three transmembrane segments, a re-entrant pore loop, an extracellular loop between the transmembrane segments that are structurally not well known, and an intracellular C-terminus, which are different in size depending on the subunit and provide multiple sites of interaction with many intracellular proteins. Figure 1 shows a basic structure of GluN1/GluN2 subunits that forms the binding site for memantine, Mg2+ and ketamine.
Mg2+ blocks the NMDA receptor channel in a voltage-dependent manner. The channels are also highly permeable to Ca2+. Activation of the receptor depends on glutamate binding, D-serine or glycine binding at its GluN1-linked binding site and AMPA receptor-mediated depolarization of the postsynaptic membrane, which relieves the voltage-dependent channel block by Mg2+. Activation and opening of the receptor channel thus allows the flow of K+, Na+ and Ca2+ ions, and the influx of Ca2+ triggers intracellular signaling pathways. Allosteric receptor binding sites for zinc, proteins and the polyamines spermidine and spermine are also modulators for the NMDA receptor channels.
The GluN2B subunit has been involved in modulating activity such as learning, memory, processing and feeding behaviors, as well as being implicated in number of human derangements. The basic structure and functions associated with the NMDA receptor can be attributed to the GluN2B subunit. For example, the glutamate binding site and the control of the Mg2+ block are formed by the GluN2B subunit. The high affinity sites for glycine antagonist are also exclusively displayed by the GluN1/GluN2B receptor.
GluN1/GluN2B transmembrane segments are considered to be the part of the receptor that forms the binding pockets for uncompetitive NMDA receptor antagonists, but the transmembrane segments structures are not fully known as stated above. It is claimed that three binding sites within the receptor, A644 on the GluNB subunit and A645 and N616 on the GluN1 subunit, are important for binding of memantine and related compounds as seen in figure 2.
The NMDA receptor forms a heterotetramer between two GluN1 and two GluN2 subunits, two obligatory GluN1 subunits and two regionally localized GluN2 subunits. A related gene family of GluN3 A and B subunits have an inhibitory effect on receptor activity. Multiple receptor isoforms with distinct brain distributions and functional properties arise by selective splicing of the GluN1 transcripts and differential expression of the GluN2 subunits.
Each receptor subunit has modular design and each structural module, also represents a functional unit:
  • The extracellular domain contains two globular structures: a modulatory domain and a ligand-binding domain. GluN1 subunits bind the co-agonist glycine and GluN2 subunits bind the neurotransmitter glutamate.
  • The agonist-binding module links to a membrane domain, which consists of three transmembrane segments and a re-entrant loop reminiscent of the selectivity filter of potassium channels.
  • The membrane domain contributes residues to the channel pore and is responsible for the receptor's high-unitary conductance, high-calcium permeability, and voltage-dependent magnesium block.
  • Each subunit has an extensive cytoplasmic domain, which contains residues that can be directly modified by a series of protein kinases and protein phosphatases, as well as residues that interact with a large number of structural, adaptor, and scaffolding proteins.
The glycine-binding modules of the GluN1 and GluN3 subunits and the glutamate-binding module of the GluN2A subunit have been expressed as soluble proteins, and their three-dimensional structure has been solved at atomic resolution by x-ray crystallography. This has revealed a common fold with amino acid-binding bacterial proteins and with the glutamate-binding module of AMPA receptors and kainate receptors.