NMDA-рецептор: структура, функции и роль в нейропластичности и нейротоксичности.

Glutamate receptor and ion channel protein found in nerve cells

The N methyl D aspartate receptor (also known as the NMDA receptor or NMDAR), 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 (or D serine). 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 (cations) to flow through the cell membrane. 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 (NMDA) 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 (Mg2+) and zinc (Zn2+) ions can bind and prevent other cations from flowing through the open ion channel. A voltage dependent flow of predominantly calcium (Ca2+), sodium (Na+), and potassium (K+) 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 (PCP), alcohol (ethanol) and dextromethorphan (DXM). 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 (nNOS) 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 (due to glutathione deficiency or other causes) 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.

While a single GluN2 subunit is found in invertebrate organisms, four distinct isoforms of the GluN2 subunit are expressed in vertebrates and are referred to with the nomenclature GluN2A through GluN2D (encoded by GRIN2A, GRIN2B, GRIN2C, GRIN2D). Strong evidence shows that the genes encoding the GluN2 subunits in vertebrates have undergone at least two rounds of gene duplication. They contain the binding site for glutamate. More importantly, each GluN2 subunit has a different intracellular C terminal domain that can interact with different sets of signaling molecules. Unlike GluN1 subunits, GluN2 subunits are expressed differentially across various cell types and developmental timepoints and control the electrophysiological properties of the NMDA receptor. In classic circuits, GluN2B is mainly present in immature neurons and in extrasynaptic locations such as growth cones, and contains the binding site for the selective inhibitor ifenprodil. However, in pyramidal cell synapses in the newly evolved primate dorsolateral prefrontal cortex, GluN2B are exclusively within the postsynaptic density, and mediate higher cognitive operations such as working memory. This is consistent with the expansion in GluN2B actions and expression across the cortical hierarchy in monkeys and humans and across primate cortex evolution.

Granule cell precursors (GCPs) of the cerebellum, after undergoing symmetric cell division in the external granule cell layer (EGL), migrate into the internal granule cell layer (IGL) where they down regulate GluN2B and activate GluN2C, a process that is independent of neuregulin beta signaling through ErbB2 and ErbB4 receptors.

NMDA receptors have been implicated by a number of studies to be strongly involved with excitotoxicity. Because NMDA receptors play an important role in the health and function of neurons, there has been much discussion on how these receptors can affect both cell survival and cell death. Recent evidence supports the hypothesis that overstimulation of extrasynaptic NMDA receptors has more to do with excitotoxicity than stimulation of their synaptic counterparts. Many of these pathways use the same protein signals, but are regulated oppositely by NMDARs depending on its location. For example, synaptic NMDA excitation caused a decrease in the intracellular concentration of p38 mitogen activated protein kinase (p38MAPK). Extrasynaptic stimulation NMDARs regulated p38MAPK in the opposite fashion, causing an increase in intracellular concentration. Experiments of this type have since been repeated with the results indicating these differences stretch across many pathways linked to cell survival and excitotoxicity.

NMDA receptors (NMDARs) critically influence the induction of synaptic plasticity. NMDARs trigger both long term potentiation (LTP) and long term depression (LTD) via fast synaptic transmission. Experimental data suggest that extrasynaptic NMDA receptors inhibit LTP while producing LTD. Inhibition of LTP can be prevented with the introduction of a NMDA antagonist. Experimentation also indicates that extrasynaptic activity is not required for the formation of LTP. In addition, both synaptic and extrasynaptic activity are involved in expressing a full LTD.

Another factor that seems to affect NMDAR induced toxicity is the observed variation in subunit makeup. NMDA receptors are heterotetramers with two GluN1 subunits and two variable subunits. Two of these variable subunits, GluN2A and GluN2B, have been shown to preferentially lead to cell survival and cell death cascades respectively. Although both subunits are found in synaptic and extrasynaptic NMDARs there is some evidence to suggest that the GluN2B subunit occurs more frequently in extrasynaptic receptors. This observation could help explain the dualistic role that NMDA receptors play in excitotoxicity. t NMDA receptors have been implicated in excitotoxicity mediated death of neurons in temporal lobe epilepsy. Despite the compelling evidence and the relative simplicity of these two theories working in tandem, there is still disagreement about the significance of these claims. Some problems in proving these theories arise with the difficulty of using pharmacological means to determine the subtypes of specific NMDARs. In addition, the theory of subunit variation does not explain how this effect might predominate, as it is widely held that the most common tetramer, made from two GluN1 subunits and one of each subunit GluN2A and GluN2B, makes up a high percentage of the NMDARs.

Excitotoxicity has been thought to play a role in the degenerative properties of neurodegenerative conditions since the late 1950s. NMDA receptors seem to play an important role in many of these degenerative diseases affecting the brain. Most notably, excitotoxic events involving NMDA receptors have been linked to Alzheimer's disease and Huntington's disease, as well as with other medical conditions such as strokes and epilepsy. Treating these conditions with one of the many known NMDA receptor antagonists, however, leads to a variety of unwanted side effects, some of which can be severe. These side effects are, in part, observed because the NMDA receptors do not just signal for cell death but also play an important role in its vitality. D serine, an antagonist/inverse co agonist of t NMDA receptors, which is made in the brain, has been shown to mitigate neuron loss in an animal model of temporal lobe epilepsy. In addition, NMDARs also require the binding of the co agonist glycine for the efficient opening of the ion channel, which is a part of this receptor. D Serine has also been found to co agonize the NMDA receptor with even greater potency than glycine. It is produced by serine racemase, and is enriched in the same areas as NMDA receptors. Removal of D serine can block NMDA mediated excitatory neurotransmission in many areas. Recently, it has been shown that D serine can be released both by neurons and astrocytes to regulate NMDA receptors. Note that D serine has also been shown to work as an antagonist / inverse co agonist for t NMDA receptors. 3,5 Dibromo L phenylalanine has been proposed a novel therapeutic drug candidate for treatment of neuropsychiatric disorders and diseases such as schizophrenia, and neurological disorders such as ischemic stroke and epileptic seizures. Other partial agonists of the NMDA receptor acting on novel sites such as rapastinel (GLYX 13) and apimostinel (NRX 1074) are now viewed for the development of new drugs with antidepressant and analgesic effects without obvious psychotomimetic activities.

Antagonists of the NMDA receptor are used as anesthetics for animals and sometimes humans, and are often used as recreational drugs due to their hallucinogenic properties, in addition to their unique effects at elevated dosages such as dissociation. When certain NMDA receptor antagonists are given to rodents in large doses, they can cause a form of brain damage called Olney's lesions. NMDA receptor antagonists that have been shown to induce Olney's lesions include ketamine, phencyclidine, and dextrorphan (a metabolite of dextromethorphan), as well as some NMDA receptor antagonists used only in research environments. So far, the published research on Olney's lesions is inconclusive in its occurrence upon human or monkey brain tissues with respect to an increase in the presence of NMDA receptor antagonists. Most NMDAR antagonists are uncompetitive or noncompetitive blockers of the channel pore or are antagonists of the glycine co regulatory site rather than antagonists of the active/glutamate site.

The NMDA receptor is regulated via nitrosylation and aminoadamantane can be used as a target directed shuttle to bring nitrogen oxide (NO) close to the site within the NMDA receptor where it can nitrosylate and regulate the ion channel conductivity.

The main problem with the development of NMDA antagonists for neuroprotection is that physiological NMDA receptor activity is essential for normal neuronal function. Complete blockade of all NMDA receptor activity results in side effects such as hallucinations, agitation and anesthesia. To be clinically relevant, an NMDA receptor antagonist must limit its action to blockade of excessive activation, without limiting normal function of the receptor. They will block healthy areas of the brain prior to having an impact on pathological areas, because healthy areas contain lower levels of agonist than pathological areas. These antagonists can be displaced from the receptor by high concentration of glutamate which can exist under excitotoxic circumstances.

Because of these adverse side effects of high affinity blockers, the search for clinically successful NMDA receptor antagonists for neurodegenerative diseases continued and focused on developing low affinity blockers. However the affinity could not be too low and dwell time not too short (as seen with Mg2+) where membrane depolarization relieves the block. The discovery was thereby development of uncompetitive antagonist with longer dwell time than Mg2+ in the channel but shorter than MK 801. That way the drug obtained would only block excessively open NMDA receptor associated channels but not normal neurotransmission. Many second generation memantine derivatives have been in development that may show even better neuroprotective effects, where the main thought is to use other safe but effective modulatory sites on the NMDA receptor in addition to its associated ion channel.

Excitotoxicity is implied to be involved in some neurodegenerative disorders such as Alzheimer's disease, Parkinson's disease, Huntington's disease and amyotrophic lateral sclerosis. Magnesium unblock is not instantaneous; to unblock all available channels, the postsynaptic cell must be depolarized for a sufficiently long period of time (in the scale of milliseconds). Therefore, the NMDA receptor functions as a "molecular coincidence detector". Its ion channel opens only when the following two conditions are met: glutamate is bound to the receptor, and the postsynaptic cell is depolarized (which removes the Mg2+ blocking the channel). This property of the NMDA receptor explains many aspects of long term potentiation (LTP) and synaptic plasticity. In a resting membrane potential, the NMDA receptor pore is opened allowing for an influx of external magnesium ions binding to prevent further ion permeation. External magnesium ions are in a millimolar range while intracellular magnesium ions are at a micromolar range to result in negative membrane potential. NMDA receptors are modulated by a number of endogenous and exogenous compounds and play a key role in a wide range of physiological (e. g., memory) and pathological processes (e. g., excitotoxicity). Magnesium works to potentiate NMDA induced responses at positive membrane potentials while blocking the NMDA channel. The use of calcium, potassium, and sodium are used to modulate the activity of NMDARs passing through the NMDA membrane. Changes in H+ concentration can partially inhibit the activity of NMDA receptors in different physiological conditions.

NMDAR antagonists like ketamine, esketamine, tiletamine, phencyclidine, nitrous oxide, and xenon are used as general anesthetics. These and similar drugs like dextromethorphan and methoxetamine also produce dissociative, hallucinogenic, and euphoriant effects and are used as recreational drugs. NMDAR targeted compounds, including ketamine, esketamine (JNJ 54135419), rapastinel (GLYX 13), apimostinel (NRX 1074), zelquistinel (AGN 241751), 4 chlorokynurenine (AV 101), and rislenemdaz (CERC 301, MK 0657), are under development for the treatment of mood disorders, including major depressive disorder and treatment resistant depression. In addition, ketamine is already employed for this purpose as an off label therapy in some clinics. Research suggests that tianeptine produces antidepressant effects through indirect alteration and inhibition of glutamate receptor activity and release of , in turn affecting neural plasticity. Tianeptine also acts on the NMDA and AMPA receptors. and has now received a limited recommendation by the UK's National Institute for Health and Care Excellence for patients who fail other treatment options. Cochlear NMDARs are the target of intense research to find pharmacological solutions to treat tinnitus. NMDARs are associated with a rare autoimmune disease, anti NMDA receptor encephalitis (also known as NMDAR encephalitis), that usually occurs due to cross reactivity of antibodies produced by the immune system against ectopic brain tissues, such as those found in teratoma. These are known as anti glutamate receptor antibodies. Compared to dopaminergic stimulants like methamphetamine, the NMDAR antagonist phencyclidine can produce a wider range of symptoms that resemble schizophrenia in healthy volunteers, in what has led to the glutamate hypothesis of schizophrenia. Experiments in which rodents are treated with NMDA receptor antagonist are today the most common model when it comes to testing of novel schizophrenia therapies or exploring the exact mechanism of drugs already approved for treatment of schizophrenia. NMDAR antagonists, for instance eliprodil, gavestinel, licostinel, and selfotel have been extensively investigated for the treatment of excitotoxicity mediated neurotoxicity in situations like ischemic stroke and traumatic brain injury, but were unsuccessful in clinical trials used in small doses to avoid sedation, but NMDAR antagonists can block Spreading Depolarizations in animals and in patients with brain injury. This use have not been tested in clinical trials yet.