Molecular determinants of the anticonvulsant felbamate binding site in the N-methyl-D-aspartate receptor

Desensitization of NMDA channels requires ligand binding to both GluN1 and GluN2 subunits to constrict the pore beside the activation gate

N-methyl-D-aspartate (NMDA) receptor channels are activated by glutamate (or NMDA) and glycine. The channels also undergo desensitization, which denotes decreased channel availability, after prolonged exposure to the activating ligands. Glycine apparently has a paradoxical negative effect on desensitization, as the increase in ambient glycine in concentrations required for channel activation would increase sustained NMDA receptor currents. We hypothesized that this classical "glycine-dependent desensitization" could be glycine-dependent activation in essence. By performing electrophysiological recordings and biophysical analyses with rat brain NMDA receptors heterogeneously expressed in Xenopus laevis oocytes, we characterized that the channel opened by "only" NMDA (in nominally glycine-free condition probably with the inevitable nanomolar glycine) would undergo a novel form of deactivation rather than desensitization, and is thus fully available for subsequent activation. Moreover, external tetrapentylammonium ions (TPentA), tetrabutylammonium ions, and tetrapropylammonium ions (TPA, in higher concentrations) block the pore and prohibit channel desensitization with a simple "foot-in-the-door" hindrance effect. TpentA and TPA have the same voltage dependence but show different flow dependence in binding affinity, revealing a common binding site at an electrical distance of ~0.7 from the outside yet differential involvement of the flux-coupling region in the external pore mouth. The smaller tetraethylammonium ion and the larger tetrahexylammonium and tetraheptylammonium ions may block the channel but could not affect desensitization. We conclude that NMDA receptor desensitization requires concomitant binding of both glycine and glutamate, and thus movement of both GluN1 and GluN2 subunits. Desensitization gate itself embodies a highly restricted pore reduction with a physical distance of ~4 Å from the charged nitrogen atom of bound tetraalkylammonium ions, and is located very close to the activation gate in the bundle-crossing region in the external pore vestibule.

Integrated network analysis reveals potentially novel molecular mechanisms and therapeutic targets of refractory epilepsies

Epilepsy is a complex neurological disorder and a significant health problem. The pathogenesis of epilepsy remains obscure in a significant number of patients and the current treatment options are not adequate in about a third of individuals which were known as refractory epilepsies (RE). Network medicine provides an effective approach for studying the molecular mechanisms underlying complex diseases. Here we integrated 1876 disease-gene associations of RE and located those genes to human protein-protein interaction (PPI) network to obtain 42 significant RE-associated disease modules. The functional analysis of these disease modules showed novel molecular pathological mechanisms of RE, such as the novel enriched pathways (e.g., "presynaptic nicotinic acetylcholine receptors", "signaling by insulin receptor"). Further analysis on the relationships between current drug targets and the RE-related disease genes showed the rational mechanisms of most antiepileptic drugs. In addition, we detected ten potential novel drug targets (e.g., KCNA1, KCNA4-6, KCNC3, KCND2, KCNMA1, CAMK2G, CACNB4 and GRM1) located in three RE related disease modules, which might provide novel insights into the new drug discovery for RE therapy.

Modulation of NMDA channel gating by Ca2+ and Cd2+ binding to the external pore mouth

NMDA receptor channels are characterized by high Ca²⁺ permeability. It remains unclear whether extracellular Ca²⁺ could directly modulate channel gating and control Ca²⁺ influxes. We demonstrate a pore-blocking site external to the activation gate for extracellular Ca²⁺ and Cd²⁺, which has the same charge and radius as Ca²⁺ but is impermeable to the channel. The apparent affinity of Cd²⁺ or Ca²⁺ is higher toward the activated (a steady-state mixture of the open and desensitized, probably chiefly the latter) than the closed states. The blocking effect of Cd²⁺ is well correlated with the number of charges in the DRPEER motif at the external pore mouth, with coupling coefficients close to 1 in double mutant cycle analyses. The effect of Ca²⁺ and especially Cd²⁺ could be allosterically affected by T647A mutation located just inside the activation gate. A prominent "hook" also develops after wash-off of Cd²⁺ or Ca²⁺, suggesting faster unbinding rates of Cd²⁺ and Ca²⁺ with the mutation. We conclude that extracellular Ca²⁺ or Cd²⁺ directly binds to the DRPEER motif to modify NMDA channel activation (opening as well as desensitization), which seems to involve essential regional conformational changes centered at the bundle crossing point A652 (GluN1)/A651(GluN2).

Iron-Catalyzed Reaction of Urea with Alcohols and Amines: A Safe Alternative for the Synthesis of Primary Carbamates

A general study of the iron-catalyzed reaction of urea with nucleophiles is here presented. The carbamoylation of alcohols allows for the synthesis of N-unsubstituted (primary) carbamates, including present drugs (Felbamate and Meprobamate), without the necessity to apply phosgene and related derivatives. Using amines as nucleophiles gave rise to the respective mono- and disubstituted ureas via selective transamidation reaction. These atom-economical transformations provide a direct and selective access to valuable compounds from cheap and readily available urea using a simple Lewis-acidic iron(II) catalyst.

A Potential Role for Felbamate in TSC- and NF1-Related Epilepsy: A Case Report and Review of the Literature

A 15-year-old girl with maternal inheritance of neurofibromatosis type 1 (NF1) and paternal inheritance of tuberous sclerosis complex (TSC) developed intractable epilepsy at age 5. Her seizures were refractory to adequate doses of four antiepileptic medications until felbamate was initiated at age 7. She has since remained seizure-free on felbamate monotherapy. Although felbamate has multiple mechanisms of action, it is thought to have its most potent antiepileptic effects through inhibition of the N-methyl-D-aspartate receptor (NMDAR). Previous studies have shown that the NMDAR is altered in varying epilepsy syndromes and notably in the cortical tubers found in TSC. The aim of this paper is to examine how felbamate monotherapy was able to achieve such robust antiepileptic effects in a unique patient and possibly offer a novel therapeutic approach to patients suffering from TSC- and NF-related epilepsy.

Differences in epileptic symptoms depending on the type of autoimmune-mediated limbic encephalitis

Limbic encephalitis (LE) is an inflammatory disease of the central nervous system that is characterized by the selective involvement of limbic structures. The clinical manifestations of LE include the acute or sub-acute onset of recent memory disorders, mental disorders and seizures. Autoimmune-mediated LE is a major type of non-infectious LE; seizure is a hallmark of this type of LE. The treatment of epilepsy, which is a key factor that affects the prognosis of LE patients, warrants special attention. Understanding the characteristics of epilepsy caused by autoimmune-mediated LE and providing the appropriate treatment will help to improve patients' outcomes. In this article, we extensively review the literature related to autoimmune-mediated LE epidemiology, mechanisms, characteristics and seizure frequency and onset, and we discuss the possible diagnosis and treatment of this disease.

Glutamate receptor pores

Glutamate receptors are ligand-gated ion channels that mediate fast excitatory synaptic transmission throughout the central nervous system. Functional receptors are homo- or heteromeric tetramers with each subunit contributing a re-entrant pore loop that dips into the membrane from the cytoplasmic side. The pore loops form a narrow constriction near their apex with a wide vestibule toward the cytoplasm and an aqueous central cavity facing the extracellular solution. This article focuses on the pore region, reviewing how structural differences among glutamate receptor subtypes determine their distinct functional properties.

Key binding interactions for memantine in the NMDA receptor

Memantine (Namenda) is prescribed as a treatment for moderate to severe Alzheimer's Disease. Memantine functions by blocking the NMDA receptor, but the key binding interactions between drug and receptor are not fully elucidated. To determine key binding interactions of memantine, we made side-by-side comparisons of IC(50) for memantine and amantadine, a structurally related drug, in the GluN1/GluN2B NMDA receptor. We identified hydrophobic binding pockets for the two methyl groups on memantine formed by the residues A645 and A644 on the third transmembrane helices of GluN1 and GluN2B, respectively. Moreover, we found that while adding two methyl groups to amantadine to produce memantine greatly improves affinity, adding a third methyl group to produce the symmetrical trimethylamantadine diminished affinity. Our results provide a better understanding of chemical-scale interactions between memantine and the NMDA channel, which will potentially benefit the development of new drugs for neurodegenerative diseases involving NMDA receptors.

Pharmacological modulation of NMDA receptor activity and the advent of negative and positive allosteric modulators

The NMDA receptor (NMDAR) family of l-glutamate receptors are well known to have diverse roles in CNS function as well as in various neuropathological and psychiatric conditions. Until recently, the types of agents available to pharmacologically regulate NMDAR function have been quite limited in terms of mechanism of action and subtype selectivity. This has changed significantly in the past two years. The purpose of this review is to summarize the many drug classes now available for modulating NMDAR activity. Previously, this included competitive antagonists at the l-glutamate and glycine binding sites, high and low affinity channel blockers, and GluN2B-selective N-terminal domain binding site antagonists. More recently, we and others have identified new classes of NMDAR agents that are either positive or negative allosteric modulators (PAMs and NAMs, respectively). These compounds include the pan potentiator UBP646, the GluN2A-selective potentiator/GluN2C and GluN2D inhibitor UBP512, the GluN2D-selective potentiator UBP551, the GluN2C/GluN2D-selective potentiator CIQ as well as the new NMDAR-NAMs such as the pan-inhibitor UBP618, the GluN2C/GluN2D-selective inhibitor QZN46 and the GluN2A inhibitors UBP608 and TCN201. These new agents do not bind within the l-glutamate or glycine binding sites, the ion channel pore or the N-terminal regulatory domain. Collectively, these new allosteric modulators appear to be acting at multiple novel sites on the NMDAR complex. Importantly, these agents display improved subtype-selectivity and as NMDAR PAMs and NAMs, they represent a new generation of potential NMDAR therapeutics.

Rhythmic delta activity represents a form of nonconvulsive status epilepticus in anti-NMDA receptor antibody encephalitis

Anti-NMDA receptor antibody encephalitis is a limbic encephalitis with psychiatric manifestations, abnormal movements, coma, and seizures. The coma and abnormal movements are not typically attributed to seizure activity, and slow activity is the most common EEG finding. We report drug-resistant nonconvulsive status epilepticus as the basis for coma in a 19-year-old woman with anti-NMDA receptor antibodies and a mediastinal teratoma. The EEG showed generalized rhythmic delta activity, with evolution in morphology, frequency, and field typical of nonconvulsive status epilepticus. The status was refractory to antiepileptic drugs, repeated drug-induced coma, resection of the tumor, intravenous steroids, rituximab, and plasmapheresis. She awoke after the addition of felbamate, and the rhythmic delta activity ceased. The rhythmic delta activity described with coma in anti-NMDA receptor antibody encephalitis may represent a pattern of status epilepticus in some patients. Felbamate, which has NMDA receptor antagonist activity, should be studied as a therapeutic agent in this condition.

Novel N-methyl-D-aspartate receptor antagonists: a review of compounds patented since 2006

IMPORTANCE OF THE FIELD

The NMDA receptor is a complex ligand gated, voltage-dependent ion channel. It has been a drug target for > 25 years for neurological and psychiatric indications. Whereas the initial optimism to turn preclinically active compounds rapidly into drugs for human use was dampened, new insights into cellular receptor localization, role of subunits and receptor operation have kept the interest alive to modulate this receptor for therapeutic intervention.

AREAS COVERED IN THIS REVIEW

The article describes the NMDA receptor antagonists patented since 2006. Also included are novel NMDA receptor ligands potentially useful for positron emission tomography imaging.

WHAT THE READER WILL GAIN

The first section summarizes the current status of NMDA receptor pharmacology. This serves as a base for the next sections discussing the patented compounds with respect to their mode of action, potency and, in some cases, drugability.

TAKE HOME MESSAGE

The most important recent strategies aiming for inhibition of NMDA receptor-mediated neurotransmission avoid for safety reasons full receptor blockade but allow a low degree of normal receptor function. Approaches pursued by the latest patents comprise blocking the channel with compounds of low affinity, antagonizing receptor activity by highly potent NR2B ligands, partial agonism at the glutamate or glycine-binding site and improvement of pharmacokinetic properties of well established, safe antagonists by deuteration.

Ionic flow enhances low-affinity binding: a revised mechanistic view into Mg2+ block of NMDA receptors

The N-methyl-d-aspartate receptor (NMDAR) channel is one of the major excitatory amino acid receptors in the mammalian brain. Since external Mg(2+) blocks the channel in an apparently voltage-dependent fashion, this ligand-gated channel displays intriguing voltage-dependent control of Na(+) and Ca(2+) permeability and thus plays an important role in synaptic physiology. We found that the essential features of Mg(2+) block could not be solely envisaged by binding of a charged blocker in the membrane electric field. Instead, the blocking effect of Mg(2+) is critically regulated by, and quantitatively correlated with, the relative tendency of outward and inward ionic fluxes. The 'intrinsic' affinity of Mg(2+) to the binding sites, however, is low (in the millimolar range) in the absence of net ionic flow at 0 mV. Besides, extracellular and intracellular Mg(2+) blocks the channel at distinct sites of electrical distances 0.7 and 0.95 from the outside, respectively. The two sites are separated by a high energy barrier for the movement of Mg(2+) (but not Na(+) or the other ions), and functionally speaking, each could accommodate 1.1 and 0.8 coexisting permeating ions, respectively. Mg(2+) block of the ionic flow thus is greatly facilitated by the flux-coupling effect or the ionic flow (the preponderant direction of permeant ion movement) per se, as if the poorly permeable Mg(2+) is 'pushed' against a high energy barrier by the otherwise permeating ions. Extracellular and intracellular Mg(2+) block then is in essence 'use dependent', more strongly inhibiting both Na(+) and Ca(2+) fluxes with stronger tendencies of influx and efflux, respectively. In conclusion, although permeant ions themselves could compete with Mg(2+), the flow or the tendency of movement of the permeant ions may actually enhance rather than interfere with Mg(2+) block, making the unique current-voltage relationship of NMDAR and the molecular basis of many important neurobiological phenomena.

The activation gate and gating mechanism of the NMDA receptor

The NMDA receptor opens in response to binding of NMDA and glycine. However, it remains unclear where and how gating of the NMDA receptor pore is accomplished. We show that different point mutations between S645 and I655 (thus including the highly conserved SYTANLAAF motif) of M3c in NR2B lead to constitutively open channels. The current through these constitutively open channels are readily blocked by external Mg2+ and MK-801 [(+)-5-methyl-10,11-dihydro-5H-dibenzo [a,d] cyclohepten-5,10-imine maleate]. Also, the open-channel blocker MK-801 can no longer be trapped in these channels when NMDA and glycine are washed off. Moreover, M3c residues at or below A651(NR2B, A7 in SYTANLAAF) react with external methanethiosulfonate (MTS) reagents approximately 500 to 1000-fold faster in the presence than in the absence of agonists NMDA and glycine. In fact, the MTS modification rate shows exactly the same NMDA concentration dependence as channel activation. In contrast, those residues external to A651 are always modified with similar kinetics whether NMDA and glycine are present or not. Interestingly, MTS modification of A651C(NR2B) holds the channel constitutively open. Mutations of A651(NR2B) into arginine, tryptophan, or phenylalanine, and similar mutations of the corresponding A652 in NR1 also lead to constitutively open channels. Double-mutant cycle analysis further shows that the effects of A652(NR1) and A651(NR2B) mutations are evidently non-additive (i.e., cooperative) if mutated into residues with large side chains or with compensatory charges [e.g., A652E(NR1)+A651R(NR2B)]. The side chain of A7 thus plays a determinant role in the intersubunit distance at this level, which is directly responsible for the activation gate and activation-deactivation gating of the NMDA receptor.