Characterization of the gating conformational changes in the felbamate binding site in NMDA channels

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.

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).

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.

NMDA receptor binding in focal epilepsies

OBJECTIVE

To demonstrate altered N-methyl-d-aspartate (NMDA) receptor availability in patients with focal epilepsies using positron emission tomography (PET) and [(18)F]GE-179, a ligand that selectively binds to the open NMDA receptor ion channel, which is thought to be overactive in epilepsy.

METHODS

Eleven patients (median age 33 years, 6 males) with known frequent interictal epileptiform discharges had an [(18)F]GE-179 PET scan, in a cross-sectional study. MRI showed a focal lesion but discordant EEG changes in two, was non-localising with multifocal EEG abnormalities in two, and was normal in the remaining seven patients who all had multifocal EEG changes. Individual patient [(18)F]GE-179 volume-of-distribution (VT) images were compared between individual patients and a group of 10 healthy controls (47 years, 7 males) using Statistical Parametric Mapping.

RESULTS

Individual analyses revealed a single cluster of focal VT increase in four patients; one with a single and one with multifocal MRI lesions, and two with normal MRIs. Post hoc analysis revealed that, relative to controls, patients not taking antidepressants had globally increased [(18)F]GE-179 VT (+28%; p<0.002), and the three patients taking an antidepressant drug had globally reduced [(18)F]GE-179 VT (-29%; p<0.002). There were no focal abnormalities common to the epilepsy group.

CONCLUSIONS

In patients with focal epilepsies, we detected primarily global increases of [(18)F]GE-179 VT consistent with increased NMDA channel activation, but reduced availability in those taking antidepressant drugs, consistent with a possible mode of action of this class of drugs. [(18)F]GE-179 PET showed focal accentuations of NMDA binding in 4 out of 11 patients, with difficult to localise and treat focal epilepsy.

The differential contribution of GluN1 and GluN2 to the gating operation of the NMDA receptor channel

The Ν-methyl-D-aspartate (NMDA) receptor channel is an obligatory heterotetramer formed by two GluN1 and two GluN2 subunits. However, the differential contribution of the two different subunits to channel operation is not clear. We found that the apparent affinity of glycine to GluN1 (K gly ∼ 0.6 μM) is much higher than NMDA or glutamate to GluN2 (K NMDA ∼ 36 μM, K glu ∼ 4.8 μM). The binding rate constant (derived from the linear regression of the apparent macroscopic binding rates) of glycine to GluN1 (∼9.8 × 10(6) M(-1) s(-1)), however, is only slightly faster than NMDA to GluN2 (∼4.1 × 10(6) M(-1) s(-1)). Accordingly, the apparent unbinding rates of glycine from activated GluN1 (time constant ∼2 s) are much slower than NMDA from activated GluN2 (time constant ∼70 ms). Moreover, the decay of NMDA currents upon wash-off of both glycine and NMDA seems to follow the course of NMDA rather than glycine unbinding. But if only glycine is washed off, the current decay is much slower, apparently following the course of glycine unbinding. The apparent binding rate of glycine to the fully deactivated channel, in the absence of NMDA, is roughly the same as that measured with co-application of both ligands, whereas the apparent binding rate of NMDA to the fully deactivated channel in the absence of glycine is markedly slower. In this regard, it is interesting that the seventh residue in the highly conserved SYTANLAAF motif (A7) in GluN1 and GluN2 are so close that they may interact with each other to control the dimension of the external pore mouth. Moreover, specific mutations involving A7 in GluN1 but not in GluN2 result in channels showing markedly enhanced affinity to both glycine and NMDA and readily activated by only NMDA, as if the channel is already partially activated. We conclude that GluN2 is most likely directly responsible for the activation gate of the NMDA channel, whereas GluN1 assumes a role of more global control, especially on the gating conformational changes in GluN2. Structurally, this intersubunit regulatory interaction seems to involve the SYTANLAAF motif, especially the A7 residue.

Astroglial d-serine is the endogenous co-agonist at the presynaptic NMDA receptor in rat entorhinal cortex

Presynaptic NMDA receptors facilitate the release of glutamate at excitatory cortical synapses and are involved in regulation of synaptic dynamics and plasticity. At synapses in the entorhinal cortex these receptors are tonically activated and provide a positive feedback modulation of the level of background excitation. NMDA receptor activation requires obligatory occupation of a co-agonist binding site, and in the present investigation we have examined whether this site on the presynaptic receptor is activated by endogenous glycine or d-serine. We used whole-cell patch clamp recordings of spontaneous AMPA receptor-mediated synaptic currents from rat entorhinal cortex neurones in vitro as a monitor of presynaptic glutamate release. Addition of exogenous glycine or d-serine had minimal effects on spontaneous release, suggesting that the co-agonist site was endogenously activated and likely to be saturated in our slices. This was supported by the observation that a co-agonist site antagonist reduced the frequency of spontaneous currents. Depletion of endogenous glycine by enzymatic breakdown with a bacterial glycine oxidase had little effect on glutamate release, whereas d-serine depletion with a yeast d-amino acid oxidase significantly reduced glutamate release, suggesting that d-serine is the endogenous agonist. Finally, the effects of d-serine depletion were mimicked by compromising astroglial cell function, and this was rescued by exogenous d-serine, indicating that astroglial cells are the provider of the d-serine that tonically activates the presynaptic NMDA receptor. We discuss the significance of these observations for the aetiology of epilepsy and possible targeting of the presynaptic NMDA receptor in anticonvulsant therapy.

The bundle crossing region is responsible for the inwardly rectifying internal spermine block of the Kir2.1 channel

Inward rectifier potassium channels conduct K(+) across the cell membrane more efficiently in the inward than outward direction in physiological conditions. Voltage-dependent and flow-dependent blocks of outward K(+) currents by intracellular polyamines (e.g., spermine (SPM)) have been proposed as the major mechanisms underlying inward rectification. In this study, we show that the SPM blocking affinity curve is shifted according to the shift in K(+) reversal potential. Moreover, the kinetics of SPM entry to and exit from the binding site are correlatively slowed by specific E224 and E299 mutations, which always also disrupt the flux coupling feature of SPM block. The entry rates carry little voltage dependence, whereas the exit rates are e-fold decelerated per ∼15 mV depolarization. Interestingly, the voltage dependence remains rather constant among WT and quite a few different mutant channels. This voltage dependence offers an unprecedented chance of mapping the location (electrical distance) of the SPM site in the pore because these kinetic data were obtained along the preponderant direction of K(+) current flow (outward currents for the entry rate and inward currents for the exit rate) and thus contamination from flow dependence should be negligible. Moreover, double mutations involving E224 and A178 or M183 seem to alter the height of the same asymmetrical barrier between the SPM binding site and the intracellular milieu. We conclude that the SPM site responsible for the inward rectifying block is located at an electrical distance of ∼0.5 from the inside and is involved in a flux coupling segment in the bundle crossing region of the pore. With preponderant outward K(+) flow, SPM is "pushed" to the outmost site of this segment (∼D172). On the other hand, the blocking SPM would be pushed to the inner end of this segment (∼M183-A184) with preponderant inward K(+) flow. Moreover, E224 and E299 very likely electrostatically interact with the other residues (e.g., R228, R260) in the cytoplasmic domain and then allosterically keep the bundle crossing region in an open conformation appropriate for the flux coupling block of SPM.

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

The antiepileptic effect of felbamate (FBM) is ascribable to gating modification of NMDA receptors. Using site-directed mutagenesis and electrophysiological studies, we found that single-point mutations of four pairs of homologous residues in the external vestibule of the receptor pore, namely V644(NR1)-L643(NR2B) (the two inner pairs) and T648(NR1)-T647(NR2B) (the two outer pairs), significantly decrease FBM binding. Moreover, double mutations involving either the inner or the outer pair always show cooperative (nonadditive) effects on FBM binding, whereas double mutations involving both inner and outer pairs always show additive (noncooperative) effects. Most interestingly, triple mutations of any three of the four critical residues essentially abolish the effect of FBM. These findings indicate that T648(NR1)/T647(NR2B) and V644(NR1)/L643(NR2B) act cooperatively to contribute directly to the "outer binding region" and "inner binding region" in the FBM binding site, respectively. The outer and inner binding regions, however, seem to contribute independently to FBM binding. We conclude that residues L643 and T647 in NR2B as well as homologous residues V644 and T648 in NR1 are the major, and very likely the exclusive, molecular determinants constituting the FBM binding site in the NMDA receptor.

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.

Extracellular proton-modulated pore-blocking effect of the anticonvulsant felbamate on NMDA channels

Felbamate (FBM) is a potent nonsedative anticonvulsant whose clinical effect is chiefly related to gating modification (and thus use-dependent inhibition) rather than pore block of N-methyl-D-aspartate (NMDA) channels at pH 7.4. Using whole-cell recording in rat hippocampal neurons, we examined the effect of extracellular pH on FBM action. In sharp contrast to the findings at pH 7.4, the inhibitory effect of FBM on NMDA currents shows much weakened use-dependence at pH 8.4. Moreover, FBM neither accelerates the activation kinetics of the NMDA channel, nor enhances the currents elicited by very low concentrations of NMDA at pH 8.4. These differential effects of FBM between pH 7.4 and 8.4 are abolished in the mutant NMDA channels which lack proton sensitivity. Most interestingly, the inhibitory effect of FBM becomes flow-dependent and is evidently stronger in inward than in outward NMDA currents at pH 8.4. These findings indicate that FBM has a significantly more manifest pore-blocking effect on the NMDA channel at pH 8.4 than at pH 7.4. FBM therefore acts as an opportunistic pore blocker modulated by extracellular proton, suggesting that the FBM binding site is located at the junction of a widened and a narrow part of the ion conduction pathway. Also, we find that the inhibitory effect of FBM on NMDA currents is antagonized by external but not internal Na+, and that increase of external Na+ decreases the binding rate without altering the unbinding rate of FBM. These findings indicate that the FBM binding site faces the extracellular rather than the intracellular solution, and coincides with the outmost ionic (e.g., Na+) site in the NMDA channel pore. We conclude that the FBM binding site very likely is located in the external pore mouth, where extracellular proton, Na+, FBM, and NMDA channel gating have an orchestrating effect.