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

Glucose-6-phosphate dehydrogenase alleviates epileptic seizures by repressing reactive oxygen species production to promote signal transducer and activator of transcription 1-mediated N-methyl-d-aspartic acid receptors inhibition

The pathogenesis of epilepsy remains unclear; however, a prevailing hypothesis suggests that the primary underlying cause is an imbalance between neuronal excitability and inhibition. Glucose-6-phosphate dehydrogenase (G6PD) is a key enzyme in the pentose phosphate pathway, which is primarily involved in deoxynucleic acid synthesis and antioxidant defense mechanisms and exhibits increased expression during the chronic phase of epilepsy, predominantly colocalizing with neurons. G6PD overexpression significantly reduces the frequency and duration of spontaneous recurrent seizures. Furthermore, G6PD overexpression enhances signal transducer and activator of transcription 1 (STAT1) expression, thus influencing N-methyl-d-aspartic acid receptors expression, and subsequently affecting seizure activity. Importantly, the regulation of STAT1 by G6PD appears to be mediated primarily through reactive oxygen species signaling pathways. Collectively, our findings highlight the pivotal role of G6PD in modulating epileptogenesis, and suggest its potential as a therapeutic target for epilepsy.

Role of NMDA Receptor in High-Pressure Neurological Syndrome and Hyperbaric Oxygen Toxicity

Professional divers exposed to pressures greater than 11 ATA (1.1 MPa) may suffer from high-pressure neurological syndrome (HPNS). Divers who use closed-circuit breathing apparatus and patients and medical attendants undergoing hyperbaric oxygen therapy (HBOT) face the risk of CNS hyperbaric oxygen toxicity (HBOTx) at oxygen pressure above 2 ATA (0.2 MPa). Both syndromes are characterized by reversible CNS hyperexcitability, accompanied by cognitive and motor deficits, and N-methyl-D-aspartate receptor (NMDAR) plays a crucial role in provoking them. Various NMDAR subtypes respond differently under hyperbaric conditions. The augmented currents observed only in NMDAR containing GluN2A subunit increase glutamatergic synaptic activity and cause dendritic hyperexcitability and abnormal neuronal activity. Removal of the resting Zn2+ voltage-independent inhibition exerted by GluN2A present in the NMDAR is the major candidate for the mechanism underlying the increase in receptor conductance. Therefore, this process should be the main target for future research aiming at developing neuroprotection against HPNS and HBOTx.

Short-term postsynaptic plasticity facilitates predictive tracking in continuous attractors

Introduction

The N-methyl-D-aspartate receptor (NMDAR) plays a critical role in synaptic transmission and is associated with various neurological and psychiatric disorders. Recently, a novel form of postsynaptic plasticity known as NMDAR-based short-term postsynaptic plasticity (STPP) has been identified. It has been suggested that long-lasting glutamate binding to NMDAR allows for the retention of input information in brain slices up to 500 ms, leading to response facilitation. However, the impact of STPP on the dynamics of neuronal populations remains unexplored.

Methods

In this study, we incorporated STPP into a continuous attractor neural network (CANN) model to investigate its effects on neural information encoding in populations of neurons. Unlike short-term facilitation, a form of presynaptic plasticity, the temporally enhanced synaptic efficacy resulting from STPP destabilizes the network state of the CANN by increasing its mobility.

Results

Our findings demonstrate that the inclusion of STPP in the CANN model enables the network state to predictively respond to a moving stimulus. This nontrivial dynamical effect facilitates the tracking of the anticipated stimulus, as the enhanced synaptic efficacy induced by STPP enhances the system's mobility.

Discussion

The discovered STPP-based mechanism for sensory prediction provides valuable insights into the potential development of brain-inspired computational algorithms for prediction. By elucidating the role of STPP in neural population dynamics, this study expands our understanding of the functional implications of NMDAR-related plasticity in information processing within the brain.

Conclusion

The incorporation of STPP into a CANN model highlights its influence on the mobility and predictive capabilities of neural networks. These findings contribute to our knowledge of STPP-based mechanisms and their potential applications in developing computational algorithms for sensory prediction.

Glutamate excitotoxicity: Potential therapeutic target for ischemic stroke

Glutamate-mediated excitotoxicity is an important mechanism leading to post ischemic stroke damage. After acute stroke, the sudden reduction in cerebral blood flow is most initially followed by ion transport protein dysfunction and disruption of ion homeostasis, which in turn leads to impaired glutamate release, reuptake, and excessive N-methyl-D-aspartate receptor (NMDAR) activation, promoting neuronal death. Despite extensive evidence from preclinical studies suggesting that excessive NMDAR stimulation during ischemic stroke is a central step in post-stroke damage, NMDAR blockers have failed to translate into clinical stroke treatment. Current treatment options for stroke are very limited, and there is therefore a great need to develop new targets for neuroprotective therapeutic agents in ischemic stroke to extend the therapeutic time window. In this review, we highlight recent findings on glutamate release, reuptake mechanisms, NMDAR and its downstream cellular signaling pathways in post-ischemic stroke damage, and review the pathological changes in each link to help develop viable new therapeutic targets. We then also summarize potential neuroprotective drugs and therapeutic approaches for these new targets in the treatment of ischemic stroke.

Cadmium activates AMPA and NMDA receptors with M3 helix cysteine substitutions

AMPA and NMDA receptors are ligand-gated ion channels that depolarize postsynaptic neurons when activated by the neurotransmitter L-glutamate. Changes in the distribution and activity of these receptors underlie learning and memory, but excessive change is associated with an array of neurological disorders, including cognitive impairment, developmental delay, and epilepsy. All of the ionotropic glutamate receptors (iGluRs) exhibit similar tetrameric architecture, transmembrane topology, and basic framework for activation; conformational changes induced by extracellular agonist binding deform and splay open the inner helix bundle crossing that occludes ion flux through the channel. NMDA receptors require agonist binding to all four subunits, whereas AMPA and closely related kainate receptors can open with less than complete occupancy. In addition to conventional activation by agonist binding, we recently identified two locations along the inner helix of the GluK2 kainate receptor subunit where cysteine (Cys) substitution yields channels that are opened by exposure to cadmium ions, independent of agonist site occupancy. Here, we generate AMPA and NMDA receptor subunits with homologous Cys substitutions and demonstrate similar activation of the mutant receptors by Cd. Coexpression of the auxiliary subunit stargazin enhanced Cd potency for activation of Cys-substituted GluA1 and altered occlusion upon treatment with sulfhydryl-reactive MTS reagents. Mutant NMDA receptors displayed voltage-dependent Mg block of currents activated by agonist and/or Cd as well as asymmetry between Cd effects on Cys-substituted GluN1 versus GluN2 subunits. In addition, Cd activation of each Cys-substituted iGluR was inhibited by protons. These results, together with our earlier work on GluK2, reveal a novel mechanism shared among the three different iGluR subtypes for prying open the gate that controls ion entry into the pore.

The Mechanism of NMDA Receptor Hyperexcitation in High Pressure Helium and Hyperbaric Oxygen

Professional divers exposed to pressures greater than 1.1 MPa may suffer from the high pressure neurological syndrome (HPNS). Divers who use closed-circuit breathing apparatus face the risk of CNS hyperbaric oxygen toxicity (HBOTox). Both syndromes are characterized by reversible CNS hyperexcitability, accompanied by cognitive and motor deficits. Previous studies have demonstrated that the hyperexcitability of HPNS is induced mainly by NMDA receptors (NMDARs). In our recent studies, we demonstrated that the response of NMDARs containing GluN1 + GluN2A subunits was increased by up to 50% at high pressure (HP) He, whereas GluN1 + GluN2B NMDARs response was not affected under similar conditions. Our aim was to compare the responses of both types of NMDARs under HBOTox conditions to those of HP He and to reveal their possible underlying molecular mechanism(s). The two combinations of NMDARs were expressed in Xenopus laevis oocytes, placed in a pressure chamber, voltage-clamped, and their currents were tested at 0.1 (control) -0.54 MPa 100% O₂ or 0.1-5.1 MPa He pressures. We show, for the first time, that NMDARs containing the GluN2A subunit exhibit increased responses in 100% O₂ at a pressure of 0.54 MPa, similar to those observed in 5.1 MPa He. In contrast, the GluN1 + GluN2B response is not sensitive to either condition. We discovered that neither condition produced statistically significant changes in the voltage-dependent Mg²⁺ inhibition of the response. The averaged IC₅₀ remained the same, but a higher [Mg²⁺] _o was required to restore the current to its control value. The application of TPEN, a Zn²⁺ chelator, in control, HP He and HBOTox conditions, revealed that the increase in GluN1 + GluN2A current is associated with the removal of the high-affinity voltage-independent Zn²⁺ inhibition of the receptor. We propose that HPNS and HBOTox may share a common mechanism, namely removal of Zn²⁺ from its specific binding site on the N-terminal domain of the GluN2A subunit, which increases the pore input-conductance and produces larger currents and consequently a hyperexcitation.

Potentiation of NMDA-Mediated Responses by Amyloid-β Peptide 1-40 in Rat Sympathetic Preganglionic Neurons

The abnormal accumulation of amyloid-β peptides (Aβ) is one of the main characteristics of Alzheimer's disease (AD). Cerebro- and cardiovascular diseases may be the risk factors for developing AD. The effect of Aβ on central sympathetic control of cardiovascular function remains unclear. The present study examines the acute effects of Aβ oligomers on the function of NMDA receptors, a subtype of ionotropic glutamate receptors, in rat sympathetic preganglionic neurons (SPNs). In the in vitro electrophysiological study, Aβ1-40 but not Aβ1-42 applied by superfusion for 5 min significantly potentiated NMDA-induced depolarizations in SPNs of neonatal rat spinal cord slice preparation. Application of Aβ1-40 had little effects on AMPA-induced depolarizations or GABA-induced hyperpolarizations. Treatment with a selective protein kinase C (PKC) inhibitor applied together with Aβ1-40 blocked the augmentation by Aβ1-40 of NMDA-induced depolarizations. Western blot analysis showed an increase in the levels of phosphoserine 896, selectively regulated by PKC, without significant changes in phosphoserine 897 on GluN1 subunits in lateral horn areas of spinal cord slices following treatment with Aβ1-40. In the in vivo study, intrathecal injection of Aβ1-40 (0.2 nmol) potentiated the pressor effects induced by NMDA (2 nmol) injected intrathecally in urethane-anesthetized rats. These results suggest that different fragments of Aβ may have differential effects on the NMDA receptor function and the selective augmentation of NMDA receptor function by Aβ1-40 may involve PKC-dependent mechanisms in sympathetic preganglionic neurons.

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.

The effect of high pressure on the NMDA receptor: molecular dynamics simulations

Professional divers exposed to ambient pressures above 11 bar develop the high pressure neurological syndrome (HPNS), manifesting as central nervous system (CNS) hyperexcitability, motor disturbances, sensory impairment, and cognitive deficits. The glutamate-type N-methyl-D-aspartate receptor (NMDAR) has been implicated in the CNS hyperexcitability of HPNS. NMDARs containing different subunits exhibited varying degrees of increased/decreased current at high pressure. The mechanisms underlying this phenomenon remain unclear. We performed 100 ns molecular dynamics (MD) simulations of the NMDAR structure embedded in a dioleoylphosphatidylcholine (DOPC) lipid bilayer solvated in water at 1 bar, hydrostatic 25 bar, and in helium at 25 bar. MD simulations showed that in contrast to hydrostatic pressure, high pressure helium causes substantial distortion of the DOPC membrane due to its accumulation between the two monolayers: reduction of the Sn-1 and Sn-2 DOPC chains and helium-dependent dehydration of the NMDAR pore. Further analysis of important regions of the NMDAR protein such as pore surface (M2 α-helix), Mg²⁺ binding site, and TMD-M4 α-helix revealed significant effects of helium. In contrast with previous models, these and our earlier results suggest that high pressure helium, not hydrostatic pressure per se, alters the receptor tertiary structure via protein-lipid interactions. Helium in divers’ breathing mixtures may partially contribute to HPNS symptoms.

Impaired Cognitive Function and Altered Hippocampal Synaptic Plasticity in Mice Lacking Dermatan Sulfotransferase Chst14/D4st1

Chondroitin sulfate (CS) and dermatan sulfate (DS) proteoglycans (PGs) are major extracellular matrix (ECM) components of the central nervous system (CNS). A large body of evidence has shown that CSPGs/DSPGs play critical roles in neuronal growth, axon guidance, and plasticity in the developing and mature CNS. It has been proposed that these PGs exert their function through specific interaction of CS/DS chains with its binding partners in a manner that depends on the sulfation patterns of CS/DS. It has been reported that dermatan 4-O-sulfotransferase-1 (Chst14/D4st1) specific for DS, but not chondroitin 4-O-sulfotransferase-1 (Chst11/C4st1) specific for CS, regulates proliferation and neurogenesis of neural stem cells (NSCs), indicating that CS and DS play distinct roles in the self-renewal and differentiation of NSCs. However, it remains unknown whether specific sulfation profiles of DS has any effect on CNS plasticity. In the present study, Chst14/D4st1-deficient (Chst14^−/−) mice was employed to investigate the involvement of DS in synaptic plasticity. First, behavior study using Morris Water Maze (MWM) showed that the spatial learning and memory of Chst14^−/− mice was impaired when compared to their wild type (WT) littermates. Corroborating the behavior result, long-term potentiation (LTP) at the hippocampal CA3-CA1 connection was reduced in Chst14^−/− mice compared to the WT mice. Finally, the protein levels of N-Methyl-D-aspartate (NMDA) receptor, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor, postsynaptic density 95 (PSD95), growth associated protein 43 (GAP-43), synaptophysin (SYN) and N-ethylmaleimide sensitive factor (NSF) which are important in synaptic plasticity were examined and Chst14/D4st1 deficiency was shown to significantly reduce the expression of these proteins in the hippocampus. Further studies revealed that Akt/mammalian target rapamycin (mTOR) pathway proteins, including protein kinase B (p-Akt), p-mTOR and p-S6, were significantly lower in Chst14^−/− mice, which might contribute to the decreased protein expression. Together, this study reveals that specific sulfation of DS is critical in synaptic plasticity of the hippocampus and learning and memory, which might be associated with the changes in the expression of glutamate receptors and other synaptic proteins though Akt/mTOR pathway.

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).