Potentiation of convergent synaptic inputs onto pyramidal neurons in somatosensory cortex: dependence on brain wave frequencies and NMDA receptor subunit composition

The t-N-methyl-d-aspartate receptor: Making the case for d-Serine to be considered its inverse co-agonist

The N-methyl-d-aspartate receptor (NMDAR) is an enigmatic macromolecule that has garnered a good deal of attention on account of its involvement in the cellular processes that underlie learning and memory, following its discovery in the mid twentieth century (Baudry and Davis, 1991). Yet, despite advances in knowledge about its function, there remains much more to be uncovered regarding the receptor's biophysical properties, subunit composition, and role in CNS physiology and pathophysiology. The motivation for this review stems from the need for synthesizing new information gathered about these receptors that sheds light on their role in synaptic plasticity and their dichotomous relationship with the amino acid d-serine through which they influence the pathogenesis of neurodegenerative diseases like temporal lobe epilepsy (TLE), the most common type of adult epilepsies (Beesley et al., 2020a). This review will outline pertinent ideas relating structure and function of t-NMDARs (GluN3 subunit-containing triheteromeric NMDARs) for which d-serine might serve as an inverse co-agonist. We will explore how tracing d-serine's origins blends glutamate-receptor biology with glial biology to help provide fresh perspectives on how neurodegeneration might interlink with neuroinflammation to initiate and perpetuate the disease state. Taken together, we envisage the review to deepen our understanding of endogenous d-serine's new role in the brain while also recognizing its therapeutic potential in the treatment of TLE that is oftentimes refractory to medications.

Visualizing the triheteromeric N-methyl-D-aspartate receptor subunit composition

N-methyl-D-aspartate receptors (NMDARs) are one of three ligand-gated ionotropic channels that transduce the effects of neurotransmitter glutamate at excitatory synapses within the central nervous system. Their ability to influx Ca2+ into cells, unlike mature AMPA or kainate receptors, implicates them in a variety of processes ranging from synaptic plasticity to cell death. Many of the receptor's capabilities, including binding glutamate and regulating Ca2+ influx, have been attributed to their subunit composition, determined putatively using cell biology, electrophysiology and/or pharmacology. Here, we show that subunit composition of synaptic NMDARs can also be readily visualized in acute brain slices (rat) using highly specific antibodies directed against extracellular epitopes of the subunit proteins and high-resolution confocal microscopy. This has helped confirm the expression of triheteromeric t-NMDARs (containing GluN1, GluN2, and GluN3 subunits) at synapses for the first time and reconcile functional differences with diheteromeric d-NMDARs (containing GluN1 and GluN2 subunits) described previously. Even though structural information about individual receptors is still diffraction limited, fluorescently tagged receptor subunit puncta coalesce with precision at various magnifications and/or with the postsynaptic density (PSD-95) but not the presynaptic active zone marker Bassoon. These data are particularly relevant for identifying GluN3A-containing t-NMDARs that are highly Ca2+ permeable and whose expression at excitatory synapses renders neurons vulnerable to excitotoxicity and cell death. Imaging NMDAR subunit proteins at synapses not only offers firsthand insights into subunit composition to correlate function but may also help identify zones of vulnerability within brain structures underlying neurodegenerative diseases like Temporal Lobe Epilepsy.

GluN3 Subunit Expression Correlates with Increased Vulnerability of Hippocampus and Entorhinal Cortex to Neurodegeneration in a Model of Temporal Lobe Epilepsy

Temporal lobe epilepsy (TLE) is the most common type of epilepsy in adults that is often refractory to anti-epileptic medication therapy. Neither the pathology nor the etiology of TLE are fully characterized, although recent studies have established that the two are causally related. TLE pathology entails a stereotypic pattern of neuron loss in hippocampal and parahippocampal regions, predominantly in CA1 subfield of the hippocampus and layer 3 of the medial entorhinal area (MEA), deemed hallmark pathological features of the disease. Through this work, we address the contribution of glutamatergic N-methyl-D-aspartate receptors (NMDARs) to the pathology (vulnerability and pattern of neuronal loss), and by extension to the pathophysiology (Ca²⁺ induced excitotoxicity), by assaying the spatial expression of their subunit proteins (GluN1, GluN2A, GluN2B and GluN3A) in these regions using ASTA (area specific tissue analysis), a novel methodology for harvesting brain chads from hard-to-reach regions within brain slices for Western blotting. Our data suggest gradient expression of the GluN3A subunit along the mid-lateral extent of layer 3 MEA and along the CA1-subicular axis in the hippocampus, unlike GluN1 or GluN2 subunits which are uniformly distributed. Incorporation of GluN3A in the subunit composition of conventional diheteromeric (d-) NMDARs yield triheteromeric (t-) NMDARs which by virtue of their increased selectivity for Ca²⁺ render neurons vulnerable to excitotoxic damage. Thus, the expression profile of this subunit sheds light on the spatial extent of the pathology observed in these regions and implicates the GluN3 subunit of NMDARs in hippocampal and entorhinal cortical pathology underlying TLE.

A Model for Predicting Cation Selectivity and Permeability in AMPA and NMDA Receptors Based on Receptor Subunit Composition

Glutamatergic AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) and NMDA (N-methyl-D-aspartate) receptors are implicated in diverse functions ranging from synaptic plasticity to cell death. They are heterotetrameric proteins whose subunits are derived from multiple distinct gene families. The subunit composition of these receptors determines their permeability to monovalent and/or divalent cations, but it is not entirely clear how this selectivity arises in native and recombinantly-expressed receptor populations. By analyzing the sequence of amino acids lining the selectivity filters within the pore forming membrane helices (M2) of these subunits and by correlating subunit stoichiometry of these receptors with their ability to permeate Na⁺ and/or Ca²⁺, we propose here a mathematical model for predicting cation selectivity and permeability in these receptors. The model proposed is based on principles of charge attractivity and charge neutralization within the pore forming region of these receptors; it accurately predicts and reconciles experimental data across various platforms including Ca²⁺ permeability of GluA2-lacking AMPARs and ion selectivity within GluN3-containing di- and tri-heteromeric NMDARs. Additionally, the model provides insights into biophysical mechanisms regulating cation selectivity and permeability of these receptors and the role of various subunits in these processes.

N-Methyl-D-Aspartate (NMDA) receptor modulators: a patent review (2015-present)

INTRODUCTION

- The NMDA receptor is implicated in various diseases including neurodegenerative, neurodevelopmental and mood disorders. However, only a limited number of clinically approved NMDA receptor modulators are available. Today, apparent NMDA receptor drug development strategies entail 1) exploring the unknown chemical space to identify novel scaffolds; 2) using the clinically available NMDA receptor modulators to expand the therapeutic indication space; 3) and to trace physiological functions of the NMDA receptor.

AREAS COVERED

- The current review reflects on the functional and pharmacological facets of NMDA receptors and the current clinical status quo of NMDA receptor modulators. Patent literature covering 2015 till April 2020 is discussed with emphasis on new indications.

EXPERT OPINION

- Supporting evidence shows that subtype-selective NMDA receptor antagonists show an improved safety profile compared to broad-spectrum channel blockers. Although GluN2B-selective antagonists are by far the most extensively investigated subtype-selective modulators, they have shown only modest clinical efficacy so far. To overcome the limitations that have hampered the clinical development of previous subtype-selective NMDA receptor antagonists, future studies with improved animal models that better reflect human NMDA receptor pathophysiology are warranted. The increased availability of subtype-selective probes will allow target engagement studies and proper dose finding in future clinical trials.

The GluN3 subunit regulates ion selectivity within native N-methyl-d-aspartate receptors

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The GluN3 subunit is the least understood of all subunits that make up functional NMDARs in the brain.

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We show through ion substitution experiments that NMDARs containing GluN3 are more permeable to Ca²⁺ than those containing just GluN1 and GluN2.

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We attribute these differences to their ability to screen for Ca²⁺ over Na⁺.

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Subunit-dependent cation selectivity represents a hitherto unrealized mechanism for finer control of Ca²⁺ influx enhancing the repertoire of synaptic NMDARs.

Glutamatergic N-methyl-d-aspartate receptors (NMDARs) are heterotetrameric proteins whose subunits are derived from three gene families, GRIN1 (codes for GluN1), GRIN2 (GluN2) and GRIN3 (GluN3). In addition to providing binding sites for glutamate and the co-agonist glycine, these subunits in their di (d-) and tri (t-) heteromeric configurations regulate various aspects of receptor function in the brain. For example, the decay kinetics of NMDAR-mediated synaptic currents depend on the type of GluN2 subunit (GluN2A-GluN2D) in the receptor subunit composition. While much is known about the contributions of GluN1 and GluN2 to d-NMDAR function, we know comparatively little about how GluN3 influences the function of t-NMDARs composed of one or more subunits from each of the three gene families. We report here that in addition to altering kinetics and voltage-dependent properties, the GluN3 subunit endows these receptors with ion selectivity wherein influx of Ca²⁺ is preferred over Na⁺. This became apparent in the process of assessing Ca²⁺ permeability through these receptors and is of significance given that NMDARs are generally believed to be nonselective to cations and increased selectivity can lead to enhanced permeability. This was true of two independent brain regions where t-NMDARs are expressed, the somatosensory cortex, where both receptor subtypes are expressed at separate inputs onto single neurons, and the entorhinal cortex, where they are co-expressed at individual synaptic inputs. Based on this data and the sequence of amino acids lining selectivity filters within these subunits, we propose GluN3 to be a regulatory subunit for ion selectivity in t-NMDARs.

Colocalization of distinct NMDA receptor subtypes at excitatory synapses in the entorhinal cortex

The subunit composition of N-methyl-d-aspartate receptors (NMDARs) at synaptic inputs onto a neuron can either vary or be uniform depending on the type of neuron and/or brain region. Excitatory pyramidal neurons in the frontal and somatosensory cortices (L5), for example, show pathway-specific differences in NMDAR subunit composition in contrast with the entorhinal cortex (L3), where we now show colocalization of NMDARs with distinct subunit compositions at individual synaptic inputs onto these neurons. Subunit composition was deduced electrophysiologically based on alterations of current-voltage relationship ( I-V) profiles, amplitudes, and decay kinetics of minimally evoked, pharmacologically isolated, NMDAR-mediated excitatory postsynaptic currents by known subunit-preferring antagonists. The I-Vs were outwardly rectifying in a majority of neurons assayed (~80%), indicating expression of GluN1/GluN2/GluN3-containing triheteromeric NMDARs ( t-NMDARs) and of the conventional type, reversing close to 0 mV with prominent regions of negative slope, in the rest of the neurons sampled (~20%), indicating expression of GluN1/GluN2-containing diheteromeric NMDARs ( d-NMDARs). Blocking t-NMDARs in neurons with outwardly rectifying I-Vs pharmacologically unmasked d-NMDARs, with all responses antagonized using D-AP5. Coimmunoprecipitation assays of membrane-bound protein complexes isolated from the medial entorhinal area using subunit-selective antibodies corroborated stoichiometry and together suggested the coexpression of t- and d-NMDARs at these synapses. Colocalization of functionally distinct NMDAR subtypes at individual synaptic inputs likely enhances the repertoire of pyramidal neurons for information processing and plasticity within the entorhinal cortex. NEW & NOTEWORTHY The subunit composition of a N-methyl-d-aspartate (NMDA) receptor, which dictates most aspects of its function, can vary between neurons in different brain regions and/or between synaptic inputs onto single neurons. Here we demonstrate colocalization of tri- and diheteromeric-NMDA receptors at the same/single synaptic input onto excitatory neurons in the entorhinal cortex. Synaptic colocalization of distinct NMDAR subtypes might endow entorhinal cortical neurons with the ability to encode distinct patterns of neuronal activity through single synapses.