Novel toxicity of the unedited GluR2 AMPA receptor subunit dependent on surface trafficking and increased Ca2+-permeability

RNA Editing by ADAR Adenosine Deaminases in the Cell Models of CAG Repeat Expansion Diseases: Significant Effect of Differentiation from Stem Cells into Brain Organoids in the Absence of Substantial Influence of CAG Repeats on the Level of Editing

Expansion of CAG repeats in certain genes is a known cause of several neurodegenerative diseases, but exact mechanism behind this is not yet fully understood. It is believed that the double-stranded RNA regions formed by CAG repeats could be harmful to the cell. This study aimed to test the hypothesis that these RNA regions might potentially interfere with ADAR RNA editing enzymes, leading to the reduced A-to-I editing of RNA and activation of the interferon response. We studied induced pluripotent stem cells (iPSCs) derived from the patients with Huntington's disease or ataxia type 17, as well as midbrain organoids developed from these cells. A targeted panel for next-generation sequencing was used to assess editing in the specific RNA regions. Differentiation of iPSCs into brain organoids led to increase in the ADAR2 gene expression and decrease in the expression of protein inhibitors of RNA editing. As a result, there was increase in the editing of specific ADAR2 substrates, which allowed identification of differential substrates of ADAR isoforms. However, comparison of the pathology and control groups did not show differences in the editing levels among the iPSCs. Additionally, brain organoids with 42-46 CAG repeats did not exhibit global changes. On the other hand, brain organoids with the highest number of CAG repeats in the huntingtin gene (76) showed significant decrease in the level of RNA editing of specific transcripts, potentially involving ADAR1. Notably, editing of the long non-coding RNA PWAR5 was nearly absent in this sample. It could be stated in conclusion that in most cultures with repeat expansion, the hypothesized effect on RNA editing was not confirmed.

Spinophilin-dependent regulation of GluN2B-containing NMDAR-dependent calcium influx, GluN2B surface expression, and cleaved caspase expression

N-methyl-d-aspartate receptors (NMDARs) are calcium-permeable ion channels that are ubiquitously expressed within the glutamatergic postsynaptic density. Phosphorylation of NMDAR subunits defines receptor conductance and surface localization, two alterations that can modulate overall channel activity. Modulation of NMDAR phosphorylation by kinases and phosphatases regulates the amount of calcium entering the cell and subsequent activation of calcium-dependent processes. The dendritic spine enriched protein, spinophilin, is the major synaptic protein phosphatase 1 (PP1) targeting protein. Depending on the substrate, spinophilin can act as either a PP1 targeting protein, to permit substrate dephosphorylation, or a PP1 inhibitory protein, to enhance substrate phosphorylation. Spinophilin limits NMDAR function in a PP1-dependent manner. Specifically, we have previously shown that spinophilin sequesters PP1 away from the GluN2B subunit of the NMDAR, which results in increased phosphorylation of Ser-1284 on GluN2B. However, how spinophilin modifies NMDAR function is unclear. Herein, we utilize a Neuro2A cell line to detail that Ser-1284 phosphorylation increases calcium influx via GluN2B-containing NMDARs. Moreover, overexpression of spinophilin decreases GluN2B-containing NMDAR activity by decreasing its surface expression, an effect that is independent of Ser-1284 phosphorylation. In hippocampal neurons isolated from spinophilin knockout animals, there is an increase in cleaved caspase-3 levels, a marker of calcium-associated apoptosis, compared with wildtype mice. Taken together, our data demonstrate that spinophilin regulates GluN2B containing NMDAR phosphorylation, channel function, and trafficking and that loss of spinophilin enhances neuronal cleaved caspase-3 expression.

A novel approach for vital visualization and studying of neurons containing Ca2+ -permeable AMPA receptors

Calcium-permeable AMPA receptors (CP-AMPARs) play a pivotal role in brain functioning in health and disease. They are involved in synaptic plasticity, synaptogenesis, and neuronal circuits development. However, the functions of neurons expressing CP-AMPARs and their role in the modulation of network activity remain elusive since reliable and accurate visualization methods are absent. Here we developed an approach allowing the vital identification of neurons containing CP-AMPARs. The proposed method relies on evaluating Ca²⁺ influx in neurons during activation of AMPARs in the presence of NMDAR and KAR antagonists, and blockers of voltage-gated Ca²⁺ channels. Using this method, we studied the properties of CP-AMPARs-containing neurons. We showed that the overwhelming majority of neurons containing CP-AMPARs are GABAergic, and they are distinguished by higher amplitudes of the calcium responses to applications of the agonists. Furthermore, about 30% of CP-AMPARs-containing neurons demonstrate the presence of GluK1-containing KARs. Although CP-AMPARs-containing neurons are characterized by more significant Ca²⁺ influx during the activation of AMPARs than other neurons, AMPAR-mediated Na⁺ influx is similar in these two groups. We revealed that neurons containing CP-AMPARs demonstrate weak GABA(A)R-mediated inhibition because of the low percentage of GABAergic synapses on the soma of these cells. However, our data show that weak GABA(A)R-mediated inhibition is inherent to all GABAergic neurons in the culture and cannot be considered a unique feature of CP-AMPARs-containing neurons. We believe that the suggested approach will help to understand the role of CP-AMPARs in the mammalian nervous system in more detail.

GluR2Q and GluR2R AMPA Subunits are not Targets of lypd2 Interaction

A large family of prototoxin-like molecules endogenous to mammals, Ly6 proteins have been implicated in the regulation of cell signaling processes across multiple species. Previous work has shown that certain members of the Ly6 family are expressed in the brain and target nicotinic acetylcholine receptor and potassium channel function. Structural similarities between Ly6 proteins and alpha-neurotoxins suggest the possibility of additional ionotropic receptor targets. Here, we investigated the possibility of lypd2 as a novel regulator of AMPA receptor (AMPAR) function. In particular, we focused on potential interactions with the Q/R isoforms of the GluR2 subunit, which have profound impacts on AMPAR permeability to calcium during neuronal stimulation. We find that although lypd2 and GluR2 share overlapping expression patterns in the mouse hippocampus, there was no interaction between lypd2 and either GluR2Q or GluR2R isoform. These results underscore the importance of continuing to investigate novel targets for Ly6 interaction and regulation.

A new mouse line with reduced GluA2 Q/R site RNA editing exhibits loss of dendritic spines, hippocampal CA1-neuron loss, learning and memory impairments and NMDA receptor-independent seizure vulnerability

Calcium (Ca²⁺)-permeable AMPA receptors may, in certain circumstances, contribute to normal synaptic plasticity or to neurodegeneration. AMPA receptors are Ca²⁺-permeable if they lack the GluA2 subunit or if GluA2 is unedited at a single nucleic acid, known as the Q/R site. In this study, we examined mice engineered with a point mutation in the intronic editing complementary sequence (ECS) of the GluA2 gene, Gria2. Mice heterozygous for the ECS mutation (named GluA2^+/ECS(G)) had a ~ 20% reduction in GluA2 RNA editing at the Q/R site. We conducted an initial phenotypic analysis of these mice, finding altered current-voltage relations (confirming expression of Ca²⁺-permeable AMPA receptors at the synapse). Anatomically, we observed a loss of hippocampal CA1 neurons, altered dendritic morphology and reductions in CA1 pyramidal cell spine density. Behaviourally, GluA2^+/ECS(G) mice exhibited reduced motor coordination, and learning and memory impairments. Notably, the mice also exhibited both NMDA receptor-independent long-term potentiation (LTP) and vulnerability to NMDA receptor-independent seizures. These NMDA receptor-independent seizures were rescued by the Ca²⁺-permeable AMPA receptor antagonist IEM-1460. In summary, unedited GluA2(Q) may have the potential to drive NMDA receptor-independent processes in brain function and disease. Our study provides an initial characterisation of a new mouse model for studying the role of unedited GluA2(Q) in synaptic and dendritic spine plasticity in disorders where unedited GluA2(Q), synapse loss, neurodegeneration, behavioural impairments and/or seizures are observed, such as ischemia, seizures and epilepsy, Huntington’s disease, amyotrophic lateral sclerosis, astrocytoma, cocaine seeking behaviour and Alzheimer’s disease.

Evaluating a Gene-Environment Interaction in Amyotrophic Lateral Sclerosis: Methylmercury Exposure and Mutated SOD1

PURPOSE OF REVIEW

Gene-environment (GxE) interactions likely contribute to numerous diseases, but are often difficult to model in the laboratory. Such interactions have been widely hypothesized for amyotrophic lateral sclerosis (ALS); recent controlled laboratory studies are discussed here and hypotheses related to possible mechanisms of action are offered. Using methylmercury exposure and mutated SOD1 to model the impacts of such an interaction, we interpret evidence about their respective mechanisms of toxicity to interrogate the possibility of additive (or synergistic) effects when combined.

RECENT FINDINGS

Recent work has converged on mechanisms of calcium-mediated glutamate excitotoxicity as a likely contributor in one model of a gene-environment interaction affecting the onset and progression of ALS-like phenotype. The current experimental literature on mechanisms of metal-induced neuronal injury and their relevant interactions with genetic contributions in ALS is sparse, but we describe those studies here and offer several integrative hypotheses about the likely mechanisms involved.

Advances in cellular models to explore the pathophysiology of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS), the most common adult-onset motor neuron disorder, is fatal for most patients less than 3 years from when the first symptoms appear. The aetiologies for sporadic and most familial forms of ALS are unknown, but genetic factors are increasingly recognized as causal in a subset of patients. Studies of disease physiology suggest roles for oxidative stress, glutamate-mediated excitotoxicity or protein aggregation; how these pathways interact in the complex pathophysiology of ALS awaits elucidation. Cellular models are being used to examine disease mechanisms. Recent advances include the availability of expanded cell types, from neuronal or glial cell culture to motoneuron-astrocyte co-culture genetically or environmentally modified. Cell culture experiments confirmed the central role of glial cells in ALS. The recent adaptation of induced pluripotent stem cells (iPSC) for ALS modeling could allow a broader perspective and is expected to generate new hypotheses, related particularly to mechanisms underlying genetic factors. Cellular models have provided meaningful advances in the understanding of ALS, but, to date, complete characterization of in vitro models is only partially described. Consensus on methodological approaches, strategies for validation and techniques that allow rapid adaptation to new genetic or environmental influences is needed. In this article, we review the principal cellular models being employed in ALS and highlight their contribution to the understanding of disease mechanisms. We conclude with recommendations on means to enhance the robustness and generalizability of the different concepts for experimental ALS.

Ca2+-permeable AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptors and dopamine D1 receptors regulate GluA1 trafficking in striatal neurons

Regulation of striatal medium spiny neuron synapses underlies forms of motivated behavior and pathological drug seeking. A primary mechanism for increasing synaptic strength is the trafficking of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) into the postsynapse, a process mediated by GluA1 AMPAR subunit phosphorylation. We have examined the role of converging glutamate and dopamine inputs in regulating biochemical cascades upstream of GluA1 phosphorylation. We focused on the role of Ca(2+)-permeable AMPARs (CPARs), which lack the GluA2 AMPAR subunit. Under conditions that prevented depolarization, stimulation of CPARs activated neuronal nitric oxide synthase and production of cGMP. CPAR-dependent cGMP production was sufficient to induce synaptic insertion of GluA1, detected by confocal microscopy, through a mechanism dependent on GluA1 Ser-845 phosphorylation. Dopamine D1 receptors, in contrast, stimulate GluA1 extra synaptic insertion. Simultaneous activation of dopamine D1 receptors and CPARs induced additive increases in GluA1 membrane insertion, but only CPAR stimulation augmented CPAR-dependent GluA1 synaptic insertion. This incorporation into the synapse proceeded through a sequential two-step mechanism; that is, cGMP-dependent protein kinase II facilitated membrane insertion and/or retention, and protein kinase C activity was necessary for synaptic insertion. These data suggest a feed-forward mechanism for synaptic priming whereby an initial stimulus acting independently of voltage-gated conductance increases striatal neuron excitability, facilitating greater neuronal excitation by a subsequent stimulus.

Calcium-permeable AMPA receptors in neonatal hypoxic-ischemic encephalopathy (Review)

Hypoxic-ischemic encephalopathy (HIE) is an important cause of brain injury in the newborn and may result in long-term devastating consequences. Excessive stimulation of glutamate receptors (GluRs) is a pivotal mechanism underlying ischemia-induced selective and delayed neuronal death. Although initial studies focused on N-methyl-D-aspartic acid (NMDA) receptors as critical mediators in HIE, subsequent studies supported a more central role for α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors (AMPARs), particularly Ca²⁺-permeable AMPARs, in brain damage associated with hypoxia-ischemia. This study reviewed the important role of Ca²⁺-permeable AMPARs in HIE and the future potential neuroprotective strategies associated with Ca²⁺-permeable AMPARs.

Synergistic inhibition of survival, proliferation, and migration of U87 cells with a combination of LY341495 and Iressa

Glioblastomas exploit various molecular pathways to promote glutamate- dependent growth by activating the AMPA (2-amino-3-(3-hydroxy-5-methyl-isoxazol-4-yl) propanoic acid) receptor, the group II metabotropic glutamate receptor, mGluR, and the epidermal growth factor receptor, EGFR. We hypothesized that targeting more than one of these pathways would be more effective in inhibiting glutamate-dependent growth. Using a model of U87 cell line, we show that blocking glutamate release by Riluzole inhibits cell proliferation. Glutamate-dependent growth is effectively inhibited by a combination of Iressa, an inhibitor of EGFR activation and LY341495, a group II mGluR inhibitor. Treatment of U87 cells with a combination of Iressa and LY341495 inhibits proliferation as indicated by Ki-67 staining, induces apoptosis and inhibits migration of U87 cells more effectively than the treatment by Iressa or LY341495 alone. These results demonstrate that a combinatorial therapy with Iressa and LY341495 is more effective due to synergistic effects of these drugs in inhibiting the growth of glioblastoma.

The essential role of AMPA receptor GluR2 subunit RNA editing in the normal and diseased brain

α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors are comprised of different combinations of GluA1–GluA4 (also known asGluR1–GluR4 and GluR-A to GluR-D) subunits. The GluA2 subunit is subject to RNA editing by the ADAR2 enzyme, which converts a codon for glutamine (Gln; Q), present in the GluA2 gene, to a codon for arginine (Arg; R) found in the mRNA. AMPA receptors are calcium (Ca²⁺)-permeable if they contain the unedited GluA2(Q) subunit or if they lack the GluA2 subunit. While most AMPA receptors in the brain contain the edited GluA2(R) subunit and are therefore Ca²⁺-impermeable, recent evidence suggests that Ca²⁺-permeable AMPA receptors are important in synaptic plasticity, learning, and disease. Strong evidence supports the notion that Ca²⁺-permeable AMPA receptors are usually GluA2-lacking AMPA receptors, with little evidence to date for a significant role of unedited GluA2 in normal brain function. However, recent detailed studies suggest that Ca²⁺-permeable AMPA receptors containing unedited GluA2 do in fact occur in neurons and can contribute to excitotoxic cell loss, even where it was previously thought that there was no unedited GluA2.This review provides an update on the role of GluA2 RNA editing in the healthy and diseased brain and summarizes recent insights into the mechanisms that control this process. We suggest that further studies of the role of unedited GluA2 in normal brain function and disease are warranted, and that GluA2 editing should be considered as a possible contributing factor when Ca²⁺-permeable AMPA receptors are observed.

Profound downregulation of the RNA editing enzyme ADAR2 in ALS spinal motor neurons

Amyotrophic lateral sclerosis (ALS) is the most common adult-onset fatal motor neuron disease. In spinal motor neurons of patients with sporadic ALS, normal RNA editing of GluA2, a subunit of the L-α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor, is inefficient. Adenosine deaminase acting on RNA 2 (ADAR2) specifically mediates RNA editing at the glutamine/arginine (Q/R) site of GluA2 and motor neurons expressing Q/R site-unedited GluA2 undergo slow death in conditional ADAR2 knockout mice. Therefore, investigation into whether inefficient ADAR2-mediated GluA2 Q/R site-editing occurs universally in motor neurons of patients with ALS would provide insight into the pathogenesis of ALS. We analyzed the extents of GluA2 Q/R site-editing in an individual laser-captured motor neuron of 29 ALS patients compared with those of normal and disease control subjects. In addition, we analyzed the enzymatic activity of three members of the ADAR family (ADAR1, ADAR2 and ADAR3) in ALS motor neurons expressing unedited GluA2 mRNA and those expressing only edited GluA2 mRNA. Q/R site-unedited GluA2 mRNA was expressed in a significant proportion of motor neurons from all of the ALS cases examined. Conversely, motor neurons of the normal and disease control subjects expressed only edited GluA2 mRNA. ADAR2, but not ADAR1 or ADAR3, was significantly downregulated in all the motor neurons of ALS patients, more extensively in those expressing Q/R site-unedited GluA2 mRNA than those expressing only Q/R site-edited GluA2 mRNA. These results indicate that ADAR2 downregulation is a profound pathological change relevant to death of motor neurons in ALS.

Ca permeable AMPA channels in diseases of the nervous system

Since the discovery and molecular characterization of Ca(2+)-permeable AMPA channels just over two decades ago, a large body of evidence has accumulated implicating contributions of these unusual glutamate activated channels to selective neurodegeneration in certain conditions, including ischemia and amyotrophic lateral sclerosis. Factors likely involved in their contributions to disease include their distinct patterns of expression in certain neuronal populations, their upregulation via various mechanisms in response to disease associated stresses, and their high permeability to Zn(2+) as well as to Ca(2+). However, full characterization of their contributions to certain diseases as well as development of therapeutics has been limited by the lack of selective and bioavailable blockers of these channels that can be employed in animals or humans. This review summarizes some of the clues that have emerged over recent years to the contributions of these channels in disease.

Exposure of neurons to excitotoxic levels of glutamate induces cleavage of the RNA editing enzyme, adenosine deaminase acting on RNA 2, and loss of GLUR2 editing

AMPA receptors are glutamate receptors that are tetramers of various combinations of GluR1-4 subunits. AMPA receptors containing GluR1, 3 and 4 are Ca2+ permeable, however, AMPA receptors containing even a single subunit of GluR2 are Ca2+ impermeable. Most AMPA receptors are Ca2+ impermeable due to the presence of GluR2. GluR2 confers special properties on AMPA receptors through the presence of arginine at the pore apex; other subunits (GluR1, 3, 4) contain glutamine at the pore apex and allow Ca2+ influx. Normally, an RNA editing step changes DNA-encoded glutamine to arginine, introduces arginine in the GluR2 pore apex. GluR2 RNA editing is carried out by an RNA-dependent adenosine deaminase (ADAR2). Loss of GluR2 editing leads to the formation of highly excitotoxic AMPA channels [Mahajan and Ziff (2007) Mol Cell Neurosci 35:470-481] and is shown to contribute to loss of motor neurons in amyotrophic lateral sclerosis (ALS). Relatively higher levels of Ca2+-permeable AMPA receptors are found in motor neurons and this has been correlated with lower GluR2 mRNA levels. However, the reason for loss of GluR2 editing is not known. Here we show that exposure of neurons to excitotoxic levels of glutamate leads to specific cleavage of ADAR2 that leads to generation of unedited GluR2. We demonstrate that cleaved ADAR2 leads to a decrease or loss of GluR2 editing, which will further result in high Ca2+ influx and excitotoxic neuronal death.

ADAR2-dependent RNA editing of GluR2 is involved in thiamine deficiency-induced alteration of calcium dynamics

BACKGROUND

Thiamine (vitamin B1) deficiency (TD) causes mild impairment of oxidative metabolism and region-selective neuronal loss in the central nervous system (CNS). TD in animals has been used to model aging-associated neurodegeneration in the brain. The mechanisms of TD-induced neuron death are complex, and it is likely multiple mechanisms interplay and contribute to the action of TD. In this study, we demonstrated that TD significantly increased intracellular calcium concentrations [Ca2+]i in cultured cortical neurons.

RESULTS

TD drastically potentiated AMPA-triggered calcium influx and inhibited pre-mRNA editing of GluR2, a Ca2+-permeable subtype of AMPA receptors. The Ca2+ permeability of GluR2 is regulated by RNA editing at the Q/R site. Edited GluR2 (R) subunits form Ca2+-impermeable channels, whereas unedited GluR2 (Q) channels are permeable to Ca2+ flow. TD inhibited Q/R editing of GluR2 and increased the ratio of unedited GluR2. The Q/R editing of GluR2 is mediated by adenosine deaminase acting on RNA 2 (ADAR2). TD selectively decreased ADAR2 expression and its self-editing ability without affecting ADAR1 in cultured neurons and in the brain tissue. Over-expression of ADAR2 reduced AMPA-mediated rise of [Ca2+]i and protected cortical neurons against TD-induced cytotoxicity, whereas down-regulation of ADAR2 increased AMPA-elicited Ca2+ influx and exacerbated TD-induced death of cortical neurons.

CONCLUSIONS

Our findings suggest that TD-induced neuronal damage may be mediated by the modulation of ADAR2-dependent RNA Editing of GluR2.

Development of calcium-permeable AMPA receptors and their correlation with NMDA receptors in fast-spiking interneurons of rat prefrontal cortex

Abnormal influx of Ca(2+) is thought to contribute to the neuronal injury associated with a number of brain disorders, and Ca(2+)-permeable AMPA receptors (CP-AMPARs) play a critical role in the pathological process. Despite the apparent vulnerability of fast-spiking (FS) interneurons in neurological disorders, little is known about the CP-AMPARs expressed by functionally identified FS interneurons in the developing prefrontal cortex (PFC). We investigated the development of inwardly rectifying AMPA receptor-mediated currents and their correlation with NMDA receptor-mediated currents in FS interneurons in the rat PFC. We found that 78% of the FS interneurons expressed a low rectification index, presumably Ca(2+)-permeable AMPARs, with only 22% exhibiting AMPARs with a high rectification index, probably Ca(2+) impermeable (CI). FS interneurons with CP-AMPARs exhibited properties distinct from those expressing CI-AMPARs, although both displayed similar morphologies, passive membrane properties and AMPA currents at resting membrane potentials. The AMPA receptors also exhibited dramatic changes during cortical development with significantly more FS interneurons with CP-AMPARs and a clearly decreased rectification index during adolescence. In addition, FS interneurons with CP-AMPARs exhibited few or no NMDA currents, distinct frequency-dependent synaptic facilitation, and protracted maturation in short-term plasticity. These data suggest that CP-AMPARs in FS interneurons may play a critical role in neuronal integration and that their characteristic properties may make these cells particularly vulnerable to disruptive influences in the PFC, thus contributing to the onset of many psychiatric disorders.

Dorsal horn alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor trafficking in inflammatory pain

Activation of synaptic N-methyl-D-aspartic acid receptor and its intracellular downstream signals in dorsal horn neurons of spinal cord contribute to central sensitization, a mechanism that underlies the development and maintenance of pain hypersensitivity in persistent pain. However, the molecular process of this event is not understood completely. Recently, new studies suggest that peripheral inflammatory insults drive changes in alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor subunit trafficking via N-methyl-D-aspartic acid receptor-triggered activation of protein kinases in dorsal horn and raise the possibility that such changes might contribute to central sensitization in persistent pain. This review presents current evidence regarding the changes that occur in the trafficking of dorsal horn alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor subunits GluR1 and GluR2 under persistent inflammatory pain conditions and discusses the potential mechanisms by which such changes participate in the development and maintenance of inflammatory pain.

AMPA and metabotropic excitoxicity explain subplate neuron vulnerability

Cerebral hypoxia-ischemia results in unique patterns of injury during development owing to selective vulnerability of specific cell populations including subplate neurons. To evaluate the contribution of glutamate excitotoxicity, we studied enriched cultures of subplate neurons in comparison with cortical neurons, deriving expression profiles for glutamate receptor subunits by microarray and immunoblot. The excitotoxic potency of specific glutamate receptors was tested with selective agonists and antagonists. After 1 week in culture, subplate neurons are more sensitive to oxygen-glucose deprivation than cortical neurons, confirming in vivo observations. Subplate and cortical neurons are equally sensitive to glutamate and insensitive to NMDA. Subplate neurons are more sensitive than cortical neurons to AMPA and express twofold less GluR2. Subplate neurons express significantly more mGluR3, a receptor proposed to be protective. Despite this increased expression, group II mGluR agonists increase subplate neuron death and antagonists lessen glutamate excitotoxicity, suggesting a novel mechanism for subplate vulnerability.