Extracellular Calcium Regulates Postsynaptic Efficacy through Group 1 Metabotropic Glutamate Receptors

Bursts of synaptic transmission are known to induce transient depletion of Ca2+ within the synaptic cleft. Although Ca2+ depletion has been shown to lower presynaptic release probability, effects on the postsynaptic cell have not been reported. In this study, we show that physiologically relevant reductions in extracellular Ca2+ lead to a decrease in synaptic strength between synaptically coupled layer 2/3 cortical pyramidal neurons. Using quantal analysis and mEPSP analysis, we demonstrate that a lowered extracellular Ca2+ produces a reduction in the postsynaptic quantal size in addition to its known effect on release probability. An elevated Mg2+ level can prevent this reduction in postsynaptic efficacy at subphysiological Ca2+ levels. We show that the calcium-dependent effect on postsynaptic quantal size is mediated by group 1 metabotropic glutamate receptors, acting via CaMKII (Ca2+/calmodulin-dependent protein kinase II) and PKC. Therefore, physiologically relevant changes in extracellular Ca2+ can regulate information transfer at cortical synapses via both presynaptic and postsynaptic mechanisms

The Subtype of GluN2 C-terminal Domain Determines the Response to Excitotoxic Insults

It is currently unclear whether the GluN2 subtype influences NMDA receptor (NMDAR) excitotoxicity. We report that the toxicity of NMDAR-mediated Ca(2+) influx is differentially controlled by the cytoplasmic C-terminal domains of GluN2B (CTD(2B)) and GluN2A (CTD(2A)). Studying the effects of acute expression of GluN2A/2B-based chimeric subunits with reciprocal exchanges of their CTDs revealed that CTD(2B) enhances NMDAR toxicity, compared to CTD(2A). Furthermore, the vulnerability of forebrain neurons in vitro and in vivo to NMDAR-dependent Ca(2+) influx is lowered by replacing the CTD of GluN2B with that of GluN2A by targeted exon exchange in a mouse knockin model. Mechanistically, CTD(2B) exhibits stronger physical/functional coupling to the PSD-95-nNOS pathway, which suppresses protective CREB activation. Dependence of NMDAR excitotoxicity on the GluN2 CTD subtype can be overcome by inducing high levels of NMDAR activity. Thus, the identity (2A versus 2B) of the GluN2 CTD controls the toxicity dose-response to episodes of NMDAR activity

Comment on "Drug Screening for ALS Using Patient-Specific Induced Pluripotent Stem Cells"

Egawa et al. recently showed the value of patient-specific induced pluripotent stem cells (iPSCs) for modeling amyotrophic lateral sclerosis in vitro. Their study and our work highlight the need for complementary assays to detect small, but potentially important, phenotypic differences between control iPSC lines and those carrying disease mutations

Neurochemical Changes in the Mouse Hippocampus Underlying the Antidepressant Effect of Genetic Deletion of P2X7 Receptors.

Recent investigations have revealed that the genetic deletion of P2X7 receptors (P2rx7) results in an antidepressant phenotype in mice. However, the link between the deficiency of P2rx7 and changes in behavior has not yet been explored. In the present study, we studied the effect of genetic deletion of P2rx7 on neurochemical changes in the hippocampus that might underlie the antidepressant phenotype. P2X7 receptor deficient mice (P2rx7-/-) displayed decreased immobility in the tail suspension test (TST) and an attenuated anhedonia response in the sucrose preference test (SPT) following bacterial endotoxin (LPS) challenge. The attenuated anhedonia was reproduced through systemic treatments with P2rx7 antagonists. The activation of P2rx7 resulted in the concentration-dependent release of [3H]glutamate in P2rx7+/+ but not P2rx7-/- mice, and the NR2B subunit mRNA and protein was upregulated in the hippocampus of P2rx7-/- mice. The brain-derived neurotrophic factor (BDNF) expression was higher in saline but not LPS-treated P2rx7-/- mice; the P2rx7 antagonist Brilliant blue G elevated and the P2rx7 agonist benzoylbenzoyl ATP (BzATP) reduced BDNF level. This effect was dependent on the activation of NMDA and non-NMDA receptors but not on Group I metabotropic glutamate receptors (mGluR1,5). An increased 5-bromo-2-deoxyuridine (BrdU) incorporation was also observed in the dentate gyrus derived from P2rx7-/- mice. Basal level of 5-HT was increased, whereas the 5HIAA/5-HT ratio was lower in the hippocampus of P2rx7-/- mice, which accompanied the increased uptake of [3H]5-HT and an elevated number of [3H]citalopram binding sites. The LPS-induced elevation of 5-HT level was absent in P2rx7-/- mice. In conclusion there are several potential mechanisms for the antidepressant phenotype of P2rx7-/- mice, such as the absence of P2rx7-mediated glutamate release, elevated basal BDNF production, enhanced neurogenesis and increased 5-HT bioavailability in the hippocampus

Mitochondrial calcium uniporter Mcu controls excitotoxicity and is transcriptionally repressed by neuroprotective nuclear calcium signals

The recent identification of the mitochondrial Ca(2+) uniporter gene (Mcu/Ccdc109a) has enabled us to address its role, and that of mitochondrial Ca(2+) uptake, in neuronal excitotoxicity. Here we show that exogenously expressed Mcu is mitochondrially localized and increases mitochondrial Ca(2+) levels following NMDA receptor activation, leading to increased mitochondrial membrane depolarization and excitotoxic cell death. Knockdown of endogenous Mcu expression reduces NMDA-induced increases in mitochondrial Ca(2+), resulting in lower levels of mitochondrial depolarization and resistance to excitotoxicity. Mcu is subject to dynamic regulation as part of an activity-dependent adaptive mechanism that limits mitochondrial Ca(2+) overload when cytoplasmic Ca(2+) levels are high. Specifically, synaptic activity transcriptionally represses Mcu, via a mechanism involving the nuclear Ca(2+) and CaM kinase-mediated induction of Npas4, resulting in the inhibition of NMDA receptor-induced mitochondrial Ca(2+) uptake and preventing excitotoxic death. This establishes Mcu and the pathways regulating its expression as important determinants of excitotoxicity, which may represent therapeutic targets for excitotoxic disorders

Structural basis of subtype-selective competitive antagonism for GluN2C/2D-containing NMDA receptors.

N-Methyl-D-aspartate receptors (NMDARs) play critical roles in the central nervous system. Their heterotetrameric composition generates subtypes with distinct functional properties and spatio-temporal distribution in the brain, raising the possibility for subtype-specific targeting by pharmacological means for treatment of neurological diseases. While specific compounds for GluN2A and GluN2B-containing NMDARs are well established, those that target GluN2C and GluN2D are currently underdeveloped with low potency and uncharacterized binding modes. Here, using electrophysiology and X-ray crystallography, we show that UBP791 ((2S*,3R*)-1-(7-(2-carboxyethyl)phenanthrene-2-carbonyl)piperazine-2,3-dicarboxylic acid) inhibits GluN2C/2D with 40-fold selectivity over GluN2A-containing receptors, and that a methionine and a lysine residue in the ligand binding pocket (GluN2D-Met763/Lys766, GluN2C-Met736/Lys739) are the critical molecular elements for the subtype-specific binding. These findings led to development of UBP1700 ((2S*,3R*)-1-(7-(2-carboxyvinyl)phenanthrene-2-carbonyl)piperazine-2,3-dicarboxylic acid) which shows over 50-fold GluN2C/2D-selectivity over GluN2A with potencies in the low nanomolar range. Our study shows that the L-glutamate binding site can be targeted for GluN2C/2D-specific inhibition