Opposite effects of Zn on the in vitro binding of [3H]LY354740 to recombinant and native metabotropic glutamate 2 and 3 receptors

A specific role for hippocampal mossy fiber's zinc in rapid storage of emotional memories

We investigated the specific role of zinc present in large amounts in the synaptic vesicles of mossy fibers and coreleased with glutamate in the CA3 region. In previous studies, we have shown that blockade of zinc after release has no effect on the consolidation of spatial learning, while zinc is required for the consolidation of contextual fear conditioning. Although both are hippocampo-dependent processes, fear conditioning to the context implies a strong emotional burden. To verify the hypothesis that zinc could play a specific role in enabling sustainable memorization of a single event with a strong emotional component, we used a neuropharmacological approach combining a glutamate receptor antagonist with different zinc chelators. Results show that zinc is mandatory to allow the consolidation of one-shot memory, thus being the key element allowing the hippocampus submitted to a strong emotional charge to switch from the cognitive mode to a flashbulb memory mode. Individual differences in learning abilities have been known for a long time to be totally or partially compensated by distributed learning practice. Here we show that contextual fear conditioning impairments due to zinc blockade can be efficiently reduced by distributed learning practice.

Altered distribution of mGlu2 receptors in β-amyloid-affected brain regions of Alzheimer cases and aged PS2APP mice

Altered glutamatergic synaptic transmission is among the key events defining the course of Alzheimer's disease (AD). mGlu2 receptors, a subtype of group II metabotropic glutamate receptors, regulate (as autoreceptors) fast synaptic transmission in the CNS via the controlled release of the excitatory amino acid glutamate. Since their pharmacological manipulation in rodents has been reported to affect cognition, they are potential drug targets for AD therapy. We examined the fate of these receptors in cases of AD as well as in aging PS2APP mice--a proposed model of the disease. In vitro binding of [(3)H]LY354740, a selective group II agonist (with selective affinity for mGlu2 receptors, under the assay conditions used) and quantitative radioautography revealed a partial, but highly significant, loss of receptors in amyloid-affected discrete brain regions of AD cases and PS2APP mice. Among the mouse brain regions affected were, above all, the subiculum but also frontolateral cortex, dentate gyrus, lacunosum moleculare and caudate putamen. In AD, significant receptor losses were registered in entorhinal cortex and lacunosum moleculare (40% and 35%, respectively). These findings have implications for the development of selective ligands for symptomatic therapy in AD and for its diagnosis.

Mutagenesis and molecular modeling of the orthosteric binding site of the mGlu2 receptor determining interactions of the group II receptor antagonist (3)H-HYDIA

Binding of the mGlu2/3 antagonist HYDIA in the closed conformation model of mGlu2 causes repulsive interactions with Y216 in lobe II of the binding pocket, preventing closure of the VFT.Modulation of metabotropic glutamate 2/3 receptors represents a promising target for the treatment of neuropsychiatric disorders such as schizophrenia and depression. The novel mGlu2/3 ligand HYDIA ((1S,2R,3R,5R,6S)-2-amino-3-hydroxy-bicyclo[3.1.0]hexane-2,6-dicarboxylic acid) is a conformationally restricted and hydroxylated glutamate analogue. HYDIA is a potent and selective competitive antagonist of L-glutamate at the mGlu2/3 receptors in spite of being structurally very similar to the bicyclic LY354740, which is a potent and selective mGlu2/3 agonist. By comparing these two ligands, this study delineate the interaction mode of (3)H-HYDIA at the mGlu2 receptor, using both mutagenesis studies and computational modeling. Binding of HYDIA in the closed conformation model of mGlu2 results in repulsive interaction with the Y216 residue, preventing closure of the binding pocket and thus receptor activation. Consequently, HYDIA is proposed to bind in an open conformation model of mGlu2. Mutation of the structurally important Y216 residue in the binding site caused complete loss of affinity of both (3)H-LY354740 and (3)H-HYDIA. T168 in lobe I was shown to have an important role in HYDIA binding, and in the open conformation model this residue is interacting with the amino group of HYDIA. The Y144 residue in lobe I is shown to be engaged in both receptor interlobe binding and ligand interaction. Receptor mutations at this position (Y144G, Y144S and Y144A) showed dramatic impact on binding affinity and functional effect of HYDIA. The mGlu2 receptor mutants with increased structural flexibility at this position, which is crucial for pocket closure, were clearly preferred. These studies highlight the unique properties of the novel (3)H-HYDIA ligand and provide further support to our understanding of binding and signal transduction mechanisms of the mGlu2 receptor.

Phosphorylation of Tau at S422 is enhanced by Abeta in TauPS2APP triple transgenic mice

Amyloid beta peptides and microtubule-associated protein Tau are misfolded and form aggregates in brains of Alzheimer's disease patients. To examine their specific roles in the pathogenesis of Alzheimer's disease and their relevance in neurodegenerative processes, we have created TauPS2APP triple transgenic mice that express human mutated Amyloid Precursor Protein, presenilin 2 and Tau. We present a cross-sectional analysis of these mice at 4, 8, 12 and 16 months of age. By comparing with single transgenic Tau mice, we demonstrate that accumulation of Abeta in TauPS2APP triple transgenic mice impacts on Tau pathology by increasing the phosphorylation of Tau at serine 422, as determined by a novel immunodetection method that is able to reliably measure phospho-Tau species in transgenic mouse brains. The TauPS2APP triple transgenic mouse model will be very useful for studying the effect of new therapeutic paradigms on amyloid deposition and downstream neurofibrillary tangle development.

The group II metabotropic glutamate receptor 3 (mGluR3, mGlu3, GRM3): expression, function and involvement in schizophrenia

Group II metabotropic glutamate receptors (mGluRs) comprise mGluR2 (mGlu2; encoded by GRM2) and mGluR3 (mGlu3; encoded by GRM3) and modulate glutamate neurotransmission and synaptic plasticity. Here we review the expression and function of mGluR3 and its involvement in schizophrenia. mGluR3 is expressed by glia and neurons in many brain regions and has a predominantly presynaptic distribution, consistent with its role as an inhibitory autoreceptor and heteroceptor. mGluR3 splice variants exist in human brain but are of unknown function. Differentiation of mGluR3 from mGluR2 has been problematic because of the lack of selective ligands and antibodies; the available data suggest particular roles for mGluR3 in long-term depression, in glial function and in neuroprotection. Some but not all studies find genetic association of GRM3 polymorphisms with psychosis, with the risk alleles also being associated with schizophrenia-related endophenotypes such as impaired cognition, cortical activation and glutamate markers. The dimeric form of mGluR3 may be reduced in the brain in schizophrenia. Finally, preclinical findings have made mGluR3 a putative therapeutic target, and now direct evidence for antipsychotic efficacy of a group II mGluR agonist has emerged from a randomised clinical trial in schizophrenia. Together these data implicate mGluR3 in aetiological, pathophysiological and pharmacotherapeutic aspects of the disorder.

Ion dependence of ligand binding to metabotropic glutamate receptors

The ionic requirements for ligand binding to metabotropic glutamate receptors were carried out on secreted truncated receptors containing only the extracellular ligand binding domains of the receptors. The influence of ions on agonist binding was examined in mGluR1, mGluR3, and mGluR4 representing Group I, II, and III metabotropic glutamate receptors, respectively. [(3)H]Quisqualic acid binding to mGluR1 required the presence of calcium (or magnesium) ions but not sodium or chloride ions while [(3)H]DCG-IV binding to mGluR3 was dependent upon both cations and anions. [(3)H]L-AP4 binding to mGluR4 required chloride ions but not monovalent or divalent cations. The EC(50) for chloride facilitation of L-AP4 binding to mGluR4 was 63mM; this value is approximately one-half of the normal resting extracellular chloride concentration. These results demonstrate that metabotropic glutamate receptor subtypes require different complements of ions for ligand binding and suggest that natural physiological fluctuations in synaptic ion concentrations may regulate receptor binding and activation.