Co-agonist site of NMDA receptors: From synaptic dynamics to novel signalling mechanisms – COAGO

L’étude du cerveau et de ses connexions nerveuses se fait sur des modèles animaux de type rongeur. L’activité des récepteurs NMDA est mesurée par l’intermédiaire d’enregistrements électrophysiologiques à des manipulations pharmacologiques permettant d’identifier la nature et la composition de ces récepteurs. Nous combinons ces approches avec l’imagerie cellulaire par nanoparticule qui permet de visualiser la mobilité des récepteurs d’intérêt à la surface des cellules.

Nos travaux effectués au cours des derniers 18 mois indiquent que les récepteurs NMDA sont dans un 1er temps modulés par la glycine jusqu’à la 2e semaine de développement du cerveau avant que la D-sérine ne remplace la glycine dans ce rôle. Ce changement de coagoniste intervient en parallèle à la modification de la composition des récepteurs NMDA en terme de sous-unités. Nous avons donc étudié si ces deux phénomènes étaient reliés. Il apparait que les récepteurs NMDA n’ont pas une préférence pour l’un ou l’autre des coagoniste en fonction de leur composition. Inversement, nos données indiquent que la présence de glycine ou de D-sérine influence le sous-type de récepteur NMDA présent à la synapse.

La glycine et la D-sérine ont une origine différente. Nos travaux antérieurs ont démontré que la D-sérine était produite par les cellules astrogliales présentes dans l’environnement synaptique. Il semblerait donc qu’à partir de la 2e semaine de développement du cerveau de la souris, les astrocytes entre en jeu et jouent un rôle clé dans l’activation des récepteurs NMDA et des processus qui en dépendent comme l’apprentissage et la mémoire. Nous comptons poursuivre notre étude en disséquant les mécanismes moléculaires responsables de ce changement de coagoniste et du contrôle de la composition des récepteurs NMDA

Article en préparation

The function of the human brain and its capacity for experience-dependent change hinges on the dynamics of chemical synapses. At excitatory synapses, glutamate mediates fast transmission by acting on AMPA, kainate and NMDA receptor (NMDAR) ion channels. NMDARs are of particular interest because of their unique ability to flux large amounts of calcium that allows them to inducing long-lasting changes in synaptic strength. Normal activation of NMDARs is required for learning and memory, while both over-activation and hypo-function are involved in several disorders such as stroke, chronic pain, Alzheimer’s disease and schizophrenia.

Consistent with their critical role in physiological processes and the need for precise receptor regulation, NMDARs contain numerous sites for fine-tuning of receptor function by biologically relevant factors. These include the “glycine-site”, which binds the NMDAR co-agonist D-serine or glycine. Intriguingly, D-serine is abundant in the forebrain and preferentially localize to glutamatergic synapses. We have much information about the actions of D-serine/glycine on recombinant NMDARs but little on receptors embedded in the native synaptic environment. D-serine is proposed to be released by astrocytes that ensheath glutamatergic synapses. However, the nature of receptor modulation depends on the temporal dynamics and concentration profile of the modulator at the synapse, two parameters that remain poorly defined. Furthermore, for D-serine and glycine, it is still unclear which of the two co-agonists preferentially bind to synaptic NMDARs and how dynamic occupancy of the co-agonist binding site is during neuronal activity. In that context, a major breakthrough was recently made by members of our consortium. We showed that synaptic and extrasynaptic hippocampal CA1 NMDARs were gated by different endogenous co-agonists, that such a difference was matching the subunit composition of the receptors and that glycine and D-serine were impacting NMDAR membrane diffusion differentially in a subunit-dependent and activity-independent manner, pointing to a possible novel non-canonical (i.e. non-ionotropic) signaling pathway for these receptors.

In this project, building on our recent findings, we aim to further study how NMDAR activity is regulated through its glycine-site under native conditions. In particular, we plan to determine the nature of the endogenous co-agonist at different synapses and at different developmental stages to establish the relationship between receptor subunit composition and the identity of this co-agonist. We also aim at characterizing the synaptic profiles of glycine and D-serine in and outside the synaptic cleft under different conditions of neuronal activity and microenvironment. Finally we plan to investigate the molecular process by which co-agonist binding affects NMDAR signaling and influences the dynamics of receptor membrane trafficking. For that purpose, we propose to combine electrophysiological recordings with original genetically-encoded biosensors as well as quantum dot imaging of single receptor. The proposed experiments should provide a detailed description of the nature and the impact of the endogenous co-agonists on NMDAR functions in the context of synaptic transmission, synaptic plasticity, receptor membrane trafficking and excitotoxicity.

This project should provide critical novel information on the endogenous modulation of NMDARs, and more generally, on the physiology of the excitatory synapse and its deregulation during pathological events.