The first step towards the achievement of these goals was to understand the role of non-conventional NMDARs in synaptic function, and more specifically in synaptic plasticity. For that, we first studied the role of NMDARs in specific forms of long-term synaptic plasticity, depending on their synaptic localization. To this end, we have developed and initiated the characterization of a transgenic mouse line, that were invalidated for the gene encoding the necessary subunit of the NMDAR in excitatory cells. Using the quadruple patch-clamp recording technique coupled to 2-photon laser-scanning microscopy, we discovered that presynaptic NMDARs were necessary for the induction of a specific form of synaptic plasticity, namely timing-dependent long-term depression, whereas postsynaptic NMDARs were necessary for the “opposite” form of plasticity, namely timing-dependent long-term potentiation. In addition, these mouse lines also enabled us to establish that NMDARs play a crucial role in neuronal morphology, and in particular in the maintenance and plasticity of dendritic spines.
Then, using pharmacological approaches, we probed for the mode of signalling of involved in these two forms of synaptic plasticity. We revealed that timing-dependent long-term depression relies on non-ionotropic presynaptic NMDAR signalling (Figure Summary) whereas timing-dependent long-term potentiation relies on ionotropic postsynaptic NMDAR signalling. We have then deciphered further the signalling cascade mediating timing-dependent long-term depression. Using transgenic lines combined with pharmacology, we showed that non-ionotropic NMDARs determine timing-dependent long-term depression by recruiting the presynaptic JNK2 protein, which interacts with Syntaxin1a (Figure Summary). We have also demonstrated that this signalling pathway is required for timing-dependent long-term depression, regardless of the neuronal firing frequency during the induction (Figure Summary). Overall, our results show that NMDARs may have a specific mode of operation, depending on their synaptic localization as well as their signaling mode, to regulate specific forms of synaptic plasticity.
We now aim at testing whether these mechanisms are dysregulated in mouse model for Fragile X syndrome. In this line, we have started to study the main mouse model for Fragile X syndrome, the Fmr1 KO mouse line. Characterization is ongoing, and our preliminary data on synaptic plasticity have not led to any significant conclusions. Another step is to verify whether the signalling pathways downstream non-ionotropic NMDARs are dysregulated in the mouse model for Fragile X syndrome, thus by using proteomic and transcriptomic approaches. This will be done in the Host laboratory during the second phase of this MSC Action.
