Scaffold remodeling: Dynamics and function of mGlu5 receptosome in synaptic plasticity – SYN CITY

Understanding how proteins are activated as free molecules or part of complexes is an essential biological issue that will offer new possibilities to target exclusively the therapeutically relevant signaling pathway of a given receptor. Currently, the molecular detail of the dynamics of these interactions and the roles that they play in various cellular functions are poorly defined because of the dearth of methods for acutely and specifically controlling the binding interactions. We are developing new technologies allowing to highlight in living neurons the spatio-temporal dynamics of protein interactions. We also engineer tools to force or disrupt the interactions of interest to repair the communication between neurons in mouse models of brain pathologies.

The efficacy of the communication between neurons is altered in mental retardation. We have developed tools to study interactions between actors of the neuronal communication in a mouse model of mental retardation, and compare them to control mice. We identified interaction deficiencies between 2 main partners controlling communication in physiological conditions. By repairing these interactions, we can improve the neuronal communication.

Identifying single receptor function played by association with specific protein partners will allow to propose dysfunction-specific adapted therapy.

3 publications on the development and optimization of technologies to follow protein complex dynamics are published. 2 other articles demonstrate the specific different functions played by one receptor when associated with distinct partners. One of these interactions would trigger mental retardation.

At brain synapses, scaffolding proteins function not only as anchors but also as signaling proteins for neurotransmitter receptors. As synapses are dynamic structures, it is a fundamental issue to study the dynamics of such synaptic receptor scaffolds and their role in neurotransmission. Indeed, membrane receptors are associated with scaffolding proteins that link them to their intracellular signal transduction pathways and cytoskeleton. Such receptosome, is a relatively stable structure, but exchange of individual adaptor proteins can occur at a short time scale and in a highly regulated manner, which provides fine-tuning, speed, and specificity of the receptor signaling. Therefore, understanding how proteins are activated as free molecules or part of complexes is an essential biological concern.

NMDA and mGlu5 receptors-mediated long term synaptic plasticity at the hippocampal CA1-Shaffer collateral synapse is believed to be the molecular basis for hippocampal learning and memory. NMDA and mGlu5 receptors interact with a scaffolding complex containing PSD95-GKAP-Shank-Homer proteins. This example represents an excellent model to understand the functional consequences of a dynamic remodeling of protein-protein interaction within the complex. Not only Homer protein–containing complex physically links together in the postsynaptic density these metabotropic and ionotropic Glutamate receptors, but we recently demonstrated that the remodeling of this scaffold governs functional cross-talk between the two types of Glutamate receptors to control the synaptic transmission and excitability in hippocampal neurons. Moreover, neurological disorders displaying aberrant neuronal plasticity (such as mental retardation) result from mutations of these scaffolding proteins and / or alteration of receptor - scaffolding protein interactions, which thereby offer exciting opportunities for therapeutic intervention.

The aim of the present project is to characterize the spatio-temporal dynamics of protein-protein interactions and various stoichiometries of proteins within mGlu5 receptosome in neuronal dendritic spines, and to establish the functional significance of these oligomers remodeling in the physiological synaptic plasticity and in mental retardation.
The first goal requires the implementation of innovative technologies in living neurons, including Bioluminescent Resonance Energy Transfer (BRET) imaging combined to super-resolution 3D-Structured Illumination Microscopy (3D-SIM) to study the occurrence and dynamic of protein-protein interactions, but also two photon Fluorescence Fluctuation Microscopies (FFM) to depict variations in number and stoichiometry of the complexes in real time during plasticity.
The second goal encompasses the development of molecular tools to favor or prevent the remodeling of scaffolding complexes. The functional role of protein-protein interactions within the mGlu5 receptosome will then be analyzed studying 1) the synaptic activity and possible changes in long-term synaptic efficacy in hippocampal neurons 2) hippocampus-related mice behavior in wild type mice and mouse model of mental retardation. Reinstatement of defective scaffold remodeling in pathological conditions will aim to restore physiological synaptic activity and plasticity.
We will focus on Homer-complex remodeling given its obvious involvement in controlling synaptic plasticity, but we will also perform a gene profiling in hippocampus CA1-pyramidal cell type to identify other newly translated mRNA enabling hippocampus-dependent learning and memory processes. This screen will propose new therapeutic targets (proteins from the PSD) to counteract learning and memory deficiencies in mental retardation.
This study will bring the first clear demonstration of the existence and functional relevance of scaffold remodeling in space and time to control synaptic plasticity. It will also propose new strategies to tackle such issues for other multimeric oligomers.