AdheSyn

Deciphering the molecular mechanisms that control the formation and organization of synapses between specific neurons remains a major challenge, not only in fundamental neurobiology but also for understanding brain disorders. Even in a simple animal like the nematode Caenorhabditis elegans, where neuronal connectivity has been mapped at single-synapse resolution, the molecular composition of each synapse and the mechanisms underlying their specification remain largely unknown. In this proposal, we aim to characterize new molecular mechanisms involved in the functional and structural organization of synaptic complexes in C. elegans, with a focus on adhesion molecules of the immunoglobulin superfamily, known as IgLONs. IgLONs form an evolutionarily conserved family, characterized by their ability to interact with one another through immunoglobulin-like domains. These interactions occur in trans, between two adjacent cells. The expression profile of IgLONs in the mammalian central nervous system, and their detection at synapses by biochemical approaches suggested a synaptic role. However, this hypothesis remained to be substantiated at the molecular level. Using C. elegans, we have demonstrated for the first time that IgLONs establish in vivo trans-synaptic interactions that control synaptic assembly. The most striking aspect of our findings is that the IgLON located on the postsynaptic membrane physically binds to a nicotinic acetylcholine receptor (nAChR), enabling its recruitment to synapses. Our data position this tripartite adhesion complex (IgLON-IgLON-nAChR) at the core of synaptic assembly. However, our current molecular model does not yet explain how this complex multimerizes, how it interacts with presynaptic molecules, or how it is targeted to specific synapses. To address these questions, we aim to identify new components of this complex and elucidate the genetic and structural mechanisms involved. We will use genetic, cell biology, in vivo imaging, structural biochemistry, and biophysical approaches to: 1) Identify and characterize new components acting with IgLON complexes at synapses. Some of these components have already been identified through genetic and biochemical approaches, and preliminary analyses suggest they play essential synaptic roles. 2) Analyze the structure of the macromolecular complex involving IgLONs. The IgLON-nAChR interaction was identified through 3D structural prediction and in vivo data. We propose to characterize its structure using cryo-electron microscopy. Additionally, we will conduct biophysical and structural analyses of interactions between IgLONs and their newly identified synaptic partners. 3) Explore the diversity of IgLON complexes at distinct synapses. Cell adhesion molecules exhibit versatile functions, depending on their interaction partners within a given cellular context. We have detected IgLONs at other synaptic sites in C. elegans, paving the way for the characterization of IgLON complexes with distinct molecular compositions from the previously identified complex. Our approach integrates the complementary expertise of B. Pinan-Lucarre, a cell biologist of the synapse in C. elegans, and E. Özkan, a structural biologist with a longstanding interest in IgLONs in invertebrates. Together, our teams are uniquely positioned to decipher the molecular determinants governing precise and selective synaptic assemblies, supporting synapse formation, maintenance, and function.