Abstract
Cognitive symptoms observed in neurodevelopmental pathologies such as schizophrenia or autism spectrum disorders are highly debilitating and poorly controlled by currently available treatments. Accordingly, there is still an urgent need for new therapeutic approaches. It has become clear in the last few years that these cognitive deficits are caused by neurodevelopmental alterations. The 5-HT6 receptor has emerged as one promising targets for the treatment of these deficits in light of its early expression during brain development, its role in key neurodevelopmental processes such as neuronal migration ad neurite growth and the pro-cognitive effects of antagonists in various cognitive tasks in rodents. In order to identify novel molecular substrates of the control of cognition and neural development by 5-HT6 receptors, our team has recently characterized the receptor interactome thanks to complementary proteomic strategies. Further functional studies revealed that the Cdk5 pathway under the control of 5-HT6 receptor plays a crucial role in in the migration and positioning of cortical pyramidal neurons as well as in the initiation of neurite outgrowth. The new protein partners of the receptor identified also include G protein-regulated inducer of neurite outgrowth 1 (GPRIN1), a Cdk5 substrate, known to promote neurite outgrowth. This thesis aims at characterizing the impact of the 5-HT6 receptor/GPRIN1 interaction upon receptor activity and neuronal differentiation in a neuroblastoma cell line (NG108-15 cells) commonly used as a cellular model for studying molecular mechanisms underlying neuronal differentiation. I showed that GPRIN1 interacts with a receptor sequence comprising the 22 N-terminal residues of its C-terminal domain via a mechanism depending on its association with activated Gαs protein and regulated by receptor phosphorylation at Ser350 by Cdk5, thus demonstrating that the receptor recruits either Cdk5 or GPRIN1 in a dynamic manner. I then showed that the physical interaction between GPRIN1 and the 5-HT6 receptor stabilizes an active receptor conformation able to activate Gs and the production of cAMP in an agonist-independent manner and exhibiting a low apparent affinity for inverse agonists. This interaction also promotes neurite extension and branching both in NG108-15 cells and primary cultured neurons originating from different regions of the mouse brain. These effects upon neuronal differentiation are mediated by agonist-independent activation of the Gs/adenylyl cyclase/PKA pathway. Collectively, my results indicate that neuronal differentiation under the control of 5-HT6 receptors requires a complex sequence of signaling mechanisms that depends on the sequential association of the receptor with Cdk5 and GPRIN1: while the 5-HT6/Cdk5 interaction is required for the initiation of neurite growth, neurite extension and branching depends on receptor association with GPRIN1. Finally, I demonstrated that sub-chronic administration to mice depleted of Mu opioid receptor (Oprm1-/- mice, preclinical model of autism spectrum disorders - ASDs -) improves a range of primary symptoms, including alteration of social cognition and stereotypic behaviors, suggesting that 5-HT6 receptor blockade might be a relevant strategy for the treatment of some behavioral symptoms in ASDs. Collectively, this thesis highlights the role of the 5-HT6 receptor interactome and of its dynamics in neuronal differentiation and the establishment of neuronal connectivity and opens new perspectives for the treatment of psychiatric disorders of neuro-developmental origin, such as ASDs.