Constitutive activity of Gs-coupled serotonin receptors: from underlying mechanisms to pathophysiological outcomes

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An important conceptual evolution in G protein-coupled receptor (GPCR) pharmacology is the demonstration that they can couple to and activate G proteins in absence of agonists. This constitutive activity has been established in native cells or tissues for several GPCRs and can represent up to 50% of maximal agonist-dependent activity, suggesting its high physiological relevance. However, constitutive activity is often neglected in the understanding of GPCR pathophysiological influence and has mostly been demonstrated toward G protein-dependent signaling. Whether GPCRs can also constitutively activate non-canonical (G protein-independent) signaling pathways remains to be established.To address these issues, the project will use three Gs-coupled serotonin (5-HT) receptors, including 5-HT4, 5-HT6 and 5-HT7 receptors, that display a high level of constitutive activity as GPCR models, and pursue the two following complementary objectives: 1) Deciphering the mechanisms underlying their constitutive activity at both canonical and non-canonical (e.g., mTOR and Rho-GTPase activation, amyloid precursor protein cleavage) signaling pathways, with a special focus on the role of receptor interactome, phosphorylation and compartmentation.2) Characterizing the pathophysiological outcomes of agonist-independent activation of these receptors. We will specifically explore its impact on neuronal development under physiological and pathological conditions with a particular focus on neurodegenerative disorders.The ultimate goal of the ContAct_5-HT proposal thus is (i) to demonstrate that GPCR canonical and non-canonical signaling is not only modulated by the binding to an extracellular ligand, but also by the receptor association with interacting partners, acting as positive or negative allosteric modulators, and (ii) to establish the importance of constitutive GPCR activity in health and disease and elucidate its relevance for drug discovery. This transdisciplinary project relies on the longstanding experience of both partner teams in serotonin receptor pharmacology, signaling and functions, and on their complementary technical skills in proteomics and mouse behavior (Partner #2) and state-of-the-art imaging technologies, including confocal and 2-photon FRET and TIRF-FRET imaging (Partner #1). Both partner teams have been collaborating for many years to decipher signal transduction engaged by serotonin receptors and mechanisms controlling their functional status in various biological models. This collaboration has been very fruitful (6 joint publications since 2002) and generated preliminary results that serve as proof of concept of the current proposal, which strongly guarantees the success of this project.