5-HT6 Receptor: A New Player Controlling the Development of Neural Circuits

5-HT6 receptor (5-HT6R) is a G protein-coupled receptor that has recently emerged as a new regulator of neural development. In addition to the canonical Gs adenylyl cyclase pathway, recent proteomics approaches reveal that 5-HT6R is able to engage key developmental signaling pathways controlling neuronal circuit formation, neuronal connectivity, and psychiatric-relevant behaviors. For example, at early stages of neuronal development, expression of 5-HT6R constitutively regulates the activity of the cyclin-dependent kinase (Cdk)5 and, through this mechanism, controls cellular processes involved in circuit formation, including neuronal migration and neurite outgrowth. In addition to the Cdk5 pathway, 5-HT6R modulates a variety of key developmental targets such as Fyn, Jab1, and mammalian target of rapamycin (mTOR). Engagement of developmental pathways through 5-HT6R pharmacological manipulation has led to interesting new therapeutic perspectives in the field of psychiatric-related disorders. Indeed, 5-HT6R blockade can rescue a pathological overactivation of the mTOR pathway induced by early life insults in rodents and normalizes the associated social and episodic memory deficits. Here, we review recent evidence supporting the notion that 5-HT6R is at the interface of key developmental signaling pathways and a novel actor in the orchestration of neural circuit formation

The serotonin 6 receptor controls neuronal migration during corticogenesis via a ligand-independent Cdk5-dependent mechanism

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The serotonin 6 receptor controls neuronal migration during corticogenesis via a ligand-independent Cdk5-dependent mechanism

The formation of a laminar structure such as the mammalian neocortex relies on the coordinated migration of different subtypes of excitatory pyramidal neurons in specific layers. Cyclin-dependent kinase 5 (Cdk5) is a master regulator of pyramidal neuron migration. Recently, we have shown that Cdk5 binds to the serotonin 6 receptor (5-HT6R), a G protein-coupled receptor (GPCR). Here, we investigated the role of 5-HT6R in the positioning and migration of pyramidal neurons during mouse corticogenesis. We report that constitutive expression of 5-HT6R controls pyramidal neuron migration through an agonist-independent mechanism that requires Cdk5 activity. These data provide the first in vivo evidence of a role for constitutive activity at a GPCR in neocortical radial migration

The 5-HT6 receptor interactome: New insight in receptor signaling and its impact on brain physiology and pathologies

International audienceThe serotonin (5-HT)6receptor is a Gs-coupled receptor exclusively expressed in the central nervous system. Highest receptor densities are found in brain regions implicated in mnemonic functions where the receptor is primarily but not exclusively locatedin the primary cilium of neurons. The 5-HT6receptor continues to raise particular interest for neuropharmacologists,given the pro-cognitive effects of antagonistsin a wide range of cognitive impairment paradigms in rodents and human. The 5-HT6receptor also finely controls key neuro-developmental processes including neuron migration and differentiation. However, itsinfluence upon neurodevelopment and cognition is not solely mediated by its coupling to the Gs-adenylyl cyclase pathway, suggestingalternative signal transduction mechanisms. This prompted studies aimed at characterizing the receptor interactome that identified 125 candidate receptor partners, making the 5-HT6receptor one of the G protein-coupled receptors with the most extensivelycharacterized interactome. These studies showed that the receptor localization at the plasma membrane and, consequently, its signal transduction, arefinely modulated by several receptor partners. They demonstrated that prefrontal 5-HT6receptorsengage the mTOR pathway to compromise cognition in neurodevelopmental models of schizophrenia, and a role of the 5-HT6-mTORpathway in temporal epilepsy.Finally, they revealed that the receptor activates Cdk5 signalingin an agonist-independent manner through a mechanism involving receptor phosphorylation by the associated Cdk5 and highlighted its key rolein the migration ofneurons and neurite growth. These new receptor-operated signalingmechanisms should be considered inthefuture development of drugs acting on5-HT6receptor

Impact of 5-HT₆ Receptor Subcellular Localization on Its Signaling and Its Pathophysiological Roles

The serotonin (5-HT)6 receptor still raises particular interest given its unique spatio-temporal pattern of expression among the serotonin receptor subtypes. It is the only serotonin receptor specifically expressed in the central nervous system, where it is detected very early in embryonic life and modulates key neurodevelopmental processes, from neuronal migration to brain circuit refinement. Its predominant localization in the primary cilium of neurons and astrocytes is also unique among the serotonin receptor subtypes. Consistent with the high expression levels of the 5-HT6 receptor in brain regions involved in the control of cognitive processes, it is now well-established that the pharmacological inhibition of the receptor induces pro-cognitive effects in several paradigms of cognitive impairment in rodents, including models of neurodevelopmental psychiatric disorders and neurodegenerative diseases. The 5-HT6 receptor can engage several signaling pathways in addition to the canonical Gs signaling, but there is still uncertainty surrounding the signaling pathways that underly its modulation of cognition, as well as how the receptor’s coupling is dependent on its cellular compartmentation. Here, we describe recent findings showing how the proper subcellular localization of the receptor is achieved, how this peculiar localization determines signaling pathways engaged by the receptor, and their pathophysiological influence

Classification and Signaling Characteristics of 5-HT Receptors: Towards the Concept of 5-HT Receptosomes

International audienceSerotonin (5-hydroxytryptamine, 5-HT) is one of the extracellular messengers capable of activating the largest number of receptors (17 receptors belonging to seven classes—5-HT1-7—defined first on a pharmacological basis and then on their gene sequence). Alternative splicing and RNA editing add to this diversity. Most 5-HT receptors belong to the G protein–coupled receptor (GPCR) family except for 5-HT3 receptors that are ionic channels. Their activation gives rise to a huge complexity of intracellular signaling events that depend or not on G protein activation and underlie the diversity of physiological functions of 5-HT. Assembly of 5-HT receptors into heteromers with distinct 5-HT receptors or other GPCRs, as well as their physical association with protein partners further increase the versatility of serotonergic signaling and the complexity of its regulation. These interactions also offer novel opportunities for therapeutic interventions in disorders related to dysfunctions of serotonergic systems

Novel and atypical pathways for serotonin signaling

International audienceSerotonin (5-HT) appeared billions of years before 5-HT receptors and synapses. It is thus not surprising that 5-HT can control biological processes independently of its receptors. One example is serotonylation, which consists of covalent binding of 5-HT to the primary amine of glutamine. Over the past 20 years, serotonylation has been involved in the regulation of many signaling mechanisms. One of the most striking examples is the recent evidence that serotonylation of histone H3 constitutes an epigenetic mark. However, the pathophysiological role of histone H3 serotonylation remains to be discovered. All but one of the 5-HT receptors are G-protein-coupled receptors (GPCRs). The signaling pathways they control are finely tuned, and new, unexpected regulatory mechanisms are being uncovered continuously. Some 5-HT receptors (5-HT 2C , 5-HT 4 , 5-HT 6 , and 5-HT 7) signal through mechanisms that require neither G-proteins nor β-arrestins, the two classical and almost universal GPCR signal transducers. 5-HT 6 receptors are constitutively activated via their association with intracellular GPCR-interacting proteins (GIPs), including neurofibromin 1, cyclin-dependent kinase 5 (Cdk5), and G-protein-regulated inducer of neurite outgrowth 1 (GPRIN1). Interactions of 5-HT 6 receptor with Cdk5 and GPRIN1 are not concomitant but occur sequentially and play a key role in dendritic tree morphogenesis. Furthermore, 5-HT 6 receptor-mediated G-protein signaling in neurons is different in the cell body and primary cilium, where it is modulated by smoothened receptor activation. Finally, 5-HT 2A receptors form heteromers with mGlu 2 metabotropic glutamate receptors. This heteromerization results in a specific phosphorylation of mGlu 2 receptor on a serine residue (Ser 843) upon agonist stimulation of 5-HT 2A or mGlu 2 receptor. mGlu 2 receptor phosphorylation on Ser 843 is an essential step in engagement of G i/o signaling not only upon mGlu 2 receptor activation but also following 5-HT 2A receptor activation, and thus represents a key molecular event underlying functional crosstalk between both receptors

Dynamic interactions of the 5-HT 6 receptor with protein partners control dendritic tree morphogenesis

International audienceThe serotonin (5-hydroxytrypatmine) receptor 5-HT6 (5-HT6R) has emerged as a promising target to alleviate the cognitive symptoms of neurodevelopmental diseases. We previously demonstrated that 5-HT6R finely controls key neurodevelopmental steps, including neuronal migration and the initiation of neurite growth, through its interaction with cyclin-dependent kinase 5 (Cdk5). Here, we showed that 5-HT6R recruited G protein-regulated inducer of neurite outgrowth 1 (GPRIN1) through a Gs-dependent mechanism. Interactions between the receptor and either Cdk5 or GPRIN1 occurred sequentially during neuronal differentiation. The 5-HT6R-GPRIN1 interaction enhanced agonist-independent, receptor-stimulated cAMP production without altering the agonist-dependent response in NG108-15 neuroblastoma cells. This interaction also promoted neurite extension and branching in NG108-15 cells and primary mouse striatal neurons through a cAMP-dependent protein kinase A (PKA)-dependent mechanism. This study highlights the complex allosteric modulation of GPCRs by protein partners and demonstrates how dynamic interactions between GPCRs and their protein partners can control the different steps of highly coordinated cellular processes, such as dendritic tree morphogenesis

Spatiotemporal dynamics of 5-HT6 receptor ciliary localization during mouse brain development

International audienceThe serotonin 5-HT6 receptor (5-HT6R) is a promising target to improve cognitive symptoms of psychiatric diseases of neurodevelopmental origin, such as autism spectrum disorders and schizophrenia. However, its expression and localization at different stages of brain development remain largely unknown, due to the lack of specific antibodies to detect endogenous 5-HT6R. Here, we used transgenic mice expressing a GFP-tagged 5-HT6R under the control of its endogenous promoter (Knock-in) as well as embryonic stem cells expressing the GFP-tagged receptor to extensively characterize its expression at cellular and subcellular levels during development. We show that the receptor is already expressed at E13.5 in the cortex, the striatum, the ventricular zone, and to a lesser extent the subventricular zone. In adulthood, it is preferentially found in projection neurons of the hippocampus and cerebral cortex, in striatal medium-sized spiny neurons, as well as in a large proportion of astrocytes, while it is expressed in a minor population of interneurons. Whereas the receptor is almost exclusively detected in the primary cilia of neurons at embryonic and adult stages and in differentiated stem cells, it is located in the somatodendritic compartment of neurons from some brain regions at the neonatal stage and in the soma of undifferentiated stem cells. Finally, knocking-out the receptor induces a shortening of the primary cilium, suggesting that it plays a role in its function. This study provides the first global picture of 5-HT6R expression pattern in the mouse brain at different developmental stages. It reveals dynamic changes in receptor localization in neurons at the neonatal stage, which might underlie its key role in neuronal differentiation and psychiatric disorders of neurodevelopmental origin

Physical interaction between neurofibromin and serotonin 5-HT 6

International audienceActive G protein-coupled receptor (GPCR) conformations not only are promoted by agonists but also occur in their absence, leading to constitutive activity. Association of GPCRs with intracellular protein partners might be one of the mechanisms underlying GPCR constitutive activity. Here, we show that serotonin 5 hydroxytryptamine 6 (5-HT6) receptor constitutively activates the Gs/adenylyl cyclase pathway in various cell types, including neurons. Constitutive activity is strongly reduced by silencing expression of the Ras-GTPase activating protein (Ras-GAP) neurofibromin, a 5-HT6 receptor partner. Neurofibromin is a multidomain protein encoded by the NF1 gene, the mutation of which causes Neurofibromatosis type 1 (NF1), a genetic disorder characterized by multiple benign and malignant nervous system tumors and cognitive deficits. Disrupting association of 5-HT6 receptor with neurofibromin Pleckstrin Homology (PH) domain also inhibits receptor constitutive activity, and PH domain expression rescues 5-HT6 receptor-operated cAMP signaling in neurofibromin-deficient cells. Furthermore, PH domains carrying mutations identified in NF1 patients that prevent interaction with the 5-HT6 receptor fail to rescue receptor constitutive activity in neurofibromin-depleted cells. Further supporting a role of neurofibromin in agonist-independent Gs signaling elicited by native receptors, the phosphorylation of cAMP-responsive element-binding protein (CREB) is strongly decreased in prefrontal cortex of Nf1+/− mice compared with WT mice. Moreover, systemic administration of a 5-HT6 receptor inverse agonist reduces CREB phosphorylation in prefrontal cortex of WT mice but not Nf1+/− mice. Collectively, these findings suggest that disrupting 5-HT6 receptor–neurofibromin interaction prevents agonist-independent 5-HT6 receptor-operated cAMP signaling in prefrontal cortex, an effect that might underlie neuronal abnormalities in NF1 patients