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The 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.

The atypical chemokine receptor 3 (ACKR3) is known to regulate cell migration, but the underlying mechanisms are unclear. Here, the authors show, from an interactome analysis, ACKR3 association with the gap junction protein Connexin 43 in vivo and ACKR3-mediated inhibition of astrocyte gap junctional communication.

Abstract Cannabis abuse during adolescence confers an increased risk for developing later in life cognitive deficits reminiscent of those observed in schizophrenia, suggesting common pathological mechanisms that remain poorly characterized. In line with previous findings that revealed a role of 5‐HT6 receptor‐operated mTOR activation in cognitive deficits of rodent developmental models of schizophrenia, we show that chronic administration of ∆9‐tetrahydrocannabinol (THC) to mice during adolescence induces a long‐lasting activation of mTOR in prefrontal cortex (PFC), alterations of excitatory/inhibitory balance, intrinsic properties of layer V pyramidal neurons, and long‐term depression, as well as cognitive deficits in adulthood. All are prevented by administrating a 5‐HT6 receptor antagonist or rapamycin, during adolescence. In contrast, they are still present 2 weeks after the same treatments delivered at the adult stage. Collectively, these findings suggest a role of 5‐HT6 receptor‐operated mTOR signaling in abnormalities of cortical network wiring elicited by THC at a critical period of PFC maturation and highlight the potential of 5‐HT6 receptor antagonists as early therapy to prevent cognitive symptom onset in adolescent cannabis abusers.

Graphical Abstract

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.

Salt bridge (SB, double-charge-assisted hydrogen bonds) formation is one of the strongest molecular non-covalent interactions in biological systems, including ligand–receptor complexes. In the case of G-protein-coupled receptors, such an interaction is formed by the conserved aspartic acid (D3.32) residue and the basic moiety of the aminergic ligand. This study aims to determine the influence of the substitution pattern at the basic nitrogen atom and the geometry of the amine moiety at position 4 of 1H-pyrrolo[3,2-c]quinoline on the quality of the salt bridge formed in the 5-HT6 receptor and D3 receptor. To reach this goal, we synthetized and biologically evaluated a new series of 1H-pyrrolo[3,2-c]quinoline derivatives modified with various amines. The selected compounds displayed a significantly higher 5-HT6R affinity and more potent 5-HT6R antagonist properties when compared with the previously identified compound PZ-1643, a dual-acting 5-HT6R/D3R antagonist; nevertheless, the proposed modifications did not improve the activity at D3R. As demonstrated by the in silico experiments, including molecular dynamics simulations, the applied structural modifications were highly beneficial for the formation and quality of the SB formation at the 5-HT6R binding site; however, they are unfavorable for such interactions at D3R.

In addition to the canonical Gs adenylyl cyclase pathway, the serotonin type 6 receptor (5-HT6R) recruits additional signaling pathways that control cognitive function, brain development, and synaptic plasticity in an agonist-dependent and independent manner. Considering that aberrant constitutive and agonist-induced active states are involved in various pathological mechanisms, the development of biased ligands with different functional profiles at specific 5-HT6R-elicited signaling pathways may provide a novel therapeutic perspective in the field of neurodegenerative and psychiatric diseases. Based on the structure of SB-258585, an inverse agonist at 5-HT6R-operated Gs and Cdk5 signaling, we designed a series of 1-(arylsulfonyl-isoindol-2-yl)piperazine derivatives and synthesized them using a sustainable mechanochemical method. We identified the safe and metabolically stable biased ligand 3g, which behaves as a neutral antagonist at the 5-HT6R-operated Gs signaling and displays inverse agonist activity at the Cdk5 pathway. Inversion of the sulfonamide bond combined with its incorporation into the isoindoline scaffold switched the functional profile of 3g at Gs signaling with no impact at the Cdk5 pathway. Compound 3g reduced the cytotoxicity of 6-OHDA and produced a glioprotective effect against rotenone-induced toxicity in C8-D1A astrocyte cell cultures. In view of these findings, compound 3g can be considered a promising biased ligand to investigate the role of the 5-HT6R-elicited Gs and Cdk5 signaling pathways in neurodegenerative diseases.

Abstract Cognitive deficits in schizophrenia severely compromise quality of life and are poorly controlled by current antipsychotics. While 5‐HT6 receptor blockade holds special promise, molecular substrates underlying their control of cognition remain unclear. Using a proteomic strategy, we show that 5‐HT6 receptors physically interact with several proteins of the mammalian target of rapamycin (mTOR) pathway, including mTOR. Further, 5‐HT6 receptor activation increased mTOR signalling in rodent prefrontal cortex (PFC). Linking this signalling event to cognitive impairment, the mTOR inhibitor rapamycin prevented deficits in social cognition and novel object discrimination induced by 5‐HT6 agonists. In two developmental models of schizophrenia, specifically neonatal phencyclidine treatment and post‐weaning isolation rearing, the activity of mTOR was enhanced in the PFC, and rapamycin, like 5‐HT6 antagonists, reversed these cognitive deficits. These observations suggest that recruitment of mTOR by prefrontal 5‐HT6 receptors contributes to the perturbed cognition in schizophrenia, offering new vistas for its therapeutic control.