The serotonin6 receptor (5-HT6R) is a promising target for treating cognitive deficits of schizophrenia often linked to alterations of neuronal development. This receptor controls neurodevelopmental processes, but the signaling mechanisms involved remain poorly understood. Using a proteomic strategy, we show that 5-HT6Rs constitutively interact with cyclin-dependent kinase 5 (Cdk5). Expression of 5-HT6Rs in NG108-15 cells induced neurite growth and expression of voltage-gated Ca(2+) channels, two hallmarks of neuronal differentiation. 5-HT6R-elicited neurite growth was agonist independent and prevented by the 5-HT6R antagonist SB258585, which behaved as an inverse agonist. Moreover, it required receptor phosphorylation at Ser350 by Cdk5 and Cdc42 activity. Supporting a role of native 5-HT6Rs in neuronal differentiation, neurite growth of primary neurons was reduced by SB258585, by silencing 5-HT6R expression or by mutating Ser350 into alanine. These results reveal a functional interplay between Cdk5 and a G protein-coupled receptor to control neuronal differentiation.

Functional interplay between ionotropic and metabotropic receptors frequently involves complex intracellular signaling cascades. The group I metabotropic glutamate receptor mGlu5a co-clusters with the ionotropic N-methyl-D-aspartate (NMDA) receptor in hippocampal neurons. In this study, we report that a more direct cross-talk can exist between these types of receptors. Using bioluminescence resonance energy transfer in living HEK293 cells, we demonstrate that mGlu5a and NMDA receptor clustering reflects the existence of direct physical interactions. Consequently, the mGlu5a receptor decreased NMDA receptor current, and reciprocally, the NMDA receptor strongly reduced the ability of the mGlu5a receptor to release intracellular calcium. We show that deletion of the C terminus of the mGlu5a receptor abolished both its interaction with the NMDA receptor and reciprocal inhibition of the receptors. This direct functional interaction implies a higher degree of target-effector specificity, timing, and subcellular localization of signaling than could ever be predicted with complex signaling pathways.Presynaptic glutamate release activates multiple receptors that are assembled onto the postsynaptic membrane by scaffolding proteins (1) and determine the neuronal response. Activation of the glutamate ionotropic receptors of the ␣-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid subtype is responsible for the fast excitatory postsynaptic currents. Concomitant activation of glutamate ionotropic receptors of the N-methyl-D-aspartate (NMDA) 2 subtype mediates a slow excitatory postsynaptic current (2) that can trigger bidirectional modifications of synaptic strength: long-term potentiation (3) or long-term depression (4). The metabotropic glutamate (mGlu) receptors are subdivided into three groups based on sequence homology, preferred associated second messenger systems, and pharmacological properties (group I, mGlu1 and mGlu5; group II, mGlu2 and mGlu3; and group III, mGlu4 and mGlu6 -mGlu8). mGlu1a and mGlu5a/b receptor subtypes are mainly postsynaptic and positively coupled to G q proteins, stimulation of phospholipase C (5), protein kinase C, and mobilization of intracellular calcium via inositol 1,4,5-trisphosphate receptors. These receptors can also be synaptically activated by glutamate and participate in the modulation of postsynaptic ␣-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor currents (4, 6 -8). Electrophysiological experiments have shown functional interplay between mGlu1a/5 and NMDA receptors in various structures of the brain. Indeed, activation of mGlu5 receptors can enhance NMDA-evoked responses in the hippocampus, striatum, cortex, and spinal cord (9 -18), suggesting that the functional interactions between the mGlu and NMDA receptors are of widespread significance.Because both NMDA and mGlu receptors are important players in synaptic transmission, there has been considerable interest in identifying the mechanisms underlying the functional interactions between these two classes of glutamate r...

Synaptic activity–dependent remodeling of the glutamate receptor scaffold complex generates a negative feedback loop that limits further NMDA receptor activation.