Serotonin transport and serotonin transporter-mediated antidepressant recognition are controlled by 5-HT_2B receptor signaling in serotonergic neuronal cells

The plasma membrane 5-HT transporter (SERT) is the major protagonist in regulating extracellular 5-HT concentration and constitutes the target of drugs used to treat a host of metabolic and psychiatric disorders. The exact mechanisms sustaining SERT function still remain elusive. The present work exploits the properties of the 1C11 neuroectodermal progenitor, which acquires, upon 4 days of differentiation, a functional SERT within an integrated serotonergic phenotype to investigate regulatory mechanisms involved in SERT onset and functions. We show that poly(A) addition precedes SERT mRNA translation on day 2 of the serotonergic program. The newly translated transporter molecules immediately bind cocaine. Day 4 must be awaited to monitor antidepressant recognition and 5-HT uptake. Because external 5-HT reduces both 5-HT transport and SERT antidepressant binding, we identify 5-HT_2B receptors as key players in controlling the overall 5-HT transport system. In the absence of external 5-HT, 5-HT_2B receptor coupling to NO production ensures SERT phosphorylation to basal level and maximal 5-HT uptake. In the presence of 5-HT, the 5-HT_2B receptor-PKC coupling promotes additional phosphorylations of both SERT and Na⁺,K⁺-ATPase α-subunit, impairing the electrochemical gradient necessary to 5-HT uptake. SERT hyperphosphorylation also affects antidepressant recognition. Finally, such 5-HT_2B receptor-mediated control of SERT activity operates in primary neurons from raphe nuclei. Altogether, our data shed new light on the 5-HT-driven post-translational modifications involved in the control of SERT activity.