Functional characterization of rhesus embryonic stem cell-derived serotonin neurons

Optimal function of the serotonin system is essential for mental health and its role in psychopathologies is undisputed. Enhancing the ability to study primate serotonin neurons in culture would facilitate understanding of intracellular signaling pathways that mediate the action of drugs and other epigenetic or developmental factors impacting human mental health. We were the first group to report differentiation of the non-human primate rhesus monkey embryonic stem cell (ESC) line 366.4 into cultures of serotonin neurons. In this study, we optimized yield and obtained functional characteristics of the derived serotonin neurons. Sequential treatments of ESC 366.4 during expansion stage with fibroblast growth factor 4 and sonic hedgehog markedly increased the yield of serotonin neurons. These serotonin neurons propagated action potentials and expressed GABA receptors. Also, for the first time we demonstrate that these ESC-derived serotonin neurons exhibit functional high-affinity transporter sites, as well as high-affinity 5HT_1A binding sites, which are essential targets of common psychoactive drugs. Finally, to test the generality of this method, we utilized another rhesus ESC line, ORMES-22, which efficiently differentiated into serotonin neurons. Together, these findings demonstrate the feasibility of our protocol to direct different primate ESC lines to serotonin neurons with physiological characteristics, which makes them a useful in vitro model system.