Human neuromodulatory assembloids to study serotonin signaling and disease

Neuromodulators influence critical functions of the developing brain and regulate behavioral states. Dysfunction of these systems is often involved in neuropsychiatric disease and many drugs for these conditions act on these signaling pathways. Recent advances in stem cell biology have made it possible to derive a wide range of cells across the developing human nervous system in organoids and to functionally integrate them into assembloids, however they currently do not systematically incorporate neuromodulation. Here, we generate a midbrain-hindbrain organoid (hMHO) from human induced pluripotent (hiPS) cells and fuse them with cortical organoids (hCO) to form neuromodulatory assembloids (hNMA). We focus on serotonin (5-hydroxytryptamine, 5-HT) as one key neuromodulator and find characteristic gene expression patterns and properties of the 5-HT neurons in the hMHO. In hNMA, 5-HT neurons project into hCO, release 5-HT and change cortical network activity. To explore the utility of this system, we study 22q11.2 deletion syndrome (22q11.2DS), a common microdeletion associated with high risk for neuropsychiatric disease and defects in 5-HT signaling. We found differences in 5-HT dynamics in hNMA from patients that can be rescued by a selective serotonin reuptake inhibitor (SSRI). Taken together, hNMA can be used to study human 5-HT dynamics and uncover disease phenotypes which would facilitate therapeutic development.