Trisomy 21 impairs synchronized activity and connectivity in developing human down syndrome cortical excitatory neuron networks

Down syndrome (DS) is the most common genetic cause of intellectual disability, affecting one in 700 live births worldwide, and is caused by trisomy of the human chromosome 21 (Hsa21). Here, we investigated whether trisomy 21 alters the development of human excitatory cortical neuronal networks, and whether that could contribute to the neurodevelopmental phenotypes of DS. By recapitulating cerebral cortex developmentin vitrowith Trisomy 21 and control isogenic and non-isogenic human induced pluripotent stem cells (hiPSC), we assessed the impact of Hsa21 triplication on activity and connectivity in developing excitatory cortical neural networks. Our analysis revealed a significant decrease in neuronal activity in TS21 neurons early in development. Unlike euploid cortical excitatory neurons, TS21 neurons failed to develop synchronised bursting activityin vitroandin vivofollowing transplantation into the mouse forebrain. Viral transynaptic tracing identified significant reduction of neuronal connectivity in TS21 neuronal networksin vitro, suggesting that reduced network connectivity contributes to the reduction of synchronised bursting. Furthermore, TS21 neurons displayed significantly reduced expression of voltage-gated potassium channels, with single-neuron recordings confirming the absence of hyperpolarization-activated currents. This points to a functional loss of the potassium/sodium hyperpolarization-activated cyclic nucleotide-gated channel 1 (HCN1) in TS21 neurons. Together, these findings indicate that Hsa21 trisomy leads to alterations in ion channel composition, spontaneous synchronised activity, and neuronal connectivity in cortical excitatory neuron networks, which likely contribute to the neurodevelopmental features of DS.