Grafted human pluripotent stem cell‐derived cortical neurons integrate into adult human cortical neural circuitry

Several neurodegenerative diseases cause loss of cortical neurons, leading to sensory, motor, and cognitive impairments. Studies in different animal models have raised the possibility that transplantation of human cortical neuronal progenitors, generated from pluripotent stem cells, might be developed into a novel therapeutic strategy for disorders affecting cerebral cortex. For example, we have shown that human long‐term neuroepithelial‐like stem (lt‐NES) cell‐derived cortical neurons, produced from induced pluripotent stem cells and transplanted into stroke‐injured adult rat cortex, improve neurological deficits and establish both afferent and efferent morphological and functional connections with host cortical neurons. So far, all studies with human pluripotent stem cell‐derived neurons have been carried out using xenotransplantation in animal models. Whether these neurons can integrate also into adult human brain circuitry is unknown. Here, we show that cortically fated lt‐NES cells, which are able to form functional synaptic networks in cell culture, differentiate to mature, layer‐specific cortical neurons when transplanted ex vivo onto organotypic cultures of adult human cortex. The grafted neurons are functional and establish both afferent and efferent synapses with adult human cortical neurons in the slices as evidenced by immuno‐electron microscopy, rabies virus retrograde monosynaptic tracing, and whole‐cell patch‐clamp recordings. Our findings provide the first evidence that pluripotent stem cell‐derived neurons can integrate into adult host neural networks also in a human‐to‐human grafting situation, thereby supporting their potential future clinical use to promote recovery by neuronal replacement in the patient's diseased brain.