Generation of human nucleus basalis organoids with functional nbM-cortical cholinergic projections in transplanted assembloids.

The nucleus basalis of Meynert (nbM), the major cholinergic output of the basal forebrain, regulates cortical modulation, learning, and memory. Dysfunction of the nbM-cortical cholinergic pathway is implicated in neurodegenerative and neurodevelopmental disorders, including Alzheimer's disease (AD) and Down syndrome (DS). Here, we generated human nbM organoids (hnbMOs) from human pluripotent stem cells (hPSCs) containing functional cholinergic projection neurons. Then we reconstructed long-distance cholinergic projections from nbM to the cortex by co-culturing hnbMOs with human fetal brains and transplanting hnbMOs into immunodeficient mice. We further established nbM-cortical assembloids by fusing hnbMOs with human cortical organoids (hCOs). We also established a human-specific cholinergic projection system in transplanted assembloids. Using viral tracing and functional assays, we validated that cholinergic neurons send projections into hCOs and form synaptic connections. Moreover, we captured projection deficits in DS-derived assembloids, demonstrating the utility of this model for studying nbM-related neural circuits and neurological disorders.