iPS-cell-derived microglia promote brain organoid maturation via cholesterol transfer.

Microglia are specialized brain-resident macrophages that arise from primitive macrophages colonizing the embryonic brain 1 . Microglia contribute to multiple aspects of brain development, but their precise roles in the early human brain remain poorly understood owing to limited access to relevant tissues 2-6 . The generation of brain organoids from human induced pluripotent stem cells recapitulates some key features of human embryonic brain development 7-10 . However, current approaches do not incorporate microglia or address their role in organoid maturation 11-21 . Here we generated microglia-sufficient brain organoids by coculturing brain organoids with primitive-like macrophages generated from the same human induced pluripotent stem cells (iMac) 22 . In organoid cocultures, iMac differentiated into cells with microglia-like phenotypes and functions (iMicro) and modulated neuronal progenitor cell (NPC) differentiation, limiting NPC proliferation and promoting axonogenesis. Mechanistically, iMicro contained high levels of PLIN2 + lipid droplets that exported cholesterol and its esters, which were taken up by NPCs in the organoids. We also detected PLIN2 + lipid droplet-loaded microglia in mouse and human embryonic brains. Overall, our approach substantially advances current human brain organoid approaches by incorporating microglial cells, as illustrated by the discovery of a key pathway of lipid-mediated crosstalk between microglia and NPCs that leads to improved neurogenesis.