iPSC-derived microglia export cholesterol to neuronal cells in human brain organoids and promote their maturation
Microglia are specialised brain-resident macrophages that arise from primitive macrophages colonising the embryonic brain. Microglia contribute to multiple aspects of brain development, but their precise roles in early human brain remain poorly understood due to limited access to relevant tissues. The generation of brain organoids from induced human pluripotent stem cells (iPSC) recapitulates some key features of human embryonic brain development, but current approaches do not incorporate microglia and thus are lacking. Here, we generated microgliasufficient brain organoids by co-culturing brain organoids with primitive-like macrophages generated from the same human iPSC (iMac). In organoid co-cultures, 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 weretaken up by NPC in the organoids. We also detected PLIN2+ lipid droplet-loaded microglia in mouse and human embryonic brain. Overall, our approach significantly advances current human brain organoid approaches by incorporating microglial Overall design: 8 samples;  3 cerebral organoids and 3 co-cultured cerebral organoids on day 18 of co-culture, iPSC-dreived macrophages (iMac) on day 18 of mono-culture