Elp1 is required for myeloid differentiation and brain repopulation.

Haematopoietic stem and progenitor cell (HSPC) transplantation-mediated replacement of microglia-the reconstitution of the central nervous system microglial niche with donor-derived myeloid cells-represents a promising strategy for brain cell therapies1-7. However, the cell-intrinsic molecular regulators of HSPC-derived myeloid lineage commitment in the brain have remained largely uncharacterized. This is partially because the scarcity of primary HSPCs has rendered large-scale genetic interrogation intractable. Here we explored whether ex vivo expansion of primary cultured HSPCs (cHSPCs) enables unbiased genetic interrogation of haematopoietic differentiation. We established that cHSPCs can be genetically manipulated and readily differentiate into myeloid cells and repopulate microglia niches of the brain after transplantation. Moreover, primary neonatal astrocytes contained strong signals to induce myeloid specification of cHSPCs in vitro. To identify genes that regulate glia-induced myeloid differentiation, we successfully screened around 20,000 individual genetic perturbations and characterized genes involved in the regulation of cell signalling, cell cycle, mitochondrial function and protein translation. Among these genes was elongator acetyltransferase complex subunit 1 (Elp1), which suppressed the unfolded protein response during myeloid specification and was essential for the generation but not maintenance of myeloid cells. Our work establishes a platform to investigate the molecular mechanisms of brain myeloid biology.