The Identification, Differentiation, and Genetic Regulation of mrc1+ microglia in the developing zebrafish brain

Central nervous system (CNS) construction during development relies on the coordination of specialized cell types, including microglia, the resident immune cells of the CNS. Microglia colonize the brain during development and aid in neuronal network construction, phagocytosis of CNS debris, and response to CNS injury. Traditionally, microglia express pu1 and are derived from erythromyeloid progenitors in the embryonic yolk sac. While much is known about pu1+ microglia, it is increasingly recognized that microglia are a heterogeneous population of cells, and we know much less about the various subpopulations of microglia outside of pu1+ cells. Here, we utilize in vivo imaging of larval zebrafish to identify a novel population of microglia in the developing brain that expresses mannose receptor type C, type 1a (mrc1a), is dependent on lymphangiogenesis, and can be generated from lymphatic endothelial cells (LECs). mrc1a+ microglia colonize the brain earlier than previously described microglia and are not dependent on traditional microglia determinants like pu1 or Csf1r. Using photoconvertible proteins to track LECs at the brain borders, we discover that mrc1a+ LECs can produce mrc1a+ microglia from at least 3-11 days post fertilization. We show that brain border LECs are bipotential and can produce both microglia as well as additional LECs that remain localized to the brain borders. Finally, we discover that sox17 regulates mrc1a+ microglia as a maternal effect gene, where animals lacking maternally-contributed sox17 display deficits in microglia abundance. Maternal zygotic sox17 mutants have a reduction in yolk sac microglia precursors, and we can rescue yolk sac precursor as well as microglia abundance with exogenous sox17. We discover 3 additional genes that regulate mrc1a+ microglia: f11r.1, gas6, and mpp1. We go on to show that sox17 mutants have reduced mpp1 transcript in mrc1a+ microglia, and propose these two genes may be working together to regulate embryonic microglia abundance. Together, these data advance our understanding of the mrc1a+ subpopulation of microglia and challenge the current view that microglia are solely derived from erythromyeloid progenitors in the embryonic yolk sac, with the ultimate goal of informing therapies for neurodevelopmental and neurodegenerative diseased in which microglia are implicated.