A Cre-deleter specific for embryo-derived brain macrophages reveals distinct features of microglia and border macrophages

Genetic tools to target microglia specifically and efficiently from the early stages of the embryonic development are missing. Here we present Crybb1-Cre, a new constitutive Cre deleter producing a nearly complete recombination of embryonic brain macrophages (microglia and embryonic BAMs) by the perinatal period, with limited recombination leakage in peripheral myeloid cells. Using this tool, in combination with Flt3-Cre lineage tracer, scRNA-seq analysis and confocal imaging we were able to accurately fate-map embryonic derived versus monocyte derived BAMs in the mouse cortex and identified specific surface markers to resolve either population. Furthermore, we used Crybb1-Cre to delete the transcription factor SMAD4. Using multiomics analysis combining single-cell RNA-seq and ATAC-seq we showed that SMAD4-deficient microglia underwent an arrest of their homeostatic maturation and at the same time acquired a BAMs specification signature. By contrast, BAMs development was unaffected. Therefore, SMAD4 drives a transcriptional and epigenetic program that is indispensable for the commitment of brain macrophages to the microglia fate. This data provides a valuable resource to study molecular underpinnings of microglia development. Overall design: Immune cells were isolated from the brains of Smad4 F/F and Smad4 cKO mice and subjected to multi-omic profiling using 10X genomics reagents. Immune cells were isolated from the brains of WT and 5XFAD mice and subjected to scRNAseq using 10X genomics reagents.