Programming of Resident Macrophages by the Brain Environment Following Transplantation

PROJECT SUMMARY A striking feature of microglia, the brain's resident macrophages, is their ability to adapt in response to changes in the brain environment. Microglial state changes occur in development and nearly all diseases, often linked to harmful or helpful functions. A better understanding of the regulatory mechanisms underlying microglia state

change will therefore improve understanding of brain diseases, and uncover new therapeutic targets. Dozens of disease reactive states have been identified, but little is known about how microglia transition between them. Even microglial “homeostasis” is a state actively maintained by brain environmental signals, and lost in a culture

dish. We are experts in the isolation and manipulation of microglia, and created a unique model for intracranial transplantation of microglia and other macrophages following genetic microglia depletion. After transplantation, macrophages engraft the brain and over 14 days undergo dramatic changes in gene expression. In preliminary

data, we harvested transplanted macrophages at several timepoints, and by single cell RNA sequencing (scRNAseq), measured the progressive acquisition of microglial identity over time. With this highly controlled in vivo model, we will generate a comprehensive fingerprint of how transplanted microglia are

programmed by the brain environment, and use it to identify the genes, pathways, regulatory networks likely to be responsible. In aim 1A, we will capture the environmental programming of cultured microglia after transplant, using paired single cell RNA/ATACseq to identify intermediate states, and to predict the external

signals, transcription factors, receptors, pathways and networks responsible. In aim 1B, we will compare transplantation with all combinations of donor and host sex, in order to determine its role in microglia identity specification. Finally, since blood infiltrating macrophages can resemble microglia but remain a distinct cell type,

aim 2 will measure their programming after transplantation, to better understand why they cannot become microglia. In summary, completion of these aims will fill knowledge gaps about the regulation of microglial state and identity, create a new foundational data resource, and substantiate an R01 proposal to causally test identity

regulators uncovered here.