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A Cre-deleter specific for embryo-derived brain macrophages reveals distinct features of microglia and border
Simone Brioschi1, Julia A Belk2, Vincent Peng1
1Department of Pathology and Immunology, Washington University School of Medicine in Saint Louis, Saint Louis, MO, USA.
Abstract:
Genetic tools to target microglia specifically and efficiently from the early stages of embryonic development are lacking. We generated a constitutive Cre line controlled by the microglia signature gene Crybb1 that produced nearly complete recombination in embryonic brain macrophages (microglia and border-associated macrophages [BAMs]) by the perinatal period, with limited recombination in peripheral myeloid cells. Using this tool in combination with Flt3-Cre lineage tracer, single-cell RNA-sequencing analysis, and confocal imaging, we resolved embryonic-derived versus monocyte-derived BAMs in the mouse cortex. Deletion of the transcription factor SMAD4 in microglia and embryonic-derived BAMs using Crybb1-Cre caused a developmental arrest of microglia, which instead acquired a BAM specification signature. By contrast, the development of genuine BAMs remained unaffected. Our results reveal that SMAD4 drives a transcriptional and epigenetic program that is indispensable for the commitment of brain macrophages to the microglia fate and highlight Crybb1-Cre as a tool for targeting embryonic brain macrophages.
Insights
Researchers developed a new genetic tool, Crybb1-Cre, to precisely target early brain macrophages. This tool revealed that SMAD4 is essential for microglia development, preventing them from adopting a border-associated macrophage fate.
Area of Science:
- Neuroscience
- Developmental Biology
- Immunology
Background:
- Targeting microglia specifically during early embryonic development is challenging due to a lack of efficient genetic tools.
- Understanding the distinct origins and developmental trajectories of brain macrophages is crucial for neuroscience research.
Purpose of the Study:
- To generate and validate a novel genetic tool for targeting embryonic brain macrophages.
- To investigate the role of the transcription factor SMAD4 in microglia development and fate commitment.
Main Methods:
- Development of a constitutive Cre line (Crybb1-Cre) targeting microglia and embryonic-derived border-associated macrophages (BAMs).
- Utilized Flt3-Cre lineage tracing, single-cell RNA-sequencing, and confocal imaging to analyze macrophage populations.
- Investigated the effects of SMAD4 deletion in microglia and embryonic-derived BAMs.
Main Results:
- Crybb1-Cre achieved high recombination efficiency in embryonic brain macrophages by the perinatal period with minimal peripheral myeloid cell recombination.
- SMAD4 deletion in microglia and embryonic-derived BAMs resulted in developmental arrest of microglia, leading to a BAM specification signature.
- Genuine monocyte-derived BAM development was unaffected by SMAD4 deletion.
Conclusions:
- SMAD4 is critical for the transcriptional and epigenetic programming required for microglia fate commitment.
- The Crybb1-Cre mouse line is a valuable tool for targeting embryonic brain macrophages, including microglia and embryonic-derived BAMs.
- This study elucidates a key molecular mechanism governing microglia development and differentiation.
