Reestablishing adult microglia homeostasis reveals origin of self-renewal, sequential maturation programs, and proximal spatial re-colonization
Lihong Zhan1, Grietje Krabbe1, Fei Du3, Ian Jones2, Meredith C. Reichert1, Maria Telpoukhovskaia1, Lay Kodama1, Chao Wang1, Seo-hyun Cho1, Faten Sayed1, Yaqiao Li1, David Le1, Yungui Zhou1, Yin Shen2, Brian West4, Li Gan1*,
1Gladstone Institutes of Neurological Diseases, Department of Neurology; San Francisco, USA; 2Institute for Human Genetics and Department of Neurology, University of California-San Francisco, San Francisco, USA; 3Department of Geography, University of Wisconsin-Madison, Madison, USA; 4Plexxikon Inc., Berkeley, California, USA
Microglia are resident immune cells that play critical roles in maintaining normal physiology of the central nervous system. Disruption of microglia homeostasis is a common theme covered in virtually all forms of neurodegenerative diseases. Therefore, how microglia homeostasis is regulated forms an important biological question that is relevant to human neurological diseases as well. The intrinsic mechanisms pertaining to microglia's capacity to restore homeostasis is fascinating but largely elusive. Here we applied microglia depletion/repopulation model with CSF1R inhibitor PLX5622 to investigate the restoration process underlying microglia homeostatic regulation. Remarkably, we found that microglia homeostatic restoration follows a series of choreographical events that involve re-entry of cell cycle, inflammatory resolution, and re-establishment of microglia maturity. Early stage adult newborn microglia adopted an immature transcriptome phenotype that partially overlapped with the transcriptome signature from the postnatal day 4 microglia. Using lineage-tracing tools, we showed that the replenished microglia pool is entirely derived through self-renewal, not from nestin progenitors as previously reported. This proliferative process also partially depends on NFKB signaling. The repopulated microglia then underwent maturation process that involves apoptotic egress of over-proliferated cells, inflammatory resolution, and re-expression of microglia maturation regulator MafB as well as other mature markers such as TMEM119 and P2RY12. Finally, using multi-color labeling from brainbow reporter, we showed that newborn microglia recolonize the parenchyma forming distinctive clusters that hold stable territorial boundaries, suggesting proximal migratory nature of adult microgliogenesis.