Temporary depletion resets microglia phagocytic activity and protects against long-term memory deficits after therapeutic cranial irradiation
Xi Feng1,2,3, Russell Huie1,2, Karen Krukowski1,2, Sonali Gupta1,2, Maria Serena Paladini1,2, Adam Ferguson1,2,3, Nalin Gupta3,4 and Susanna Rosi1,2,3
1Brain and Spinal Injury Center; 2Department of Physical Therapy and Rehabilitation Science; 3Department of Neurological Surgery; 4Department of Pediatrics, University of California at San Francisco
Microglia are the predominant immune cells in the central nervous system (CNS). They constantly survey the CNS to clear damaged cells and pathogens in order to maintain normal brain functions. Recent studies suggest that microglia are also key regulators of synaptic remodeling during development and in the adult brain. We previously demonstrated that microglia depletion can prevent dendritic spine loss and memory deficits induced by whole-brain irradiation. To understand the underlying mechanisms, here, we used a colony-stimulating factor 1 receptor (CSF-1R) inhibitor, PLX5622, to deplete microglia (>90%) in the CNS at the time of therapeutic cranial irradiation. Ten days later, PLX5622 treatment was withdrawn and microglia were allowed to repopulate for 3 weeks. We then performed transcriptome analysis on repopulated microglia using RNA sequencing and compared the transcriptome of the repopulated microglia with those from irradiated animals. Gene ontology analyses revealed that repopulated microglia were spared from radiation-induced responses; this included biological processes involved in cell cycle arrest, DNA damage repair and mitotic regulation. Notably, a subset of phagocytic and lysosome markers were activated in microglia after irradiation, and were “reset” to normal level in repopulated microglia. Using an in vivo phagocytosis assay, we demonstrated that microglia had elevated activity in engulfing injected synaptosomes after irradiation, which corresponded with decreased dendritic spine density in hippocampal neurons. Strikingly, repopulated microglia had significantly lower engulfment of synaptosomes when compared to microglia in irradiated brains. Furthermore, no dendritic spine loss was measured in the hippocampi of irradiated brains when microglia were depleted. Based on these results, we summarize that microglia depletion resets their phagocytic activity towards synapses, therefore prevents hippocampal-dependent memory deficits induced by therapeutic cranial irradiation.