Aging-like changes in the transcriptome of irradiated microglia

Whole brain irradiation remains important in the management of brain tumors. Although necessary for improving survival outcomes, cranial irradiation also results in cognitive decline in long‐term survivors. A chronic inflammatory state characterized by microglial activation has been implicated in radiation‐induced brain injury. We here provide the first comprehensive transcriptional profile of irradiated microglia. Fluorescence‐activated cell sorting was used to isolate CD11b+ microglia from the hippocampi of C57BL/6 and Balb/c mice 1 month after 10 Gy cranial irradiation. Affymetrix gene expression profiles were evaluated using linear modeling and rank product analyses. One month after irradiation, a conserved irradiation signature across strains was identified, comprising 448 and 85 differentially up‐ and downregulated genes, respectively. Gene set enrichment analysis demonstrated enrichment for inflammation, including M1 macrophage‐associated genes, but also an unexpected enrichment for extracellular matrix and blood coagulation‐related gene sets, in contrast previously described microglial states. Weighted gene coexpression network analysis confirmed these findings and further revealed alterations in mitochondrial function. The RNA‐seq transcriptome of microglia 24‐h postradiation proved similar to the 1‐month transcriptome, but additionally featured alterations in apoptotic and lysosomal gene expression. Reanalysis of published aging mouse microglia transcriptome data demonstrated striking similarity to the 1‐month irradiated microglia transcriptome, suggesting that shared mechanisms may underlie aging and chronic irradiation‐induced cognitive decline. GLIA 2015;63:754–767 Main Points The irradiated mouse microglia transcriptome is unique, featuring changes in extracellular matrix, inflammation and mitochondria‐related genes. Strong enrichment for irradiation‐induced changes is observed in the aging microglia transcriptome.