IRF8 Is a Critical Transcription Factor for Transforming Microglia into a Reactive Phenotype

Abstract

SUMMARY Microglia become activated by multiple types of damage in the nervous system and play essential roles in neuronal pathologies. However, how micro-glia transform into reactive phenotypes is poorly understood. Here, we identify the transcription factor interferon regulatory factor 8 (IRF8) as a critical regulator of reactive microglia. Within the spinal cord, IRF8 expression was normally low; however, the expression was markedly upregulated in microglia, but not in neurons or astrocytes, after peripheral nerve injury (PNI). IRF8 overexpression in cultured microglia promoted the transcription of genes associated with reactive states; conversely, IRF8 deficiency prevented these gene expressions in the spinal cord following PNI. Furthermore, IRF8-deficient mice were resistant to neuropathic pain, a common sequela of PNI, and transferring IRF8-over-expressing microglia spinally to normal mice produced pain. Therefore, IRF8 may activate a program of gene expression that transforms microglia into a reactive phenotype. Our findings provide a newly observed mechanism for microglial activation.

IRF8 is a transcriptional determinant for microglial motility

“…Mouse primary cultured microglia were prepared as described previously [5,35]. Briefly, a mixed glial culture was prepared from neonatal IRF8 knockout, hetero-knockout, and wildtype mice and maintained for 10-16 days in DMEM with 10 % fetal bovine serum [5,36].…”

“…Briefly, a mixed glial culture was prepared from neonatal IRF8 knockout, hetero-knockout, and wildtype mice and maintained for 10-16 days in DMEM with 10 % fetal bovine serum [5,36]. Immediately before experiments, microglia were collected by a gentle shake as the floating cells over the mixed glial culture.…”

“…The protein extracts from whole-cell lysates of cultured microglial cells were prepared in accordance with methods described previously [5]. Aliquots (0.5-10 μg) were subjected to a 10 % polyacrylamide gel electrophoresis, and proteins were transferred electrophoretically to PVDF membranes.…”

“…They actively move their branched processes to sense pathological alterations or disturbances to ultimately maintain CNS homeostasis [1][2][3][4]. Once abnormalities are detected, microglia dramatically transform into a reactive phenotype through a progressive series of cellular and molecular changes, including morphological hypertrophy, proliferation, and the expression of various genes [3,5]. Following these changes, activated microglia destroy infectious agents, remove cell debris, and promote tissue repair [1,6,7], that is, microglia form the first line of defense in the CNS.…”

“…We have previously reported that IRF8 is expressed specifically in microglia in the CNS and plays a central role in regulating microglial gene expression [5]. Furthermore, recent studies have also shown that IRF8 is required for the full maturation of microglia [17] in addition to its physiological functions, including phagocytosis and cytokine production [18,19].…”