Microglia M1/M2 polarization contributes to electromagnetic pulse-induced brain injury

The development of electronic technology has attracted attention on the biological effects of electromagnetic fields (EMFs) and electromagnetic pulse (EMP). It remains controversial whether EMP irradiation is neurotoxic or beneficial for recovery from injuryies such as cerebral ischemia. Microglia is innate immune cells in the brain, exhibiting either neurotoxicity or neuroprotection effect during various central nervous system diseases, depending on their activation into a classical (M1) or alternative (M2) phenotype, respectively. The Toll-like receptor-4 (TLR4), myeloid differentiation factor 88 (MyD88) and nuclear factor kappa B (NFκB) pathway is important for microglia activation. In this study, we investigated the effect of EMP on neuronal apoptosis and microglia polarization in vivo and in vitro, using an EMP of 400 kV/m and 1 hertz for 200 pulses. Short EMP irradiation (≤24 h) resulted in microglial conversion from the resting to the M1-type state, activation of the TLR4/MyD88/NFκB pathway, higher levels of inflammatory cytokines including interleukin (IL)-6, IL-1β and tumor necrosis factor-α, as well as neuronal apoptosis induction. In contrast, long EMP irradiation (3 days) resulted in microglial activation into the M2-type, decreased apoptosis and inflammatory mediator production, and increased levels of the neuroprotective effectors IL-10, transforming growth factor beta, and brain-derived neurotrophic factor. EMP induces both neuronal damage and neuronal recovery by influencing the switch of M1/M2 polarization and the TLR4/MyD88/NFκB pathway.