Mitigation of helium irradiation-induced brain injury by microglia depletion

Background

Cosmic radiation exposures have been found to elicit cognitive impairments involving a wide-range of underlying neuropathology including elevated oxidative stress, neural stem cell loss, and compromised neuronal architecture. Cognitive impairments have also been associated with sustained microglia activation following low dose exposure to helium ions. Space-relevant charged particles elicit neuroinflammation that persists long-term post-irradiation. Here, we investigated the potential neurocognitive benefits of microglia depletion following low dose whole body exposure to helium ions.

Methods

Adult mice were administered a dietary inhibitor (PLX5622) of colony stimulating factor-1 receptor (CSF1R) to deplete microglia 2 weeks after whole body helium irradiation (⁴He, 30 cGy, 400 MeV/n). Cohorts of mice maintained on a normal and PLX5622 diet were tested for cognitive function using seven independent behavioral tasks, microglial activation, hippocampal neuronal morphology, spine density, and electrophysiology properties 4–6 weeks later.

Results

PLX5622 treatment caused a rapid and near complete elimination of microglia in the brain within 3 days of treatment. Irradiated animals on normal diet exhibited a range of behavioral deficits involving the medial pre-frontal cortex and hippocampus and increased microglial activation. Animals on PLX5622 diet exhibited no radiation-induced cognitive deficits, and expression of resting and activated microglia were almost completely abolished, without any effects on the oligodendrocyte progenitors, throughout the brain. While PLX5622 treatment was found to attenuate radiation-induced increases in post-synaptic density protein 95 (PSD-95) puncta and to preserve mushroom type spine densities, other morphologic features of neurons and electrophysiologic measures of intrinsic excitability were relatively unaffected.

Conclusions

Our data suggest that microglia play a critical role in cosmic radiation-induced cognitive deficits in mice and, that approaches targeting microglial function are poised to provide considerable benefit to the brain exposed to charged particles.

Attenuation of neuroinflammation reverses Adriamycin-induced cognitive impairments

Numerous clinical studies have established the debilitating neurocognitive side effects of chemotherapy in the treatment of breast cancer, often referred as chemobrain. We hypothesize that cognitive impairments are associated with elevated microglial inflammation in the brain. Thus, either elimination of microglia or restoration of microglial function could ameliorate cognitive dysfunction. Using a rodent model of chronic Adriamycin (ADR) treatment, a commonly used breast cancer chemotherapy, we evaluated two strategies to ameliorate chemobrain: 1) microglia depletion using the colony stimulating factor-1 receptor (CSF1R) inhibitor PLX5622 and 2) human induced pluripotent stem cell-derived microglia (iMG)-derived extracellular vesicle (EV) treatment. In strategy 1 mice received ADR once weekly for 4 weeks and were then administered CSF1R inhibitor (PLX5622) starting 72 h post-ADR treatment. ADR-treated animals given a normal diet exhibited significant behavioral deficits and increased microglial activation 4–6 weeks later. PLX5622-treated mice exhibited no ADR-related cognitive deficits and near complete depletion of IBA-1 and CD68⁺ microglia in the brain. Cytokine and RNA sequencing analysis for inflammation pathways validated these findings. In strategy 2, 1 week after the last ADR treatment, mice received retro-orbital vein injections of iMG-EV (once weekly for 4 weeks) and 1 week later, mice underwent behavior testing. ADR-treated mice receiving EV showed nearly complete restoration of cognitive function and significant reductions in microglial activation as compared to untreated ADR mice. Our data demonstrate that ADR treatment elevates CNS inflammation that is linked to cognitive dysfunction and that attenuation of neuroinflammation reverses the adverse neurocognitive effects of chemotherapy.