Cranial irradiation disrupts homeostatic microglial dynamic behavior

Cranial irradiation causes cognitive deficits that are in part mediated by microglia, the resident immune cells of the brain. Microglia are highly reactive, exhibiting changes in shape and morphology depending on the function they are performing. Additionally, microglia processes make dynamic, physical contacts with different components of their environment to monitor the functional state of the brain and promote plasticity. Though evidence suggests radiation perturbs homeostatic microglia functions, it is unknown how cranial irradiation impacts the dynamic behavior of microglia over time. Here, we paired in vivo two-photon microscopy with a transgenic mouse model that labels cortical microglia to follow these cells and determine how they change over time in cranial irradiated mice and their control littermates. We show that a single dose of 10 Gy cranial irradiation disrupts homeostatic cortical microglia dynamics during a 1-month time course. We found a lasting loss of microglial cells following cranial irradiation, coupled with a modest dysregulation of microglial soma displacement at earlier timepoints. The homogeneous distribution of microglia was maintained, suggesting microglia rearrange themselves to account for cell loss and maintain territorial organization following cranial irradiation. Furthermore, we found cranial irradiation reduced microglia coverage of the parenchyma and their surveillance capacity, without overtly changing morphology. Our results demonstrate that a single dose of radiation can induce changes in microglial behavior and function that could influence neurological health. These results set the foundation for future work examining how cranial irradiation impacts complex cellular dynamics in the brain which could contribute to the manifestation of cognitive deficits.

Cortical microglia dynamics are conserved during voluntary wheel running

We present the first demonstration of chronic in vivo imaging of microglia in mice undergoing voluntary wheel running. We find that healthy mice undergoing voluntary wheel running have similar microglia dynamics, morphologies, and responses to injury when compared to sedentary mice. This suggests that exercise over a period of 1 mo does not grossly alter cortical microglial phenotypes and that exercise may exert its beneficial effects on the brain through other mechanisms. Future work examining how microglia dynamics may be altered during exercise in disease or injury models could provide further insights into the therapeutic benefit of exercise.NEW & NOTEWORTHY We demonstrate the first use of chronic in vivo imaging of microglia over time during physical exercise. We found that microglia movement, morphology, and process motility were remarkably stable during voluntary wheel running (VWR). Additionally, microglia in running mice respond similarly to laser ablation injury compared to sedentary mice. These findings indicate that VWR does not induce changes in microglia dynamics in healthy adults. Exercise may elicit positive effects on the brain through other mechanisms.