Microglial depletion and repopulation did not affect hippocampal neurogenesis after whole brain irradiation

Abstract

Abstract Background Ablation of hippocampal neurogenesis is thought to be one reason behind the lifelong cognitive complications that childhood brain cancer survivors face after cranial radiotherapy. Microglia react strongly immediately after an irradiation insult, promoting a neuroinflammatory microenvironment that can prove detrimental for the neurogenic niche. The depletion and repopulation of microglia as a therapeutic strategy was recently shown to ameliorate cognitive deficits in a traumatic brain injury model, but this approach has not been explored in the context of hippocampal neurogenesis after radiation therapy. Methods Cx3cr1 CreERt2 − YFP/+ Rosa26 DTA/+, a transgenic mouse model that allows for endogenous expression of diptheria toxin A in targeted cells in a Cre-mediated manner was utilised to deplete microglia from the brain. Postnatal day 18 (P18) mice were injected with tamoxifen for 3 consecutive days to activate the Cre recombinase and were then subjected to an 8 Gy single dose of cranial irradiation on P21. The numbers of proliferating cells and immature neurons were assessed through quantification of Ki67+ and DCX+ cells. Results In Cx3cr1CreERt2 − YFP/+Rosa26DTA/+ transgenic mice, microglia could be 97% depleted, remained absent for at least 7 days, and had fully repopulated the brain 10 days after three tamoxifen injections. Following irradiation, the lack of microglia led to delayed removal of cellular debris. Microglia depletetion itself caused increased concentrations of CCL2, presumably to recruit bloodstream monocytes, and irradiation caused synergistically higher and sustained levels of CCL2 compared to controls. The absence of microglia at the time of irradiation did not affect neurogenesis, as judged by the numbers of Ki67+ and DCX+ cells, neither 7, nor 10 days after the last tamoxifen injection, when microglia were still absent or had repopulated, respectively. Lastly, long-term assessment of neurogenesis, 6 weeks after irradiation, as judged by Ki67+ and DCX + cells and granule cell volume, was not different between micoglia depleted and control mice. Conclusions The transgenic mouse model described here is an effective tool to study the effects of microglia depletion.Microglia depletion prior to cranial irradiation did not rescue neurogenesis, neither short-, nor long-term, possibly, at least partly, because of the increased levels of CCL2.