Impact of aging and senescence on phenotype and function of murine brain microglia in vitro and in vivo

Senescent cells are cells which lost their proliferation potential but are hyperfunctional and secrete many potentially detrimental cytokines and growth factors. Senescence of macrophages, so called macroph-aging, seems to be especially involved in several age-related diseases like Alzheimer or atherosclerosis. The role of microglia in brain aging as well as the factors regulating their response to pathogens are not well understood. The aims of this study were: 1) to develop an in vitro model to better characterize the senescent phenotype of murine brain microglia; 2) to determine the impact of aging and senescence on phenotype and function of microglia; 3) to evaluate the impact of pathogen-dose, maturation state, aging and senescence on microglial response in vitro. We found that microglia develop a senescent phenotype after long-term culture characterized by telomere shortening, increased expression of p16Ink4a, p21/p53, and altered response to the prototypical pathogen-associated molecular pattern LPS. Aged microglia in vivo showed increased p16 expression, enhanced activation and dysfunctional profiles, however did not show typical senescence-associated changes like DNA damage or p21/p53 pathway activation. In order to further characterize the impact of aging on the immune response from murine microglia, we tested the effect of increasing doses of typical PAMPs on naive, adult and aged microglia. Naive microglia showed a dose-dependent stimulation with adaptive responses, while adult or aged microglia did not exhibit a similar pattern. Reduced responses after a second stimulation, so called tolerance occurred in all groups. In conclusion, we found that aging of microglia was followed by cellular dysfunction but not by typical senescent changes as shown in other studies. Response of microglia to pathogen molecules was found to be dose-dependent, maturation and age-dependent but not pathogen specific.