Early-life stress impacts the adult microglia transcriptome under basal and inflammatory conditions

Early-life stress (ELS) leads to increased vulnerability to psychiatric illness including cognitive impairment later in life. Neuroinflammatory processes have been implicated in ELS-induced negative health outcomes, but, how ELS impacts microglia, the macrophages of the central nervous system, is unknown. Here, we determined the effects of ELS induced by limited bedding and nesting material during the first week of life (P2 – P9) on the morphology and gene expression of microglia from young (postnatal day (P)9) and adult (P200) mice. ELS led to a change in the proportion of morphological microglia subtypes in adulthood, associated with immune reactivity. At the transcriptional level, whereas at P9 no ELS mediated changes were detected, at P200 ELS induced transcriptional changes in microglia genes involved in the immune response and protein ubiquitination. Additionally, ELS altered microglia gene expression changes during development from P9 to P200 and in response to LPS at P200, in both cases marked by GO-terms associated with the immune response. Concluding, these data show that ELS has persistent effects on the morphology and gene expression program of microglia and results in an altered transcriptional response to a systemic LPS challenge. Overall design: To determine acute and long-term effects of ELS on microglia, we exposed three different cohorts of mice to control conditions or ELS from postnatal day (P)2 to P9. The first cohort of CTL and ELS-exposed mice was injected with PBS or LPS (5 mg/kg) by the age of 3-5 months of age and was used for morphological characterization of microglia 24 hours after PBS or LPS injection (CTR-PBS (n=5), ELS-PBS (n=4), CTR-LPS (n=10), ELS-LPS (n=6). The second cohort was sacrificed at P9 and in order to investigate how ELS affects the microglia inflammatory response the third cohort was allowed to mature until P200, at which time they were injected with either PBS or LPS (1 mg/kg) (Figure 1A). To determine microglia transcriptional changes, hippocampal microglia were isolated at P9 and P200, and the P200 animals were injected with either PBS or LPS. Thus, for microglia transcriptomics we have the following experimental groups: P9: CTR (n=12), P9: ELS (n=12), P200: CTR-PBS (n=7), P200: ELS-PBS (n=6), P200: CTR-LPS (n=7), P200: ELS-LPS (n=5).