Consequences of early-life stress for microglia throughout life

Stress exposure during early-life (ES) is associated with later cognitive impairments and is hypothesized to enhance the progression of Alzheimer’s disease (AD)-related β-amyloid (Aβ) neuropathology. In addition, neuroinflammation can play a role in the progression of Aβ neuropathology and there is recent evidence that ES altered the inflammatory profile in the hippocampus of wild type and APP/PS1 mice, an AD mouse model that develops Aβ neuropathology and exhibits (chronic) neuroinflammatory activation as the result of this. How ES affects microglial functioning is not well understood to date. We investigated if ES alters microglial functioning in vitro and questioned if ES modulates microglia and the expression of microglial regulatory factors in vivo , under basal conditions in wild type mice and in the APP/PS1 mouse model. Mice were exposed to ES consisting of limited bedding and nesting material from postnatal day (P)2 to P9. Primary hippocampal glial cultures were prepared from P9 Ctrl and ES pups and used to study in vitro functional changes in the (micro)glial inflammatory response to lipopolysaccharide (LPS), and the phagocytic responses to either exposure with beads or with Aβ1-42. The expression of microglial regulatory factors was further studied in the hippocampus of ES-exposed pups and adult wild type and APPswe/PS1dE9 mice at ages 4 and 10 months. In addition, the proliferation and clustering of microglia was studied in the hippocampus of Ctrl and ES APP/PS1 mice. LPS exposure elicited an inflammatory response in primary glial cultures, a response that was exacerbated in cultures derived from ES mice. Next to this, preliminary data on the in vitro phagocytosis assays indicated that microglia from ES mice tended to internalize more beads than Ctrl microglia, while exposure to Aβ1-42 failed to elicit such an increase in internalization. The hippocampal mRNA expression profile showed that ES reduced the mRNA expression of the inflammasome component NLRP3, and of fractalkine CX3CL1 at P9. In contrast, expression of the microglial receptor CX3CR1 was enhanced in ES-exposed mice at 10 months of age. In addition, 10-month-old APP/PS1 mice had an elevated expression of NLRP3 and CCR2, whereas ES exposure in these mice lowered expression of these factors. ES further reduced the number of dividing microglia in the hippocampus of APP/PS1 mice, but not the numbers of clustered microglia at Aβ plaques. Our findings support the concept that ES can lead to a sensitized response of microglia. The in vitro inflammatory and phagocytic response were enhanced in ES-derived cultures. In addition, ES induced a dysregulation in microglia at P9 and the chronic inflammatory stimulation induced by Aβ accumulation in the brains of APP/PS1 mice led to an abnormal, dampened inflammatory response in 10-month-old ES APP/PS1 mice. These data highlight the lasting nature of ES effects on inflammatory mediators and microglial programming. They raise the question whether an early modulation of the neuroinflammatory signaling changes in ES-exposed individuals can at least in part prevent the later-life risks associated with such exposure, for example in relation to AD.