Generation of a microglial developmental index in mice and in humans reveals a sex difference in maturation and immune reactivity

Abstract

Evidence suggests many neurological disorders emerge when normal neurodevelopmental trajectories are disrupted, i.e. when circuits or cells do not reach their fully mature state. Microglia play a critical role in normal neurodevelopment and are hypothesized to contribute to brain disease. We used whole transcriptome profiling with Next Generation sequencing of purified developing microglia to identify a microglial developmental gene expression program involving thousands of genes whose expression levels change monotonically (up or down) across development. Importantly, the gene expression program was delayed in males relative to females and exposure of adult male mice to LPS, a potent immune activator, accelerated microglial development in males. Next, a microglial developmental index (MDI) generated from gene expression patterns obtained from purified mouse microglia, was applied to human brain transcriptome datasets to test the hypothesis that variability in microglial development is associated with human diseases such as Alzheimer’s and autism where microglia have been suggested to play a role. MDI was significantly increased in both Alzheimer’s Disease and in autism, suggesting that accelerated microglial development may contribute to neuropathology. In conclusion, we identified a microglia-specific gene expression program in mice that was used to create a microglia developmental index, which was applied to human datasets containing heterogeneous cell types to reveal differences between healthy and diseased brain samples, and between males and females. This powerful tool has wide ranging applicability to examine microglial development within the context of disease and in response to other variables such as stress and pharmacological treatments.

Microglia from offspring of dams with allergic asthma exhibit epigenomic alterations in genes dysregulated in autism

“…When specific enrichment was examined for MSigDB immune signatures, DMRs were found to be enriched for gene sets associated with LPS treatment and other types of immune cell activators (Table S3). Furthermore, genes with transcription start sites +/−5kb of DMRs (424 DMRs, total with 209 hyper-methylated DMRs and 215 hypo-methylated DMRs) (Table S5) were significantly enriched for genes responsive to LPS, and other immune stimuli in microglia (Figure 3D) (Hickman et al, 2013; Holtman et al, 2015; Hanamsagar et al, 2017). Together these findings indicate that regions sensitive to MAA methylation differences may contain critical regulatory regions responsive to immune stimuli in microglia.…”

“…D. Fisher’s Exact Test Odds Ratios for gene list overlap between genes with transcription start sites +/− 5kb from the MAA DMRs. DMR associated genes significantly overlapped genes from the microglial (MG) sensome (Hickman et al, 2013), primed microglia, genes up-regulated acutely in response to LPS (Holtman et al, 2015), and genes up-regulated in microglia two hours after LPS in males and females at postnatal day 60 (P60) (Hanamsagar et al, 2017). See Table S7 for full lists and statistics.…”

Environment matters: microglia function and dysfunction in a changing world

“…Notably, MIA with the viral mimic poly I:C can disrupt the pre-microglial developmental stage in offspring and push the microglial transcription state to a more advanced developmental stage [47]. Similarly, we have demonstrated that LPS challenge in late adolescent mice accelerates microglial transcriptional maturation, but only in males [48]. Impacts on microglial transcriptional development likely have significant adverse consequences on neuronal development (e.g.…”

“…so-called “activation”, which remains largely descriptive. We have recently demonstrated a close positive correlation between developmental changes in microglial gene expression and morphology, in mice [48]. Specifically, global transcriptional changes in microglia over normal development correlate strongly with changes in microglial morphology in the same individual.…”

“…Further, accumulating data suggests that environmental exposures increases vulnerability in the offspring by accelerating microglial development [47,48]. Impacts on microglial transcriptional development likely have significant adverse consequences on neuronal development (e.g.…”