Microglia are resident immune cells that play critical roles in maintaining the normal physiology of the central nervous system (CNS). Remarkably, microglia have an intrinsic capacity to repopulate themselves after acute ablation. However, the underlying mechanisms that drive such restoration remain elusive. Here, we characterized microglial repopulation both spatially and temporally following removal via treatment with the colony stimulating factor 1 receptor (CSF1R) inhibitor PLX5622. We show that microglia were replenished via self-renewal, with no contribution from nonmicroglial lineages, including Nestin+ progenitors and the circulating myeloid population. Interestingly, spatial analyses with dual-color labeling revealed that newborn microglia recolonized the parenchyma by forming distinctive clusters that maintained stable territorial boundaries over time, indicating the proximal expansive nature of adult microgliogenesis and the stability of microglia tiling. Temporal transcriptome profiling at different repopulation stages revealed that adult newborn microglia gradually regain steady-state maturity from an immature state that is reminiscent of the neonatal stage and follow a series of maturation programs, including nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) activation, interferon immune activation, and apoptosis. Importantly, we show that the restoration of microglial homeostatic density requires NF-κB signaling as well as apoptotic egress of excessive cells. In summary, our study reports key events that take place from microgliogenesis to homeostasis reestablishment.

Aging is the predominant risk factor for neurodegenerative diseases. One key phenotype as the brain ages is an aberrant innate immune response characterized by proinflammation. However, the molecular mechanisms underlying aging-associated proinflammation are poorly defined. Whether chronic inflammation plays a causal role in cognitive decline in aging and neurodegeneration has not been established. Here we report a mechanistic link between chronic inflammation and aging microglia and a causal role of aging microglia in neurodegenerative cognitive deficits. We showed that SIRT1 is reduced with the aging of microglia and that microglial SIRT1 deficiency has a causative role in aging-or tau-mediated memory deficits via IL-1␤ upregulation in mice. Interestingly, the selective activation of IL-1␤ transcription by SIRT1 deficiency is likely mediated through hypomethylating the specific CpG sites on IL-1␤ proximal promoter. In humans, hypomethylation of IL-1␤ is strongly associated with chronological age and with elevated IL-1␤ transcription. Our findings reveal a novel epigenetic mechanism in aging microglia that contributes to cognitive deficits in aging and neurodegenerative diseases.

SignificanceLate-onset Alzheimer’s disease is the most common form of dementia. A rare hemizygous variant in a microglial-expressed gene, Triggering Receptor Expressed on Myeloid Cells 2 (TREM2), significantly increases risk for late-onset Alzheimer’s disease. This variant is thought to cause loss of function, inducing TREM2 haploinsufficiency. The ramifications of TREM2 haploinsufficiency on microglial function and tau pathology are major gaps in the field. We find that, in contrast to the protective effects of complete TREM2 deficiency, TREM2 haploinsufficiency exacerbates tau pathology, inflammation, and atrophy at a late stage of disease in a mouse model of tauopathy. The differential effects of partial and complete loss of TREM2 are important considerations for TREM2-targeted therapeutic strategies.