The high level of 5-hydroxymethylcytosine (5hmC) present in neuronal genomes suggests that mechanisms interpreting 5hmC in the central nervous system (CNS) may differ from those present in embryonic stem cells. Here we present quantitative, genome-wide analysis of 5hmC, 5-methylcytosine (5mC) and gene expression in differentiated CNS cell types in vivo. We report that 5hmC is enriched in active genes, and that surprisingly strong depletion of 5mC is observed over these regions. The contribution of these epigenetic marks to gene expression depends critically on cell type. We identify methyl-CpG binding protein 2 (MeCP2) as the major 5hmC binding protein in the brain, and demonstrate that MeCP2 binds 5hmC and 5mC containing DNA with similar high affinities. The Rett Syndromecausing mutation R133C preferentially inhibits 5hmC binding. These findings support a model in which 5hmC and MeCP2 constitute a cell specific epigenetic mechanism for regulation of chromatin structure and gene expression.

Microglia, the brain’s resident macrophages, help to regulate brain function by removing dying neurons, pruning non-functional synapses, and producing ligands that support neuronal survival 1 . Here we show that microglia are also critical modulators of neuronal activity and associated behavioural responses in mice. Microglia respond to neuronal activation by suppressing neuronal activity, and ablation of microglia amplifies and synchronizes the activity of neurons, leading to seizures. Suppression of neuronal activation by microglia occurs in a highly region-specific fashion and depends on the ability of microglia to sense and catabolize extracellular ATP, which is released upon neuronal activation by neurons and astrocytes. ATP triggers the recruitment of microglial protrusions and is converted by the microglial ATP/ADP hydrolysing ectoenzyme CD39 into AMP; AMP is then converted into adenosine by CD73, which is expressed on microglia as well as other brain cells. Microglial sensing of ATP, the ensuing microglia-dependent production of adenosine, and the adenosine-mediated suppression of neuronal responses via the adenosine receptor A 1 R are essential for the regulation of neuronal activity and animal behaviour. Our findings suggest that this microglia-driven negative feedback mechanism operates similarly to inhibitory neurons and is essential for protecting the brain from excessive activation in health and disease.

The rapid elimination of dying neurons and nonfunctional synapses in the brain is carried out by microglia, the resident myeloid cells of the brain. Here we show that microglia clearance activity in the adult brain is regionally regulated and depends on the rate of neuronal attrition. Cerebellar, but not striatal or cortical, microglia exhibited high levels of basal clearance activity, which correlated with an elevated degree of cerebellar neuronal attrition. Exposing forebrain microglia to apoptotic cells activated gene-expression programs supporting clearance activity. We provide evidence that the polycomb repressive complex 2 (PRC2) epigenetically restricts the expression of genes that support clearance activity in striatal and cortical microglia. Loss of PRC2 leads to aberrant activation of a microglia clearance phenotype, which triggers changes in neuronal morphology and behavior. Our data highlight a key role of epigenetic mechanisms in preventing microglia-induced neuronal alterations that are frequently associated with neurodegenerative and psychiatric diseases.

Microglia rapidly respond to changes in neural activity and inflammation to regulate synaptic connectivity. The extracellular signals, particularly neuron-derived molecules, that drive these microglial functions at synapses remains a key open question. Here, whisker lesioning, known to dampen cortical activity, induces microglia-mediated synapse elimination. We show that this synapse elimination is dependent on the microglial fractalkine receptor, CX3CR1, but not complement receptor 3, signaling. Further, mice deficient in the CX3CR1 ligand (CX3CL1) also have profound defects in synapse elimination. Single-cell RNAseq then revealed that Cx3cl1 is cortical neuron-derived and Adam10 , a metalloprotease that cleaves CX3CL1 into a secreted form, is upregulated specifically in layer IV neurons and microglia following whisker lesioning. Finally, inhibition of Adam10 phenocopies Cx3cr1 −/− and Cx3cl1 −/− synapse elimination defects. Together, these results identify novel neuron-to-microglia signaling necessary for cortical synaptic remodeling and reveal context-dependent immune mechanisms are utilized to remodel synapses in the mammalian brain.