Abstract One of the long-held mysteries in chromatin signaling is the excessive number of enzymes that install or remove a single chromatin mark. The most extreme case is histone H3 lysine 4 (H3K4me) methylation, a hallmark of transcriptionally active chromatin areas. H3K4me can be placed by six writer enzymes and removed by six eraser enzymes, most of which are expressed broadly in all cell types of the brain and other tissues. Strikingly, 10 of these 12 enzymes are responsible for monogenic forms of neurodevelopmental disorders. These enzymes thus have non-redundant yet poorly understood roles in cognitive development and function. Why do our brains need this many enzymes for the single histone mark? Cognitive function critically relies on a neural network's capability to rewire in response to sensory inputs. However, the same network needs to maintain its excitability within an optimal range; otherwise, the continuous sensory inputs or lack thereof could lead to excessive neuronal activity or inactivity, harmful to the brain functions. Synaptic scaling is believed to be a way to meet the two competing demands. In response to sustained hyperactivity, neurons scale down the receptivity to excitatory neurotransmitters. Conversely, prolonged network inactivity causes neurons to scale up and increase synaptic efficacy. Prior studies revealed that transcription in response to activity shifts is necessary for synaptic scaling. We now know several chromatin regulators essential in the process. However, we know very little about the roles of histone H3K4me enzymes in synaptic scaling. Our goal is to determine the roles of this prominent family of histone H3K4me enzymes in synaptic scaling. The research team combines expertise in chromatin biology (Iwase lab.) and synaptic plasticity (Sutton lab.) to attain this goal. We will test the hypothesis that H3K4me writer enzymes and their functional interaction with H3K4me eraser enzymes delineate distinct facets of synaptic scaling. To this end, the research team will 1) Generate the division of labor map of the H3K4me writer enzymes in synaptic scaling, 2) Explore the molecular mechanisms by which KMT2A, an H3K4me writer enzyme, governs induction of synaptic scaling. 3) Determine the functional interactions between H3K4me writer and eraser enzymes in synaptic plasticity and behavior. Completion of the proposed research will reveal how H3K4me signaling stabilizes the neural ensemble. In addition, our work will shed light on how the evolving brains have coopted expanded families of H3K4me enzymes to achieve an intricate balance of network excitability. The obtained knowledge will be a foundation on which evidence-based therapeutics can develop for H3K4me-related neurodevelopmental disorders and many other cognitive deficits with chromatin dysregulation.