Hypermyelination Improves Strength and Detection of Neuronal Activity in the CA1 Hippocampus and Facilitates Neuroprotection in FusOLcKO Mice

Loss of oligodendrocytes (OLs) and myelin impairs cortical neuronal firing and network stability, whereas enhancement of oligodendrogenesis improves electrophysiological stability in cortex and, to a lesser extent, hippocampus. OLs exhibit regional heterogeneity, especially in their ability to synthesize cholesterol, a critical driver of myelin wrapping and ensheathment of axons. Conditional depletion of the Fused in sarcoma (Fus) gene in OLs, referred to as FusOLcKO, increases cholesterol biosynthesis, myelin thickness, and tissue cholesterol content. We examine whether this hypermyelination alters extracellular recordings across the layers of visual cortex and the underlying hippocampal CA1 over 16 weeks. In FusOLcKO mice, visually-evoked single‐unit detectability and firing rate in CA1 increased relative to wild-type littermates, whereas cortical recordings showed no improvement. At the population level, FusOLcKO cortex exhibited reduced firing rates and lower functional connectivity, indicating altered network dynamics. Post-mortem histology revealed higher neuron density in recorded cortex and greater excitatory synapse density in CA1 of FusOLcKO mice suggesting region-specific neuroprotection and synaptic strengthening. These results demonstrate that cholesterol-driven hypermyelination enhances chronic hippocampal recordings while disrupting cortical network communication. Our study highlights myelin s region‐dependent roles in supporting single‐cell reliability, tuning population dynamics, and maintaining circuit integrity under chronic perturbation.