Cortical cholesterol deficit and reduced network excitability in ApoE-Deficient mice

Apolipoprotein E (ApoE) is a key regulator of cholesterol transport in both the periphery and the central nervous system and is the major genetic risk factor for late-onset Alzheimer's disease. Although ApoE deficiency is known to elevate circulating cholesterol levels, its consequences for brain cholesterol homeostasis, synaptic integrity, and cortical network excitability remain incompletely understood. In the present study, ApoE knockout (ApoE^-/-) and wild-type (WT) mice were used to evaluate systemic and brain cholesterol levels, synaptic protein expression, and cortical electrophysiological activity. ApoE deficiency resulted in significantly elevated serum cholesterol levels compared with wild-type mice. In contrast, cortical cholesterol levels were significantly reduced in ApoE^-/- mice, whereas hippocampal cholesterol levels remained unchanged. Consistent with altered cortical lipid homeostasis, synaptophysin (SYP) and postsynaptic density protein-95 (PSD-95) levels were significantly decreased in ApoE-deficient animals. At the functional level, ApoE deficiency was associated with reduced basal cortical network activity, reflected by lower electrocorticographic (ECoG) power. Moreover, pharmacological excitation with 4-aminopyridine (4-AP) produced a markedly attenuated increase in cortical excitability in ApoE-deficient mice. Inflammatory markers, tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6), showed no differences between WT and ApoE^-/- mice. Similarly, oxidative stress markers measured in cortex and hippocampus did not indicate a consistent increase in oxidative damage. These findings demonstrate that ApoE deficiency is associated with a multilevel disruption characterized by elevated peripheral cholesterol, reduced cortical cholesterol availability, synaptic protein loss, and diminished cortical network excitability under both basal and hyperexcitable conditions. The results highlight a potential link between impaired brain cholesterol homeostasis and altered synaptic network function in the cortex of ApoE-deficient mice.