Apolipoprotein E4 and synaptic dysfunction in Alzheimer's disease: Mechanisms and therapeutic implications.

Alzheimer's disease (AD) is characterized by progressive cognitive decline, with synaptic dysfunction as the strongest correlate of clinical symptoms. The apolipoprotein E ε4 (ApoE4) allele is the most potent genetic risk factor for late-onset AD. Beyond its roles in amyloid-β aggregation and tau hyperphosphorylation, ApoE4 disrupts synaptic integrity by perturbing lipid metabolism, neuroimmune regulation, mitochondrial dynamics, and activity-dependent plasticity. These ApoE4-driven mechanisms impair presynaptic vesicle trafficking, destabilize postsynaptic receptor and scaffolding networks (including PSD-95, SynGAP, and Shank3), and accelerate complement- and microglia-mediated synaptic pruning. Collectively, these processes converge to destabilize neuronal circuits and drive early cognitive decline. In this review, we synthesize current evidence on the molecular mechanisms by which ApoE4 compromises synaptic function, with particular emphasis on lipid microdomain instability, mitochondrial failure, and the collapse of postsynaptic density proteins. We also discuss therapeutic strategies to enhance synaptic resilience, including modulation of glutamatergic transmission, restoration of lipid homeostasis, augmentation of neurotrophic signaling, and regulation of microglial activity. Targeting synaptic preservation in APOE ε4 carriers holds promise as a disease-modifying approach to mitigate cognitive decline in AD.