A role for apolipoprotein E4 (ApoE4) in the pathogenesis of depressive symptoms and Alzheimer's disease.

Depression is a chronic mental disorder that is often difficult to diagnose and mostly left untreated. Apolipoprotein E (ApoE) is a lipid transport protein that plays a central role in the metabolism of plasma lipoproteins and the transport of lipids within tissues. Among its three major isoforms, only ApoE4 has a unique structural variant that confers significant neurotoxicity not present in the other two subtypes. ApoE4 not only disrupts lipid metabolism but also interferes with neuroimmune regulation and mitochondrial dynamics, thereby impairing synaptic integrity. It is a major genetic risk factor for Alzheimer's disease (AD) and accelerates its age of onset. Although developing depression in midlife may be a risk factor for AD, the underlying mechanisms by which ApoE4 influences depression or depression-related behaviors associated with AD remain fragmented, and systematic integrative reviews on this topic are scarce. We integrate existing evidence to elucidate the role of ApoE4 in depression or AD-associated prodromal depressive-like behaviors, with a focus on how its dysfunction disrupts cellular lipid homeostasis, impairs synaptic plasticity, and damages mitochondrial function. We also explored specific therapeutic strategies targeting the pathological defects of ApoE4 that enhance synaptic resilience, and utilizing conformational modulating small molecules such as EZ-482 and ALZ-801 to treat depressive symptoms in early AD. Thus, these findings indicate that ApoE4 may influence the progression of depressive phenotypes through multiple pathological pathways, making it a promising therapeutic target for treating depression comorbid with early AD.