Apolipoprotein D

Apolipoprotein D (ApoD) is a glycosylated protein of the lipocalin family that transports small hydrophobic ligands such as cholesterol, arachidonic acid, and steroids^[1][2]. It is expressed predominantly in the nervous system, including glia and neurons, but is also present in other tissues and plasma bound to high-density lipoproteins (HDL), influencing lipid metabolism^[3][1][4]. Mechanistically, ApoD modulates oxidative stress by binding arachidonic acid and cholesterol, preventing formation of neurotoxic products such as F₂-isoprostanes and 7-ketocholesterol^[5][4]. In disease models, ApoD expression increases in neurodegenerative disorders including Alzheimer's, Parkinson’s, and motor neuron disease, supporting a neuroprotective function^[1][4]. ApoD also preserves blood-brain barrier integrity by inhibiting endothelial CD36 signaling, particularly when hypoglycosylated, and promotes neurological recovery after ischemic injury^[6]. Compared with other apolipoproteins, ApoD exhibits tissue-specific expression, multiple isoforms, and unique ligand-binding properties, which distinguish it from related family members such as ApoA-I or ApoE^[7][8]. Experimental applications exploit its antioxidant and neuroprotective functions, including addition of human ApoD to ApoD-deficient astrocytes to improve survival and modulate reactivity^[4]. ApoD polymorphisms have been associated with metabolic phenotypes such as obesity, insulin resistance, and hepatic lipid alterations, highlighting its relevance beyond the nervous system^[9][10]. These characteristics make ApoD a multifunctional biomarker and therapeutic target in both neurobiology and metabolic research^[11][12].