ApoE is Secreted as a Lipid Nanoparticle by Mammalian Cells: Implications for Alzheimer's Disease Pathogenesis

The brain is the most cholesterol-rich organ in the body, and ApoE is the main lipid carrier protein in the brain. Although very little, if any, ApoE exists in its apoprotein form in physiological fluids, recombinant ApoE is typically prepared in a lipid-free state to study its physiological functions. We describe a lipid nanoparticle (LNP) form of ApoE as a primary extracellular product of the eukaryotic protein export system. Whereas the apoprotein is the dominant secreted product when the APOE gene is overexpressed in mammalian cells, an LNP form of ApoE is also observed. The LNP form is, however, the major secreted product from unmodified CCF-STTG1 astrocytoma cells. The C-terminal domain of ApoE plays a key role in LNP biosynthesis as the ApoE3 W210* truncation mutant is secreted without lipidation. Secreted ApoE LNPs are markedly better substrates than the apoprotein itself for further growth via the action of ATP-dependent lipid pumps. Compared to ApoE3 or the Alzheimer's disease-protective ApoE2 variant, the recovered yield of the LNP form of the disease-predisposing ApoE4 variant is higher. Intriguingly, the LNP yield of the rare disease-protective R251G variant of ApoE4 is comparable to that of ApoE3 and ApoE2. Analogous to the well-documented intracellular biosynthesis of ApoB-containing LNPs, the biogenesis and pathophysiological relevance of the LNP form of ApoE warrant further investigation.