Apolipoprotein J and Alzheimer's amyloid β solubility

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Abstract
Apolipoprotein J (apoJ) has been found associated with soluble amyloid β (sAβ) in plasma and cerebrospinal fluid in normal individuals and co-deposited with fibrillar Aβ in Alzheimer's cerebrovascular and parenchymal lesions. Although studies in vitro and in vivo indicate that apoJ is a major carrier protein for sAβ, its role in the fibrillogenesis process is not known. We report herein that apoJ in its native high-density lipoprotein lipidic environment is fully active to interact with Aβ peptides. Furthermore, apoJ prevents aggregation and polymerization of synthetic Aβ in vitro. The interaction was stable for at least 14 days at 37 °C in physiologic buffers, and the peptide retrieved after complex dissociation at low pH retained its inherent aggregation properties. In addition, the binding to apoJ protects synthetic Aβ from proteolytic degradation; both Aβ1–42 and Aβ1–40 were more resistant to proteolysis by trypsin and chymotrypsin when complexed to apoJ. The data suggest that the interaction may preclude sAβ aggregation in biological fluids and point to a protecting role of apoJ for complexed Aβ species.