Cerebrovascular smooth muscle cells internalize Alzheimer amyloid beta protein via a lipoprotein pathway: implications for cerebral amyloid angiopathy.

Cerebral amyloid angiopathy (CAA) is caused by the cerebrovascular deposition of Alzheimer amyloid beta protein (Abeta) and shows an increased incidence in carriers of the apolipoprotein E (APOE) epsilon4 genotype. To study the pathogenesis of CAA, primary cultures of human and canine smooth muscle cells from leptomeningeal vessels were incubated with fluorescein- and biotin-conjugated amyloid beta-protein. In the presence of human serum or cerebrospinal fluid, A beta1-40 and Abeta1-42 were rapidly internalized and appeared within endosomal and lysosomal vesicles. The accumulation of intracellular Abeta was enhanced by chloroquine and blocked by cycloheximide and brefeldin A and pretreatment with trypsin, suggesting that the internalization of Abeta occurs by receptor-mediated endocytosis. The internalization of Abeta was also inhibited by lipoprotein-deficient serum or by incubation with the 39-kd receptor-associated protein, indicating that Abeta is internalized via a receptor of the low-density lipoprotein receptor family. A lipoprotein pathway was confirmed by colocalization of cell surface-bound or internalized Abeta with APOE and low-density lipoprotein receptor-related protein. We propose a pathogenetic model of CAA, in which Abeta-APOE-complexes contained within the cerebrospinal fluid or the extracellular fluid of the brain are internalized and accumulated in cerebrovascular smooth muscle cells. Such a model could explain the preferential localization of CAA to the outer and middle layers of cortical and leptomeningeal arterioles, while indicating a mechanism by which the APOE genotype might determine the risk of CAA.