Mechanisms of U87 Astrocytoma Cell Uptake and Trafficking of Monomeric versus Protofibril Alzheimer’s Disease Amyloid-b Proteins

A significant hallmark of Alzheimer’s disease is the formation of senile plaques in the brain due to the unbalanced levels of amyloid-beta (Ab). However, although how Ab is produced from amyloid precursor proteins is well understood, little is known regarding the clearance and metabolism of various Ab aggregates from the brain. Similarly, little is known regarding how astrocytes internalize and degrade Ab, although astrocytes are known to play an important role in plaque maintenance and Ab clearance. The objective of this study is to investigate the cellular mechanisms that mediate the internalization of soluble monomeric versus oligomeric Ab by astrocytes. We used a combination of laser confocal microscopy and genetic and pharmacological experiments to dissect the internalization of sAb42 and oAb42 and their postendocytic transport by U87 human brain astrocytoma cell line. Both Ab42 species were internalized by U87 cells through fluid phase macropinocytosis, which required dynamin 2. Depleting LDL receptor-related protein 1 (LRP1) decreased sAb42 uptake more significantly than that of oAb42. We finally show that both Ab42 species were rapidly transported to lysosomes through an endolytic pathway and subjected to proteolysis after internalization, which had no significant toxic effects to the U87 cells under relatively low concentrations. We propose that macropinocytic sAb42 and oAb42 uptake and their subsequent proteolytic degradation in astroglial cells is a significant mechanism underlying Ab clearance from the extracellular milieu. Understanding the molecular events involved in astrocytic Ab internalization may identify potentia