Kinetic analysis of aggregated amyloid-beta peptide clearance in adult bone-marrow-derived macrophages from APP and CCL2 transgenic mice.

Accumulating evidence suggests that bone-marrow (BM)-derived mononuclear phagocytes have an important role in the clearance of soluble and aggregated amyloid-beta peptides (Abeta) in Alzheimer's disease (AD) brains. However, the exact kinetics of Abeta clearance in mononuclear phagocytes derived from transgenic animal models of AD expressing beta-amyloid precursor protein (APP) mutants have been poorly characterized. We have examined whether CCL2 and APP expression affects the clearance of Abeta in conjunction with our control, acetylated low-density lipoprotein (AcLDL), using primary cultured BM-derived macrophages derived from adult APP, CCL2, APP/CCL2, and control littermates. Pulse-chase analysis demonstrated three distinct destinations for Abeta40 and AcLDL: intracellular retention, degradation, and secretion. As predicted, 50% of Abeta remained intracellularly contained even 5 days after pulse, while 40% of degraded and 14% of nondegraded Abeta were secreted. APP/CCL2 macrophages show reduced intracellular Abeta retention, along with enhanced secretion of both degraded and nondegraded Abeta. Abeta accumulation in aggresome is also partially reduced in APP/CCL2 macrophages as compared to other APP, CCL2, or control groups, suggesting impaired sorting of aggregated Abeta in aggresomes. The degradation of intracranially injected (125)I-Abeta40 aggregates was also enhanced in adult APP/CCL2 mice as compared to APP littermates in vivo. These data suggest that APP and CCL2 synergistically enhance BM-derived macrophage-mediated clearance of Abeta. In contrast, the clearance of AcLDL by BM-derived macrophages was not significantly enhanced by the presence of either APP or CCL2.