Scavenging of Alzheimer's amyloid beta-protein by microglia in culture

Deposits of amyloid beta-protein (A beta) form the cores of the pathological plaques which characterize Alzheimer's disease. The mechanism of formation of the deposits is unknown; one possibility is failure of a clearance mechanism that would normally remove the protein from brain parenchyma. This study has investigated the capacity of the central nervous system (CNS) phagocytes, microglia cells, to clear exogenous A beta 1-42 from their environment. Cultured microglia from adult rat CNS have a high capacity to remove A beta from serum-free medium, shown by immunoblotting experiments. A beta from incubation medium was attached to the cell surface and could be identified by immunocytochemistry at the light or electron microscopic (EM) level; by EM, A beta also appeared in phagosome-like intracellular vesicles. Light microscopic immunocytochemistry combined with computer-assisted image analysis showed that cells accumulated A beta within 24 hr. from culture medium containing from 1 to 20 micrograms/ml A beta. Microglial accumulation of A beta was substantially reduced in the presence of fetal bovine serum. Addition of the protease inhibitor leupeptin to incubation medium with serum resulted in accumulation of A beta in a membrane-bound intracellular compartment, but not at the cell surface. The increase in intracellular accumulation in the presence of the protease inhibitor indicates a microglial capacity for intracellular degradation of A beta in the absence of inhibition. The change from predominantly cell-surface accumulation in serum-free medium to predominantly intracellular accumulation with serum may be explained by the presence in serum of carrier proteins that complex with A beta and target it to cell surface receptors capable of stimulating endocytosis. Microglia were also cultured on unfixed cryostat sections of human brain tissue containing Alzheimer's plaques. Very little A beta from the tissue was accumulated by the cells, although cultured microglia were found in direct contact with anti-A beta immunopositive plaques. Possibly A beta in tissue sections was complexed with other proteins which either inhibited its uptake by microglia or enhanced its proteolysis, preventing cellular accumulation of immunostainable A beta. The results indicate that cultured microglia effectively remove A beta from tissue culture medium and from the surface of the dish and concentrate monomer and aggregates of A beta either on the cell surface or intracellularly. This process may be modified by proteins present in Alzheimer's brain sections.