title: Scavenging of Alzheimer's amyloid beta-protein by microglia in culture.
abstract: 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.
authors.affiliation: Department of Anatomy, University of Mississippi Medical Center, Jackson 39216, USA.
authors.forename: M D
authors.initials: MD
authors.lastname: Ard
authors.forename: G M
authors.initials: GM
authors.lastname: Cole
authors.forename: J
authors.initials: J
authors.lastname: Wei
authors.forename: A P
authors.initials: AP
authors.lastname: Mehrle
authors.forename: J D
authors.initials: JD
authors.lastname: Fratkin
mesh_terms: D000544:Alzheimer Disease / Q000378:metabolism*; D016229:Amyloid beta-Peptides / Q000378:metabolism*; D000818:Animals; D001923:Brain Chemistry / Q000502:physiology; D002478:Cells, Cultured; D016895:Culture Media, Serum-Free; D003720:Densitometry; D006801:Humans; D007091:Image Processing, Computer-Assisted; D015151:Immunoblotting; D007150:Immunohistochemistry; D017628:Microglia / Q000378:metabolism*; D051381:Rats; D013116:Spinal Cord / Q000166:cytology
chemical_list: D016229:Amyloid beta-Peptides; D016895:Culture Media, Serum-Free