Influence of Lipoproteins on Microglial Degradationof Alzheimer’s Amyloid Beta-ProteinGREG M. COLE1*AND MARCH D. ARD21Sepulveda VA Medical Center, Sepulveda, California 913432Department of Anatomy, University of Mississippi Medical Center, Jackson, Mississippi 39216KEY WORDS apolipoprotein E; microglia; A!ABSTRACT Amyloid !-protein (A!), the major component of plaques in Alzheimer’s disease, isa small hydrophobic protein that is carried on apolipoprotein E (ApoE)- and ApoJ-containinglipoprotein particles in plasma and cerebrospinal fluid (CSF). Microglia, the scavenger cells of theCNS, take up and degrade A! via lipoprotein receptors including scavenger receptors A and B, andpossibly via other receptors. Lipoproteins, ApoE, and ApoJ influence the uptake and degradation ofA! in vitro and in vivo. Differences in ApoE-E4, -E3, and -E2 isoforms with respect to A! bindingto lipoproteins and delivery to cells, including microglia, may contribute to the increased risk ofAlzheimer’s disease for people with an APOE4 genotype and to risk reduction with APOE2. Microsc.Res. Tech. 50:316 –324, 2000.© 2000 Wiley-Liss, Inc.INTRODUCTIONExtracellular plaques of amyloid !-protein (A!) andother associated proteins, distributed in specific brainregions, are a definitive characteristic of Alzheimer’sdisease, along with intracellular neurofibrillary tan-gles and loss of synapses. A! is a normally secretedbyproduct of amyloid precursor protein metabolismthat is normally rapidly degraded in a steady-stateequilibrium with production (Savage et al., 1998). Ac-cumulations of aggregated A! in solution or depositedin plaques may be damaging to neurons, either bydirect toxicity or through inflammatory, neurotoxic se-cretions of microglia cells. Accrual of plaques appearsto reach an equilibrium during the disease, based inpart on the demonstration by Hyman et al. (1993) thatwhereas a higher total area of A! immunoreactivity intemporal cortex distinguishes Alzheimer’s disease fromnormal brain tissue, there is no correlation of amountof A! immunoreactivity with the duration of illness.Rather than plaque deposition continuing uncheckedthroughout the course of the disease, it appears thatturnover occurs, so that as amyloid is deposited, apercentage of it is also removed. Consistent with theidea of turnover of plaque material during the disease,the form of A! found in plaques changes from predom-inantly A!1–42 at early stages (diffuse plaques) topredominantly A!1– 40 at later stages (senile plaques)(Dickson, 1997; Nakamura et al., 1997). In contrast,SDS insoluble, formic acid extractable amyloid, accu-mulates progressively and correlates with cognitive de-cline (Naslund et al., 2000).Microglia are likely agents of A! aggregation andamyloid removal. Activated, HLA-DR positive micro-glia are closely associated with plaques (Dickson et al.,1993; Mattiace et al., 1990; Styren et al., 1990;Tooyama et al., 1990; for review see Kalaria, 1999).They are the phagocytic scavenger cells of the CNS,and they have multiple receptors for and take up bothsoluble and fibrillar A! in vitro (Ard et al., 1996; Cole etal., 1999; Paresce et al., 1996, 1997; Shaffer et al.,1995). Similarly, in vivo after injection of A! into hip-pocampus, microglia loaded with A!-immunoreactivematerial are found migrating toward ventricles andblood vessels (Frautschy et al., 1992).Increasing amyloid removal by microglia is a poten-tial therapeutic target in Alzheimer’s disease. This willrequire understanding of both microglial endocytosisand degradation of A!, which may serve as a salutaryclearance mechanism, and microglial inflammatory re-actions to A! uptake. This review focuses on endocyto-sis and degradation of A! and its modulation by li-poproteins.Lipoproteins are carriers of A! in biological fluids(Biere et al., 1996; Fagan et al., 1999; Koudinov et al.,1994, 1996; Koudinov and Koudinova, 1997; LaDu etal., 1995), and this discovery has led to interest in theinfluence of lipoprotein on the microglial-A! interac-tion. The observed effects of the E4 allele of apolipopro-tein E (APOE4) on risk for Alzheimer’s disease, reduc-ing the age of onset (Saunders et al., 1993) and increas-ing the amount of amyloid plaque burden inAlzheimer’s brain tissue (Gearing et al., 1996; Mann etal., 1997; Schmechel et al., 1993), could result directlyfrom a lipoprotein role in A! trafficking and clearance.In keeping with such a role, polymorphism in an ApoEreceptor, LRP, has recently been shown to be associ-ated with Alzheimer’s disease (Hollenbach et al., 1998).Lipoproteins may act at several levels as they affectaggregation of soluble A! into fibrils, binding of A! tocell surface receptors for endocytosis, lysosomal degra-dation of A!, and the inflammatory response.Contract grant sponsor: VA Merit; Contract grant number: AG13741; Contractgrant sponsor: Elizabeth and Thomas Plott Foundation*Correspondence to: Dr. Greg M. Cole, VAGLA Medical Center, GRECC 11E,16111 Plummer Street, Sepulveda, CA 91343. E-mail: [email protected] 16 February 2000; accepted in revised form 15 March 2000MICROSCOPY RESEARCH AND TECHNIQUE 50:316 –324 (2000)© 2000 WILEY-LISS, INC.A! CIRCULATES BOUND TO LIPOPROTEINSLike other nonpolar or hydrophobic peptides, A! inaqueous body fluids attaches to a carrier protein orlipoprotein particle for solubilization. Studies of A! inplasma by LaDu et al. (1995) and by Koudinov et al.(1994) showed that A! circulating in plasma is boundto lipoprotein particles, mainly to HDL. In one set ofexperiments in which radiolabeled A! was added toplasma, serum albumin as well as lipoprotein wasfound to bind A! (Biere et al., 1996). Transthyretin hasalso been reported as an A! carrying protein (Schwar-zman et al., 1994). The discovery of lipoprotein carriersfor A! was quickly followed by fractionation of CSF andidentification of native A! in lipoprotein fractions,leading to the conclusion that A! circulates in CSF asa component of lipoprotein particles, mainly ApoE- andApoJ-containing lipoproteins similar in size and den-sity to plasma HDL (Fagan et al., 1999; Koudinov et al.,1996).Lipoprotein is produced by astrocytes, secreted asrelatively lipid-poor apolipoprotein and lipid particles(LaDu et al., 1998). A! itself in a transfected cell line issecreted with lipid as part of a lipoprotein particle(Koudinov and Koudinova, 1997). Therefore, A! is mostlikely bound to lipoprotein in brain parenchyma, as inCSF. This is an important point