Apolipoprotein E isoforms and apolipoprotein AI protect from amyloid precursor protein carboxy terminal fragment-associated cytotoxicity Izumi Maezawa,* Lee-Way Jin,* Randall L. Woltjer,* Nobuyo Maeda, George M. Martin,* Thomas J. Montine* and Kathleen S. Montine* *Department of Pathology, University of Washington, Seattle, Washington, USA  Department of Pathology, University of North Carolina, Chapel Hill, North Carolina, USA Abstract Inheritance of the apolipoprotein (APO) E gene e4 or e2 allele alters the risk of developing Alzheimer disease (AD), while increased alpha-tocopherol (AT) intake appears to lower the risk of AD. As APOE is a major apolipoprotein in the CNS and AT in vivo is transported in lipoproteins, we tested the hypo- thesis that CNS lipoproteins, as modeled by relevant con- centrations of high density lipoprotein (HDL), and AT would interact to suppress neurotoxicity in a cell culture model of amyloid beta (Ab)- related toxicity. These cells conditionally express C99-derived peptides, proposed to be a key step in AD pathogenesis; this expression is closely associated with subsequent cell death. We found that physiologic concentrations of lipoproteins present in the CNS protected from C99-associated toxicity and provided evidence for two mechanisms of protection. The first was AT-independent, APOE isoform-dependent, and most potent for the APOE2 isoform. The second was a synergistic protection afforded by a combination of APOAI, or less so APOE, and AT. These data provide a novel explanation for the apparent AD-pro- tective effect of inheriting an e2 APOE allele, and suggest that optimizing AT enrichment of CNS lipoproteins or devising APOAI mimetics may augment AT efficacy in treating AD. Keywords: alpha-tocopherol, apolipoproteins, APO, C99, HDL. J. Neurochem. (2004) 91, 1312–1321. Alzheimer’s disease (AD) is an adult–onset neurodegenera- tive syndrome that can be inherited as a rare autosomal dominant trait or more commonly appears sporadically, probably as the confluence of genetic susceptibilities and environmental factors. Substantial evidence indicates that the etiology of AD is related to proteolytic products from the C-terminus of the amyloid precursor protein (APP) (Selkoe 2002; Walsh et al. 2002). Cleavage of APP by b-secretase generates C99, a peptide consisting of the 99 C-terminal amino acids of APP. Further proteolytic cleavage produces a number of peptides, several of which, as with C99 itself, have been shown to be neurotoxic in vitro and in vivo (Oster- Granite et al. 1996; Fraser et al. 1997; McPhie et al. 1997; McPhie et al. 2001). Which species is the primary neuro- toxin in vivo remains to be clarified, although greatest attention has been focused on one particular family of C99 fragments, the amyloid beta (Ab) peptides, especially Ab 42 . Much of the data examining the neurotoxicity of Ab peptides has been performed in cell culture with extracellular application of micromolar concentrations of artificially aggregated synthetic peptides, likely yielding an array of assembly forms that may or may not reflect what occurs Received May 15, 2004; revised manuscript received August 13, 2004; accepted August 13, 2004. Address correspondence and reprint requests to Kathleen S. Montine, Department of Pathology, University of Washington, Box 359645, Harborview Medical Center, 325 9th Ave, Seattle, WA 98104, USA. E-mail: kmontine@u.washington.edu Abbreviations used: AD, Alzheimer’s disease; APO, apolipoprotein; APP, amyloid precursor protein; AT, alpha-tocopherol; Ab, amyloid beta; CM, conditioned medium; DMPC, dimyristoylphosphatidylcholine; HDL, high density lipoprotein; HDL-AT sat , HDL enriched with 0.28 nmoles AT/lg HDL; HDL fixed -AT, HDL at 0.75 lg/mL with unspecified AT enrichment; HDL fixed -AT 1/10sat , HDL fixed enriched with 0.028 nmoles AT/lg HDL; HDL fixed -AT 1/2sat , HDL fixed enriched with 0.14 nmoles AT/lg HDL; HDL fixed -AT sat , HDL fixed enriched with 0.28 nmoles AT/lg HDL; HMWC, high molecular weight complexes; LDL, low density lipoprotein; PBS, phosphate-buffered saline; Tet, tet- racycline; Tet – , cell culture medium without Tet; Tet + , cell culture medium containing 1 lg/mL Tet; tHDL, trypsinized HDL; TR, targeted replacement; 6E10, monoclonal antibody directed against Ab 1)17 ; 4G8, monoclonal antibody directed against Ab 17)24 . Journal of Neurochemistry , 2004, 91, 1312–1321 doi:10.1111/j.1471-4159.2004.02818.x 1312 Ó 2004 International Society for Neurochemistry, J. Neurochem. (2004) 91, 1312–1321