procedures were performed in accordance with the protocols approved by the Institutional Committee for Use and Care of Laboratory Animals of the University of South Florida, under animal protocol number R3336. Primary Neuronal Culture and Neuron/COS7 cell Coculture experiments Neuron/COS7 cell co-culture experiments were performed as described. Primary hippocampal neuronal cultures from embryonic day 18 rats were used for synapse formation assays. Transfected COS7 cells were added to the transfected neurons at 9-11 days in vitro. After 1 day of co-culture, cells were fixed in cold 100% methanol and incubated with primary antibodies and secondary antibodies in PBS with 1% goat serum. 92 Random images were collected using a LSM510 confocal microscope. Confocal z-series image stacks encompassing entire neurons were analyzed using MetaMorph software. Synapse formation was determined by quantifying co-immunostaining between the presynaptic marker synaptophysin and contacting axons of transfected COS7 cells. Presynaptic specializations were normalized to the number of COS7 cells in the entire field. Results were averaged for each condition. Cell lines and culture conditions COS7 cells were maintained in Opti-MEMH with 10% fetal bovine serum in a 5% CO2 incubator. The cells were transiently transfected with 0.5 mg of plasmid in FuGENE 6 according to the manufacturer’s protocol and cultured 24 hr in DMEM containing 10% FBS. For co-transfections, cells were similarly transfected with 0.5 mg of each plasmid in FuGENE 6 and cultured 24 hr in DMEM with 10% FBS. Antibodies We used antibodies anti-HA, anti-X11a, anti-Flag, anti-PSD-95, antiGFP, b-actin, anti-ApoEr2, antiGluA1, anti-GluA2, anti-c-myc, and anti-synaptophysin. Primary neuron culture and transfection Hippocampal neurons from embryonic day 189 SpragueDawley rats were cultured at 150 cells/mm2 as described. Neurons were transfected at 12, 14 or 16 days in vitro with GFP-ApoEr2, ApoEr2-HA, ApoEr2-myc, ApoEr2 deletion constructs with C-terminal myc tag, PSD-95-Flag, X11a-Flag, GFP or empty vector by lipofectamine 2000 according to manufacturer’s instructions. Transcription of each insert was driven by the CMV promoter. Synaptosome fractionation Synaptosome fractionation was performed as described, with minor modifications. Adult rat brains were briefly homogenized in 0.32 M sucrose, 4 mM HEPES-NaOH, pH 7.3 with protease inhibitors, and centrifuged at AG1024 custom synthesis 10006g for 10 min to recover the supernatant S1 and the pellet P1. S1 fraction was centrifuged at 12,0006g for 15 min to obtain the pellet P2 and the supernatant S2. The P2 fraction 
was osmotically shocked by diluting with double-distilled water and further centrifuged at 25,0006g for 20 min to generate the pellet LP1 and the supernatant LS1. The LS1 fraction was subjected to ultracentrifugation to obtain the synaptic vesicle fraction LP2. LP1 was detergent extracted in buffer B, and then centrifuged at 33,0006g for 20 min. The pellet LP1P was resuspended and applied to a discontinuous sucrose gradient consisting of 1.0 M, 1.5 M, and 2.0 M sucrose layers. After ultracentrifugation, the PSD fraction was recovered at the interface between 1.5 and 2.0 M sucrose. Immunocytochemistry Hippocampal cultured neurons were fixed either in 4% paraformaldehyde or in methanol at 220uC for 10 min. Antibodies for immunostaining were incubated in GDB buffer. Cell surface expression levels of ApoEr2 were performed as described. Live neuronal cultures were briefl