An ABCA1-independent pathway for recycling a poorly lipidated 8.1 nm apolipoprotein E particle from glia

• Published in: Journal of lipid research Vol. 52; no. 9; pp. 1605 - 1616
• Main Authors: Fan, Jianjia, Stukas, Sophie, Wong, Charmaine, Chan, Jennifer, May, Sharon, DeValle, Nicole, Hirsch-Reinshagen, Veronica, Wilkinson, Anna, Oda, Michael N., Wellington, Cheryl L.
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 01.09.2011; The American Society for Biochemistry and Molecular Biology; Elsevier
• Subjects: ABCA1 protein; Animals; apoE; Apolipoprotein E; Apolipoproteins E - chemistry; Apolipoproteins E - genetics; Apolipoproteins E - metabolism; ATP Binding Cassette Transporter 1; ATP-Binding Cassette Transporters - genetics; ATP-Binding Cassette Transporters - metabolism; ATP-binding protein; Brain; Brain - cytology; Brain - metabolism; bromides; Cells, Cultured; Cholesterol; Cholesterol - metabolism; Density gradients; Glia; Humans; Lipid Metabolism; Lipids; lipoprotein; Lipoprotein (low density) receptors; Lipoproteins; Lipoproteins (low density); Mice; Mice, Inbred C57BL; Mice, Inbred DBA; Mice, Knockout; Neuroglia - chemistry; Neuroglia - cytology; Neuroglia - metabolism; Neurological diseases; Neuronal-glial interactions; Particle Size; Potassium; Receptor mechanisms; Receptors, LDL - genetics; Receptors, LDL - metabolism; Recycling; Secretion; Ultracentrifugation; apoE; lipoprotein; recycling
• ISSN: 0022-2275; 1539-7262
• DOI: 10.1194/jlr.M014365

Lipid transport in the brain is coordinated by glial-derived lipoproteins that contain apolipoprotein E (apoE) as their primary protein. Here we show that apoE is secreted from wild-type (WT) primary murine mixed glia as nascent lipoprotein subspecies ranging from 7.5 to 17 nm in diameter. Negative-staining electron microscropy (EM) revealed rouleaux, suggesting a discoidal structure. Potassium bromide (KBr) density gradient ultracentrifugation showed that all subspecies, except an 8.1 nm particle, were lipidated. Glia lacking the cholesterol transporter ABCA1 secreted only 8.1 nm particles, which were poorly lipidated and nondiscoidal but could accept lipids to form the full repertoire of WT apoE particles. Receptor-associated-protein (RAP)-mediated inhibition of apoE receptor function blocked appearance of the 8.1 nm species, suggesting that this particle may arise through apoE recycling. Selective deletion of the LDL receptor (LDLR) reduced the level of 8.1 nm particle production by approximately 90%, suggesting that apoE is preferentially recycled through the LDLR. Finally, apoA-I stimulated secretion of 8.1 nm particles in a dose-dependent manner. These results suggest that nascent glial apoE lipoproteins are secreted through multiple pathways and that a greater understanding of these mechanisms may be relevant to several neurological disorders.