Unique Lipoproteins Secreted by Primary Astrocytes From Wild Type, apoE (−/−), and Human apoE Transgenic Mice

Abstract

Composition of central nervous system lipoproteins affects the metabolism of lipoprotein constituents within the brain. The ⑀4 allele of apolipoprotein E (apoE) is a risk factor for Alzheimer's disease via an unknown mechanism(s). As glia are the primary central nervous system cell type that synthesize apoE, we characterized lipoproteins secreted by astrocytes from wild type (WT), apoE (؊/؊), and apoE transgenic mice expressing human apoE3 or apoE4 in a mouse apoE (؊/؊) background. Nondenaturing size exclusion chromatography demonstrates that WT, apoE3, and apoE4 astrocytes secrete particles the size of plasma high density lipoprotein (HDL) composed of phospholipid, free cholesterol, and protein, primarily apoE and apoJ. However, the lipid:apoE ratio of particles containing human apoE is significantly lower than WT. ApoE localizes across HDL-like particle sizes. ApoJ localizes to the smallest HDL-like particles. ApoE (؊/؊) astrocytes secrete little phospholipid or free cholesterol despite comparable apoJ expression, suggesting that apoE is required for normal secretion of astrocyte lipoproteins. Further, particles were not detected in apoE (؊/؊) samples by electron microscopy. Nondenaturing immunoprecipitation experiments indicate that apoE and apoJ reside predominantly on distinct particles. These studies suggest that apoE expression influences the unique structure of astrocyte lipoproteins, a process further modified by apoE species.The composition and type of lipoproteins present in the brain have implications not only for lipid delivery but also for the transport of apolipoprotein (apo) 1 and other lipoprotein constituents within the central nervous system (CNS). Glia, in particular astrocytes, are the primary cell type in the CNS that synthesize apoE (1, 2), whereas apoJ is expressed by astrocytes and neurons (3-5). We have previously observed that primary rat astrocytes secrete discoidal particles the size of large plasma high density lipoproteins (HDLs) that contain apoE and apoJ (6). As a ligand for lipoprotein receptors, apoE helps to regulate plasma lipid and cholesterol metabolism. This process may also be operating in the parenchyma of the brain, as neural cells express a variety of receptors in the low density lipoprotein receptor family (7-10). The role of apoJ in lipid transport in both the periphery and within the CNS is less clear, and gp330 (megalin), the only known receptor for mammalian apoJ (11), appears to be expressed only by ependymal and endothelial cells in the brain (12, 13). Thus, lipoprotein secretion by isolated glial cells may provide a system in which to further dissect the role of apoE and apoJ in lipoprotein synthesis, secretion and function in the brain. In terms of function, several lines of evidence suggest that apoE and apoJ may be involved in neural homeostasis beyond their capacity to transport lipid. Both apoE and apoJ increase in response to different brain insults (3, 14 -16). In addition, apoE and apoJ may play a role in the pathogenesis of Alzheimer's disease (AD), as...

Production and characterization of astrocyte-derived human apolipoprotein E isoforms from immortalized astrocytes and their interactions with amyloid-β

“…Purification and size analysis of apoE-containing lipoproteins secreted by immortalized astrocytes Primary astrocytes secrete nascent murine and human apoE into apoE-containing HDLs that are disc-like in appearance since they contain cholesterol and phospholipid but no cholesteryl-ester core (DeMattos et al, 2001b;Fagan et al, 1999;LaDu et al, 1998). To determine if the apoE2-, apoE3-, and apoE4-secreting Table 1 Quantitation of the level of apoE present in serum-free conditional media from immortalized astrocytes derived from APOE2, APOE3, and APOE4 knock-in mice Cells were grown in 6-well plates.…”

“…Glia (astrocytes and microglia) are cells in the CNS that produce the majority of apoE (Boyles et al, 1985;LaDu et al, 1998;Nakai et al, 1996;Pitas et al, 1987a;Stone et al, 1997), with astrocytes appearing to produce the preponderance of this protein. It has been demonstrated that astrocytes secrete apoE in nascent HDL-like lipoprotein particles, which contain cholesterol and phospholipid but lack a cholesteryl-ester core (DeMattos et al, 2001b;Fagan et al, 1999;LaDu et al, 1998). Since the physiological activity of apolipoproteins and their interactions with other molecules (e.g., Ah and receptors) can be markedly influenced by the size, composition, and type of lipoprotein particle with which they are associated, it is important to study the properties and interactions of apoE derived from the CNS or CNS cells.…”

No association between ApoE polymorphism and febrile seizures

“…We focused on ApoE as a candidate gene for three reasons: (1) ApoE is located on chromosome 19q thought to contain susceptibility genes associated with febrile seizures, (2) three different genotypes (ApoE2, ApoE3, ApoE4), linked to single nucleotide polymorphisms, have been identified in humans, and (3) apolipoprotein E found in the brain is synthesized in astrocytes and plays a major role in the maintenance of lipid homeostasis and vesicular trafficking [20,21]. In Europeans free of any neurological disorders, the distribution of different alleles is stable, with an ApoE2 frequency of 0.068, an ApoE3 frequency of 0.813 and an ApoE4 frequency of 0.119 [22].…”

ApoE and cholesterol in schizophrenia and bipolar disorder: comparison of grey and white matter and relation with APOE genotype

“…A doublet can be observed at about 36kDa, in addition to a second band at about 80kDa (arrowheads). It has been suggested that doublets may reflect apoE sialylation, 8,31 whereas the band at about 80kDa represents an apoE complex not dissociated by SDS. 31,32 No qualitative differences in staining were observed among the 3 genotypes.…”

“…Doublets at 36kDa have been described previously and are thought to result from apoE sialylation. 8,31 In addition, apoE complexes that are not dissociated by SDS have been reported at a molecular weight of about 80kDa. 31,32 APOE genotyping: polymerase chain reaction-restriction fragment length polymorphism…”