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

Anne M Fagan,David M Holtzman,Gregory Munson,Tanya Mathur,Danielle Schneider,Louis K Chang,Godfrey S Getz,Catherine A Reardon,John Lukens,Javeed A Shah,Mary Jo Ladu

doi:10.1074/jbc.274.42.30001

Anne M Fagan, David M Holtzman + Show 9 more

Open Access

https://doi.org/10.1074/jbc.274.42.30001

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Abstract

Composition of central nervous system lipoproteins affects the metabolism of lipoprotein constituents within the brain. The epsilon4 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.

Highlights

Composition of central nervous system lipoproteins affects the metabolism of lipoprotein constituents within the brain
Astrocytes were cultured from wild type, apolipoprotein E (apoE) (Ϫ/Ϫ), and apoE transgenic mice in which human apoE3 or apoE4 is immunosorbent assay; EM, electron microscopy; HDL, high density lipoprotein; IP, immunoprecipitation; PAGE, polyacrylamide gel electrophoresis; PL, phospholipid; TC, total cholesterol; TG, triglyceride; WT, wild type
Our results suggest that apoE expression by glial cells, primary cultures of mouse astrocytes, is required for the normal secretion of HDL-like lipoprotein particles by these cells

Summary

Introduction

Composition of central nervous system lipoproteins affects the metabolism of lipoprotein constituents within the brain. 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 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. Astrocytes were cultured from wild type, apoE (Ϫ/Ϫ), and apoE transgenic mice in which human apoE3 or apoE4 is immunosorbent assay; EM, electron microscopy; HDL, high density lipoprotein; IP, immunoprecipitation; PAGE, polyacrylamide gel electrophoresis; PL, phospholipid; TC, total cholesterol; TG, triglyceride; WT, wild type. Our data show that expression of apoE by astrocytes is required for normal lipoprotein secretion by these cells and that apoE species appears to influence lipoprotein composition

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