The Low Density Lipoprotein Receptor Gene Family

doi:10.1074/jbc.273.48.32213

The Low Density Lipoprotein Receptor Gene Family

DIFFERENTIAL EXPRESSION OF TWO α₂-MACROGLOBULIN RECEPTORS IN THE BRAIN*

From the ^‡Department of Molecular Genetics, Biocenter and University of Vienna, Dr. Bohrgasse 9/2, A-1030 Vienna, Austria,^¶Friedrich Miescher Institute, P. O. Box 2543, 4002 Basel, Switzerland, the ^‖Department of Medical Chemistry, University of Vienna, Währingerstrasse 10, A-1090, Vienna, Austria, and ^**Institute of Neurology, University of Vienna, Schwarzspanierstrasse 17, A-1090, Vienna, Austria

Abstract

LR7/8B is a member of the low density lipoprotein receptor gene family that is specifically synthesized in the brain. Here we have functionally expressed in 293 cells the splice variant harboring eight ligand binding repeats (LR8B). As assessed by confocal microscopy, the expressed receptor is localized to the plasma membrane. Importantly, in cell binding experiments, we demonstrate that this protein is a receptor for activated α₂-macroglobulin. Because to date low density lipoprotein receptor-related protein (LRP) has been shown to be the only α₂-macroglobulin receptor in brain, we became interested in the expression pattern of both proteins at the cellular level in the brain. LR7/8B is expressed in large neurons and Purkinje cells of the cerebellum and in cells constituting brain barrier systems such as the epithelial cells of the choroid plexus, the arachnoidea, and the endothelium of penetrating blood vessels. Anti-LR7/8B antibody stains the plasma membrane, dendrites, and vesicular structures close to the cell membrane of neurons, especially of Purkinje cells. In contrast, LRP is present in patchy regions around large neurons and most prominently in the glomeruli of the stratum granulare of the cerebellum. This suggests that, contrary to LR7/8B, LRP is expressed in synaptic regions of the neurons; furthermore, there is a striking difference in the expression patterns of LR7/8B and LRP in the choroid plexus. Whereas LRP shows baso-lateral and apical localization in the epithelial cells, LR7/8B is restricted to the apical cell aspect facing the cerebrospinal fluid. Finally, these studies were extended to cultured primary rat neurons, where double immunofluorescence labeling with anti-LR7/8B and anti-microtubuli-associated protein 2 (MAP2) confirmed the somatodendritic expression of the receptor. Based upon these data, we propose that LR7/8B is involved in the clearance of α₂-macroglobulin·proteinase complexes and/or of other substrates bound to α₂-macroglobulin from the cerebrospinal fluid and from the surface of neurons.

Footnotes

↵* These studies were supported by Austrian Science Foundation Grants P11692 (to J. N.) and P11694 (to W. J. S.) and European Community Grants BMH4-CT96-0162 (to M. H., H. L., and A. M.) and BMH3-CT93-1088 (to W. J. S).The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
↵§ Present address: Gladstone Cardiovascular Research Institute, P. O. Box 418100, San Francisco, CA 94141-9100.
↵‡ To whom correspondence should be addressed. Tel.: ++43-1-4277-61808; Fax: ++43-1-4277-9618; E-mail:JNIMPF{at}mol.univie.ac.at.
Abbreviations:

LDL(R)

low density lipoprotein (receptor)

VLDL

very low density lipoprotein

LR7/8B

brain-specific LDLR relative with 7 or 8 ligand binding repeats

LRP

LDLR-related protein

RAP

receptor-associated protein

apo

apolipoprotein

apoER2

apoE receptor 2

α₂M

α₂-macroglobulin

α₂M*

activated α₂M

GST

glutathione S- transferase

CHAPS

3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonic acid

CHO

Chinese hamster ovary

PBS

phosphate-buffered saline

TBS

Tris-buffered saline

MAP2

microtubuli-associated protein 2.
- Received July 20, 1998.
- Revision received September 2, 1998.
The American Society for Biochemistry and Molecular Biology, Inc.