Very Low Density Lipoprotein Receptor and Alzheimer’s Disease

Summary

The very low density lipoprotein receptor (VLDLR) is a recently discovered member of a protein family that includes the low density lipoprotein (LDL) receptor and the LDL receptor-related protein/α ₂-macroglobulin receptor (LRP). The VLDLR specifically binds apolipoprotein E (apoE)-containing lipoproteins, including VLDL. Since antibodies to apoE immunostain senile plaques, neurofibrillary tangles and certain neuronal populations in autopsy brains from Alzheimer’s disease patients, and since the message of this new apoE-binding receptor is abundant in the brain tissue, we examined the possibility that the receptor is involved in the pathogenesis of Alzheimer’s disease, in which apoE has been implicated.

We first examined cell types that express the VLDLR in rat and human brain tissue by immunohistochemistry using monoclonal antibodies that recognize amino- and carboxyl-terminal portions of human and rat VLDLR, respectively. Immunoreactivity was found in most of the neurons throughout rat and human brains. Some astrocytes were also stained. We then examined primary cultured cells from human fetal brain. Intense staining was observed in most of the neurons. Glial fibrillary acidic protein (GFAP) positive astrocytes were weakly stained. To confirm the latter finding, pure astrocyte cultures were prepared by shaking off the loosely attached neurons and were examined by immunoblotting and reverse transcriptase (RT) polymerase chain reaction (PCR) methods. In immunoblotting experiments of cell extracts, the VLDLR antibodies identified a 130-kDa protein, whose apparent molecular weight was similar to that observed in extracts of cells transfected with the human VLDLR cDNA and tissue extracts of autopsied brain tissue. In RT-PCR analysis, total RNA from cultured astrocytes provided two PCR products, corresponding, respectively, to the portions of the VLDLR transcript (type I) and a variant transcript (type II) that lacks 84 nucleotides. These data demonstrated that the VLDLR was expressed both in neurons and astrocytes in brain and in culture.

With these data in mind, we examined formalin-fixed, paraffin-embedded brain tissues (hippocampus and frontal lobe) from 18 Alzheimer’s disease patients and 15 age-matched controls by a standard immunoperoxidase method using the VLDLR antibodies. In Alzheimer’s disease cases, intense staining was observed in a large number of granule cell neurons and pyramidal neurons of hippocampus, with less intense staining in neurons of neocortex. Staining was restricted to neuronal perikarya and proximal processes, and its pattern was cytoplasmic and granular. Neuronal staining was weak in control brains, when compared to Alzheimer’s disease brains. The VLDLR immunoreactivity was also seen in reactive astrocytes and dystrophic neurites in senile plaques. Senile plaques having the VLDLR immunoreactivity were stained with apoE antibody in the adjacent sections.

Taken together with a report of similar immunohistochemical findings for another apoE-binding lipoprotein receptor, the LRP, our data support a no tion that apoE-binding lipoprotein receptors play roles in the formation of senile plaques. Since both apoE and βPP, a precursor protein that contains an amyloidogenic Aβ sequence, appear to be internalized through receptor-mediated endocytosis in neurons and processed in a specific, yet unidentified manner, it is of interest to speculate that common neuronal subcellular organelles might serve as sites where the two proteins can interact, both of which are implicated in AD and Aβ amyloidogenesis, respectively.