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The Journal of Neuroscience, April 1, 1999, 19(7):2464-2473
A Test of the Cytosolic Apolipoprotein E Hypothesis Fails to
Detect the Escape of Apolipoprotein E from the Endocytic Pathway into
the Cytosol and Shows that Direct Expression of Apolipoprotein E in the
Cytosol is Cytotoxic
Ronald B.
DeMattos,
Fayanne E.
Thorngate, and
David L.
Williams
Department of Pharmacological Sciences, University Medical Center,
State University of New York at Stony Brook, Stony Brook, New York
11794
Genetic evidence indicates that apolipoprotein E4 (apoE4) is a risk
factor for the development of Alzheimer's disease. A controversial hypothesis proposes that apoE, a typical secretory protein, accesses the neuronal cytosol in which apoE3, but not apoE4, protects tau from
hyperphosphorylation. However, no conclusive evidence for the presence
of apoE in the cytosolic compartment has been presented. We designed a
novel assay to test whether apoE can access the cytosol via escape from
the endocytic pathway by incorporating a nuclear localization signal
(NLS) into apoE. Control experiments demonstrated that apoE plus NLS
(apoE+NLS) is chaperoned to the nucleus if it reaches the
cytosolic compartment. When exogenous apoE+NLS was endocytosed by
neuronal cells, no nuclear apoE was detected, indicating that apoE
remains within the endocytic pathway and does not escape into the
cytosol. Furthermore, we show that direct cytosolic expression of apoE
is cytotoxic. These data argue that effects of apoE on the neuronal
cytoskeleton and on neurite outgrowth are not mediated via cytosolic
interactions but rather by actions originating at the cell surface.
Key words:
Alzheimer's disease; apoE; cytosolic; cytotoxic; endocytosis; microtubules; nuclear localization assay; tau
Copyright © 1999 Society for Neuroscience 0270-6474/99/1972464-10$05.00/0
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