Proteolysis of
apolipoprotein E (
apoE) may be involved in the pathogenesis of
Alzheimer's disease (AD). We previously identified aspartic
protease(s) as possibly contributing to the proteolysis of
apoE in human brain homogenates. The current study used biochemical and immunohistochemical methods to examine whether
cathepsin D (catD) and
cathepsin E (catE), candidate aspartic
proteases, may be involved in
apoE proteolysis. CatD was found to proteolyze both
lipid-free recombinant full-length human
apoE and lipidated human plasma full-length
apoE (
apoE4/
dipalmitoylphosphatidylcholine-reconstituted discs). CatE was found to proteolyze
lipid-free recombinant human
apoE to a much greater extent than lipidated
apoE. This proteolysis, as well as proteolysis of human
apoE added to brain homogenates from
apoE-deficient mice, was inhibited by
pepstatin A (an aspartic
protease inhibitor), but not by
phenylmethanesulfonyl fluoride (a
serine protease inhibitor). The major
apoE fragment obtained with catD included the receptor-binding domain and had an apparent molecular weight similar to that found in human brain homogenates. There was little immunoreactivity for catE in AD brain tissue sections. In contrast, qualitative and quantitative analyses of immunostained sections of the frontal cortex revealed that catD and
apoE are colocalized in a subset of predominantly dense-core
neuritic plaques and in some neurofibrillary tangles. A positive correlation was observed between estimated duration of illness and the percentage of
apoE-positive plaques that were also catD-positive. These results suggest that aspartic
proteases, catD in particular, may be involved in proteolysis of
apoE and perhaps contribute to the generation of
apoE fragments previously implicated in AD pathology.