18 September 2001. A cholesterol-modifying enzyme not previously implicated
in Alzheimer's disease modifies production of the pathogenic Aβ peptide,
report researchers led by Dora Kovacs at Massachusetts General Hospital in a
paper in the October Nature Cell Biology published online today. The researchers
found that blocking this enzyme lowers Aβ production in mutant cell lines
and primary neurons. They suggest that pharmacological inhibitors, developed
originally for atherosclerosis, might prove useful for testing the enzyme as
a novel target in AD.
Among the hypothesis that are being advanced to explain Alzheimer's disease,
one implicating elevated cholesterol has picked up steam in the last few years.
Epidemiological studies suggest a decreased incidence in people who take cholesterol-lowering
statin drugs to treat or prevent heart disease (Wolozin
et al.) More than a dozen studies have shown that manipulating cholesterol
in cultured cells and animal models alters processing of APP, the parent protein
from which Aβ is cleaved (Bodovitz
and Klein, Fassbender
But these studies largely treated cellular cholesterol as one entity. Kovacs
and colleagues examined which of the separate pools of cholesterol present in
different intracellular compartments correlates best with Aβ production.
They focused on acyl-coenzyme A:cholesterol acyltransferase (ACAT) found in
the endoplasmic reticulum (ER). ACAT moves an acyl group from coenzyme A to
free cholesterol, which resides in membranes, whereas cholesteryl-esters form
lipid droplets in the cytoplasm. ACAT senses the free cholesterol concentration
in the ER and keeps it relatively constant, shunting excess free cholesterol
into lipid droplets and replenishing depleted cholesterol by hydrolyzing the
ester bond in the lipid store.
Kovacs et al. used three approaches to show that cholesteryl-ester levels correlate
with total Aβ and Aβ42 generation (the latter forms fibrils faster and
is considered more toxic.) First, they measured Aβ levels in CHO cell lines
that overexpressed APP and had mutations in cholesterol homeostasis. Cells with
elevated cholesteryl-ester levels produced more Aβ than did wild-type CHO
cells, while cells that lacked cholesteryl-esters but had elevated free cholesterol
produced almost no Aβ.
Second, Kovacs et al used competitive inhibitors of ACAT to show that Aβ
levels decreased along with cholesteryl-ester levels. For example, a 45 percent
decrease in the ester and parallel 42 percent increase in free cholesterol caused
by a 10-micromolar dose of one of the inhibitors reduced the secretion of total
Aβ by 30 percent and of Aβ42 by 50 percent. Manipulating free cholesterol
alone did not affect Aβ.
Third, they depleted cholesterol in the mutant cell lines by growing them in
lipoprotein-deficient medium (cells import cholesterol in the form of lipoprotein
particles). Again, a reduction in Aβ generation accompanied a similar reduction
in cholesteryl-ester levels, but not free cholesterol levels. Finally, the scientists
reproduced their results in human neuroglioma cells and primary neurons.
"Our main finding that cholesteryl-esters correlate with Aβ is unexpected
because the esters are in the cytoplasm, whereas APP, presenilin, and BACE-1
are in membranes. We have to provide a mechanistic explanation for this," says
On this front, intriguing hints abound amid conflicting evidence, Kovacs adds.
The basic routes of cholesterol trafficking into and out of cells and within
cells, including neurons, are becoming clear (Simons
and Ikonen, Dietschy
and Turley). Free cholesterol, which makes membranes rigid, occurs in an
increasing gradient along the intracellular protein production and secretion
pathway. The ER contains little cholesterol to remain fluid as it accommodates
protein synthesis and folding, the Golgi and transgolgi network contain more,
and the cell membrane still more, probably to make it stiff enough to separate
the intracellular and extracellular environments. At the same time, the outer
membrane leaflets of the Golgi and cell membranes also contain dense patches
called lipid rafts, where even higher cholesterol concentrations help pack sphingolipid
molecules. Lipid rafts sort, concentrate, and distribute proteins. APP, Aβ,
as well as the presenilin complex have been found in these rafts but their role,
if any, in AD remains mysterious.
Kovacs' ongoing work focuses on trying to find out whether there is a direct
physical association between ACAT, cholesteryl esters, APP, and any of the secretases
known to cut it, and to pinpoint precisely in which cellular compartment such
interactions would occur. Alternatively, ACAT activity might somehow affect
the conformation or accessibility of APP for processing. "The biology of these
membrane compartments is not at all clear yet," says Kovacs.
Are the present findings relevant clinically? Answering that question will
require treating transgenic mouse models with ACAT inhibitors to see if they,
much like statins, metal chelation, and Aβ vaccination schemes, can decrease
the formation of amyloid in the brain.-Gabrielle Strobel.
Reference:Puglielli L, Konopka G, Pack-Chung E, Ingano LA, Berezovska O, Hyman BT, Chang TY, Tanzi RE, Kovacs DM. Acyl-coenzyme A:cholesterol acyltransferase
modulates the generation of the amyloid-b-peptide. Nat Cell Biol. 2001 Oct;3(10):905-12. Abstract