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Butyrylcholinesterase in the life cycle of amyloid plaques.

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Butyrylcholinesterase in the Life Cvcle of
Amyloid Plaques
A. L. Guillozet, BA,* J. F. Smiley, PhD,* D. C. Mash, PhD,t and M.-M. Mesulam, MD*
Deposits of diffuse P-amyloid (AP) may exist in the brain for many years before leading to neuritic degeneration and
dementia. The factors that contribute to the putative transformation of the AP amyloid from a relatively inert to a
pathogenic state remain unknown and may involve interactions with additional plaque constituents. Matching brain
sections from 2 demented and 4 nondemented subjects were processed for the demonstration of AP immunoreactivity,
butyrykhoiinesterase (BChE) enzyme activity, and thioflavine S binding. Additional sections were processed for the
concurrent demonstration of two or three of these markers. A comparative analysis of multiple cytoarchitectonic areas
processed with each of these markers indicated that AP plaque deposits are likely to undergo three stages of maturation,
ie, a “diffuse” thioflavine S-negative stage, a thioflavine S-positive (ie, compact) but nonneuritic stage, and a compact
neuritic stage. A multiregional analysis showed that BChE-positive plaques were not found in cytoarchitectonic areas or
cortical layers that contained only the thioflavine S-negative, diffuse type of AP plaques. The BChE-positive plaques
were found only in areas containing thioflavine S-positive compact plaques, both neuritic and nonneuritic. Within such
areas, almost all (>!%yo) BChE-containing plaques bound thioflavine S, and almost all (93%) thioflavine S plaques
contained BChE. These results suggest that BChE becomes associated with amyloid plaques at approximately the same
time that the AP deposit assumes a compact P-pleated conformation. BChE may therefore participate in the transformation of AP from an initially benign form to an eventually malignant form associated with neuritic tissue degeneration
and clinical dementia.
Guillozet AL, Smiley JF, Mash DC, Mesulam M-M. Butyrylcholinesterase in the
life cycle of amyloid plaques. Ann Neurol 199?;42:909-918
The extracellular accumulation of the P-amyloid (A@)
peptide in the form of insoluble amyloid plaques is the
sine qua non for the diagnosis of Alzheimer’s disease
(AD) [ I , 21. The A@ peptide originates from a much
larger, membrane-spanning amyloid precursor protein
(APP). Depending on the site of protease action, the
AP moiety that is split off from the APP may be
“short” (39-40 amino acids) or “long” (42-43 amino
acids). The dominantly inherited forms of AD show
that mutations of the APP gene on chromosome 21 are
sufficient to cause disease [3-81, probably because they
promote an increased production of the more insoluble
long form of the AP peptide [9, 101. The presenilin 1
and 2 mutations on chromosomes 14 and 1 cause a
dominantly inherited form of AD because they also
promote an increased production of the longer species
of AP peptide [ll-141. Furthermore, Down’s syndrome (trisomy 21) almost invariably leads to the
emergence o f A D patholog relatively early in life [15],
probably because it leads to an increased production of
APP. Last, the apolipoprotein E4 allele is thought to
act as a major risk factor for developing AD, because it
may make AP more insoluble and therefore promote
its deposition in the form of plaques. These observations have led to the widespread assumption that the
deposition of AP constitutes a primary and decisive
common denominator in the pathogenesis of all forms
of AD.
Other observations, however, suggest that there is no
simple one-to-one relationship between amyloid and
dementia. First, amyloid deposits are ubiquitous in the
course of aging and may be seen in demented as well as
nondemented individuals [ 16-18]. Second, the density
of amyloid deposits shows no direct correlation with
the severity of cognitive deficits ([19], but see Cummings and associates [20] for a different conclusion).
Third, as shown in Down’s syndrome, AP plaques may
exist in the brain for decades before being associated
with dementia [3, 15, 211.
It is therefore reasonable to assume that some amyloid deposits are relatively benign whereas others are
pathogenic and that only the latter lead to the tissue
From the *Cognitive Neurology and Alzheimer Disease Center,
Northwestern University Medical School, Chicago, IL; and TDepartment of Neurology, University of Miami School of Medicine,
Miami, FL.
Address correspondence to Dr Guillozet, Cognitive Neurology and
Alzheimer Disease Center, Northwestern University, 320 E. Superior Ave, Searle 11-478, Chicago, IL 6061 1.
Received May 23. 1997, and in revised forin Aug 19. Accepted for
publication Aug 19, 1997.
Copyright 0 1997 by the American Neurological Association 909
injury and dementia characteristic of AD. In keeping
with such a possibility, amyloid deposits display considerable heterogeneity; they may be enriched in the
short or long form of the AP peptide, they may form
diffuse or compact (thioflavine S positive) plaques, and
they may or may not be associated with neuritic degeneration.
One morphological criterion of plaques that is intimately associated with dementia is the neuritic component. The neuritic component indicates that the plaque
is associated with a degeneration of axons and dendrites. The severity of dementia is correlated with the
number of neuritic plaques but not with the total
number of all plaques [22-24). Furthermore, neuritic
plaques are rare in nondemented individuals but very
frequent in those with AD. We do not fully understand why plaques lead to neuritic degeneration and
dementia in only a subgroup of elderly individuals (ie,
those with AD) despite the very frequent deposition of
amyloid in individuals older than age 65. Conceivably,
some individuals produce predominantly diffuse and
nonneuritic plaques that do not lead to dementia
whereas others may produce a type of AP that promotes the formation of neuritic plaques and clinical
dementia. An alternate possibility is that all amyloid
deposits may be similar at the time of deposition but
may undergo a process of maturation. There may be
individual variations in the rate of this maturational
process, so that elderly nondemented individuals may
represent those who die before the maturation process
has had a chance to run its full course. According to
this scenario, which has guided the design of the experiment reported below, the amyloid deposits in nondemented elderly subjects would represent a preliminary stage in the overall pathophysiology of the
neurotoxic degeneration that leads to AD.
The determinants of plaque pathogenicity are poorly
understood and could depend either on intrinsic properties of the deposited AP or on the effect of “companion” molecules with which it may subsequently become
associated. Almost all dominantly inherited forms of
AD appear to promote the formation of the long rather
than short form of the AP peptide [9, 1 1 , 12, 25-27].
The long form of AP has been shown in vitro to form
insoluble fibrils more readily than the short form and
also to have more potent cytotoxic properties [28-311.
It is, however, also the species of AP that is deposited
initially in nondemented individuals [32] and in
Down’s syndrome before the emergence of the dementia [33].Thus, whereas it appears that the long form of
the AP peptide may play an important role in “seedi n g amyloid plaque formation [34], it does not, by
itself, represent a marker of pathogenicity.
Amyloid plaques can also be distinguished on the
basis of the companion molecules with which they become associated. Numerous companion molecules have
910 Annals of Neurology
Vol 42
No 6
December 1997
been reported. They include a,-antichymotrypsin [35,
361, heparin sulfate proteoglycan [37],serum amyloid
P [38, 391, protease nexin-1 [40],complement factors
[38, 41-47], apolipoprotein E [48-501, apolipoprotein
J [51, 521, acetylcholinesterase (AChE), and butyrylcholinesterase (BChE) [53-551.
Some of these companion molecules could conceivably influence the transformation of the plaque from a
benign to a malignant form. One potential pattern for
such a companion molecule would be to have a preferential association with the more mature rather than
early stages of amyloid plaques. This report focuses on
the relationship of BChE to AP plaques and summarizes evidence showing that BChE is likely to be inserted into the amyloid plaque at an advanced stage of
maturation, at a time when the plaque is becoming associated with pathogenic properties.
Subjects and Methods
Collection and Processing of Tissue
The entire right cerebral hemispheres from 2 AD and 4 nondemented controls subjects were used. Tissue was collected
with death-to-fixation intervals of 2.5 to 28 hours, and, in
most cases, less than 6 hours (Table 1). On removal at autopsy, the right hemispheres were coronally sectioned into 1to 2-cm slabs and fixed in 4% paraformaldehyde in 0.1 M
phosphate, pH 7.4, for 30 hours at 4°C. They were then
cryoprotected in ascending concentrations (lo%, 20%, 30%,
and 40%) of a sucrose solution in 0.1 M phosphate, pH 7.4,
containing 0.02% sodium azide. The tissue slabs were then
frozen with dry ice and sectioned at 40 km with a sliding
microtome. Serially collected sections were saved in numbered wells containing 0.1 M phosphate, pH 7.4,with
0.02% sodium azide. In all brains, adjacent sections were
processed with cresyl violet, AP iinmunohistochemistry,
thioflavine S histofluorescence, and BChE enzyme histochemistry. Comparative regional analysis of the distribution
of these labels was done in sections through the frontal, parietal, temporal, and occipital lobes of each brain. Additional
sets of adjacent sections were processed for Aj3 immunohistochemistry and for double labeling with BChE and thioflavine S, and for triple labeling with BChE, Aj3, and thioflavine S.
Table 1. Subjects
Time from
Present in Plaques
Death to
Fixation (hr) AP ‘Ihioflavine S BChE
= P-amyloid; BChE = butyrylcholinesterase; AD = Alzheimer’s
Classift'cation of Specimens
The specimens were selected to cover a wide spectrum of age
and pathology. They were classified as AD or nondemented
control on the basis of clinical and pathological criteria. Both
AD subjects and 3 of the 4 control subjects (excluding Case
1) were clinically evaluated within 1 year before death with a
CERAD (Consortium to Establish a Registry for Alzheimer's
Disease) or a modified CERAD battery [56, 571. The fourth
control subject (Case 1) died at the age of 61 with no known
history of dementia. Brains were included as having AD only
if they met postmortem CERAD criteria [I] and lacked
other pathological alterations other than gliosis and cortical
microinfarcts. In addition to neuritic plaques, the AD cases
also had dense neurofibrillary tangle accumulations in all
limbic and association cortices whereas tangles were relatively
rare and confined to few limbic areas of high predilection in
control cases.
Histochemical Detection of BChE
Tissue was mounted on chrome alum-coated slides and
allowed to dry overnight and processed by modified
Karnovsky-Roots method [58]. Slides were incubated for 2
to 4 hours at 20°C in a dilute (10%) Karnovsky-Roots solution followed by metal ion diaminobenzidine (DAB) inten'
sification in the presence of H,O,. The incubation solution
(pH 6.8) contained 100 m M maleic acid (disodium salt), 0.5
mM sodium citrate, 0.3 mM cupric sulfate, 0.05 m M potassium ferricyanide, and 0.336 m M butyrylthiocholine iodide.
Tissue was rinsed in 0.1 M Tris-HC1, p H 7.6 (Tris buffer),
and then intensified for 10 minutes in 0.008% (wt/vol) cobalt chloride in Tris buffer. The reaction product was visualized by using 0.5 mg/ml DAB in Tris buffer and 0.01%
hydrogen peroxide. Sections were rinsed in Tris buffer and
coverslipped using Permount mounting media. The presence
of cholinesterase activity was visualized as a blue-brown reaction product. T o determine that the reaction product was
due to hydrolysis of the substrate by BChE, control sections
were preincubated for 1 hour in lop4 M isooctamethyl pyrophosphoramide (iso-OMPA) (a selective BChE inhibitor)
in 100 mM maleic acid (disodium salt), and lo-* M isoOMPA was added to the Karnovsky-Roots solution.
Thioflavine S Labeling
Sections were mounted onto chrom alum-coated slides and
allowed to dry overnight. Slides were placed in a 1:1 mixture
of chloroform and ethanol for 1 hour. They were then rehydrated and placed in a 1% thioflavine S solution for 15 minutes, followed by a 10-second wash in 80% ethanol to remove excess stain. Slides were then rinsed in distilled H,O
and coverslipped using Apathy's mounting media.
Sections adjacent to those stained for BChE and thioflavine
S were processed for AP immunohistochemistry using the
well-characterized rabbit antibody R1280 [59-611, which
was a gift of D. J. Selkoe. Immunolabeling was done as previously described [62]. In brief, tissue was permeabilized in
0.4% Triton X-100, and endogenous peroxidase activity was
blocked with 3% H,O,. Nonspecific binding was blocked
with 3% normal goat serum in 0.01 M phosphate with 0.9%
NaCl (PBS). Sections were incubated in primary antibody
for 24 to 30 hours at 4"C, washed in PBS, and then incubated in biotinylated secondary antibody (Vector Laboratories, Burlingame, CA) for approximately 18 hours. The tissue
was then incubated for 2 hours in an avidin-biotin-peroxidase complex (ABC Elite kit, Vector Laboratories) followed
by incubation in 12% urea for 1 hour to reduce background.
Immunoreactivity was visualized with 0.05% DAB in
0.01% H,O, in 0.05 M Tris buffer, p H 7.4.
In control sections, the primary antibody was replaced by a nonimmune
IgG .
Consecutive Labeling Experiments
T o determine whether BChE and thioflavine S were present
in the same plaques, some sections were consecutively processed for these two labels. Sections were first processed for
BChE histochemistry as described above except that 0.5
mg/ml aminoethylcarbazole (AEC) in 0.1 M sodium acetate
buffer, p H 5.2 (acetate buffer), was used as the chromogen.
AEC was substituted for the DAB solution because it was
easily removed with ethanol, facilitating subsequent unhindered labeling of the same tissue with thioflavine S. After the
initial BChE labeling, the tissue was rinsed with sodium acetate buffer and coverslipped using a 1:1 mixture of glycerol
and sodium acetate buffer. Slides were placed on a microscope stage that was electronically attached to a HewlettPackard X-Y plotter. Selected areas with BChE-labeled
plaques, visualized as a red reaction product, were then photographed and their location recorded with the plotter. The
tissue was then uncoverslipped, and the BChE label removed
in ethanol. Slides were then stained with thioflavine S as described above. After thioflavine S labeling, the sections were
replotted and the identical areas rephotographed.
To confirm that the deposits labeled with BChE and thioflavine S were in fact AP immunoreactive, some sections
were processed for three consecutive labeling procedures. After each procedure, selected areas were photographed and
plotted as described above for double-labeled material. Sections were first stained for BChE, using AEC as the chromogen. After removal of the AEC with ethanol, AP immunohistochemistry was done as described above, except that
the ABC kit was replaced with an avidin-biotin-alkaline
phosphatase kit (Vector Laboratories) and visualized with the
blue alkaline phosphatase substrate (Vector Blue, Vector
Laboratories). Endogenous alkaline phosphatase activity was
blocked by adding levamisole of the substrate solution (10
pllml). The tissue was then processed for thioflavine S labeling as described above. The alkaline phosphatase reaction
product was readily solubilized by the chloroform/ethanol solution in which slides were incubated before thioflavine S
Control experiments were done to confirm that thoflavine
S staining was neither hindered nor augmented by prior
staining for BChE and/or AP immunoreactivity. Adjacent
sections were processed simultaneously with the consecutively labeled sections, except that one or both of the labeling
procedures before thioflavine S labeling was omitted. Using a
low-power objective, a point was marked at the same area of
cortex on all relevant adjacent sections. Thioflavine S plaques
were then counted by positioning a 500 X 500-pm ocular
Guillozet et al: BChE in Plaques
Fig 1. Plaques can be divided into three different maturational stages based on P-anyloid (A@)immunohistoche~ni~t~
(on the le$)
and thioflvine S histojuorescence (on the rigbt). (A and B) Case 2, temporal cortex. D z f i e plaques are labeled witb antibodies
directed against AP but do not stain witb tbiojlavine S. (C and 0) A 100-year-old nondemented woman, temporal cortex. Compact, nonneuritic plaques stain with A@ antibodies and witb thioflvine S (E and F) Case G, temporal cortex. Compact neuritic
plaques stain f i r both A@ and tbiojlavine S and they contain dystrophic neurites (arrows), which the compact nonneuritic plaques
grid just beneath the pial surface and counting all thioflavine
S-positive plaques in 10 contiguous widths of the ocular
grid, moving laterally beneath the pial surface. The results
showed that there was no significant difference between the
thioflavine S-positive plaque densities in the four conditions.
Annals of Neurology
Vol 42
No 6
December 1997
AP Immunobistocbemisty
Plaques were visualized with AP immunohistochemistry in all 6 cases examined. In 2 of the control brains
Fig 2. Bu yvylcbolinesterase (BCbE)
in the temporal cortex o f Alzbeimeri
diseae (AD) patients and controls.
(A) BCbE staining in a 71-year-old
BCbE is
subject with A D (Case
found in plaques (open arrow), tangles
(curved arrow), dystrophic neurites
(arrowheads), and glia (single arrow).
(B) BCbE staining in an 89-year-old
nondemented individual (Case 2).
BCbE staining is limited to the glia
(arrow). Scale bar = 100 p m .
(Cases 1 and 2, see Table I), AP was detected only in
portions of the temporal and frontal lobes. In 2 other
control brains (Cases 3 and 4),AP was detected
throughout the cerebral cortex. The regional and laminar variations in the distribution of AP followed patterns that we previously described [ 181. Association
cortex was consistently more severely affected than primary sensory or motor cortex. In the 2 AD brains
(Cases 5 and 6), AP was distributed throughout the
cortex, resembling the regional distribution patterns
seen in Cases 3 and 4. In Case 5, widespread amyloid
angiopathy was also detected. The presence of dystrophic neurites could not be ascertained with AP immunohistochemistry.
Thioflavine S
Thioflavine S yields histofluorescence when it binds to
P-pleated structures. It provides a very sensitive marker
for detecting P-pleated (compact) amyloid deposits and
also paired helical filaments in neurofibrillary tangles,
dystrophic neurites, and neuropil threads. In Cases 1
and 2, the thioflavine S did not bind to plaques, indicating that the AP-immunoreactive plaques did not
have a @pleated conformation and that they were
therefore in a “diffuse” rather than compact form. In
Cases 3 and 4,numerous thioflavine S-positive, compact plaques were found, and these rarely contained
dystrophic neurites. In the AD cases (Cases 5 and 6),
numerous thioflavine S plaques were found, most of
Guillozet et al: BChE in Plaques
Fig 3. Regional comparison of tbiofavine S histofluorescence and butyrylcbolinesterase (BCbE) histochemistry in adjacent sections
j o m the perirbinal cortex of a 30-year-old nondemented individual (Case 4). (A) p-Amyloid (Ap) immunobistocbemisq reveals
AP deposits in superficial and deeper layers (solid and open arrows, respectively). (B) Tbiojlavine S stains the plaques found in the
superficial (white arrow) but not deeper layers. The curved arrow points t o a tangle. (C) BChE staining of an adjacent section.
Only the layers that bind tbiofavine S contain BCbE positive plagues (arrow).
Table 2. BCbE and Tbiofavine S-Positive Plaques
Subject No.
Thioflavine S-positive plaques (n)
BChE positive (Yo)
BChE negative (%)
BChE-positive plaques (n)
Thioflavine S positive (”0)
Thioflavine S negative (Yo)
231 (90)
26 (10)
231 (98)
4 (2)
388 (93)
29 (7)
388 (100)
0 (0)
145 (100)
0 (0)
255 (90)
27 (10)
257 (96)
10 (4)
788 (93)
10 (1)
790 (99)
10 (1)
145 (100)
0 (0)
which contained thioflavine S-positive dystrophic neurites.
These comparisons of adjacent sections labeled for
AP immunoreactivity and thioflavine S fluorescence
confirmed the presence of three types of plaques (Fig
I), diffuse plaques in which the aniyloid deposits did
not bind thioflavine S (Cases 1 and 2), “compact but
nonneuritic” plaques in which thioflavine S bound to
the amyloid (Cases 3 and 4), and “compact neuritic”
plaques in which both the amyloid and the dystrophic
neurites were labeled by thioflavine S (Cases 5 and 6 ) .
Butyrylcholinesterase Histochemistry
In all 6 cases, BChE was found in the neuroglia of the
white matter and deep layers of the cortex, as described
in previous studies [55]. Cases 1 and 2, which contained thioflavine S-negative AP plaques, also lacked
914 Annals of Neurology Vol 42
No 6
December 1997
BChE activity in the plaque deposits. In Cases 3
through 6, BChE could also be found in plaques and
was especially prominent in the plaque core. It was also
found in tangles and dystrophic neurites in the two
AD brains (Fig 2). In cases that contained BChEpositive plaques, the plaques were limited to the areas
and layers that contained thioflavine S-positive plaques
but were absent in areas and layers that contained thioflavine S-negative AP plaques (Fig 3). In comparisons
of BChE- and thioflavine S-labeled adjacent sections,
we were unable to find areas that contained one label
but not the other.
A consecutive double-labeling strategy was used to
determine if the same plaques express BChE and thioflavine S. In Cases 3 through 6, areas that contained
BChE-positive plaques were photographed. The reaction product was then removed with ethanol and the
Fig 4. Consecutive labeling of tissue from 2 nondemented individuals, ages 83 (lefi column) and 90 years (middle column), and
from 1 demented individual, age 70 years (right column) revealed three different stages of plaque maturation. The tissue was Frst
stainedfor P-amyloid fAP) immunohistochemis~(A, L3, and G). The reaction product was then removed, and the sections were
stained f o r butyrylcholinesterase (BChE) histochemisty (C, F, and I). The reaction product was once again removed, and the tissue
was stained for thioflavine S histojluorescence (B, E, and H). Dzfise plaques (A-c)are stained with antibodies directed against
AP (arrow) but f i i l to bind thiojlavine S and contain no BChE activity. Compact nonneuritic plaques (D-fl stain for all three
markers. Single and double arrows mark the same plaques seen with the three different stains. Compact neuritic plaques (G-I)
stain with all three markers and contain dystrophic neurites (arrowhead). Single and double arraws mark the same plaques seen
with the three different stains. These observations show that BChE is almost invariably associated with compact plaques indtpendent
of neuritic state. Scale bar = 100 pm.
slides labeled with thioflavine S and rephotographed.
Quantitative comparisons showed that BChEcontaining plaques were almost invariably thioflavine S
positive (>98%, Table 2). Conversely, in the same areas, nearly all thioflavine S-positive plaques contained
BChE activity (93%). To demonstrate that these
BChE and thioflavine S plaques were AP immunoreactive, some sections were processed consecutively for
all three labels (Fig 4). These experiments demonstrated clearly that BChE- and thioflavine S-positive
plaques were also AP immunoreactive.
In keeping with previously published reports, we found
that deposits of AP-immunoreactive plaques were seen
in the brains of demented as well as nondemented elderly individuals. Amyloid deposits that consisted almost entirely of diffuse (thioflavine S negative) plaques
were found only in tissue from nondemented subjects.
Compact (thioflavine S positive) plaques were seen in
tissue from demented as well as nondemented specimens. Only tissue from AD subjects contained numerous plaques that were both compact and neuritic.
Guillozet et al: BChE in Plaques
These results are consistent with the hypothesis that
AP deposits in the brain undergo three stages of maturation, ie, an initial stage of diffuse amyloid deposits
characterized by the inability to bind thioflavine S, an
intermediate stage of compact but nonneuritic deposits
in which the AP is able to bind thioflavine S, and a
final stage of compact neuritic deposits in which both
AP and dystrophic neurites bind thioflavine S. It is the
neuritic stage that is the most closely associated with
clinical dementia. Because thioflavine S binds to Ppleated material, we can assume that compact plaques
are P-pleated, whereas diffuse plaques are not. PPleated amyloid has been shown in vitro to be more
toxic [63], suggesting that compact amyloid is more
toxic than diffuse amyloid.
We found that BChE-containing plaques were almost invariably of the compact variety. Thioflavine Snegative diffuse plaques did not express BChE, whereas
thioflavine S-positive neuritic as well as nonneutitic
plaques did. This selective association raises the possibility that BChE may be inserted into the plaque many
years after the initial AP deposition and that it may
participate in the maturation of initially inert AP deposits into compact plaques associated with neuritic
degeneration and clinical dementia. Naturally, we cannot rule out the possibility that BChE deposition is a
consequence of plaque maturation. These results are
consistent with our previous work that showed that the
proportional area of amyloid plaques expressing BChE
activity is higher in the AD brain than in age-matched
nondemented control brains [64]. This plaque-bound
BChE appears to be secreted by neuroglia, a process
that can also be invoked to explain the origin of nearly
all companion molecules associated with AP plaques.
The expression of BChE increases substantially in
AD. The normal cerebral cortex contains very little
BChE, located mostly in deep cortical neuron and neuroglia. In AD, cortical BChE increases dramatically
[65], and all histopathological markers of AD, including amyloid plaques, neurofibrillary tangles, and vessels
with amyloid angiopathy, express BChE enzyme activity [55, 64, 66-68]. We have previously shown that
BChE-producing deep cortical neuroglia are more
dense in AD than in control brains, and also in those
areas of control brains that have a greater vulnerability
to the neuronal pathology of AD (eg, entorhinal cortex) than in areas that have a lower vulnerability (eg,
primary somatosensory cortex) [55].
The way in which BChE might influence the pathogenesis of AD remains poorly understood. Incubation
of synthetic AP with low doses of BChE has been
shown to block aggregation of the AP into large fibrils
[69]. The resulting aggregates consist of smaller fibrils
and amorphous aggregates that may be related to an
increase in toxicity [70]. Through such a mechanism,
the presence of BChE may enhance AP neurotoxicity.
916 Annals of Neurology
Vol 42 No 6
December 1997
BChE is a hydrolytic enzyme whose principal physiological function is unknown. It is encoded by a gene
on chromosome 3, is synthesized by the liver, and is
one of the most abundant of all serum proteins. Although BChE and AChE are products of two distinct
genes, both enzymes participate in the hydrolysis of
acetylcholine. In addition to hydrolytic properties, trophic [71-731, cell adhesion [72-741, and peptidasic
175, 761 functions have also been attributed to AChE
and BChE. In fact, we found that the enzymatic activity of BChE associated with plaques and tangles of AD
is specifically inhibited by carboxypeptidase A inhibitors [77]. These nontraditional properties of AChE and
BChE are mediated either through the catalytic triad
involved in hydrolytic functions or through nonenzymatic properties associated with the peripheral sites of
these molecules. Such putative peptidasic or trophic
properties of BChE may participate in the transformation of the amyloid deposit from a diffuse to a compact or from a nonneuritic to a neuritic stage.
Of all the companion molecules that are associated
with the AP plaques (see introductory section), only
BChE has thus far been shown to be present at the
later but not initial stages of plaque maturation, although some complement proteins associated with the
inflammatory cascade may also display a similar temporal relationship. These two molecules may therefore
have a greater likelihood of being causally involved in
the pathogenic transformation of the plaque. It is noteworthy that nonsteroidal anti-inflammatory agents are
thought to plaf an important role in decreasing the
risk of developing AD [78]. It is tempting to suggest
that BChE inhibitors may also play a similar role.
Polymorphisms of BChE are common [79] and it will
be interesting to see if some of these influence the risk
for AD.
Note Added in Proof
An article by Lehmann and associates [80] reported that the
allelic frequency of the gene for the K variant of BChE
(BChE-K) was higher in late-onset AD than in nondemented
controls and non-AD dementias. In this study, the relationship was confined to carriers of the €4 allele of ApoE, suggesting that she BChE-K gene, or a nearby gene in chromosome 3, acts in synergy with ApoE4 as a susceptibility factor
for late-onset AD.
This study was supported in part by an Alzheimer’s Disease Center
granr (LP30 AG13854), NS20285, rhe Alzheimer‘s Association
(IIRG-94-039), and the Pine Family Foundation. The Miami Brain
Endowment Bank is supported by a grant from the NIA (AG
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