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New Orleans: Symposium Probes Why Synapses Are Suffering
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November 2003. Are you sometimes tempted to view APP and presenilin
merely as raw material and machine for Aβ production, respectively? If
so, think again, because that narrow spotlight is opening up to a
broader understanding of how these two proteins function and might
contribute to Alzheimer’s in their own right, as it were. At the annual
meeting of the Society for Neuroscience held earlier this month in New
Orleans, Jie Shen of Brigham and Women’s Hospital in Boston led
a symposium on synaptic changes in Alzheimer’s disease, a topic that
overlaps in large part with new areas of investigation for APP and PS.
The event drew a packed auditorium, in part, perhaps, because the
original Richard Morris of water maze fame was on hand, as
well. (Developed in the 1980s for the study of hippocampal memory, this
mouse swimming pool has become de rigueur for any Alzheimer’s
mouse lab trying to prove their favorite intervention is meaningful in
vivo.) New functions proposed for APP and PS included axonal transport,
formation of the synapse, and synaptic transmission and its related
signal transduction pathways. One provocative upshot of this new
data—briefly debated at the symposium—was that presenilin mutations may
cause memory loss and neurodegeneration not just through a toxic gain
of function—the widely accepted notion—but in a more complex manner
that involves also the loss of separate physiological functions of PS
in learning and memory. Here’s a summary of the symposium
presentations, plus another talk that tied in tau with axonal
transport.
Presenilin: A Memory Molecule?
Shen first reminded the audience that synaptic loss remains the
best early correlate of cognitive impairment in AD, and is accompanied
by a “dying back” of dendrites and axons, and by aberrant sprouting
(for a recent review, see Hashimoto et al., 2003). Of the known familial AD mutations, only 16 are in APP, and they are rare (see APP mutations directory; see also ARF related news story). By contrast, the literature contains about 143 mutations in presenilins 1 and 2 (see PS mutations directory; for the latest additions, see Tedde et al., 2003),
and they are far more common. Do they cause AD through a gain of
function or a loss of function? Speaking for the former are their
dominant inheritance pattern and evidence that they increase Aβ42
levels. At the same time, however, PS mutations are scattered
throughout the coding sequence of the two genes, and this is consistent
with a partial loss of function, Shen said.
Shen took a genetic approach to understand the normal function
of presenilin in the postnatal brain. Since PS 1 and 2 are important
for embryonic development, knockout mice die too early for the study of
these proteins' roles later in life. Shen and colleagues, therefore,
generated a line of neuron-specific conditional PS1 knockout mice,
which are normal anatomically, have lower Aβ production, and show a
surprisingly subtle spatial memory deficit (see ARF related news story).
Next, Shen’s group crossed these conditional PS1 knockouts with
APPswe+717e transgenic mice made in Lennart Mucke’s lab and asked
whether inactivating PS1 could prevent the amyloid pathology in these
mice without side effects. (Besides exploring the normal role of PS,
this experiment mimics genetically a γ-secretase inhibitor treatment
for AD; see also ARF related New Orleans story.)
As expected, these double mutants have neither amyloid pathology nor
the related gliosis and inflammation. However, collaborative studies
with Alfredo Kirkwood’s group at Johns Hopkins University
showed that eliminating PS1 did not rescue the LTP impairment. It did
not completely rescue the behavioral deficits of the APP transgenics,
either, Shen's and Morris' groups found (see also 295.13). The double
mutants temporarily performed better on an associative memory task, but
by the age of six months, had fallen behind again, plus they performed
poorly in spatial reference tests. “This mixed phenotype highlights the
importance of comprehensive testing of AD models using multiple
behavioral paradigms,” said Shen.
Up to this point, PS2 could compensate for PS1, potentially
muddying the results. To shut this escape hatch, Shen’s lab crossed
their conditional PS1 knockout to a PS2 knockout strain from Karen
Duff’s group, creating mice that express PS1 until a month after birth
and then continue life without any presenilin in their excitatory
forebrain neurons. These mice, too, seem fine at two months of age, but
go downhill from there, Shen told the audience. At this age, prior to
any pathology, the mice show mild deficits in associative and spatial
memory, as well as in short-term plasticity and LTP. The latter
prompted the researchers to investigate NMDA receptor-mediated
responses and found reductions there, as well. What’s more, NMDA
receptor and CaMKinase II levels in synaptoneurosome preparations were
reduced. Postdoc Carlos Saura presented these data in a slide talk
(239.2).
By six months, the mice were severely impaired in spatial learning and
memory, failing to learn conditioned fear or the Morris water maze. The
mice also had shrunken white matter, and by nine and 16 months (few
mice survive to this age), they suffer progressive neurodegeneration,
with reductions in synaptophysin levels, dendritic number and
complexity, number of cortical neurons, and cortical volume. Gliosis
accompanies this process, Shen reported. What molecular webs might underlie this profound phenotype?
Shen’s suspicion fell on the CREB/CREM pathway, in part because prior
work has established it as a mediator of synaptic plasticity,
hippocampus-dependent learning, and in part because mice mutant in
these factors show impaired LTP and neurodegeneration (see Pittenger et al., 2002; Chen et al., 2003).
Indeed, at two months, these mice had downregulated a list of genes
whose expression depends on CREB, including BDNF, c-fos, and others.
Total levels of CREB itself or its phosphorylated version were
unchanged, but levels of CREB-binding protein (CBP) were down,
suggesting that presenilin is somehow necessary to maintain normal
levels of CBP. Tau was hyperphosphorylated and p25 upregulated.
Shen invited the audience to ponder a working model for memory
impairment in which problems with presenilin function would, as one of
the earliest steps, lead to impaired synaptic plasticity through a
downregulation of the CREB pathway. While at first blush these data
seem to flatly contradict the amyloid hypothesis, Shen explained that
the story is more complex than that. "There is truth to both sides as
some PS mutations clearly increase Aβb42 generation. I think that,
depending on the downstream targets, PS mutations can lead to the
disease through both loss and gain of function," she concluded.
Richard Morris, of the University of Edinburgh,
Scotland, noted that finding a suitable behavioral model for AD is a
formidable task because the clinical diagnosis of this heterogeneous
disease requires memory impairment plus at least one other cognitive
disturbance, for example aphasia or deficits in decision-making. Even
just the memory component features differences in working, episodic,
remote, and semantic memory, Morris explained, with striking deficits
in episodic memory early in the disease. How can one model that? By
now, numerous mouse models
show behavioral deficits (for example, the Tg2576, App23, CRND8, and
SAMP8 strains), most of which are assessed using the water maze and the
radial arm maze introduced by Gary Arendash and Dave Morgan at the
University of South Florida in Tampa. In earlier studies of the PDAPP mouse, with Guiquan Chen at
the University of Edinburgh and Karen Chen and others at Elan
Biopharmaceuticals in south San Francisco, Morris tried to distinguish
between the ability to encode, store, and retrieve memories (Chen et al., 2000).
To that end, he devised a serial spatial-learning task that involves
longitudinal testing of an animal and allows for the teasing apart of
the different processes of memory. A key result of this work was that
these animals seem to forget what they had learned faster because of a
specific defect in memory retrieval. Morris noted that a decline in
excitatory transmission might underly this effect.
APP: A Synapse Builder?
Hui Zheng of Baylor College of Medicine in Houston presented
data implying a physiological role for APP and some of its family
members in synapse formation. She proposed that impairments in this
function could perhaps also contribute to the synaptic deficits in AD.
Zheng pointed out that Aβ is but one of many APP processing products,
all of which are affected by FAD mutations. She said it remains unclear
whether the synaptic damage in vivo occurs via Aβ, other APP cleavage
products, APP-related signaling pathways, or a combination of these.
Zheng added that other APP family members such as APLP1 or 2 are also
suspects, given that they are abundantly expressed in neurons and found
in dendrites.
Zheng and colleagues have further examined their APP-knockout mouse (Zheng et al., 1995).
In addition to gliosis, an LTP defect, and behavioral deficits in the
water maze, this mouse also moves around less and has a weak grip. The
mice have normal numbers of neurons, but their dendrites were
abnormally short. Then, Zheng crossed APLP2-knockout mice, which appear
normal, with the APP knockouts and found that the offspring died soon
after birth, much like a similar set of mice created in Ulrike Muller’s
lab (Heber et al., 2000).
A growing number of double knockouts of either APP and APLP1 or 2 now
exist, Zheng said, offering a chance to sort out the more subtle
synaptic functions of these proteins.
The locomotor defect and weak grip of the mice inspired Zheng
to study their neuromuscular junctions as a peripheral model of what
might be going wrong with synaptic development in the late embryonic
stage. The scientists found that the double knockouts had fewer
synaptic vesicles on the presynaptic side, as well as a defect in the
proper distribution and positioning of presynaptic vesicles opposite
acetylcholine receptors on the postsynaptic membrane. Slightly later,
these faulty synapses sprouted neurites excessively, Zheng reported.
It’s not clear yet whether the vesicle defect has to do with transport
or some other aspect of vesicle release and recycling, but it is clear
that the neuromuscular junctions were defective in electrophysiological
measures of presynaptic function. This synapse-forming role of
APP/APLP2 is the first in-vivo demonstration of a physiological
function of the APP family of proteins, Zheng said. Whether something
similar happens in central synapses, and whether this could explain the
behavioral and LTP deficits in APP-knockout mice, are open questions at
this point, she added.
Starving Synapses: Axonal Transport a Common Theme?
If the APP family and the presenilins play important physiological
roles at synapses, then surely trouble looms if their delivery dries
up. In this sense, blockages of axonal transport could provide another
mechanism of AD pathogenesis, and indeed, a number of groups have
described axonal transport defects in different neurodegenerative
diseases in recent years (see ARF related news story; ARF news; ARF news). At the symposium, Larry Goldstein
of University of California, San Diego, set the stage by describing
just how enormous a transport burden neurons face. Their “railway”
system of tracks (the microtubules) and engines (the kinesin and dynein
families of motor proteins) shuttle most of the neuron’s biosynthesis
products down the axon and other materials (such as NGF) back up to the
nucleus. Axons are narrow and easily jammed by stalled vesicles;
distances are immense. Goldstein briefly recapped his lab’s prior work
on huntingtin (see ARF related news story) and on Alzheimer’s, including Drosophila studies suggesting that APP serves as an anchor for kinesin (see ARF related news story), which mediates transport of vesicles containing APP, BACE, and γ-secretase to the synaptic terminals (see ARF related news story). This work led to a working model in which APP interacts with kinesin, perhaps with JIP-1b as a scaffold (see Inomata et al., 2003),
to transport a cargo vesicle that contains the APP processing
machinery. Transport defects could be physical, i.e., plugging of the
axon by protein aggregates, and/or could occur in a positive feedback
loop whereby stalling of vesicles would increase local processing of
APP in the axon. Increased APP or impaired kinesin function could also
set off this speculative cycle, as could endocytosis of Aβ, Goldstein
said. Goldstein then presented new experiments in mice that aimed to
test the prediction that APP overexpression should poison axonal
transport by titrating APP away from kinesin binding. Postdoc Gorazd (Goghy) Stokin
first studied the morphology of cholinergic axons in basal forebrain of
APPSwe mice made by Richard Bochelt, and found numerous axonal
swellings, some as large as 10 microns (see also 445.6). In the
electron microscope, these swellings resemble those seen in axons of
APP-overexpressing fruit flies and contain numerous vesicles and
organelles, including mitochondria. Some look as though they are
degenerating. Initial data from an ongoing analysis of human AD tissue
hint that it, too, contains similar axonal swellings, Goldstein added.
Could these axonal swellings be precursors of the dystrophic neurites
that are a hallmark of the AD brain? Next, Stokin crossed the APP transgenic mice to strains
deficient in a kinesin light chain gene to ask whether reducing the
amount of kinesin would disrupt axonal transport in mice as it did in
flies. When he looked for axonal swellings in cortex, he found none in
wild-type mice, some in the APP transgenics, and abundant swellings in
the double-transgenics. To ask whether reduced transport increases APP
processing, the scientists measured amyloid pathology in the
double-transgenic mice and found that reducing kinesin accelerated the
age-related increase in Aβ levels and plaque deposition. In preliminary
experiments, the scientists also saw the distribution of plaque
deposition shift in such a way that the fraction in the entorhinal
cortex (which projects through the perforant pathway to the dentate
gyrus of the hippocampus and is affected early in AD) appeared to
increase relative to deposition in the dentate gyrus (see ARF related news story). While these issues need further study, Goldstein argued that APP processing likely is regulated by kinesin in some way.
And, in turn, kinesin may be regulated by tau. In a separate session in New Orleans, Eva-Maria Mandelkow
talked about a new relationship between tau and the movement of APP
vesicles down the axon (336.6). She proposed that the kinase cascade
MARKK-MARK controls this process by phosphorylating tau and causing it
to come off the microtubules. Her presentation revolved around tau’s
role in limiting vesicle transport through its competition with kinesin
for microtubule binding. Overexpression of tau leads to a
redistribution of organelles back to the cell body and thus starves the
synapse, Mandelkow explained. Her lab has created conditional tau
transgenic mice to study subtle changes early in the disease process,
long before hyperphosphorylated tau detaches entirely from microtubuli,
causing them to disintegrate. While these mice are still too young to
use in studying brain aging, cortical neurons cultured from the mice
yielded some clues. These neurons lose their dendrites and axons, and
quickly die. In normal neurons, anterograde transport of APP-containing
vesicles predominates, but in the tau-overexpressing neurons,
anterograde transport slows down and retrograde transport predominates.
Interestingly, activating the MARKK-MARK pathway in these cells
restored anterograde transport, suggesting that the increased MARK
activity observed in degenerating neurons may be a protective response
by neurons trying to contain tau. Mandelkow believes that even a small
increase in tau may be sufficient to disrupt the flow of traffic to the
synapse (Timm et al., 2003).
Stepping back to take a broader view, Goldstein even wonders if
different neurodegenerative diseases could reflect divergent outcomes
of a common dysfunction such as axonal transport, much like different
types of cancer are all outcomes of common problems with cell cycle
control. A lot of future work is needed to test this leap, some of it
being a search for kinesin SNPs that cause subtle differences in
kinesin activity and could, over time, impair axonal transport. You can
view abstracts mentioned in this story at the SfN/ScholarOne website.—Gabrielle Strobel.
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Comments on Related News |
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Related News: Target BACE: Better Than Ever?
Comment by: Fred Van Leuven, ARF Advisor (Disclosure)
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Submitted 10 January 2004
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Posted 10 January 2004
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This is exactly what we predicted (Dewachter and Van Leuven, 2002) or, as the Americans would put it, "what the doctor ordered…"
The overall message of this study is loud and clear: BACE is hereby
proven to be the favorite target for drug-makers. That message rings
even more clearly because γ-secretase inhibitors now prove bad not only
for the brain, as we predicted, as well (Dewachter et al., 2002),
but also in vivo for the immune system, the intestine, and likely for
any, or even all, biological subsystems in our complex bodies that
depend on intramembranous proteolysis for essential functions (Wong et al., 2004).
Some points remain somewhat worrying or startling in this study.
First, why not use the "classic" cognitive test, i.e., the water maze?
Second, why not measure "classic" LTP instead of the somewhat exotic
cholinergic AHP? Third, and most important, is the lack of any data on
APP biochemistry. I, for one, would want to know what happens to APPs-α
and to the α-CTF; the discussion circumvents this issue. Indeed, the
lowering of Aβ by BACE-deficiency is spectacular and likely to be a
major contributor to the "rescue." Nevertheless, APPs-α is claimed by
many to be beneficial for brain, but only circumstantially demonstrated
to be essential for "neuronal well-being." Here was a chance to increase the experimental proof for that action.
References:
Dewachter I, Van Leuven F. Secretases as targets for the treatment of
Alzheimer's disease: the prospects. Lancet Neurol. 2002
Nov;1(7):409-16. Review. Abstract
Dewachter I, Reverse D, Caluwaerts N, Ris L, Kuiperi C, Van den
Haute C, Spittaels K, Umans L, Serneels L, Thiry E, Moechars D, Mercken
M, Godaux E, Van Leuven F. Neuronal deficiency of presenilin 1 inhibits
amyloid plaque formation and corrects hippocampal long-term
potentiation but not a cognitive defect of amyloid precursor protein
[V717I] transgenic mice. J Neurosci. 2002 May 1;22(9):3445-53. Abstract
View all comments by Fred Van Leuven
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Related News: Target BACE: Better Than Ever?
Comment by: Karen Chen, Dione Kobayashi (Disclosure)
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Submitted 10 January 2004
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Posted 10 January 2004
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By Dione Kobayashi and Karen Chen
The paper by Disterhoft et al. reporting cognitive and cholinergic rescue with their
BACE1 knockout mice on a mutant hAPP background is extremely exciting and is a
strong validation of BACE inhibitory strategies for Alzheimer's disease therapeutic
efforts. Other recent works over the past three years have also reported on the
behavioral effects of complete genetic removal of BACE1 from mice (Harrison et al., 2003;
Kobayashi et al., in SFN abstracts 2002, 2003). In addition to their
novel cholinergic function results, Disterhoft et al. found mild
phenotypes in
exploration in BACE1 -/- mice similar to other groups, although there
is some discrepancy regarding the ability of BACE1 deletion to rescue
cognitive
deficits due to overexpression of mutant hAPP.
It must be emphasized that not only do the various BACE1 -/- mice
differ in their hAPP mutations, level of APP overexpression, and thus
their subsequent cognitive
deficits, but these mice also have been subjected to widely divergent
behavioral tasks.
While the Y maze and social recognition tasks used by Disterhoft et al.
are both nonaversive and rely on hippocampal function, these tasks also
depend on the
motivational state of the animals, which is admittedly impaired in
spontaneous
exploration. In studies done by our group, BACE1 -/- PDAPP mice were tested in an
aversive serial spatial memory water maze paradigm that utilizes working memory
(Chen et al., 2000).
This modified water maze protocol was in effect designed to detect
subtle spatial impairments in the PDAPP mouse that could be missed when
using other spatial memory tasks, including the classic reference
memory task described by Morris in 1981. While we do report significant
progressive spatial memory deficits in our BACE1 -/- PDAPP mice, their
impairments can also be
viewed as subtle and specific. Thus, our seemingly opposing BACE1 -/-
hAPP
data may simply represent the dynamic range of changes in BACE1 -/-
hAPP mice
tested with different cognitive tasks. Alternatively, these results may
be entirely
independent, underlining the murkiness inherent in working with
transgenic animal
models. It will be interesting to watch this promising line of research develop,
as I agree with the authors wholeheartedly that further study with other
transgenic mouse lines, as well as other behavioral assessments, will be
illuminating, particularly if a conditional BACE1 -/- is developed like that
reported with presenilin-1 (Yu, 2001).
In addition, the mild behavioral phenotypes consistently reported with
BACE1 -/- alone should not be overlooked from a perspective of
understanding the role of the constituents of the APP
processing pathway in normal learning and memory.
View all comments by Karen Chen
View all comments by Dione Kobayashi
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Related News: Target BACE: Better Than Ever?
Comment by: Michael Irizarry
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Submitted 10 January 2004
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Posted 10 January 2004
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This
elegant report further supports BACE1 as a rational therapeutic target
for treating the cerebral amyloidosis of Alzheimer's disease (AD). Ohno
and colleagues eliminated BACE1 function in a mouse model of AD by
crossing Tg2576 mice—which overexpress the Swedish mutant of amyloid
precursor protein (APPSwe)—with BACE1 knockout mice. They found that
genetic elimination of BACE1 blocked cerebral amyloid β-protein (Aβ)
production, ameliorated the cognitive deficits of the APP-transgenic
mice in hippocampal-based learning tasks, and improved the cholinergic
electrophysiologic deficits in hippocampal slice preparations.
The Tg2576 APPSwe mouse model of Alzheimer's disease develops
cerebral amyloid deposits by the age of nine to 11 months [1]. In the
current study, soluble Aβ in brain was increased by 25-fold relative to
nontransgenic mice at four to six months—the age at which cognitive
deficits and electrophysiologic deficits were detected. The findings
that certain cognitive [2] and electrophysiologic deficits occur prior
to cerebral amyloid deposition suggest that soluble species of Aβ
adversely affect physiologic processes in the brain. The rapid
amelioration of behavioral deficits in APP-transgenic mice by passive
immunization with anti-Aβ antibodies [3,4] further supports soluble
forms of Aβ as a functionally toxic species in these mice. Ohno and colleagues tested the effects of eliminating brain Aβ
in the Tg2576 APPSwe mice by knockout of BACE1, which is the principle
enzyme responsible for the β-secretase cleavage of APP, the initial
step of Aβ synthesis. Overexpression of BACE1 accelerates Aβ production
in mouse brain [5,6], and BACE1 protein levels and enzymatic activity
are increased in the AD brain [7-9]. Targeting BACE1 therapeutically is
potentially less toxic than reducing γ-secretase activity since BACE1
knockout mice do not exhibit overt neurological or medical problems
[10]. (However, BACE1 null mice alone did exhibit impaired performance
in a spatial working memory task, a deficit that resolved in the
APPSwe/BACE1 null mice [1]). This paper demonstrates that behavioral
and electrophysiologic deficits in APPSwe mice can be ameliorated by
elimination of Aβ production in the APPSwe/BACE1 null mice, implicating
Aβ (or, less likely, β-cleaved APP fragments) as the source of these
deficits.
While very encouraging for BACE1 therapeutic strategies, the
usual caveats regarding translating behavioral tests in mice to the
cognitive deficits in human AD apply. Cognitive deficits in AD are most
consistently associated with the development of neuronal loss and
neurofibrillary tangle formation in corticolimbic regions, pathological
features which are not reproduced in the Tg2576 APPSwe mice. Prolonged
exposure to high levels of toxic forms of Aβ may initiate distinct
downstream effects in mice (behavioral deficits, cholinergic
electrophysiological deficits) and humans (neuronal and synaptic loss,
cholinergic deficits, neurofibrillary tangle formation). The amyloid
hypothesis postulates that reducing Aβ would prevent these downstream
effects; the results presented in this paper support this idea in the
case of certain deficits in the APPSwe transgenic mice.
References
1. Hsiao K, Chapman P, Nilsen S, Eckman C,
Harigaya Y, Younkin S, Yang F, Cole G. Correlative memory deficits,
Abeta elevation, and amyloid plaques in transgenic mice. Science. 1996
Oct 4;274(5284):99-102. Abstract
2. Westerman MA, Cooper-Blacketer D, Mariash A, Kotilinek L,
Kawarabayashi T, Younkin LH, Carlson GA, Younkin SG, Ashe KH. The
relationship between Abeta and memory in the Tg2576 mouse model of
Alzheimer's disease. J Neurosci. 2002 Mar 1;22(5):1858-67. Abstract
3. Dodart JC, Bales KR, Gannon KS, Greene SJ, DeMattos RB,
Mathis C, DeLong CA, Wu S, Wu X, Holtzman DM, Paul SM. Immunization
reverses memory deficits without reducing brain Abeta burden in
Alzheimer's disease model. Nat Neurosci. 2002 May;5(5):452-7. Abstract
4. Kotilinek LA, Bacskai B, Westerman M, Kawarabayashi T,
Younkin L, Hyman BT, Younkin S, Ashe KH. Reversible memory loss in a
mouse transgenic model of Alzheimer's disease.
J Neurosci. 2002 Aug 1;22(15):6331-5. Abstract
5. Bodendorf U, Danner S, Fischer F, Stefani M,
Sturchler-Pierrat C, Wiederhold KH, Staufenbiel M, Paganetti P.
Expression of human beta-secretase in the mouse brain increases the
steady-state level of beta-amyloid. J Neurochem. 2002
Mar;80(5):799-806. Abstract
6. Mohajeri MH, Saini KD, and Nitsch RM. Transgenic BACE
expression in mouse neurons accelerates amyloid plaque pathology. J
Neural Transm 2003.
7. Holsinger RM, McLean CA, Beyreuther K, Masters CL, Evin G.
Increased expression of the amyloid precursor beta-secretase in
Alzheimer's disease. Ann Neurol. 2002 Jun;51(6):783-6. Abstract
8. Fukumoto H, Cheung BS, Hyman BT, Irizarry MC. Beta-secretase
protein and activity are increased in the neocortex in Alzheimer
disease. Arch Neurol. 2002 Sep;59(9):1381-9. Abstract
9. Yang LB, Lindholm K, Yan R, Citron M, Xia W, Yang XL, Beach
T, Sue L, Wong P, Price D, Li R, Shen Y. Elevated beta-secretase
expression and enzymatic activity detected in sporadic Alzheimer
disease. Nat Med. 2003 Jan;9(1):3-4. No abstract available. Abstract
10. Luo Y, Bolon B, Kahn S, Bennett BD, Babu-Khan S, Denis P,
Fan W, Kha H, Zhang J, Gong Y, Martin L, Louis JC, Yan Q, Richards WG,
Citron M, Vassar R. Mice deficient in BACE1, the Alzheimer's
beta-secretase, have normal phenotype and abolished beta-amyloid
generation.
Nat Neurosci. 2001 Mar;4(3):231-2. No abstract available. Abstract
View all comments by Michael Irizarry
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Related News: Target BACE: Better Than Ever?
Comment by: Hui Zheng
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Submitted 12 January 2004
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Posted 12 January 2004
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Ohno
et al. bred the BACE1 knockout (BACE1-/-) mice onto the Tg2576 APP
transgenic background and tested the effects of BACE1 deficiency on Aβ
production, behavioral performance, and cholinergic function at a young
age (4-6 months), prior to amyloid plaque deposition. The authors
showed, quite convincingly, that inhibition of Aβ production, as a
result of BACE1 deficiency, rescued the behavioral deficit and
cholinergic impairment present in Tg2576 transgenic mice.
This result has several important implications: 1) It lends strong
support to the amyloid hypothesis; 2) it strengthens the notion that
BACE1 is a valid therapeutic target for AD intervention; and 3) it
establishes that the behavioral abnormality seen in Tg2576 mice is
caused by APP processing/Aβ production rather than APP overexpression.
However, as the authors pointed out, BACE1 deficiency leads not
only to inhibition of Aβ, but also to changes in other APP fragments
(e.g., β-CTF). Therefore, a definitive link between Aβ and functional
deficits cannot be established. In addition, the therapeutic potential
of BACE1 inhibitors has to be interpreted with caution since BACE1
knockout mice alone show impaired performance in the Y maze test (Fig.
1B). Although this impairment is neutralized on the APP-overexpressing
background, it represents an artificial condition as AD individuals do
not overexpress APP. In this regard, it is prudent to test the BACE1
knockout and BACE1-/- Tg2576 animals in other behavioral paradigms,
such as the Morris water maze or fear conditioning.
View all comments by Hui Zheng
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Related News: Target BACE: Better Than Ever?
Comment by: Jie Shen
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Submitted 12 January 2004
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Posted 12 January 2004
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This is an interesting paper, and these mice are very useful for addressing a number of issues.
The behavioral paradigms the authors chose to use are not the
strongest learning and memory tests available; more robust and better
established hippocampal-dependent learning and memory paradigms, such
as the Morris water maze and the contextual fear conditioning tests,
might have been preferable. This could be the reason that in the
spontaneous alternation Y maze, even the Tg2576 APP-transgenic mice did
not perform that poorly compared to the control. More importantly, the
Y maze results require more explanation: I wonder how the double-mutant
mice could have behaved "normally" while BACE-/- and APP-Tg mice
performed poorly (Figure 1B)? It seems a bit premature to conclude
"rescue" from such results.
Without going into details of the physiology result, Figure 2C
seems to show lower values in the double-mutant group compared to the
control, as well. Nevertheless, it is interesting and welcome that the
authors looked at the cholinergic input.
Overall, from the data shown in the paper, it appears that the
story is more complicated, and more thought-provoking, than a simple
rescue story. BACE-/- mice appear to exhibit a memory deficit (due to
loss of Aβ or other BACE
substrates?) in one behavioral test but not the other, whereas
BACE-/-xTg2586 bigenic mice appear to perform better than BACE-/- mice
(due to ???). If this result can be confirmed by other behavioral
tests, it warrants further investigation to characterize the underlying
mechanism.
Together with APP-transgenic mice crossed into the PS1
conditional KO background, the Ohno et al. mice stand to be great
models to tease out the different contributions of various fragments of
APP in brain function. This would require crossing the same APP
transgenic line into either PS1 cKO or BACE null background. More
importantly, a battery of robust learning and memory tests should be
used to test these groups of mice together using identical protocols;
only then we can compare results and make meaningful conclusions.
View all comments by Jie Shen
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Related News: Target BACE: Better Than Ever?
Comment by: Philip Wong
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Submitted 12 January 2004
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Posted 13 January 2004
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Previous
studies showed that the deletion of BACE1 abolished the production of
Aβ and BACE1 knockout mice are apparently normal. In this current
paper, Ohno et al tested whether cognitive deficits occurring in the
mutant APP mice (Tg2576) can be ameliorated in the absence of BACE1,
results that have important implication for the potential therapeutic
value of BACE1 in AD. Since developmental cognitive (as assessed by
either social recognition task or spontaneous alternation in Y maze)
and electrophysiological (hippocampal cholinergic dysfunction)
abnormalities occur prior to Ab deposition in 4-6 months old Tg2576
mice, Ohno et al. elected to examine whether such memory deficits can
be rescued in Tg2576 mice lacking BACE1. Their results demonstrating
that the deletion of BACE1 prevented these early onset behavioral
abnormalities strongly support their conclusion that increased levels
of Aβ (as opposed to increased APP levels) causes the cognitive
deficits occurring in Tg2576 animals. These authors also interpreted their findings to support the view
that the inhibition of BACE1 has therapeutic value in reversing
AD-associated cognitive deficits. However, it will be critical to test
whether age-associated spatial memory deficits (as assessed by Morris
water maze) occurring in aged Tg2576 mice can be rescued in the absence
of BACE1. It is unclear at present whether Tg2576 mice lacking BACE1
will display age-associated spatial memory abnormalities; if they
don't, such a positive outcome would strongly favor the idea that BACE1
inhibitors have the potential to ameliorate age-associated cognitive
deficits occurring in AD.
It is interesting to note that young (4-6 months of age) BACE1
knockout mice exhibit poor performance in spontaneous alternation in
the Y maze, a deficit that is also observed in young Tg2576 mice. The
reasons for this deficit are unclear. Because Tg2576 mice apparently
can rescue this cognitive abnormality in BACE1 null mice and the levels
of Aβ peptides in Tg2576;BACE1-/- mice were claimed to be similar to
wild-type levels, these authors infer that the lack of Aβ in BACE1 null
mice presumably is responsible for the behavioral deficits in the Y
maze. This interpretation would raise the critical issue as to the
origins of Ab peptides in the Tg2576;BACE1-/- mice. One uncertainty is
whether the Ab detected from brain extracts by the sandwich ELISA came
from Aβ peptides derived from the processing of APP, or from
APP-related fragments containing epitopes recognized by the antibodies
used. That no Ab peptides can be detected even when APP wild-type or
APPSwe is expressed highly in BACE1-/- neurons, as previously
demonstrated, would support the latter possibility. In this case, the lack of Aβ may not be sufficient to explain
the behavioral deficits observed in these young BACE1 knockout mice. It
is plausible that the lack of Aβ could affect other substrates of BACE1
to elicit the poor Y maze performance in the BACE1 null mice. However,
because the behavioral abnormality observed in the BACE1 knockout mice
can be rescued by the expression of APP in Tg2576 mice, this deficit is
more likely related to the APP pathway rather than due directly to
other BACE1 substrates. At any rate, further studies are necessary to
clarify the behavioral abnormalities observed in the BACE1 null mice. These results raise the possibility that the therapeutic
inhibition of BACE1 may not necessarily be free of mechanism-based
toxicities, particularly if age-associated behavioral abnormalities are
observed in BACE1-deficient mice. Thus, it would be critical to
determine whether age-associated spatial memory deficits are observed
in aged BACE1 null animals. Furthermore, the behavioral analysis of
conditional BACE1 knockout mice should further clarify this important
issue.
In summary, while the paper by Ohno et al provides compelling
evidence that early-onset cognitive and cholinergic abnormalities
occurring in young Tg2576 mice can be rescued by deleting BACE1,
critical issues regarding the impact of BACE1 on age-associated
cognitive deficits in aged mice remain to be established, as these
results will have important implications for the development of
potential therapeutics designed to inhibit BACE1 and ameliorate Aβ
amyloidosis in AD.
View all comments by Philip Wong
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Related News: Target BACE: Better Than Ever?
Comment by: Remi Quirion
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Submitted 14 January 2004
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Posted 14 January 2004
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This new report by Ohno et al. demonstrates further that BACE1, an
enzyme involved in the maturation of the APP precursor and the
generation of amyloid peptides, is a potential therapeutic target
toward the treatment of Alzheimer's disease. In mice in which the BACE1
gene was deleted, the overexpression of the human APP-695 Swedish
familial mutation failed to result in memory deficits and altered
cholinergic functions. Hence, the expression of BACE1 resulting in the
production of pathogenic amyloid peptides is apparently key to inducing
cognitive and neurochemical deficits in the model studied. Together,
these data suggest that BACE1 inhibitors could prove useful in the
treatment of AD by reducing the production of amyloid peptides and
ensuing cholinergic deficits and learning impairments. Of course, the
safety of such inhibitors would have to be established, but data
obtained in the mouse model are promising. Moreover, these data link
some of the key features of the AD brain, including amyloid peptides,
cholinergic dysfunction, and memory deficits. It would now be of interest
to investigate the effects of clinically used acetylcholinesterase
inhibitors in this animal model, as well as subtype selective
nicotinic and muscarinic receptor ligands.
View all comments by Remi Quirion
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Related News: Target BACE: Better Than Ever?
Comment by: Mary Reid
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Submitted 17 January 2004
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Posted 20 January 2004
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Jeon
et al. (1) suspect that the pyrogallol moiety on C-2 and/or C-3 of the
catechin skeleton is responsible for the increased inhibition of BACE1
by these green tea catechins.
It seems of interest that Bain et al. (2) have found that
epigallocatechin-3-gallate inhibits DYRK1A, one the the genes
considered responsible for the mental retardation of Down's syndrome.
Could we expect that therapeutic intervention with epigallocatechin 3-gallate may be beneficial for those with Down's syndrome?
Basi et al. (3) find that BACE2 suppresses Abeta production in cells that also express BACE1.
Motonaga et al. (4) report increased BACE2 levels in those with
Down's syndrome with Alzheimer's-type pathology and suggest that BACE2
is involved in this neuropathology.
May there be reason to expect that the increased BACE2 may actually be beneficial?
References:
1. Jeon SY, Bae K, Seong YH, Song KS. Green tea
catechins as a BACE1 (beta-secretase) inhibitor. Bioorg Med Chem Lett.
2003 Nov 17; 13(22): 3905-8. Abstract
2. Bain J, McLauchlan H, Elliott M, Cohen P. The specificities
of protein kinase inhibitors: an update. Biochem J. 2003 Apr 1; 371(Pt
1): 199-204. Abstract
3. Basi G, Frigon N, Barbour R, Doan T, Gordon G, McConlogue L,
Sinha S, Zeller M. Antagonistic effects of beta-site amyloid precursor
protein-cleaving enzymes 1 and 2 on beta-amyloid peptide production in
cells. J Biol Chem. 2003 Aug 22; 278(34): 31512-20. Epub 2003 Jun 11. Abstract
4. Motonaga K, Itoh M, Becker LE, Goto Y, Takashima S. Elevated
expression of beta-site amyloid precursor protein cleaving enzyme 2 in
brains of patients with Down syndrome.
Neurosci Lett. 2002 Jun 21; 326(1): 64-6. Abstract
View all comments by Mary Reid
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Related News: MARK Homologue Sparks Tau Terror in Fruit Fly
Comment by: Gerard Drewes
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Submitted 7 March 2004
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Posted 8 March 2004
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This paper describes an intriguing Drosophila
model of tau phosphorylation causing tau neurotoxicity. So far,
therapeutic approaches to tau pathology in AD did not progress beyond
the preclinical stage and were mainly directed at the inhibition of the
CDK5 and GSK3 kinases. However, the MARK pathway may offer more
promising targets. We and others have recently shown that MARKs are
activated by LKB1/Par-4 [1,2]. This may represent a neurotoxic signal
which is not specific for AD pathology, since it was just shown that
both LKB1 and MARK4 become rapidly upregulated in a murine stroke model
[3]. Since confirmation of the Drosophila model by mouse
knockouts may be difficult due to the presence of four MARK
genes—whereas flies possess only a single gene—we may need to await the
development of specific MARK inhibitors, and see whether these are able
to inhibit P-tau (and Aβ-?) induced neuronal cell death.
References:
1. Brajenovic M, Joberty G, Kuster B,
Bouwmeester T, Drewes G. Comprehensive proteomic analysis of human Par
protein complexes reveals an interconnected protein network. J Biol
Chem. 2003 Dec 15 [Epub ahead of print] Abstract
2. Lizcano JM, Goransson O, Toth R, Deak M, Morrice NA, Boudeau
J, Hawley SA, Udd L, Makela TP, Hardie DG, Alessi DR. LKB1 is a master
kinase that activates 13 kinases of the AMPK subfamily, including
MARK/PAR-1. EMBO J. 2004 Feb 25;23(4):833-43. Abstract
3. Schneider A. et al. Identification of regulated genes during
permanent focal cerebral ischaemia: characterization of the protein
kinase 9b5/MARKL1/MARK4. J Neurochem. 2004 Mar 1; 88 (5)1114-26.
View all comments by Gerard Drewes
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Related News: MARK Homologue Sparks Tau Terror in Fruit Fly
Comment by: Fred Van Leuven, ARF Advisor (Disclosure)
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Submitted 12 March 2004
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Posted 12 March 2004
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This
excellent paper draws renewed attention to the (other) central problem
in neurodegeneration in general and AD in particular: How does the tau
pathology originate? This essentially boils down to the question of
what is the initial kinase, i.e., the kinase that triggers the
phosphorylation that eventually results in hyperphosphorylation of tau
and instigates the deadly cascade ending in paired helical filaments,
neurofibrillary tangles and cell death. In that respect, tau is
definitely the prime suspect and candidate "executer" of neurons in
many neurodegenerative disorders, including AD. The pathological
definition of AD as "plaques + tangles" does not allow or permit the AD
field to escape this problem, despite the fact that amyloid attracts 10
times (my wild guess) more attention than tau. Through the work of the Mandelkow lab and many others, the
functions of MARK kinase have been defined in some detail, in terms of
phosphorylating tau and other MAPs, and in terms of neurite outgrowth
and polarization. What was missing was a definite link to pathology,
and that is provided by this paper. The authors define PAR1 kinase as
responsible for phosphorylation of serines 262 and 356 in tau, thereby
causing cells to die. Many studies have indirectly implicated MARK,
GSK-3, PKA, and CaMKII in phosphorylating these sites, but no animal
study is yet available to validate these kinases as the physiological
kinase for these sites.
So, are "S262 and S356" going to be magical for tau pathology
as "β and γ" are for amyloid pathology? At the least, these serine
residues are located in the region of tau that matters most, i.e., the
microtubule-binding domain, which, incidentally, is also the region
that is littered with mutations giving rise to the family of
tauopathies known as FTD, or frontotemporal dementia! Antibody 12E8
specifically detects pS262 and pS356 in the MTBD of tau (Seubert et al., 1995),
and it is definitely going to be popular and in demand among tauists
and perhaps baptists. As always, many questions and some caveats
remain. For one, the authors use not wild-type human tau, but the FTD
mutant tau-R406W. This might explain why the final outcome is cell
death but no tau aggregates in whatever form. Moreover, the choice of
this mutant might have been fortuitous, since it is the most C-terminal
of all known clinical mutations, closely positioned to the AD2/PHF1
epitope that is important in binding to MT and in tau aggregation (Spittaels et al., 2000;
Vandebroek et al., 2004,). Further along the path to understand it all
remains the question why no mice have yet been produced (or reported)
with overexpression or deficiency of MARK? Those who have such mice
available should come forward and inform the community what and how and
where …
View all comments by Fred Van Leuven
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