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Deficiency of Presenilin 1 Inhibits the Normal Cleavage of the Transmembrane Domain of Amyloid Precursor Protein
Deborah Watson and Kenneth Kosik led this live discussion on 30 January 1998. Readers are invited to submit additional comments by using our Comments form at the bottom of the page. Paper Under Discussion: De Strooper B, Saftig P, Craessaerts K, Vanderstichele H, Guhde G, Annaert W, von Figura K, Van Leuven F. Deficiency of Presenilin 1 Inhibits the Normal Cleavage of the Transmembrane Domain of Amyloid Precursor Protein.
Nature. 1998 Jan 22;391(6665):387-90. Abstract
 View Transcript of Live Discussion — Posted 15 February 1998
Background Text
By Deborah Watson and Kenneth S. Kosik. Dr. Bart De Strooper's responses are listed below each question.
Many independent observations have converged around the conclusion that
mutant presenilin molecules alter APP processing at the gamma-secretase
site. In particular, increased production of Aβ ending at residue Ala-42
appears to contribute to Alzheimer's disease pathology in patients carrying
any one of the numerous identified hereditary mutations in the PS1 or PS2
genes. The mechanism by which mutant presenilin causes this increase is
a matter of great interest, and the demonstration that APP co-precipitates
with presenilin suggests that a significant interaction between the PS and
APP in intracellular membranes - with the possible involvement of other
binding partners such as β or delta-catenin - may be occurring.
This concise and timely paper by de Strooper et al demonstrates
that presenilin is required for gamma-secretase processing of APP (both
the 40- and the 42-cleaving activities). The authors examined both endogenous
mouse APP and virally-introduced human APP in neuronal cultures of presenilin
1 knock-out mouse embryos and showed that levels of both Aβ and p3 dropped
sharply. The precursors of these two fragments (the p10 and p12 membrane-associated
C-terminal stubs of APP) accumulated 2- to 5-fold in cell membranes, consistent
with an arrest of gamma-secretase cleavage of APP. Both alpha- and β-secretase-cleaved
forms of secreted APP (APPs) were present at normal levels.
Discussion Questions and Dr. De Strooper's Responses
Question 1 Intriguing as the paper is, it still does not identify the mechanism
by which mutant presenilin causes increased Aβ42 production. Some hypothetical
roles for PS include: trafficking molecule; positive regulator of gamma-secretase
activity; or gamma-secretase itelf. What are others? How would they tie
in to the findings of De Strooper et al?
Bart De Strooper replies: See my answer to Dr. Robakis's comment
Question 2 Previous work has shown that the over-expression of human mutant PS1
in mice increases Aβ42 levels even in the presence of the endogenous
mouse PS1. For this reason and others PS mutations have been considered
gain of functions. How do these new observations fit with the gain of function
idea and does the observation that neurons from +/- heterozygous knockout
animals show no change in APP processing offer any additional insights?
Response: Since absence of the PS1 gene inhibits gamma secretase activity, it
seems logical to conclude that the increased Aβ42 production observed
with the clinical mutations is a gain of function. However, it is a
selective gain of function, since the mutations only increase Aβ42,
not Aβ40. It could be that the mutations cause a slight change in the
way PS1 is presenting/exposing APP to gamma secretase. It seems more
difficult to understand this result if the PS1 knockout effects are
interpreted as being the result of an APP transport deficiency. The
statement that in the +/- animals no effects are observed with APP
processing is not entirely correct, since we see a small decrease in
amyloid peptide production in these cultures as well.
Question 3 Knockout of presenilin 1 reduces gamma-secretase processing of APP,
but does not abolish it completely. Is it reasonable to expect that endogenous
PS2 might be responsible for the residual activity?
Response: That is what we suggested. The PS2 knockout, and double PS1/PS2 knockout
will give us an answer to this question. However, it is also possible
that we are looking here at the basal activity of gamma secretase
Question 4 Though multiple labs have demonstrated an interaction between APP
and PS, other labs have contested this finding. Does the decrease in gamma-secretase
processing of APP in PS-knockout neurons reflect on this issue?
Response: In my opinion our results at least suggest that there is a temporary/
functional interaction of PS1 with APP(fragments). On the other hand our
data demonstrate that PS1 is not needed for the normal maturation of APP
and that it is only later in the biosynthetic pathway (after alfa- and
β-secretase cleavage) that PS1 plays its role. The cited
publications indicate that it is mainly "immature" APP that binds to PS1
which is somewhat difficult to conciliate with the findings in our
publication.
Question 5 If we conclude that PS1 can in some way regulate gamma-secretase activity,
what is the role of the endoproteolytic cleavage in this activity?
Response: Interesting question, no answer.
Question 6 Both 40- and 42-cleaving activities were inhibited equally in the
cells lacking presenilin. Does this shed any light on whether the activities
arise from the same or different enzymes?
Response: The data suggest at least that PS1 influences both gamma secretase
activities.
Question 7 The search for proteins that interact with presenilin will now surely
intensify. What technologies are available for discovering proteins which
interact within their transmembrane domains?
Response: I would like to refer to a recent paper by Rawson, R.B., Zelenski, N.G.,
Nijhawan, D., Ye, J., Sakai, J., Hasan, M.T., Chang, T.Y., Brown, M.S.,
Goldstein, J.L. (1997) Complementation cloning of S2P, a gene encoding a
putative metalloprotease required for intramembrane claevage of SREBPs.
Molecular Cell, 1, 1-20, 1997
Question 8 Could inhibitors of presenilin activity serve as an effective therapeutic
for Alzheimer's disease?
Response: We should be careful here:
- We do not know whether PS1 function is essential in adult brain.
- We do not know whether it is sufficient to simply decrease amyloid
peptide production to stop Alzheimer's Disease (this is still a
hypothesis, not a fact).
- As we demonstrated, the inhibition of PS1 results not only in a
decrease in amyloid peptide, but also in an increase in carboxyterminal
APP fragments. I refer to the review of Dr. R. Neve on this site for
further reflections on the possible role of these fragments in the
pathogenesis of AD.
Bart De Strooper, MD, PhD
Nikolaos Robakis responds: The report by De Strooper et. al.,
(1),and the accompanying commentary by Haas and Selkoe (2), conclude
that PS1 may directly modulate g-secretase cleavage and Ab production,
and therefore, inhibition of its function in adult brain may serve as a
suitable therapeutic target in AD. The controversy over the role of Ab
peptides in the etiology of AD notwithstanding (3), a more parsimonious
explanation of the observed partial inhibition of Ab production in the
PS1-/- cells (1), is that in these cells transport of C-terminal
fragments, (the substrates for g-secretase) to the g-secretase
compartment is inhibited. This explanation would be in agreement with a
suggested role of PS1 in membrane protein trafficking(4), and with its
presence in transport vesicles(5). It also suggests that in addition to
APP, PS1 may modulate transport of a host of membrane proteins,
consistent with the embryonic lethality of the PS1-/- phenotype. If so,
then inhibition of PS1 function may do more harm than good for AD
patients.
Nikolaos K. Robakis
Professor
Mount Sinai School of medicine
New York, Ny 10029
Response to Dr. Robakis: I agree with the comments
of Dr. Robakis. We, and also Dr. C. Haass and Dr. D. Selkoe,
discuss explicitly this alternative interpretation of
our results . The analogy with SREBP/SCAP is however striking
and lessons could be drawn from this system that are very
interesting for the Alzheimer field. The recent identification
of the S2P proteinase which cleaves SREBP in the transmembrane
domain shows us for instance how a gamma-secretase could
look like.
Ken Kosik asks: Your paper shows a suggestive change in Fig. 1 g and
it could be important for the mechanism.
Bart De Strooper replies: There is indeed a suggestive effect in the
PS1+/- cultures. We did not do sufficient experiments to demonstrate
that in a statistically significant way.
Ken Kosik asks: I'd like to revisit a previous question -- Bart, why is there the discrepant responses of Aβ in the supernatant where you see a nice effect vs the Aβ in cell extracts where the effect looks minimal?
Bart De Strooper replies: This is an interesting question, and we are
puzzled about it. we are investigating this further using different APP
mutants.
Ken Kosik asks: "Bart, could you reiterate why you believe PS1 mutants are gain of function?"
Bart De Strooper replies: Well, simply spoken: when PS1 is absent,
amyloid peptide production goes down; when the mutations are there,
amyloid peptide production goes up. Of course it is a "selective" gain
of function, since only the 42 peptide goes up.
Ken Kosik asks: Isn't it unusual that so many different mutations in PS1 all lead to gain of function?
Bart De Strooper replies: This is indeed surprising, and I would suggest
that the mutations all cause a small conformational change in PS1 which
changes slightly the way in which PS1 presents APP to gamma secretase.
Carmela Abrahams asks: Do Bart's results mean that gamma secretase
cleavage occurs in ER or Golgi, thus Aβ is completely formed there?
Bart De Strooper replies: I am not sure. An alternative interpretation
of our data is that PS1 controls a transport step, eg the transport of
gammasecretase from the endoplasmic reticulum to an undefined
compartment downstream in the secretory or endocytic pathway, were
interaction with the APP fragments could occur.
Jfloring asks: Is there any reason to imagine that the overexpression
of APP may have led to the PS-1 effects? That is, what's going on with
normal APP processing? Is the double knockout plus transgene on
anyone's list?
Bart De Strooper replies: On the first part of the question: we
confirmed most of our results in a qualitative way on the endogenously
expressed mouse APP (see first figure of our paper). On the second part
of the question: the double knock out is certainly on our list. I do not
see directly whether the rescue with a clinical PS mutant could learn us
more than what we know already.
Selected References
Xia, W. et al (1997) Interaction between amyloid precursor protein and
presenilins in mammalian cells: Implications for the pathogensis of Alzheimer
disease. Proc. Natl. Acad. Sci. USA 94: 8208-13. 1997 Jul 22. Abstract.
Weidemann, A. et al (1997) Formation of stable complexes between two
Alzheimer's disease gene products: Presenilin-2 and β-amyloid precursor
protein. Nature Medicine 1997 Mar; 3: 328-332. Abstract.
Scheuner, D. et al (1996) Secreted amyloid β-protein similar to that
in the senile plaques of Alzheimer's disease is increased in vivo by the
presenilin 1 and 2 and APP mutations linked to familial Alzheimer's disease.
Nature Medicine 1996 Aug; 2: 864-870. Abstract.
Lemere, C. et al (1996) The E280A presenilin 1 Alzheimer mutation produces
increased Aβ42 deposition and severe cerebellar pathology. Nature Medicine
1996 Oct; 2: 1146-1150. Abstract.
Zhou J, Liyanage U, Medina M, Ho C, Simmons AD, Lovett M, Kosik KS:Presenilin
1 Interacts in Brain with a Novel Member of the Armadillo Family. NeuroReport
8:1489-1494, 1997 (abstract)
and erratum published 8: 2085-2090, 1997 (abstract).
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