Live Discussion: Deficiency of Presenilin 1 Inhibits the Normal Cleavage of the Transmembrane Domain of Amyloid Precursor Protein
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Live discussion held 30 January 1998.
Participants: Deborah Watson, Ken Kosik, Bart
De Strooper, Kieran Breen, Steven Barger, June Kinoshita,
Ben Wolozin, Wilma Wasco, Carmela Abrahams, Dmitry Goldgaber,
Weiming Xia, Taisuke, Rong, Walker McGraw, Minami, Jean
Floring, Chris Weihl.
Note: Transcript has been edited for clarity and accuracy.
Deborah Watson: I'd like to start by asking Dr. DeStrroper about the effects
of one copy of PS on APP processing. Is there an intermediate effect between
+/+ and -/-?
Bart De Strooper: We did not do extensive experiments on the +/- but
there might be a decrease in amyloid peptide production in the +/- cultures
Ken Kosik: But your paper shows a suggestive change in Fig. 1 g and it
could be important for the mechanism.
Steven Barger: Steve Younkin's group presented data at Neuroscience from
a PS antisense model. Their results were essentially the opposite of this
knockout model -- any speculation on the resolution?
Ken Kosik: Responding to Steve Barger: could you tell us how the effect
was the opposite?
Bart De Strooper: Hello everybody (with a lot of delay). First again
on the PS1+/- .There is as I told, a decrease in amyloid peptide production
in the PS1 +/- cutltures. We focussed however until now mostly on the PS1-/-
Steven Barger: They showed that antisense of PS1 INCREASED Aβ 42(43).
One caveat is that it was in HEK cells.
Ken Kosik: Did they prove that they completely suppressed the expression
of PS1? That's for S.Barger.
Ken Kosik: Weiming, do you have any comments about the heterozygotes?
Weiming Xia: In what aspect of heterozygotes?
Ken Kosik: Regarding Aβ production and CTF production.
Weiming Xia: We have not tried Aβ detection on hetorozygotes.
Steven Barger: No, I don't think there was complete annihilation of PS1;
antisense is almost never that complete. So, you're right, there's a quantitative
issue to deal with. But this might be a mechanistic hint, as well (?).
Deborah Watson: Do you have any more ideas, Bart, about the residual Aβ
production in PS 1 null mice?
Weiming Xia: Is there any linear relationship regarding the decrease of PS1
expression and changes of Aβ, particularly Aβ 42?
Ken Kosik: Bart, could you reiterate why you believe PS1 mutants are gain
Bart De Strooper: to Deborah Watson: either it is PS2 or it is residual activity
Rong: Can you explain why the decrease of intracellular Aβ is not
so significant Taisuke: Did you see the expression of PS2 in PS1 -/- mice?
Ken Kosik: Bart, do you agree that the decrease applies to both intracellular
and secreted a-β?
Weiming Xia: to Bart, any comments on p3?
Ken Kosik: Rong, do you think there are specific secretases for the other
Taisuke: Did you see the expression of PS2 in PS1 -/- mice?
Bart De Strooper: To everybody, It is impossible for me to really participate
in the discussion. I get disconnected every few minutes. To Wxia: p3 behaves
like β A4
Rong: To Kosik: At this moment, I don't think so. I may argue that
same γ-secretase is responsible for generating Aβ42,40,38,37,..
Ken Kosik: To Rong: does the same secretase create all these fragments? Bart, sorry for the problems you are having, but we do appreciate
Steven Barger: The gain-of-function slant is satifying since it is consistent
with the ability of PS mutants to restore development of somites, spine,
etc. in the PS KO's.
Ken Kosik: Steve, are your referring to the Sisodia data about rescue? Wilma, do you have any thoughts about whether decreasing amounts
of presenilin increase or decrease Aβ levels?
Steven Barger: Yes, to my knowledge, Sisodia's are only publicly presented
data on this issue.
Rong: To Kosik: I don't have definite answers for that, but it is possible.
Ken Kosik: Steve and others: isn't it unusual that so many different mutations
in PS1 all lead to gain of function?
Carmela Abrahams: Do Bart's results mean that γ secretase cleavage occurs
in ER or Golgi, thus Aβ is completely formed there?
Ken Kosik: 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?
Steven Barger: Yes, I certainly agree that the number of mutations doesn't
fit Occam's razor. One thing that initially occurred to me is that the
function we talk about gaining or losing may be two different functions:
1) a developmental role and 2) APP processing.
Ken Kosik: Bart, are you still with us?
Bart De Strooper: Ken, would it be possible to pool the questions and
to send them to me by email. I get only part of the discussion on my computer.
June Kinoshita: Apologies to all. Apparently Bart's server is having
problems. I will go through the questions for Bart and email them to him.
The questions and his responses will be posted on the newsgroup.
Ken Kosik: Thank you June! Steve, even applying gain of function just to APP processing
we have a problem with regard to Occam's razor.
June Kinoshita: We can use this time for discussion of these issues
among ourselves. I'm sure many interesting questions will grow out of the
Ken Kosik: Rong, do you then believe the γ secretase is not highly
specific for a particular amino acid and are any of these fragments seen
in vivo (37, 38...)?
Weiming Xia: Rong, do you think there is one γ-secretase producing Aβ
ending at different positions?
Jean Floring: Steve, would you repeat the paradox (vs Younkin's results).
My computer crashed.
Bart De Strooper: To all, I am leaving and will do as June suggested.
June Kinoshita: Thank you so much, Bart. Sorry this Brave New Cyberworld
isn't always what it's cracked up to be!
Weiming Xia: Thank you, Bart.
Ken Kosik: Thanks Bart.
Deborah Watson: Carmela, do you believe there are other sites besides ER/Golgi
where Aβ is produced--if you say endosomes could you also be more specific?
Carmela Abrahams: Nice work. Thank you Bart.
Steven Barger: Younkin used antisense to decrease PS1 levels in HEK293 cells
and showed an increase in Aβ 42(43) (in contrast to the decrease that
occurred in these KO cultures).
Carmela Abrahams: Debra, I don't think at this moment we can tell whether Aβ
is formed also in endosomes or gets there from the ER. Does anyone who
is plugged in know more?
Jean Floring: Thanks, Steve..I'll reserve my opinion about antisense. Anyone:
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?
Rong: Yes, all of these Aβ fragments have been detected in vivo.
Ken Kosik: Rong, does the concentration of the fragments differ greatly?
Rong: The are more or less associated with Aβ40 and Aβ42.
Steven Barger: To Ken: Actually, having two functions for PS might explain
why AD would involve so many mutations: the developmental role would select
(in terms of lethality) for mutations that were mild enough to retain the
developmental role. But ...
Taisuke: To Jfloring: I transfected only PS2 mutant cDNA into Neuro2a
cells, and endogenous Aβ42 was increased.
Steven Barger: ... But once this developmental selection is satisfied, there
might be many routes to loss-of-function in the APP processing role.
Ken Kosik: Steve, are you suggesting that the PS1 mutations are loss of
function with regard to APP processing?
Rong: I have to leave. It is very nice to discuss with everyone here.
Steven Barger: Yes, Ken, I think that's a possibility.
Ken Kosik: I'd like to thank everyone for participating. Please make any
last comments now that can be posted to Bart.
June Kinoshita: Thank you Ken for moderating and for putting together
the thoughtful discussion.
Steven Barger: Very intriguing work, congratulations!
June Kinoshita: Again, please check the newsgroup for questions and
responses from Bart.
Ken Kosik: Thanks everyone. Good-bye.
June Kinoshita: Bye Ken! Thanks to all for participating!
Jean Floring: Steve: what's your email address?