8 June 2000. The true identity of γ-secretase, the enzyme that frees amyloid-β
from its membrane-bound precursor, is one of the holy grails in the search for
the cause of Alzheimer's disease. A battery of circumstantial evidence has recently
focused attention on presenilin as the prime suspect. In today's Nature, Steven
J. Gardell and his associates at Merck Research Laboratories add evidence that
has some commentators saying the suspect has been apprehended.
Using an inhibitor targeted to the active site of aspartyl proteases, they
first confirmed an earlier study indicating that γ-secretase is, in fact,
an aspartyl protease. Antibodies against presenilin- 1 and presenilin-2, then
appeared to identify that aspartyl protease as presenilin, although the authors
acknowledge that their probes could be labeling a γ-secretase that is in
a complex with presenilin.
Reactions to these latest data range from conversion to cautionary. Bart De
Strooper, writing a News and Views letter accompanying the article, is now convinced.
He accepts that there are a number of details to be worked out (e.g., how can
hydrolysis of peptide bonds take place inside the hydrophobic cell membrane?),
but he concludes that "presenilin ... is the culprit."
However, for Sam Sisodia of the University of Chicago, these data can be interpreted
differently. "It's equally conceivable that presenilin is intimately associated
with the γ-secretase and that in fact, the transition-state-analogue inhibitory
site is actually blocking-inhibiting-the real enzyme, and the active site
groups that are involved in the photoactivation are hitting presenilin which
is close by. Thus, presenilin is actually an accessory factor. I think the definitive-proof
experiment that presenilin is γ-secretase has yet to be published, and that
is to reconstitute the reaction in vitro."-Hakon Heimer.
Li YM, Xu M, Lai MT, Huang Q, Castro JL, DiMuzio-Mower J,
Harrison T, Lellis C, Nadin A, Neduvelil JG, Register RB, Sardana MK, Shearman MS, Smith AL, Shi XP, Yin KC, Shafer JA, Gardell SJ. Photoactivated γ-secretase
inhibitors directed to the active site covalently label presenilin 1. Nature 2000 June 8;405(6787):689-94. Abstract
DeStrooper B. Nature 2000 June 8;405:627-8
Interview with Sangram Sisodia
ARF: Doesn't this paper confirm that presenilin is the gamma-secretase?
Sisodia: No. It's equally conceivable that presenilin is intimately
associated with the gamma-secretase and that in fact, the
transition-state-analogue inhibitory site is actually
blocking--inhibiting--the real enzyme, and the active site groups that
are involved in the photoactivation are hitting presenilin which is close
by. Thus, presenilin is actually an accessory factor. I think the definitive
proof experiment that presenilin is gamma-secretase has yet to be
published (perhaps it's been tried), and that is to reconstitute the reaction in
vitro. That is, to take the purified enzyme in a bilayer or artificial
membrane of some sort, offer it the natural substrate and ask whether
this activity can proceed.
ARF: Are there particular technical challenges to such an experiment?
Sisodia: There are always technical challenges and it's not trivial for
presenilin because it spans the membrane several times. It's very fatty
so biochemically it's hard to work with, to reinsert into artificial
bilayers. Yes, it's technically challenging, but the burden of proof really rests
on those who claim that presenilin is the gamma-secretase.
There may be another technical challenge above and beyond that, in that
presenilin may be part of a complex. One might need to reconstitute the
complex to get activity; one would then have to go after the individual
components of the complex and put those together. And that could take
I think that presenilin does a myriad of things in mammalian cells. It
affects cellular trafficking of several proteins. It affects the
processing of several different proteins and it's hard for me to imagine--and I
could be completely wrong--that it is a protease involved in gamma-secretase
processing. It would be unprecedented in biology, to begin with. But of
course, that's the way biology is: there's always something new lurking
around the corner. How you would get catalysis in the bilayer, where the
water would come from if it really is an aspartyl protease, these are
all the kinds of things that one would have to think about from a
biochemical point of view. It could very well be true--I'm not going to discount it
all outright--but I don't think the proof is there yet.
ARF: I'll quote you as being cautionary.
Sisodia: Yes. I've taken that view from the very beginning. We published
a paper in Neuron about a year and half ago which suggests that protein trafficking
of several membrane proteins is affected by loss of presenilin-1 function.
So how could this molecule, which normally resides in the very early
subcellular compartments?the endoplasmic reticulum and the early Golgi,
perhaps--affect the maturation or proteolysis of APP in very late
compartments? One has to try to understand that.