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Updated 20 September 2002
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How the Other Half Lives—Or the What, How, and Where of the AβPP Intracellular Domain
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Frank LaFerla led this live discussion on 20 September 2002. Readers are invited to submit additional comments by using our Comments form at the bottom of the page.
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 View Transcript of Live Discussion — Posted 28 August 2006
View Comments By:
Kumar Sambamurti — Posted 28 August 2006
Background Text
By Frank LaFerla
The Notch receptor (Notch) and the β-amyloid precursor protein (APP) are examples of type-I integral membrane proteins that are substrates for γ-secretase. Notch is a vital signaling molecule that regulates cell-fate determination during development. Signaling through the Notch pathway is triggered by the binding of ligands such as Delta and Jagged, which induces cleavage of Notch. A subsequent β-secretase mediated cleavage releases the Notch intracellular domain (NICD), which binds to transcription factors (e.g. Supressor of Hairless) and translocates to the nucleus, where it regulates transcription of selective genes.
Similarities between the processing of Notch and APP have prompted speculation that APP may play an analogous signaling role. Once regarded as nothing more than a throw-away fragment that happens to border the Aß sequence, the carboxyl terminal sequence of APP has emerged as a potential nuclear signaling molecule (see ARF news story). This long ignored fragment, which is referred to as the APP intracellular domain (AICD), was initially described by Passer et al., who showed that AICD-like peptides occur in both normal and Alzheimer's disease brain (Passer et al., 2000).
AICD consists of the last 50 carboxy-terminal residues of the APP protein. Surprisingly, this does not correspond to the entire sequence downstream of the gamma-secretase site, which would have led to an AICD species containing either 57 or 59 residues, depending on whether cleavage occurred at the Aß42 or Aß40 site, respectively. Several groups have recently demonstrated that an additional proteolytic event must occur (either before or after γ-secretase processing) that cleaves at a conserved valine downstream of the canonical γ-secretase cleavage sites (either by γ-secretase or another protease). (Yu et al., 2001; Moehlmann et al., 2002; Weidemann et al., 2002).
The Alzheimer's AICD fragment, like NICD, can also complex with transcription factors. Kimberly et al., 2001 have shown that the cytoplasmic domain of APP is highly labile but that it is stabilized by forming a complex with Fe65, which is then capable of entering the nucleus. Fe65, in turn, interacts with the transcription factor CP2/LSF/LBP1 and Tip60, a histone acetyltransferase. Sudhof's group recently showed that AICD complexed with Fe65 and Tip60 can potently regulate the expression of artificial expression constructs in transfected cells (Cao & Sudhof, 2001). Pimplikar's group (Gao & Pimplikar, 2001) also showed that AICD (they called it C ) exerts effects in the nucleus and found that the 59 residue long fragment, but not the 57 amino acid fragment, potently represses retinoic acid-responsive gene expression.
What role in cell signaling does AICD mediate? My lab has recently demonstrated a functional role for AICD in regulating phosphoinositide-mediated calcium signaling Leissring et al., 2002). Genetic ablation of the presenilins or pharmacological inhibition of β-secretase activity (and thereby AICD production) greatly attenuated calcium signaling in a dose-dependent and reversible manner through a mechanism involving the modulation of endoplasmic reticulum calcium stores. Cells lacking APP (and hence AICD) exhibited similar calcium signaling deficits, and-notably-these disturbances could be reversed by transfection with APP constructs containing an intact AICD, but not by constructs lacking this domain. We noted that there was a 3 hour time lag after inhibition of γ-secretase and inhibition of calcium signaling, which would be adequate time for transcriptional-mediated events. One aspect of this work that is presently unresolved is why there was no compensation in the APP null cells by the homologs APLP1 or APLP2. It could be that there is a disparity in expression levels or stability of these molecules in fibroblasts or that they don't bind efficiently to Fe65 (all of which need to be addressed).
Is modulating calcium signaling the only role for AICD? Things are hardly ever this simple as recent findings by Luciano D'Adamio and Brad Hyman's groups point to additional roles. In the first case, D'Adamio's lab has shown that AICD also binds to cytosolic Notch inhibitors Numb and Numb-like, which can represses Notch activity (Roncarati et al. 2002). Kinoshita has shown that γ-secretase generated C-terminal domain of APP may also be involved in apoptosis (Kinoshita et al., 2002).
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Comments on Live Discussion |
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Comment by: Kumar Sambamurti
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Submitted 28 August 2006
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Posted 28 August 2006
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The role of the cytoplasmic fragment generated by gamma secretase cleavage
in APP function and AD
I am glad to see that there is more interest in this "other half."
My lab has been interested in CTFgamma for a number of years now.
Click for image 1
As shown above, the cytoplasmic domain of APP are highly conserved across several
classes of vertebrates and even conserved with the APP homologue APLP2 but less
well conserved in APLP1. On the other hand, the Ab region is less conserved
as shown below. Since APLP2 can compensate for the loss of both APLP1 and APP
but APLP1 cannot (1) the regions conserved between APP and APLP2 should play
an important role in the shared essential function. Based on early studies from
Diane Levitan showing that PS1 linked FAD mutations may show partial reduction
in g-secretase and by extension CTFgamma (also AICD/ CTF-epsilon; 2). We have
summarized these data and discussed the possible...
Read more
The role of the cytoplasmic fragment generated by gamma secretase cleavage
in APP function and AD
I am glad to see that there is more interest in this "other half."
My lab has been interested in CTFgamma for a number of years now.
Click for image 1
As shown above, the cytoplasmic domain of APP are highly conserved across several
classes of vertebrates and even conserved with the APP homologue APLP2 but less
well conserved in APLP1. On the other hand, the Ab region is less conserved
as shown below. Since APLP2 can compensate for the loss of both APLP1 and APP
but APLP1 cannot (1) the regions conserved between APP and APLP2 should play
an important role in the shared essential function. Based on early studies from
Diane Levitan showing that PS1 linked FAD mutations may show partial reduction
in g-secretase and by extension CTFgamma (also AICD/ CTF-epsilon; 2). We have
summarized these data and discussed the possible roles CTFgamma may play in
the pathogenesis of AD (4)
Click for image 2
It is particularly interesting to note the results of Laferla and colleagues
showing deficits in Ca signaling in both FAD PS1 mutants as well as in mice
lacking PS1, restoration of signaling upon expression of CTFg57 (5) and demonstrating
the role of calsenilin (6). This is the second functionally relevant finding
for this fragment after transcriptional regulation (7).
(1) Sambamurti K, Greig, NH, Lahiri, DK. Advances in the Cellular and Molecular Biology of the b-Amyloid Protein in Alzheimer's Disease. Neuromolecular Medicine. See PDF
(2) Levitan D, Doyle, Brousseau, Lee, Thinakaran, Slunt, Sisodia, Greenwald. Assessment of normal and mutant human presenilin function in Caenorhabditis elegans. Proc Natl Acad Sci U S A. 1996 Dec ; 10(93):14940-4. Abstract.
Please note that we called the fragment CTFgamma when we originally reported
the fragment in our in vitro gamma secretase assay (3) primarily because it
behaves like a product of gamma secretase cleavage, not because of its exact
cleavage site. CTFepsilon makes sense to us, if we can demonstrate that an activity
(epsilon secretase) other than gamma secretase is responsible for the processing
after Abeta49. Our recent unpublished data shows that in addition to Abeta,
CTFgamma yield is increased in an in vitro assay when cells are transfected
with a mix of APH1a, APH1b, Pen2 and Nicastrin providing additional evidence
that the fragment is a product of gamma secretase cleavage. However, the term
AICD does not strictly define the fragment but only describes a region within
the APP molecule rather than a cleavage product. Thus, AICD is perhaps misleading
as full length APP, CTFalpha, CTFbeta and CTFgamma/epsilon are AICDs.
(3) http://www.jbc.org/cgi/reprint/276/1/481;
http://www.fasebj.org/cgi/content/full/14/15/2383
(4) http://www3.interscience.wiley.com/cgi-bin/fulltext?ID=98017418&PLACEBO=IE.pdf
(5) Leissring MA, Murphy MP, Mead TR, Akbari Y, Sugarman MC, Jannatipour M, Anliker B, Müller U, Saftig P, De Strooper B, Wolfe MS, Golde TE, LaFerla FM. A physiologic signaling role for the gamma -secretase-derived intracellular fragment of APP. Proc Natl Acad Sci U S A. 2002 Apr 2 ; 99(7):4697-702. Abstract;
http://www.jcb.org/cgi/reprint/149/4/793.pdf
(6) Malcolm A. Leissring, Tritia R. Yamasaki, Wilma Wasco, Joseph D. Buxbaum, Ian Parker, and Frank M. LaFerla. Calsenilin reverses presenilin-mediated enhancement of calcium signaling. Proc. Natl. Acad. Sci. USA. 2000 July 18; 97 (15): 85908593 Abstract
(7) http://www.pubmedcentral.gov/picrender.fcgi?artid=64969&action=stream&blobtype=pdf
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