Alzheimer's disease (AD) is the most common cause of a progressive decline of cognitive function in aged humans. It is thought to be a result of the formation of amyloid plaques in the brain that are largely composed of β-amyloid (Aβ), which is one of the cleavage products of the amyloid precursor protein (APP). The amyloidogenic processing of APP to produce the Aβ peptides requires sequential proteolytic cleavages by the β- and γ-secretases. APP is first cleaved by the β-secretase to produce APP-C99, and this product is a substrate for further processing by the γ-secretase that cleaves within its transmembrane domain to produce N-terminal Aβ peptides and the C-terminal APP intracellular domain (AICD). On the basis of similarities to the Notch processing pathway, it has been postulated that the AICD may play a role in gene regulation following its release in response to some form of extracellular signal. In order to better understand the production and fate of the AICD, I have investigated the potential for exploiting a cell-free system to study its generation and properties. Having generated a number of model APP-derived fragments and shown them to be efficiently membrane integrated in vitro, I went on to study AICD production. I discovered that AICD-like fragments are extremely labile when synthesised in a rabbit reticulocyte lysate system and are rapidly degraded via a metalloproteinase, most likely the insulin degrading enzyme (IDE). The in vitro stability of these model AICD-like fragments was dependent upon the precise chain length of the polypeptide and N-terminal processing may preface the activity of IDE in vitro. The rapid degradation of the AICD in vitro is in close agreement with previous in vivo studies, and taken together such data are consistent with a role for the AICD in a signalling pathway of some form. A variety of approaches were also taken to try to generate the AICD by the γ-secretase mediated cleavage of the APP-C99 fragment, a biologically relevant substrate. In no case was any evidence of such cleavage observed in vitro and hence I conclude that the endoplasmic reticulum does not possess an active form of the γ-secretase. Preliminary in vivo-based studies did provide evidence for the γ-secretase cleavage of APP-C99 fragments, consistent with current models implying that such processing takes place at the cell surface and/or in endosomes and not at the endoplasmic reticulum.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:606760 |
| Date | January 2005 |
| Creators | Duggan, Claire |
| Publisher | University of Manchester |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://www.manchester.ac.uk/escholar/uk-ac-man-scw:77486 |
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