The mechanism of γ-Secretase dysfunction in familial Alzheimer disease

VIB Center for the Biology of Disease, Leuven, Belgium.
The EMBO Journal (Impact Factor: 10.43). 04/2012; 31(10):2261-74. DOI: 10.1038/emboj.2012.79
Source: PubMed

ABSTRACT The mechanisms by which mutations in the presenilins (PSEN) or the amyloid precursor protein (APP) genes cause familial Alzheimer disease (FAD) are controversial. FAD mutations increase the release of amyloid β (Aβ)42 relative to Aβ40 by an unknown, possibly gain-of-toxic-function, mechanism. However, many PSEN mutations paradoxically impair γ-secretase and 'loss-of-function' mechanisms have also been postulated. Here, we use kinetic studies to demonstrate that FAD mutations affect Aβ generation via three different mechanisms, resulting in qualitative changes in the Aβ profiles, which are not limited to Aβ42. Loss of ɛ-cleavage function is not generally observed among FAD mutants. On the other hand, γ-secretase inhibitors used in the clinic appear to block the initial ɛ-cleavage step, but unexpectedly affect more selectively Notch than APP processing, while modulators act as activators of the carboxypeptidase-like (γ) activity. Overall, we provide a coherent explanation for the effect of different FAD mutations, demonstrating the importance of qualitative rather than quantitative changes in the Aβ products, and suggest fundamental improvements for current drug development efforts.

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Available from: Bart De Strooper, Sep 26, 2015
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    • "Next, we characterized the FAD mutations further by examining their overall effect on c-secretase activity by monitoring AICD production from the e-cleavage site by using a Luciferase-reporter gene assay [38]. In line with a recent report from Chávez-Gutiérrez et al. [26] and Bentahir et al. [22], we observed that not all FAD mutations affect AICD formation. Most mutations that we investigated showed no decrease in AICD formation, except I143T, L166P, Dexon9 and G384A, which all lowered the production (Fig. 1E). "
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    ABSTRACT: The enzyme complex γ-secretase generates amyloid β-peptide (Aβ), a 37–43-residue peptide associated with Alzheimer disease (AD). Mutations in presenilin 1 (PS1), the catalytical subunit of γ-secretase, result in familial AD (FAD). A unifying theme among FAD mutations is an alteration in the ratio Aβ species produced (the Aβ42/Aβ40 ratio), but the molecular mechanisms responsible remain elusive. In this report we have studied the impact of several different PS1 FAD mutations on the integration of selected PS1 transmembrane domains and on PS1 active site conformation, and whether any effects translate to a particular amyloid precursor protein (APP) processing phenotype. Most mutations studied caused an increase in the Aβ42/Aβ40 ratio, but via different mechanisms. The mutations that caused a particular large increase in the Aβ42/Aβ40 ratio did also display an impaired APP intracellular domain (AICD) formation and a lower total Aβ production. Interestingly, seven mutations close to the catalytic site caused a severely impaired integration of proximal transmembrane/hydrophobic sequences into the membrane. This structural defect did not correlate to a particular APP processing phenotype. Six selected FAD mutations, all of which exhibited different APP processing profiles and impact on PS1 transmembrane domain integration, were found to display an altered active site conformation. Combined, our data suggest that FAD mutations affect the PS1 structure and active site differently, resulting in several complex APP processing phenotypes, where the most aggressive mutations in terms of increased Aβ42/Aβ40 ratio are associated with a decrease in total γ-secretase activity.
    FEBS Open Bio 12/2014; 4. DOI:10.1016/j.fob.2014.04.006 · 1.52 Impact Factor
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    • "Whether PS1 mutations cause increases in Ab42 and therefore FAD by a gain-or loss-offunction mechanism remains controversial. Controversy persists because virtually all previous studies probing the nature of FAD PS1 mutations have relied on experimental manipulations that overexpress PS1 and PS1 mutants in mouse and/or nonneuronal systems (Bentahir et al., 2006; Chá vez-Gutié rrez et al., 2012; Kumar-Singh et al., 2006) or that express mutant forms of PS1 in competition with wild-type forms in cell types that may not have normal levels of expression of other key genes. For example, studies of cultured neurons completely lacking PS1 result in near absence of Ab generation suggesting that FAD PS1 mutations cause a gain of function (De Strooper et al., 1998). "
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    ABSTRACT: Presenilin 1 (PS1) is the catalytic core of γ-secretase, which cleaves type 1 transmembrane proteins, including the amyloid precursor protein (APP). PS1 also has γ-secretase-independent functions, and dominant PS1 missense mutations are the most common cause of familial Alzheimer's disease (FAD). Whether PS1 FAD mutations are gain- or loss-of-function remains controversial, primarily because most studies have relied on overexpression in mouse and/or nonneuronal systems. We used isogenic euploid human induced pluripotent stem cell lines to generate and study an allelic series of PS1 mutations, including heterozygous null mutations and homozygous and heterozygous FAD PS1 mutations. Rigorous analysis of this allelic series in differentiated, purified neurons allowed us to resolve this controversy and to conclude that FAD PS1 mutations, expressed at normal levels in the appropriate cell type, impair γ-secretase activity but do not disrupt γ-secretase-independent functions of PS1. Thus, FAD PS1 mutations do not act as simple loss of PS1 function but instead dominantly gain an activity toxic to some, but not all, PS1 functions.
    Cell Reports 11/2013; 5(4). DOI:10.1016/j.celrep.2013.10.018 · 8.36 Impact Factor
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    • "A major gap in our understanding of LRRK2 pathobiology is, therefore, a biological phenotype for LRRK2 that correlates with disease genotype. The identification of a phenotype common to all pathogenic mutations would provide a target for drug testing in a manner analogous to the use of Ab42 levels in Alzheimer therapies [13] [14]. Moreover, it would indicate a specific pathway to be studied to find markers for early diagnosis of disease, an urgent unmet need for PD [15]. "
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    ABSTRACT: LRRK2 is one of the most important genetic contributors to Parkinson's disease (PD). Point mutations in this gene cause an autosomal dominant form of PD, but to date no cellular phenotype has been consistently linked with mutations in each of the functional domains (ROC, COR and Kinase) of the protein product of this gene. In this study, primary fibroblasts from individuals carrying pathogenic mutations in the three central domains of LRRK2 were assessed for alterations in the autophagy/lysosomal pathway using a combination of biochemical and cellular approaches. Mutations in all three domains resulted in alterations in markers for autophagy/lysosomal function compared to wild type cells. These data highlight the autophagy and lysosomal pathways as read outs for pathogenic LRRK2 function and as a marker for disease, and provide insight into the mechanisms linking LRRK2 function and mutations.
    Biochemical and Biophysical Research Communications 11/2013; 441(4). DOI:10.1016/j.bbrc.2013.10.159 · 2.30 Impact Factor
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