Analysis of 138 pathogenic mutations in presenilin-1 on the in vitro production of Aβ42 and Aβ40 peptides by γ-secretase

Abstract

A hallmark of Alzheimer's disease (AD) is the aggregation of β-amyloid peptides (Aβ) into amyloid plaques in patient brain. Cleavage of amyloid precursor protein (APP) by the intramembrane protease γ-secretase produces Aβ of varying lengths, of which longer peptides such as Aβ42 are thought to be more harmful. Increased ratios of longer Aβs over shorter ones, exemplified by the ratio of Aβ42 over Aβ40, may lead to formation of amyloid plaques and consequent development of AD. In this study, we analyzed 138 reported mutations in human presenilin-1 (PS1) by individually reconstituting the mutant PS1 proteins into anterior-pharynx-defective protein 1 (APH-1)aL-containing γ-secretases and examining their abilities to produce Aβ42 and Aβ40 in vitro. About 90% of these mutations lead to reduced production of Aβ42 and Aβ40. Notably, 10% of these mutations result in decreased Aβ42/Aβ40 ratios. There is no statistically significant correlation between the Aβ42/Aβ40 ratio produced by a γ-secretase variant containing a specific PS1 mutation and the mean age at onset of patients from whom the mutation was isolated.Alzheimer's disease | γ-secretase | Aβ peptides | cleavage activity | amyloid hypothesis T he first case of Alzheimer's disease (AD) was reported more than 100 y ago, and the brain of the deceased patient contained characteristic senile plaques (1). These plaques were found to be amyloid in nature by electron microscopy (2), and analysis of amino acid sequence revealed the amyloid to be derived from amyloid precursor protein (APP) (3). APP is first cleaved by β-secretase to produce a 99-residue transmembrane fragment C99, which then undergoes additional cleavages by γ-secretase to generate a series of amyloidogenic β-amyloid peptides (Aβ) with 39-43 amino acids (4, 5). The slightly longer Aβ peptides, particularly Aβ42 and Aβ43, are thought to be more prone to aggregation than the shorter ones such as Aβ40 (6-8). Indeed, the amyloid plaques contain mostly longer Aβ peptides such as Aβ42 (9).Familial cases of AD were found to be genetically linked to missense mutations in APP (10, 11). These observations, together with prior knowledge on amyloid plaques, prompted proposition of the amyloid hypothesis, which regards the formation of β-amyloid plaques as the cause of AD through apoptosis induction of neuronal cells (12). Consistent with this hypothesis, Aβ42 was found to induce cell death (13,14). Human presenilin-1 (PS1) was cloned and found to be targeted by missense mutations in early-onset familial AD (EOFAD) (15). Remarkably, these EOFAD-linked PS1 mutants led to elevated molar ratios of Aβ42 over Aβ40 both in cell lines and in the brains of transgenic animals (16), and the plasma levels of Aβ42 and Aβ43 in FAD patients with PS1 or APP mutations were significantly increased compared with those of healthy individuals (17). These observations led to refinement of the amyloid hypothesis, which identifies increased ratios of longer Aβ peptides over shorter ones as a key early event in AD development (18). The increa...

Bidirectional regulation of Aβ levels by Presenilin 1

“…Recently, the effect of PS1 familial Alzheimer's disease (FAD) mutations on highly purified γ-secretase activity was reported (23). Interestingly, many mutations, which have been shown to increase the amount of Aβ peptides in vivo, were shown to decrease γ-secretase activity in vitro.…”

Role of Presenilin in Mitochondrial Oxidative Stress and Neurodegeneration in <em>Caenorhabditis elegans</em>

“…There is not a high degree of spatial correlation between the presence of amyloid plaques and neurodegeneration, also neurodegeneration is not observed in certain patients with significant amyloid plaque burden and significant amyloid plaques have not been found in some patients suffering from AD (Morris et al, 2014). A recent study also showed that 90% of 138 presenilin mutation that are associated with EOFAD showed reduced production of Aβ40 and Aβ42 (Sun et al, 2017), casting some doubt on the amyloid hypothesis in AD. Given the evident failure of any drug therapy targeting amyloid peptides, there is a pressing need to explore alternate hypotheses of AD pathogenesis as a means to develop a successful therapeutic intervention.…”

“…Indeed, a recent study employed a knockin approach to investigate the effects two EOFAD PSEN1 mutations have on γ-secretase function and found that these mutations abolished γ-secretase activity (Xia et al, 2015). Correspondingly, another recent study investigated 138 distinct EOFAD PSEN1 mutations and found that 90% of these mutants decreased Aβ production (Sun et al, 2017). Yet 10% of these mutations showed normal or elevated Aβ production.…”

The amyloid hypothesis is too good to be true

“…Recently, Sun et al [105] studied the activity of γ-secretase they had reconstituted in liposomes with PS1 with 138 different mutations, one by one, many of which cause Alzheimer at different age. They could not find any correlation between the amount of Aβ peptides produced, or the Aβ42/40 ratio, and the age of Alzheimer onset.…”