Abstract
Alzheimer’s disease (AD) is the most common neurodegenerative disorder characterized by progressive cognitive decline leading to dementia. The amyloid precursor protein (APP) is a ubiquitous type I transmembrane (TM) protein sequentially processed to generate the β-amyloid peptide (Aβ), the major constituent of senile plaques that are typical AD lesions. There is a growing body of evidence that soluble Aβ oligomers correlate with clinical symptoms associated with the disease. The Aβ sequence begins in the extracellular juxtamembrane region of APP and includes roughly half of the TM domain. This region contains GXXXG and GXXXA motifs, which are critical for both TM protein interactions and fibrillogenic properties of peptides derived from TM α-helices. Glycine-to-leucine mutations of these motifs were previously shown to affect APP processing and Aβ production in cells. However, the detailed contribution of these motifs to APP dimerization, their relation to processing, and the conformational changes they can induce within Aβ species remains undefined. Here, we describe highly resistant Aβ42 oligomers that are produced in cellular membrane compartments. They are formed in cells by processing of the APP amyloidogenic C-terminal fragment (C99), or by direct expression of a peptide corresponding to Aβ42, but not to Aβ40. By a point-mutation approach, we demonstrate that glycine-to-leucine mutations in the G29XXXG33 and G38XXXA42 motifs dramatically affect the Aβ oligomerization process. G33 and G38 in these motifs are specifically involved in Aβ oligomerization; the G33L mutation strongly promotes oligomerization, while G38L blocks it with a dominant effect on G33 residue modification. Finally, we report that the secreted Aβ42 oligomers display pathological properties consistent with their suggested role in AD, but do not induce toxicity in survival assays with neuronal cells. Exposure of neurons to these Aβ42 oligomers dramatically affects neuronal differentiation and, consequently, neuronal network maturation.
Highlights
The amyloid precursor protein (APP) is a ubiquitously expressed type 1 transmembrane protein (Kang et al, 1987; Selkoe, 2004) whose processing in the amyloidogenic pathway leads to the production of the β-amyloid peptides (Aβ)
In our previous work we reported that expression of C99 leads to the formation of an oligomer, which is detected in denaturating gels as a higher molecular weight band
In order to characterize the nature of this higher molecular weight band, we expressed in Chinese hamster ovary (CHO) cells different APP constructs: human APP695, C99 and C42, which corresponds to the sequence of human Aβ42, that we engineered by adding a stop codon after residue 42 of Aβ
Summary
The amyloid precursor protein (APP) is a ubiquitously expressed type 1 transmembrane protein (Kang et al, 1987; Selkoe, 2004) whose processing in the amyloidogenic pathway leads to the production of the β-amyloid peptides (Aβ). APP or PS mutations typically result in imbalanced Aβ production and an increased Aβ42/Aβ40 ratio (Selkoe, 2004) These observations led to the amyloid cascade hypothesis, predicting that the initial steps of AD, which trigger a series of pathogenic events, are related to Aβ production and clearance (Hardy and Allsop, 1991). This hypothesis remains a matter of debate (Herrup, 2015), experimental lines of evidence from cellular models, transgenic animals and patient brain samples has been overwhelming (Hardy, 2009). Endocytosis, and posttranslational modification like phosphorylation have been involved in Aβ production and accumulation (Feyt et al, 2005, 2007)
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