AβPP processing results in greater toxicity per amount of Aβ1-42 than individually expressed and secreted Aβ1-42 inDrosophila melanogaster

Abstract

The aggregation of the amyloid-β (Aβ) peptide into fibrillar deposits has long been considered the key neuropathological hallmark of Alzheimer's disease (AD). Aβ peptides are generated from proteolytic processing of the transmembrane Aβ precursor protein (AβPP) via sequential proteolysis through the β-secretase activity of β-site AβPP-cleaving enzyme (BACE1) and by the intramembranous enzyme γ-secretase. For over a decade, Drosophila melanogaster has been used as a model organism to study AD, and two different approaches have been developed to investigate the toxicity caused by AD-associated gene products in vivo. In one model, the Aβ peptide is directly over-expressed fused to a signal peptide, allowing secretion of the peptide into the extracellular space. In the other model, human AβPP is co-expressed with human BACE1, resulting in production of the Aβ peptide through the processing of AβPP by BACE1 and by endogenous fly γ-secretase. Here, we performed a parallel study of flies that expressed the Aβ1-42 peptide alone or that co-expressed AβPP and BACE1. Toxic effects (assessed by eye phenotype, longevity and locomotor assays) and levels of the Aβ1-42, Aβ1-40 and Aβ1-38 peptides were examined. Our data reveal that the toxic effect per amount of detected Aβ1-42 peptide was higher in the flies co-expressing AβPP and BACE1 than in the Aβ1-42-expressing flies, and that the co-existence of Aβ1-42 and Aβ1-40 in the flies co-expressing AβPP and BACE1 could be of significant importance to the neurotoxic effect detected in these flies. Thus, the toxicity detected in these two fly models seems to have different modes of action and is highly dependent on how and where the peptide is generated rather than on the actual level of the Aβ1-42 peptide in the flies. This is important knowledge that needs to be taken into consideration when using Drosophila models to investigate disease mechanisms or therapeutic strategies in AD research.

Mapping pathogenic processes contributing to neurodegeneration in Drosophila models of Alzheimer's disease

“…A). Flies were analysed at day 21, a time point corresponding to the median survival time previously observed for AβPP‐BACE1 flies . The majority of all TUNEL‐positive cells were observed in the medulla and the lamina (Fig.…”

“…In our previous study, longevity and locomotor analyses showed significant toxic effects for both the Aβ42 flies and AβPP‐BACE1 flies . The time frame selected for this study was 21 days, corresponding to the median survival time for the AβPP‐BACE1 flies.…”

“…Understanding the underlying mechanisms of AD toxicity is a key requirement to developing mechanism‐based therapeutic strategies, and the use of Drosophila to investigate the pathogenesis of AD has allowed scientists to achieve important goals in this research field . AD research using Drosophila frequently implies one of two approaches; either the Aβ peptides are fused to a secretion sequence and directly produced from transgenes (the Aβ fly model) or the Aβ peptides are produced by the processing of human AβPP (the AβPP‐BACE1 fly model) . In this paper, we have looked, in detail, at the pathways leading to toxicity within the two AD fly models and have highlighted differences in the underlying mechanisms of the AD‐related toxicity observed in these systems.…”

“…The more commonly used Aβ fly model has the gene encoding the Aβ 1–42 sequence cloned into the fly genome; the peptide is expressed fused to a signal sequence, resulting in secretion of the peptide to the extracellular space . In the other models, human AβPP is co‐expressed with human BACE1 allowing the production of C99 and different isoforms of the Aβ peptide (including post‐translationally modified Aβ variants) through the processing of human AβPP by human BACE1 and by endogenous fly γ‐secretase (the AβPP‐BACE1 fly model) . AD fly models have been frequently used during the last two decades to investigate Aβ toxicity, cell‐specific vulnerability and aggregation .…”

“…However, potential differences in the toxic mechanisms between the two different AD fly models have not been thoroughly investigated. Recently, we published a study where the toxic effects in these two AD fly models were studied in parallel . We found that the proteotoxic effect, defined as the reduction in median survival time divided by total amount of Aβ 1–42, is considerably higher for the AβPP‐BACE1 flies compared to the Aβ 1–42 flies, implying that the mechanisms of toxicity are different between these two AD fly models.…”