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

Abstract

Alzheimer's disease (AD) is the most common form of dementia, affecting millions of people and currently lacking available disease‐modifying treatments. Appropriate disease models are necessary to investigate disease mechanisms and potential treatments. Drosophila melanogaster models of AD include the Aβ fly model and the AβPP‐BACE1 fly model. In the Aβ fly model, the Aβ peptide is fused to a secretion sequence and directly overexpressed. In the AβPP‐BACE1 model, human AβPP and human BACE1 are expressed in the fly, resulting in in vivo production of Aβ peptides and other AβPP cleavage products. Although these two models have been used for almost two decades, the underlying mechanisms resulting in neurodegeneration are not yet clearly understood. In this study, we have characterized toxic mechanisms in these two AD fly models. We detected neuronal cell death and increased protein carbonylation (indicative of oxidative stress) in both AD fly models. In the Aβ fly model, this correlates with high Aβ1–42 levels and down‐regulation of the levels of mRNA encoding lysosomal‐associated membrane protein 1, lamp1 (a lysosomal marker), while in the AβPP‐BACE1 fly model, neuronal cell death correlates with low Aβ1–42 levels, up‐regulation of lamp1 mRNA levels and increased levels of C‐terminal fragments. In addition, a significant amount of AβPP/Aβ antibody (4G8)‐positive species, located close to the endosomal marker rab5, was detected in the AβPP‐BACE1 model. Taken together, this study highlights the similarities and differences in the toxic mechanisms which result in neuronal death in two different AD fly models. Such information is important to consider when utilizing these models to study AD pathogenesis or screening for potential treatments.

Exploring Aβ Proteotoxicity and Therapeutic Candidates Using Drosophila melanogaster

“…This eye phenotype gives a straightforward image of the toxicity, since a higher degree of toxicity correlates with a more severe rough eye phenotype. Another method to probe cell death is the quantification of apoptotic cells using a terminal deoxynucleotidyl transferase dUTP nick end labeling-(TUNEL) assay [ 20 , 21 ]. In this assay, the enzyme terminal deoxynucleotidyl transferase (TdT) labels DNA breaks that occur during apoptosis, thus making it possible to identify apoptotic DNA fragmentation in the tissue using a histochemical readout.…”

“…Oxidative stress reflects an imbalance in the cell that results in increasing manifestations of reactive oxygen species (ROS) that can lead to cellular damage [ 24 ]. An indicator for oxidative stress is protein carbonylation, which is an irreversible protein modification that can be probed by immunoblot analysis [ 20 ].…”

“…The characterization of the presence of different protein aggregates and proteotoxicity can therefore provide valuable information about the features of the toxic protein species. Various compounds can be used to stain protein aggregates in vivo, including luminescent conjugated oligothiophenes (LCOs), such as p-FTAA [ 14 , 27 , 28 ] and h-FTAA [ 29 ], Congo Red [ 15 , 30 ], thioflavin-S [ 18 , 30 , 31 ] and different antibodies specific for a certain protein or aggregate morphology [ 15 , 20 , 31 ]. To visualize the cell nuclei, counterstaining can be performed using 4′,6-diamidino-2-phenylindole (DAPI) [ 27 ], a blue fluorescent DNA stain, or ToPro3 stain [ 28 ] with a far-red fluorescence spectrum.…”

“…Another approach to study Aβ proteotoxicity in Drosophila is to generate the production of the Aβ peptide through the processing of AβPP by BACE1 and by endogenous fly γ-secretase [ 14 , 20 , 31 , 48 , 49 ]. Studies using this AβPP-BACE1 Drosophila model of AD have shown that the toxic effect per amount of detected Aβ1-42 in the fly is higher when the peptide is produced by AβPP processing compared to when expressed directly from the transgene.…”

Reduction of NgR in perforant path protects neuronal morphology and function in APP/PS1 transgenic mice

“…In AD models, neuronal death caused by programmed cell death often exhibits the following features such as, cell membrane blebbing, cell rounding, breakdown of cytoskeleton, nuclear pyknosis, cytoplasmic condensation, chromatin fragmentation and clumping, leading to the generation of apoptotic bodies [ 29 , 30 ]. APP/PS1 transgenic mice present morphological and functional changes in neurons predisposing neuronal death.…”