Coming theses from other universities

The work presented in this thesis concerns two different Drosophila models of Alzheimer’s disease (AD), namely the Aβ- and the AβPP-BACE1 fly model.

Globally, at least 55 million people suffer from dementia, wherein AD accounts for 60-70 % of all dementia cases. The prevalence of dementia is expected to increase alongside an ascending world population. Even though there are genetic risk factors, more than 90% of all AD cases are sporadic, meaning that there is no disease pattern within the family. One of the main toxic events in AD is neurodegeneration, which is defined as widespread neuronal damage in the brain, culminating in locomotor impairments, memory loss, and premature death.

Proceeding to basic biochemistry, almost all proteins must be folded correctly for them to gain their functionality. If this folding pathway is disturbed, for example, by a mutation located in the protein sequence, the protein starts to misfold and may cause harm to the cells. This is the basis for protein misfolding diseases, in which AD is categorized. The cause of AD has not yet been wholly determined, although it is generally accepted that the amyloid-β (Aβ) peptide plays a crucial role in the pathology. Aβ is derived from a larger protein called the amyloid-β precursor protein (AβPP), which is cleaved by the enzymes β-site amyloid β precursor protein cleaving enzyme 1 (BACE1) and γ-secretase. Different Aβ variants are generated due to mutations or imprecise γ-secretase cleavage. Aβ misfolds very easily and, therefore, has a high tendency to aggregate, creating larger structures such as oligomers, protofibrils, and amyloid fibrils. This diversity among Aβ aggregates makes it challenging to identify which species that participate in toxic mechanisms, although there is evidence that oligomers and protofibrils promote neurotoxicity. Luminescent conjugated oligothiophenes (LCOs) are molecular probes that emit light upon binding cross-β-sheet structures and can thus be employed in optical imaging to facilitate the characterization of aggregates.

Drosophila melanogaster, commonly known as the fruit fly, has been used to shed light on the biomedical sciences for over a century. Initially, it was used for genetic research, but its significance in neuroscientific studies has increased in later years. It is relatively simple to alter the fly genome and allow disease-related proteins to be produced in a specific tissue or cell type. Drosophila is an excellent model for examining AD because the flies exhibit surprisingly similar disease symptoms and characteristics as humans.

The first part of my thesis describes the pathogenic processes that contribute to neurodegeneration by using two different AD Drosophila models: the Aβ-model, where Aβ is cloned into the genome and directly expressed, and the AβPP-BACE1 model, where Aβ is generated through AβPP processing. Neuronal cell death and oxidative stress were seen in both fly models. However, the levels of Aβ1-42 and the effect on the endo-lysosomal system differed, suggesting that there are different toxic mechanisms responsible for the death of these flies. The Aβ peptide's inherent properties decide the aggregates' heterogeneity and toxicity. Moreover, AβPP-processing resulted in a build-up of cleaving products that possibly have a toxic impact.

The second part focuses on expressing Aβ in the fly gut instead of the brain. The selected Aβ variants are wildtype Aβ1-42, the arctic mutant of Aβ1-42 and the tandem construct T22Aβ1-42, where the two latter are very aggregation-prone. With this model, one can study toxic mechanisms behind AD and examine whether orally administered drug molecules can modify proteotoxicity. The gut-based fly model exhibits similar phenotypes as models where the central nervous system is the target site; phenotypes include reduced survival relative to control flies and the presence of aggregates and apoptotic cells. Interestingly, the expression of the Arctic mutant Aβ1-42 and T22Aβ1-42 resulted in an equal reduction in survival, but the Aβ load was significantly greater in T22Aβ1-42 flies. This indicates that the mortality rate does not correlate with the number of aggregates, which is consistent with findings from other researchers. Instead, a specific type of Aβ species likely exerts a high toxicity effect in these flies. There is a diversity among aggregates originating from the Aβ peptide, and interpreting the aggregation process is essential to understanding the toxic mechanisms behind AD. No cure for AD is currently available. However, more research in this field will provide valuable tools to combat AD.