Modeling Alzheimer's Disease in Drosophila melanogaster

Alzheimer's disease (AD) is an age-related neurodegenerative disease characterized by presence of neuritic plaque, neurofibrillary tangles, synaptic dysfunction, and synaptic loss leading to loss of memory. In developed countries like the US, increased life expectancy has caused an increase in prevalence of age-related disorders like AD. The molecular mechanism behind the etiology and the pathology of AD is still unclear. Thus, until now there is no cure for AD. Transgenic model systems are of great value for understanding the pathophysiological basis of many neurodegenerative disorders. Simple organisms like the fruit fly, Drosophila melanogaster, can be easily genetically and pharmacologically manipulated. It has proven to be a powerful model system for studying complex human neurodegenerative disorders like AD. The data generated from fly models is translatable to mammalian systems. In this thesis, we describe genetically modified fly AD models that are able to successfully recapitulate AD symptoms. Both of our models express human AD-associated proteins APP695 and BACE genes in the Drosophila central nervous system. While modeling AD-related synaptic loss we used Drosophila larval neuromuscular junction, which is glutamatergic synapse in flies. We observed that the larvae expressing APP and BACE showed defective synaptic functioning with decreased connections, altered mitochondrial localization, and decreased post-synaptic proteins. Further, the symptoms were alleviated in the larvae that were fed on the [gamma]-secretase inhibitor, L685,458. Overall, this model could recapitulate the synaptic loss and dysfunction associated with AD. The other model described in this thesis, accounts age as the prime factor in modeling AD. The temperature dependency of GAL4/UAS system was exploited in order to develop this model. Thus, the APP and BACE were expressed on a constant low level throughout the fly lifespan. We observed that flies expressing APP and BACE showed an age-dependent AD symptoms like neuronal dysfunction, loss of neuroanatomical areas associated with learning and memory, increase in amyloid load, loss of memory. We argue that the models described in this thesis will act as powerful tools for understanding AD etiology and for rapid testing of potential therapeutics. Furthermore, to aid in rapid testing of genetic and pharmacological targets, we have developed analysis software that quantifies Drosophila courtship index, a parameter used to compute the learning and memory of flies using Courtship suppression assay.