Investigating the pathogenic roles of Rab GTPases in models of tauopathy

Alzheimer’s disease (AD) is a devastating neurodegenerative disorder with no cure and limited treatment options. A primary neuropathological hallmark is the formation of intracellular neurofibrillary tangles arising from the progressive aggregation of the tau protein, leading to synaptic impairment and  eurodegeneration. Rab GTPases (Rabs) have widespread functions in intracellular trafficking and have demonstrated significant therapeutic potential in animal models of neurodegeneration. To date, however, few have been studied in the context of tau pathology. Thus, this project employed Drosophila to explore and characterise the potential modulation of tau phenotypes via overexpression of several promising Rabs. Pan-neuronal expression of the human tau R406W mutant causes several disease-relevant phenotypes, including shortened lifespan, reduced viability, impaired locomotion and increased vacuolisation in the brain due to neurodegeneration. Six Rabs were identified that significantly rescue shortened lifespan when overexpressed, the most protective of these being Rab9. These data indicate the therapeutic potential of Rab overexpression in tau pathology. Quantification of vacuolation formation in fly brains showed that Rab9 overexpression also significantly ameliorated tau-induced neurodegeneration, almost restoring control levels. For this, a novel machine learning-based method to quantify vacuolation in the brain was developed, which improves upon manual quantification methods, providing a robust and efficient metric for assess phenotypic modulation. Through immunoblotting, it was shown that tau R406W expression induced impairments in autophagosome formation and an upregulation of the lysosomal endopeptidase, cathepsin L, with a possible impairment in its cleavage to the mature form. Transcriptomic analysis of tau R406W expressing flies identified several key cellular pathways, including those associated with ribosomal and endoplasmic reticulum-related pathways that were significantly altered in their expression profiles. Overexpression of Rab9 significantly upregulated genes associated with mitochondrial-related pathways, suggesting a prominent function for Rab9 at this compartment, and a possible mechanism of rescue for tau-induced defects.