Novel modifiers of Aβ42 toxicity in Drosophila

BACKGROUND: Alzheimer’s disease (AD) is a progressive neurodegenerative disease that is characterized by two neuropathological hallmarks: neurofibrillary tangles composed of hyperphosphorylated tau protein and extracellular plaques composed of amyloid-β peptides (Aβ). Recent GWAS have linked PICALM and BIN1 to AD risk, and several studies have implicated PICALM and BIN1 in Aβ production and clearance. However, so far their roles in modulating Aβ42 toxicity remain unclear. AIMS: This study aimed 1.) to test whether the fly orthologues of PICALM and BIN1, lap and Amph, suppress Aβ42 toxicity and to elucidate the molecular mechanisms at work. 2.) to identify novel suppressors of Aβ42 toxicity by genetic screens. RESULTS: 1.) I showed that over-expression of the Drosophila PICALM orthologue, lap, ameliorated Aβ42 toxicity in an adult onset AD model without affecting Aβ levels. I then performed a small-scale targeted genetic screen of endocytic-exocytic genes and identified Rab5 as a suppressor of Aβ42 toxicity. Aβ in flies led to the accumulation of pre-synaptic vesicular glutamate transporter (VGlut). Lap directly interacted with VGlut and reduced its accumulation back to control levels. Consistent with this, Aβ caused an increase in extracellular glutamate levels, while lap expression reduced the Aβ-induced increase in extracellular glutamate levels, suggesting lap might play a role in modulating glutamatergic transmission. Amph, the homologue of another AD risk factor, BIN1, modulated post-synaptic glutamate receptor (GluRII) localization. Amph localisation was disrupted upon Aβ expression and restored upon lap over-expression. 2.) The rough eye phenotype (REP) of Drosophila allows for high-throughput screening of enhancers and suppressors of therapeutic targets in neurodegenerative diseases. Using the REP, my collaborators and I performed a genome-wide genetic screen to identify novel modifiers of Aβ toxicity. The primary hits were further validated using negative geotaxis and lifespan assays. I found 4 gene-knockdown suppressors of Aβ42 toxicity. 3 of these suppressors did not reduce Aβ42 levels. CONCLUSIONS: 1.) I propose a model where lap and Amph collaborate to modulate glutamatergic transmission and so affect disease development. 2.) I find Rab5, EndoA and Snap25 are potential gene-overexpression suppressors of Aβ42 toxicity, and CG15011, Larp, Pxn, and Sodh2 are potent gene-knockdown suppressors of Aβ42 toxicity.