Morphological and functional abnormalities in neuromuscular junctions of Drosophila melanogaster induced by human APP gene expression

Alzheimer’s disease (AD) is a neurodegenerative disorder characterized by a loss of neurocortical and hippocampal synapses that precedes amyloidosis and neurodegeneration and closely correlates with memory impairment. Mutations in the amyloid precursor protein (APP) cause familial AD and result in increased production of amyloid-β-protein (Aβ). To gain insight into the synaptic effects of APP protein in AD patients, wild-type APP, its mutant form APP-Swedish responsible for familial AD, and human beta-secretase gene were expressed in motor neurons of Drosophila melanogaster larvae. It was found that targeted expression of APP (APP-Swedish) in Drosophila larval motor neurons caused significant morphological and functional changes in neuromuscular junctions (NMJs)-a dramatic increase in the number of synaptic boutons and altered exocytosis revealed by incorporation of the styryl dye FM4-64. Analysis of the number and distribution of mitochondria showed that motor neurons overexpressing APP (APP-Swedish) had a significant reduction of functional mitochondria in the presynaptic terminal. Significant synaptic abnormalities were observed with APP (APP-Swedish) expression, as well as for double transgenes bearing APP (APP-Swedish) and human beta-secretase (BACE), which caused secretion of amyloid beta protein (Aβ). We suggest that APP participates in regulation of synaptic functions and its elevated expression leads to synaptic pathology independently from Aβ neurotoxic effects.