Using an in-vivo Drosophila model to determine the influence of age and hyperphosphorylated tau on neuronal structure and function (2024)

Abstract

Although ageing is considered the leading risk factor for Alzheimer’s Disease (AD), knowledge of what occurs to neuronal morphology and cytoskeletal structures during physiological ageing is poor. This is interesting due to the known influence of cytoskeletal destabilisation in AD pathogenesis. Further, how age-related increases in tau phosphorylation influence neuronal morphology outside of disease is yet to be uncovered. I aimed to investigate how ageing and age-related increases in tau phosphorylation influence the morphology of neurons and the contribution of cytoskeletal destabilisation to such changes. I take advantage of the short life span and superior genetic tractability of Drosophila Melanogaster to investigate how physiological ageing influences neuronal morphology and if age-related increases in tau hyperphosphorylation can recapitulate morphological abnormalities that occur with age. I have developed optical imaging protocols using high-resolution Structured-Illumination-Microscopy and confocal line scanning microscopy to document morphological changes in the somatodendritic and axonal neuronal compartments of GFP-labelled OR47B olfactory receptor neurons. Through this, I find that neuronal morphological changes accompany physiological ageing. This includes reductions in cell body, dendrite and axonal size and the appearance of dendritic and axonal swellings. Although the microtubules of these neurons appear compromised with age, dietary supplementation of a microtubule stabilising peptide (NAPVSIPQ) only partially rescues age-related morphological changes at the axonal but not somatodendritic compartments. Moreover, the expression of human tau within young OR47B neurons demonstrates morphology that closely mirrors young controls. However, the targeted expression of a phosphomimicking designer mutant of human tau in young circuits, modelling age-related increases in tau phosphorylation, phenocopies morphological changes that are seen in physiologically aged neurons. This includes reductions in cell body and axonal size, accumulation of axonal and dendritic swellings and additionally the loss of cell bodies. Cytoskeletal fortification via NAP supplementation partially recovers cell death and morphological changes at the axon but, similar to aged neurons, does not influence somatodendritic morphology. In conclusion, neurons suffer from age-related morphological abnormalities that are phenocopied in young circuits through hyperphosphorylated tau. Cytoskeletal destabilisation is one of the drivers of these morphological changes but other mechanisms such as protein accumulation/aggregation likely contribute equally. Such insults might increase vulnerability to neurodegenerative diseases and functional impairment as we age and this work has revealed important hallmarks of physiological ageing which might lead to aberrant processes. However, more must be done to understand the additional triggers and functional consequences of age and phospho-tau-induced morphological abnormalities.