Exploring the cell biology of neuronal ageing and the underlying mechanisms

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With the increase in life expectancy people live long enough to suffer from the negative effects of ageing. The single most important health problem affecting elderly people and profoundly impacting on the quality of life is mental or cognitive decline due to the decay of brain tissues. Some of the most important but unresolved questions about the deterioration of brain functions during normal aging are: What is the nature of these age-related alterations in neurons? What (patho-) mechanisms cause the alterations? Can they be prevented or treated? Answers to these questions will provide explanations for the decline in brain function during normal ageing, and be a starting point for the development of strategies to ameliorate cognitive decline. Various subcellular changes reported for neurons of aged brains include protein aggregation, cytoskeletal decay, axonal atrophy and deterioration of synapses culminating in neuronal communication deficits. We hypothesise that these changes are triggered by deficiencies in intracellular degradation systems, i.e. the ubiquitin-proteasome and auto-phagosome-lysosome systems, which are both necessary to clean cells from unwanted and damaged proteins and organelles. In addition, the TOR and IIS signalling pathways are known orchestrators of various cellular processes and have been linked to longevity at the organism level. We hypothesise that these pathways influence neuronal ageing through regulating the intracellular degradation system.Testing this hypothesis requires neuron models of ageing in which manipulations of candidate mechanisms can be combined with detailed readouts for subcellular processes. Performing such studies in mammalian species is extremely lengthy (due to age scales), laborious and expensive, and it raises ethical concerns. Here we introduce a neuron model of ageing in the brain of the fruit fly Drosophila. Drosophila shows behavioural features of ageing, such as a decline of learning performance. Correlating with this phenomenon, our model neurons display gradual build-up of morphological features that mimic prominent hallmarks of ageing in the primate brain. These include: 1) tangle-like aggregates of Tau protein reminiscent of brains affected by Alzheimer's or ageing, 2) destabilisation of axonal microtubule bundles, 3) the occurrence of dystrophic axons, and 4) the decay of synapses. On this project we will develop this repertoire of readouts further by incorporating live imaging of organelle dynamics, axonal transport, ultrastructural analysis and neuronal activity. More importantly, we will apply our readouts as a unique opportunity to test our working hypothesis. For this, we will capitalise on the highly efficient combinatorial genetics of Drosophila and use readily available genetic tools to manipulate the ubiquitin-proteasome and autophagosome-lysosome systems, as well as the TOR and IIS pathways. We will assess whether and how these manipulations affect neurons through robust quantification provided by the age-related readouts in our model, thus taking systemic knowledge about ageing mechanisms to the subcellular level of neurons.On the one hand, these studies will consolidate our model and establish its use for future studies of neuronal ageing - with highly efficient experimental means and the unique ability to perform such research fast and cost-effectively. On the other hand, we will provide mechanistic insides derived from our hypothesis. Since our model shows characteristic hallmarks known from the ageing human brain, this approach has a great potential to advance our knowledge which can then be fed into the translational pipeline.