Exploring the cell biology of neuronal ageing and the underlying mechanisms

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.

A major risk factor for neurodegeneration and cognitive decline in the elderly is ageing. This project explores how the course of ageing induces changes in neurons at the subcellular level, as a novel approach combining ageing research with cell biology.

It is increasingly clear that decay in the ageing brain is not caused by the death of neurons but by discreet changes of their properties, and we need to understand the causes of such changes and the effects they have on neuronal function. We introduce here a newly established neuron model in the brain of the fruit fly Drosophila which shows gradual build-up of characteristic subcellular changes that resemble hallmarks of the ageing human brain: deterioration of axons and synapses, cytoskeletal decay and the occurrence of protein aggregates. The mechanisms that drive such changes are not understood, neither in humans nor in Drosophila.

We propose that key changes are caused by functional decay of ubiquitin-proteasome and auto-phagosome-lysosome systems (both necessary to clean cells from unwanted and damaged proteins) which are themselves under the control of signalling pathways involved in ageing, in particular the TOR and IIS pathways.

Here we will capitalise on our new model, powerful genetic approaches of the fly and readily available genetic tools to test our hypothesis. For this we will activate or inhibit the ubiquitin-proteasome and auto-phagosome-lysosome systems and/or TOR and IIS pathways and assess changes in the appearance of the subcellular readouts for neuronal ageing. To further refine our ageing model, work on one objective will add live analyses of axonal transport, neuronal activity, organelle dynamics and ultrastructure studies as further readouts, which are expected to change with age and help to further refine our studies.

In conclusion we will deliver a genetic network that underlies ageing processes in neurons, providing potential explanations for brain decay in the elderly.

From the public health and economic perspective, the increase of longevity has led to a sharp increase in the numbers of elderly patients with age related conditions. Also it is now becoming clear that ageing is the most important risk factor in neurodegenerative diseases such as Alzheimer's and Parkinson's, predicting a sharp increase in economic burden for our society in future. However, the process of ageing remain poorly understood. An important and pressing challenge is to establish the cellular and molecular processes causing and accompanying decay with age, so that we can find therapies to prevent them.

A key event during ageing is the decline in brain function caused by discrete changes in neurons. This project addresses the nature of these changes and the mechanisms causing them. Through our basic research we expect to provide a better understanding of the processes of brain deterioration during normal ageing with important implications also for neurodegenerative diseases. We hope to provide new avenues for future research with potential long-term benefits for social well-being. This will be of interest to the commercial and private sector involved in finding ageing remedies, to other scientist, and to the general public.
Our FIRST PATHWAY TO IMPACT concerns the commercial and private sector in various ways: (1) in the context of neurodegenerative diseases, such as Alzheimer Disease (AD) and other tauopathies, there is great emphasis on the clinical development of drugs that inhibit Tau aggregations in order to prevent and cure AD. In our model of brain ageing we find aggregation of endogenous Tau as a consequence of "normal" ageing. To our knowledge, this is the first description of such a phenomenon in model organisms, and could be used as a proxy to screening for drugs in order to prevent Tau aggregation in human patients. Such screenings in our model would be highly reliable, cheap and fast, enhancing the discovery of drugs to prevent and cure AD. (2) In the context of mental or cognitive decline affecting the elderly, our work will deliver factors and mechanisms that suppress or even revert the hallmarks of neuronal ageing. Such factors are likely to represent promising targets for the development of drugs to ameliorate age related cognitive decline. I will therefore engage in discussion with companies about these two possible applications of our model.
Our SECOND PATHWAY TO IMPACT is to achieve worldwide coverage and impact of our research. For this, we will disseminate our work by a) publishing in high impact journals and contributing book chapters; b) presenting our work on national and international research conferences; c) further developing on-line resources including a tab on our lab webpage explaining the main objectives, strategies and outcomes of the project; d) initiating and contributing to scientific multi-author blogs to discuss and inform about main breakthroughs in the area of our research (including our own work).
The THIRD PATHWAY TO IMPACT regards public communication and awareness. Public opinion has an increasing impact on funding decisions, we will foster public awareness and understanding of the importance of science and research. The focus of our research on the brain is appealing to the public and generates fascinating images. To impact on the public we will: a) present our work on open days and in public lectures, and for this the University of Liverpool has long experience in delivering high quality activities on a local, national and global level; we will make frequent contributions within this framework; b) we will develop a special section for non-specialist audiences on our webpage to inform and enthuse the public about our research; c) we will use the knowledge and experience we have on the brain to provide materials and concepts for public displays or other forms of communication whenever opportunities arise.