Relating changes in synaptic function to cognitive decline during normal healthy ageing

As our society grows older, the consequences of ageing on quality of life becomes ever more important. A common feature of normal 'healthy' ageing is a gradual cognitive decline. The onset and rate of this decline differs between individuals and we currently know very little about how and why this is the case. We will do this by measuring the changes in both brain structure and function that take place during normal ageing in mice. This is a vital step in the development of any therapeutic strategies for the management of ageing. In humans, age-related changes in brain volume have commonly been observed in the prefrontal cortex. This brain area plays an important role in shaping our personality and controls 'executive' functions such as decision-making. A loss of these executive functions is likely to underlie some of the most damaging aspects of age-related cognitive decline. Until now, data obtained from non-invasive human brain imaging studies have been unable to provide any mechanistic insight into the nature of the changes. Animal studies are, therefore, vital if we are to better understand the biology of the ageing process.

In this proposal we will label nerve fibres arriving in the prefrontal cortex from different regions of the brain, including the hippocampus - a structure crucial in memory storage. We will measure the strength of these different connections using a new method that allows us to activate nerve fibres by illuminating them with blue light. We will be able to measure how both the arrangement and strength of nerve fibres change during the course of normal ageing. Importantly, these brain mapping techniques will be applied after we have recorded the electrical activity directly in the brain during behavioural tasks that require the prefrontal cortex to be active (e.g. a decision making task). In this way we will map the structure and function of nerve cells in the prefrontal cortex and directly measure whether changes in brain function are responsible for impaired cognitive performance. Importantly, we will compare the brain structure in in age-matched animals that do and do not exhibit cognitive decline. Finally, we use molecular biological tools to introduce a toxin that blocks nerve communication between cells in the prefrontal cortex. By gradually decreasing the amount of nerve communication in the prefrontal cortex, we seek to test the hypothesis that decreased connectivity in the prefrontal cortex underlies cognitive decline.

Our three laboratories are in a unique position to explore structural and functional changes in the prefrontal cortex during normal ageing. The multidisciplinary nature of this collaboration will provide data from single brain cells and neuronal networks in order to reveal the fundamental biological mechanisms responsible for the ageing. A particular focus of this proposal is to understand why the rate of ageing appears to differ between individuals, and to do this we will study and compare the brains of animals that do and do not exhibit cognitive decline. We will explore the possibility that healthy ageing is associated with a high degree of robustness in terms of the structure and function of brain connectivity. Alternatively, healthy ageing may well be characterized by compensatory changes that can preserve cognitive function and protect the brain against age-related frailty.

Cognitive impairment is a prominent feature of ageing that is correlated with structural and physiological changes within the prefrontal cortex (PFC) but little is known about the consequences of ageing at defined PFC synapses. We will explore whether local intra-cortical synaptic transmission is impaired using paired recording from PFC neurons in young, mature and old C57Bl6 mice (6, 12 & 24 months old). Long range connectivity will be examined following stereotaxic injection of adeno-associated virus (AAV) encoding GFP. After waiting ~2 weeks, we will map PFC connectivity between cortical regions and from the hippocampus, using a newly installed TissueCyte 1000 system to acquire high-resolution 2-photon tomography data. Optogenetic circuit mapping will be used to measure synaptic efficacy in vitro using the ntsr1-Cre mouse line and FLEXed Channelrhodopsin constructs to specifically target ChR2 to the axons of layer 6 neurons that project to the PFC. Non-FLEXed AAV viruses will be used to target inputs from other brain regions. Critically, these anatomical/functional mapping experiments will be performed on animals that have previously been exposed to in vivo electrophysiological and behavioural measurements to assay network behaviour and cognitive function. We have adapted a delayed response task to test working memory function in head-fixed mice running on a linear treadmill. We will make extracellular population recordings from animals performing this task and directly relate cortical population dynamics to cognitive performance. Finally, we will adapt the viral delivery technique to introduce tetanus light chain toxin to block synaptic transmission. By increasing the number of transfected neurons we will reduce the contribution of PFC inputs and, therefore, test a central hypothesis that changes in PFC connectivity underlie cognitive decline and help identify the fundamental mechanisms responsible for cognitive decline during normal ageing.

Our proposal directly addresses the BBSRC strategic priority and grand societal challenge of an ageing human population. Cognitive impairment of one of the most significant features of normal ageing that causes a disparity between lifespan and healthspan, but we understand very little about the mechanisms involved in this process. By increasing knowledge about the mechanisms of healthy brain ageing, we have the potential to generate major impacts for society and the economy. Fundamentally, ageing effects every living creature, and thus we believe our results will be of great interest to scientists, the public, and policy-makers alike.
Our plan of work brings together investigators with complementary technical expertise. Overall, we can maximize impact by combining the most advanced methods in electrophysiology, anatomy, molecular biology and behaviour, driving novel discoveries. As a multi-disclipinary team, we are uniquely placed to address mechanisms of brain ageing comprehensively at the multiple biological levels.

This work is expected to benefit (1) other scientists, (2) industrial partners, (3) government policy makers, and (4) the wider public.

(1) The impact of this research on other scientists will be seen as new research questions are generated due to a greater understanding of the normal ageing process. We will publish research papers in open access journals, and make as much of our data and analysis tools freely available via sharing portals such a Carmen Virtual Laboratory (https://portal.carmen.org.uk). We will present our results at national (British Neuroscience Association, The Physiological Society) and international (Society for Neuroscience, Federation European Neuroscience Associations) meetings to ensure that key findings are disseminated to other scientists at the earliest opportunity.

(2) Data from the ageing brain of a mammalian model system will be important for the life sciences industry in developing rational therapeutic strategies for the treatment of senescence. The data generated during this proposal will inform both the pharmaceutical and neurotechnology industries, in their development of therapeutic strategies for combating senescence. Open access publications and participation at national/international scientific meetings will ensure that the benefits of this research can potentially be realised at the earliest opportunity.

(3) Government policy concerning the allocation of resources to deal with the societal impact of ageing requires reliable scientific data. In 2002, the societal impact of ageing was estimated to have cost the UK economy £5.4 billion with 550,000 people diagnosed with dementia. An important focus of this proposal is longitudinal monitoring of ageing in a model system that could potentially target resources to 'critical periods' during the lifespan. Moreover, it is conceivable that by identifying the characteristics of normal ageing public funding could be more targeted to increase the impact of therapeutic interventions to sensitive periods of life. Understanding the characteristics of normal ageing will provide a backdrop against which it is possible to predict the effects of pathological diseases such as Alzheimer's.

(4) The wider public will benefit from an increased understanding of the fundamental mechanisms of the ageing process. We will participate in and organize opportunities for the general public to gain a conceptual understanding of the research. Prior to publication, our press office will contact media outlets to promote our research. Given the broad and general interest underpinning the topic of ageing, we expect that our results will be covered by these organisations. Imperial College holds an annual Festival weekend (in May) at South Kensington where members of the public come into the College for interactive and fun scientific displays in the campus We look forward to presenting the outcomes of this research in future years.