A zebrafish screen to identify genes affecting working memory and age-related cognitive decline.

With advances in diet and health care there has been an increase in the general age of the population. With increases in age comes an increase in age-related disease including a decline in mental ability; As animals age their memory and ability to learn new things as well as to adapt to changes in their surroundings declines. However, this physical and mental deterioration is not uniform; there are many who continue with good health well into their 80s and 90s whilst others suffer increasingly debilitating mental decline and disease. There are many factors that influence healthy ageing including diet and general life style but it is also influenced by ones initial mental ability and genetic makeup. Here we aim to identify genes that influence mental ability and age-related deterioration in memory and learning using zebrafish as the experimental system. Zebrafish have become an established system for the identification of genes affecting human development and disease as many genes and the cell biological processes the genes regulate are conserved between fish and humans. Zebrafish are an ideal system in which to search for genes affecting mental ageing as, in addition to conservation of genetic structure and neural processes, zebrafish show gradual ageing over a 2 year period and are capable of performing behavioural tasks similar to those used in mammals and humans to assess mental ability and mental decline. Above all, it is relatively easy to generate lines of fish carrying genetic mutations and many loss of function lines that can be used in our studies already exist.

Here we take advantage of the availability of lines of zebrafish that carry mutations disrupting the function of individual genes to identify genes that affect memory and learning and age-related deterioration of these processes. We shall assess individuals from 50-75 different families of fish that carry mutations in specific genes for performance in behavioral tasks commonly used to assess working memory and response times (delayed matching to sample, 5 choice serial reaction time), at 2 different ages: young (6 months) and old (2 years). Fish that perform very well or very badly in these tasks when compared to a normal population are kept and an additional 20-40 siblings from the family assessed in the same tasks. When all mutant members within a family show the same behaviour, the mutation is taken as affecting a gene that controls an aspect of memory and learning. Analysis of performance at 6 months will identify genes affecting learning, memory and response times as measures of mental ability per se. By comparing responses at 2 years with those at 6 months we will be able to identify genes affecting the rate of change in memory and response times i.e the conservation of mental processes.

The outcome of this study will be the elucidation of genes and molecular processes influencing mental ability and maintenance of mental ability in old age. It is important to gain insight into the genetics of age-related mental deterioration as it helps understanding of how environmental factors may interact with genetics to influence the rate of mental decline in old age. This knowledge helps us develop strategies to minimize the impact of ageing on society. Moreover, understanding age-related changes in mental ability sets a background against which it is possible to assess the effects of pathological disease states such as dementia.

We will identify genes affecting cognitive performance and age-related cognitive decline using forward genetic screens of zebrafish mutant lines. Zebrafish show a high degree of conservation of genetic and neural biology with mammals and can perform behavioural tasks similar to those used in humans to assess cognition and cognitive decline (e.g. delayed matching to sample, DMTS, 5 choice serial reaction time task, 5CSRTT). They are an ideal species in which to perform mutagenesis screens for genes affecting cognition and age-related cognitive decline as they show gradual senescence over a 2 year period and numerous mutant lines are already available. We shall screen existing genebreaking genetrap zebrafish mutant lines for performance in DMTS and 5CSRTT tests using a population-based approach: Starting with a population of 150-200 mutant fish, we shall identify individuals at the extremes of the population distribution curve on the assumption that individuals are there because they house a mutation affecting behaviour. We shall confirm the role of the mutation by subsequently assessing performance of 30-40 wildtype and mutant siblings in the same task. We shall determine the relative behavior of individuals from the genetrap families at 2 age points (young, 6 months and old, 2 years) to determine genes affecting i) cognitive ability (6 month performance) and ii) cognitive stability (performance at 2yr relative to 6month). We shall perform initial characterization of mutants (e.g. determine expression of markers of cellular ageing) that i) occur at either extreme of the population curve at 6 months of age or ii) show a significant shift in relative position at the two age points. As studies in fish have strong translational relevance this work will give insight into genes and cellular processes potentially affecting human cognition and age-related cognitive decline facilitating the development of ameliorating interventions.

The anticipated scientific outcome of the project is a greater understanding of genes affecting cognitive performance and age-related cognitive decline. In addition to identifying new genes causally linked to working memory and age-related memory stability, we will elucidate the cellular processes by which they act. These studies will facilitate the identification of individuals vulnerable to age-related cognitive decline and dementia and aid the development of preventative interventions and personalised treatments. Main beneficiaries of the project are:
1. Research scientists in relevant fields (both academic and industrial).
2. Clinical scientists
3. Health care professionals
4. Charities
5. Lay public (especially the smoking population)

Research Scientists. The work will benefit researchers in the areas of genetics of cognition, age-related cognitive disease and it's treatment, zebrafish researchers and basic scientists interested in the ageing process. The work benefits those interested in the genetics of cognition by identifying genes affecting learning rates, working memory and response times and by elucidating the cellular processes affected. By demonstrating the relevance of behavioural genetics research in zebrafish to human studies we provide a cost effective system and mutant lines for future studies that will benefit both researchers and funding bodies and therefore, the general public. The pharmaceutical industry is interested in using zebrafish in their drug discovery programmes. For example we work with Pfizer to develop zebrafish as a system in which to assess the abuse potential of novel compounds. Isolation of zebrafish lines that show increased vulnerability to age-related cognitive decline can reduce the number of rodents used in related studies and during the drug discovery process bringing cost benefits to the industry and contributing to the reduction, refinement and replacement of animals in science (NC3R's policy) which is a principle of UK science policy as a whole.

Clinical scientists and health care professionals. The identification of genes affecting working memory and memory stability and how they act will benefit scientists interested in developing personalised therapies for the treatment of age-related cognitive decline and dementia. Although personalised treatments are some way off, and there is currently little chance of redressing a neurodevelopmental defect in adulthood, genetic analysis may allow vulnerable individuals to be identified before development of disease and allow introduction of life style measures to minimise impacts. In the long term establishment of the cellular processes that predispose individuals to cognitive decline and effect response to treatment/ environment interactions may allow development of novel interventions.

Charities: Identification of genetic factors influencing cognitive decline and potentially dementia will be of interest to these charities. Providing a means of reducing research costs (by validating the use of zebrafish as opposed to rodents in research programmes) is of benefit to the work of these charities.

General Public. Morbidity associated with cognitive decline places a considerable burden on society. Work that helps reduce this burden contributes to both the health and wealth of the UK. In addition there are health and wealth implications of this project revolving around the potential to identify targets for novel therapeutics and to screen for potential therapeutics in zebrafish. This approach will reduce costs in the pharmaceutical industry thereby facilitating drug discovery and increasing the rate at which new drugs are brought to market, as well as the cost once there. The general public is also fascinated by the idea that their genetic make up can predict behaviour and predispose to disease. Thus outcomes of this project will be of general public interest.