As we age, blood flow becomes impaired, starving the brain of essential oxygen and nutrients and preventing the removal of waste products. The blood vessels within our brain also become leaky, allowing the entry of toxic substances from the blood causing brain tissue injury. These vascular changes can be detected up to 20 years before the clinical onset of Alzheimer's disease. The extent of blood flow reduction and degree of blood vessel leakiness correlates strongly with the onset of cognitive decline and accelerates disease pathology. We propose that drugs that can prevent or reverse damage to blood vessels will slow or even stop the development of AD (if given early enough). Recent findings using a new method to measure the expression of genes in individual cells in blood vessels isolated from post-mortem brain tissue have provided new insights into the biological processes responsible for causing blood vessel abnormalities in Alzheimer's disease. To date, however, all of these studies have focussed on end-stage disease. This may provide limited insights into the causes of vascular damage which begin prior to disease onset in Alzheimer's disease. We will therefore undertake a large study, which will measure the activity of genes within blood vessels at both early and late stages of AD, across multiple regions of the brain that are known to develop disease pathology at different stages of disease. This study will enable us to identify the genes and pathways responsible for abnormal blood vessel function likely to contribute to the onset and the development of Alzheimer's disease and will inform future studies aimed specifically at restoring vascular function in at-risk individuals. Damaged cells within blood vessels release proteins that can be detected in the cerebrospinal fluid that bathes the brain, obtained via lumbar puncture. These markers of blood vessel injury provide a window into the brain's vascular health. In complementary studies, we will measure novel markers of vascular injury in cerebrospinal fluid from living patients to determine whether these markers can be used to identify people in the very early pre-clinical stages of Alzheimer's disease. We plan to monitor the relationship between CSF levels of vascular damage and the onset of cognitive decline and markers of disease progression such as amyloid and tau levels using CSF samples from the same individuals who have been followed up and clinically assessed spanning a period of up to 20 years. These data will indicate whether vascular damage is indeed an early event in the development of the disease, as we suspect based on our preliminary work. This study will help to identify potential biomarkers that can be useful to identify the existence of vascular damage in patients in the early stages of Alzheimer's disease who will most likely benefit from drugs that can be used to restore vascular function within the brain. It will also contribute to a better understanding of the genes and molecular pathways involved in causing vascular breakdown in the early stages of Alzheimer's, which will help efforts to identify and develop novel therapeutic targets for the disease.