Is Alzheimer's disease triggered by a failure of the brain's blood supply?

Many risk factors for Alzheimer's disease (AD), such as stroke, altered blood pressure and APOE4, the main genetic risk factor for AD, are associated with a decrease in blood flow to the brain. This decrease in blood flow may trigger AD by reducing local oxygen levels in small regions of the brain, promoting production of beta amyloid, which is toxic to neurons. We want to find out whether this is the case, because then treatments could be targeted to protect against AD by increasing brain blood flow and brain oxygen levels, reducing the occurrence of the disease.

We have already found that mice expressing human APOE4 show early decreases in blood flow in some, but not all, blood vessels, and these vessels are less able to dilate to increase blood flow when nearby neurons are active and require more energy. This supports the idea that APOE4 leads to an early failure of the brain to supply enough energy to active brain tissue, and that oxygen levels in APOE4 brains are likely to become inadequate near these vessels. Now, we want to test whether this decrease in blood flow promotes the build-up of beta amyloid in tissue fed by these vessels, leading to changes in neuronal activity and behaviour.

We will test this by breeding mice that express APOE, and a form of beta amyloid that can be switched on and off, and a protein that tracks neuronal activity. In our first experiment, we will record blood vessel function, brain activity, blood oxygen levels, and amyloid plaques in these mice while they are alive. We will first identify which blood vessels don't work well in APOE4 mice, then "switch on" beta amyloid production to see if beta amyloid aggregates more around these dysfunctional blood vessels than around those where blood flow is normal, and whether the neuronal activity near these vessels changes as beta amyloid accumulates. Because beta amyloid itself impairs blood vessel function, we expect that the functioning of these blood vessels will become more and more impaired, decreasing overall oxygen levels in the brain and accelerating the build-up of beta amyloid and neuronal damage.

In a parallel experiment, we will look at post mortem tissue from these and other mice, to test whether the regions where beta amyloid first builds up already have low oxygen levels, and which enzymes and organelles within the cell might be responsible for triggering its accumulation. We will do this using antibodies against beta amyloid peptides, proteins that exist at increased levels when oxygen is low and proteins that are found in specific compartments of the cell. To check that blood vessels don't work so well just because the brain is already using less energy, we will also measure the rate at which brain slices consume oxygen and track how this is affected by APOE4 and beta amyloid.

We expect that the areas of the brain that are affected earlier in AD will have the lowest levels of oxygen and vascular function, even before beta amyloid is produced, than those that are affected later in AD. To test whether the sensitivity of some brain regions to AD is due to increased hypoxia in these areas, we will compare regions that are affected early in the disease (hippocampus and entorhinal cortex) with an area that is affected later in the disease (visual cortex). This will allow us to understand whether early alterations in brain oxygenation and vascular function are involved in the increased susceptibility of these brain regions to damage.

Finally, we will give the mice a drug that increases brain blood flow by protecting small vascular cells, called pericytes, to test if it also prevents beta amyloid accumulation and memory impairments, to see if increasing brain blood flow and oxygenation could be a useful strategy to prevent AD.

This work will discover whether a decrease in brain blood flow could trigger Alzheimer's disease and whether preventing this decrease in blood flow could be an important therapeutic strategy.

Cardiovascular risk factors and APOE4 expression increase the risk of Alzheimer's disease (AD), possibly by decreasing brain blood flow. Our data show that brain capillaries are constricted and neurovascular coupling is impaired in APOE4 mice. Such decreased cerebral blood flow could trigger a vicious cycle of beta amyloid production, accumulation and progressive impairment of brain blood flow. If this "vascular hypothesis" of AD is correct, improving cerebrovascular function at early stages should reduce beta amyloid accumulation and subsequent neurodegeneration.

We will track the time course of the development of cerebrovascular, neuronal and behavioural dysfunction in mice that carry APOE4 and produce beta amyloid in a controllable manner, testing the prediction that beta amyloid should accumulate preferentially around already dysfunctional blood vessels, before discovering if improving cerebral blood flow reduces beta amyloid accumulation. To do this, we will generate mice expressing human APOE3 or 4, and doxycycline-suppressible human APPSwe/Ind. After baseline characterisation, doxycycline will be removed, enabling production of beta amyloid. First, we will track beta amyloid accumulation and neurovascular function in vivo, using 2-photon imaging to test whether it first aggregates in tissue surrounding blood vessels that already show impaired flow and neurovascular coupling. Second, we will immunohistochemically label unaggregated forms of beta amyloid at different time points, alongside markers of the vasculature, hypoxia, beta amyloid degradation and lysosomes, to test whether production and aggregation both start in hypoxic locations. Finally, we will pharmacologically increase cerebral blood flow and test if this reduces beta amyloid accumulation and improves neurovascular reactivity and cognitive function. Our project will test whether decreased brain blood flow could trigger AD, suggesting potential new therapeutic strategies.

Our research will reveal how decreases in local blood flow caused by the main genetic risk factor for Alzheimer's disease (AD), APOE4, promote hypoxia, beta amyloid accumulation and progressive classic AD pathology, including synaptic changes and the development of cognitive deficits. Understanding these changes and their causal links will provide important insights into the onset of Alzheimer's disease. The findings will therefore be of interest to a range of stakeholders including academics, health policy groups, the public and the pharmaceutical industry, and we will maximise the impact of our research by engaging these different groups:

Academics, including neuroscientists, psychologists and pharmacologists (short, medium & longer term). Aside from capacity building, in which the postdoctoral fellow and technician will benefit from developing the advanced approaches used in these studies, the research spans multiple fields and scientific sub-disciplines (neuroscience, cardiovascular physiology, psychology) so the relevance of our findings to local, national and international academic audience will be widespread (see Academic Beneficiaries section for details).

Governmental and non-governmental research and policy organisations (NGOs; medium & longer term). Our work examines the effects of the main genetic risk factor for AD on the interactions between vascular and brain function that could initiate AD. It is of relevance for the development of public health policies to promote cardiovascular health, which could potentially reduce the incidence of AD by protecting blood vessel function. It also has implications for what might be the most promising pharmacological interventions to protect blood vessel function at pre-symptomatic stages of AD in at risk populations such as APOE4 carriers. Our results will therefore be valuable for informing evidence-based governmental policies (e.g. Department of Health/Public Health England) to promote healthy eating and an active lifestyle, and Health Care providers (NHS Choices), who may be interested in planning for early interventions in genetically at-risk people.

The general public, through an increased understanding of the impact of lifestyle on vascular and therefore cerebral and cognitive health (medium & longer term). This could enable positive individual changes and community-based support to inform and influence others. There is already widespread public understanding about the theoretical importance of good cardiovascular health. However, our work will illustrate the consequences of vascular dysfunction on brain function, so effective communications of our results will have the opportunity for significant impact in motivating people to adopt lifestyle choices that promote cardiovascular health.

Pharmaceutical industries involved in discovering novel therapeutic targets and compounds for the prevention of vascular dysfunction and neurodegenerative disease (longer term). The pharmaceutical industry will benefit from an improved understanding of how disruptions to brain blood flow promote the accumulation of beta amyloid. We predict our research will enable targeting of different processes at progressive stages of the disease, and will provide a model to test interventions at these different stages. For example, we predict that early interventions that promote vascular function (reactivity) will be effective at slowing or preventing the disease onset, while later interventions may need to be able to reverse pathological changes that have already occurred (e.g. beta amyloid build-up, vascular constriction). The effectiveness of differently timed pharmacological interventions at preventing vascular dysfunction, hypoxia and neuronal and cognitive change can all be tested in our model. Our project provides proof-of-concept for this approach and will be further developed in the future in collaboration with the Sussex Drug Discovery Centre.