Dementia isn’t just a part of ageing. It’s caused by diseases, of which Alzheimer’s disease is the most common accounting for two-thirds of cases.
As September is World Alzheimer’s Month, we’ve put together a guide to what we know about the causes of Alzheimer’s disease.
What happens in the brain in Alzheimer’s disease?
Over the course of Alzheimer’s disease, nerve cells in the brain become damaged and die.
This causes distressing symptoms such as memory loss and thinking problems, that get worse over time.
Alzheimer’s would be easier to understand if it was as simple as a chain of falling dominoes, where a single, small nudge sets off the chain reaction. Instead, we’re learning that different things contribute to the chain reaction leading to Alzheimer’s.
And, unlike dominoes, which fall in seconds, the steps that lead to Alzheimer’s take place over many decades. Getting as much detail as we can about each step is vital, as it highlights key processes that could unlock the search for life-changing treatments.
What’s driving this damage?
One of the theories that scientists have for what drives Alzheimer’s is called the ‘amyloid cascade’ and is based around a protein called amyloid.
First, amyloid sticks together to form abnormal clumps between nerve cells in the brain. These clumps, known as ‘plaques’, start a damaging chain reaction within nerve cells, which gets in the way of how our brain cells work.
This amyloid build-up has a range of unhelpful knock-on effects for brain cells. It can affect how cells use energy, how they communicate and how they clear away waste.
Amyloid also triggers our immune system to kick in. Microglia are immune cells that would usually clear up waste to maintain a healthy brain environment. But in Alzheimer’s, they are pushed to their limits and start to make things worse.
One of the ways they do this is by promoting inflammation. Inflammation is a natural process that happens when we get an injury or an infection: in the body, it’s what causes the area around a cut to go red and swell as part of healing. However, scientists are finding out that inflammation in the brain might do more harm than good, damaging healthy brain cells and their ability to fight against the toxic proteins.
Amyloid can also disrupt blood flow in the brain, by building-up in blood vessel walls and affecting the ability of blood vessels to respond to changes in demand. Blood flow is vital to the brain, providing cells with oxygen and food to function. When this delivery is disrupted, nerve cells become unhealthy, inflammation kicks in and healthy nerve cells can be damaged too.
Another protein involved in Alzheimer’s disease is Tau, which is inside our nerve cells and helps with structure and transport.
In Alzheimer’s, tau also starts to behave out of character. Scientists think that these changes may be due to the build-up of toxic amyloid too. These changes cause tau proteins to stick to each other to form tangles inside nerve cells.
Abnormal tau can spread across the brain, passing from cell to cell, like a wildfire. This spread causes further brain damage.
The appearance of tau tangles in the brain coincides with the onset of Alzheimer’s symptoms, so researchers believe it may be a key final step in the toxic chain of events that causes nerve cells to die.
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