Possible link between Alzheimer’s disease and gut microbiome found in mice – new study

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Scientists may have found an unlikely contributor to Alzheimer’s disease: bacteria in the gut. Their study, published in the journal Science, suggests that certain gut bacteria increase the brain damage seen in mice with Alzheimer’s-like disease.

In Alzheimer’s disease – the most common form of dementia – proteins in the brain accumulate abnormally. One of these proteins is called tau, and its accumulation causes the death of brain cells involved in memory, causing forgetfulness and eventually permanent memory loss.

The researchers worked with a group of mice with a genetic mutation that causes the tau protein to build up in the brain. The mice were divided into two groups: one group had a healthy gut microbiome (the collection of bacteria, viruses and fungi that live in the gut), while the other had no gut microbiome at all (these mice are called “germ-free”). By the time they reached old age, the germ-free mice had significantly less tau brain damage.

The germ-free mice also had lower levels of tau-related inflammation in the brain. There is a high level of inflammation in the brains of people with Alzheimer’s disease, which contributes to brain damage. Thus, the lower level of encephalitis may be one of the reasons for less brain damage in these mice.

One of the main causes of Alzheimer’s brain inflammation is the activation of a type of immune cell known as microglia. Microglia are known to be regulated by the gut microbiome, which may explain why germ-free mice had lower levels of active microglia.

Removing the gut microbiome is not the solution

If you’ve heard of the gut microbiome, you may know that it’s usually considered important to our overall health. So the suggestion that a lack of microbiome could reduce the severity of Alzheimer’s disease is surprising. However, if you’re wondering whether removing your gut microbiome can reduce your chances of developing the disease, you may be disappointed.

Everyone has a unique gut microbiome that is home to millions of different types of bacteria, some of which are beneficial and some that can be harmful to our health. One explanation for the study’s findings is that only a certain type of bacteria is involved in increasing brain inflammation and brain cell death caused by tau.

The researchers tested this idea by giving antibiotics to a third group of mice that had a healthy gut microbiome. The antibiotics reduced levels of inflammation in the brain as well as microglial activation, although only in male mice. Interestingly, the antibiotics only reduced certain types of gut bacteria, so it may have been one of those bacteria that helped cause brain damage in the mice.

It is not clear why the antibiotics only improved performance in male mice but not in female mice. In humans, Alzheimer’s disease affects women disproportionately. It’s not clear why, but research suggests that one reason women are more likely to develop Alzheimer’s may be due to the hormonal changes that occur during menopause.

That the gut microbiome can influence the brain is not surprising. More than a decade of research has shown that there are strong communicative links between the two, known as the gut-brain axis. The gut-brain axis helps regulate many processes in the body, including the immune system and thus inflammation.

This is not the first study to suggest a role the gut microbiome may play in the development of Alzheimer’s disease in mice. There is also evidence that the gut microbiome differs in people with Alzheimer’s compared to healthy people.

While the idea that the gut microbiome is linked to Alzheimer’s disease is compelling, it’s important to interpret the study’s findings with caution. Mice, especially those with genetic mutations, are very different from humans. Their immune systems, their genetics, and most importantly, their gut microbiome have differences.

Germ-free mice also have an underdeveloped immune system. Their gut-brain axis is also affected, and studies in germ-free mice have shown changes in their behavior and brain signaling compared to normal mice. These differences may also be why the germ-free mice in the study did not develop as severe Alzheimer’s disease.

Based on current research, it cannot be argued that bacteria in the gut microbiome contribute to Alzheimer’s disease in humans. Instead, research points to the possibility that there are specific bacteria in the gut microbiome that may increase the risk of developing Alzheimer’s disease. If it can be shown that similar bacteria exist in humans that have similar effects, the possibility that targeting them could create a new basis for treating Alzheimer’s disease is an exciting prospect.

This article has been republished from The Conversation under a Creative Commons license. Read the original article.

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Catherine Purse receives funding from the Biotechnology and Life Sciences Research Council.

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