How Common Viruses Can Cause Alzheimer's Disease
Working together, the shingles virus and herpes simplex virus (HSV) can cause inflammation and protein buildup associated with Alzheimer's disease in the brain. This mechanism appears to be responsible for the previously identified association between herpes viruses and Alzheimer's disease.
Researchers from Tufts University and the University of Oxford have shown how varicella-zoster virus and herpes simplex viruses can trigger the early stages of Alzheimer's disease. The results of the corresponding study were published in the "Journal of Alzheimer's Disease".
Widespread varicella zoster virus and HSV-1
An estimated 3.7 billion people under the age of 50 are infected with HSV-1, the virus that causes cold sores, the research team reports, citing figures from the World Health Organization ( WHO).
After the first infection, the virus usually remains asymptomatic and lies dormant in nerve cells. However, it can be reactivated and cause corresponding symptoms. The links between HSV-1 and Alzheimer's disease that have been proven so far could only be established with such reactivation of HSV-1.
Varicella zoster virus is also extremely common and around 95% of people have already been infected before the age of 20, with initial infection usually manifesting as chickenpox, the team explains. The virus can also remain in the body and cause shingles if reactivated.
Alzheimer's disease and herpes virus
Previous studies have identified a link between herpes viruses and Alzheimer's disease, but the underlying mechanisms remain unclear.
It has become clear that HSV-1 (one of the main variants of the virus) lies dormant in brain neurons and, when reactivated, can lead to the accumulation of tau and beta-amyloid proteins and the loss of neuronal function, which are typical features of Alzheimer's disease.
There is "a body of evidence linking HSV to an increased risk of Alzheimer's disease in patients," said Professor David Kaplan of Tufts University's School of Engineering.
"We know there is an association between HSV-1 and Alzheimer's disease, and there is evidence that varicella zoster viruses are involved, but we didn't know the sequence of events that trigger the viruses to trigger the disease," Kaplan continues.
Survey in the brain model
To study the cause-and-effect relationship between the virus and Alzheimer's disease, researchers created a three-dimensional human tissue culture model made up of small, six-millimeter-wide sponges of silk protein and collagen that mimic the brain. .
The sponges were seeded with neural stem cells, which grew into functioning neurons capable of relaying signals to one another in a network, just as happens in the brain.
Some of the stem cells also made glial cells, which are typically found in the brain and help keep neurons alive and functioning, the research team reports.
Varicella-zoster viruses reactivate HSV-1
In infection with varicella-zoster viruses alone, no formation of Alzheimer's characteristic tau and beta-amyloid proteins could be detected and neuron function was not impaired.
However, if the neurons already harbored the dormant HSV-1, the varicella-zoster virus led to its reactivation, accompanied by a dramatic increase in tau and beta-amyloid proteins. At the same time, neural signals began to slow down.
"It's a double whammy of two viruses that are very common and generally harmless," said study author Dana Cairns of Tufts University. The study shows that problems can arise when further exposure to varicella-zoster virus awakens dormant HSV-1.
HSV-1 activation promotes Alzheimer's disease
Professor Kaplan adds that varicella-zoster viruses are known in many clinical cases to trigger inflammation in the brain, which could eventually lead to activation of dormant HSV and increased inflammation.
Repeated cycles of HSV-1 activation could lead to further brain inflammation, plaque formation, and accumulation of neuronal and cognitive damage, the researchers explain.
Vaccination could help
It is also known that vaccination against varicella zoster viruses to prevent chickenpox and shingles also reduces the risk of dementia. The vaccine can help break the cycle of viral reactivation, inflammation and neuronal damage.
Similar mechanisms in COVID-19?
Finally, the researchers also point to the long-term neurological effects that can occur after infection with the coronavirus virus SARS-CoV-2. (see COVID-19 as a trigger for Alzheimer-type dementia?) These could be related to the fact that varicella-zoster viruses and HSV-1 can be reactivated by COVID-19. (fp)
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This text corresponds to the specifications of the specialized medical literature, medical guidelines and current studies and has been verified by health professionals.
Sources:Dana Cairns, Ruth F. Itzhaki, David L. Kaplan: Potential involvement of varicella-zoster virus in Alzheimer's disease via reactivation of quiescent herpes simplex virus type 1; in: Journal of Alzheimer's Disease (published 2022/06/20), content.iospress.com Tufts University: Common viruses may trigger onset of Alzheimer's disease (published 2022/06/29), eurekalert. org
This article contains general advice only and should not be used for self-diagnosis or treatment. It cannot substitute a visit to the doctor.