According to the results of a recent study, the SARS-CoV-2 virus responsible for Covid-19 could also infect brain cells or neurons and use the internal machinery of these cells to multiply. The study in question has not yet been published in a peer-reviewed journal, but was submitted to the bioRxiv database on September 8.
Scientists had already concluded that there was a link between the coronavirus and several brain lesions such as encephalopathies, which lead to confusion, delirium or even brain fog. However, so far, no concrete evidence has yet shown that the coronavirus can invade brain tissue.
In the new study, scientists tried to determine whether the virus actually had the ability to enter neurons. To do this, they analyzed the brain tissue of three patients who died from Covid-19, and carried out experiments on mice infected with the virus, as well as on organoids.
What did the experiments consist of?
Organoids are “groups of cells grown in the lab to mimic the 3D structure of brain tissue.” In experiments with organoids, the researchers observed that the virus could trigger an infection by entering neurons through a protein called ACE2 which is found on the surface of cells. By analyzing the tissue with an electron microscope, they found that the virus was multiplying in the cells. This shows that he has succeeded in hijacking the internal machinery of neurons to make new copies of himself.
On the other hand, infection of brain cells also causes the death of many other neurons that are nearby. Scientists say infected cells may steal oxygen from surrounding cells so they can continue to make new viruses.
According to Akiko Iwasaki, lead author of the study and immunologist at Yale University, it is still unclear whether the same phenomenon occurs in those infected. However, there is some evidence that it is quite possible. They discovered during the study of autopsied tissues that certain neurons in the wrinkled cerebral cortex were infected with SARS-CoV-2 and that “small strokes” had taken place near the infected cells. According to Iwasaki, this could indicate that infected cells are stealing oxygen from neighboring cells, as was the case during experiments with organoids.
Regarding the body’s immune response to infection, it turns out that the autopsied brain tissue did not contain many immune cells. The authors indicated that the coronavirus may be able to elude the body’s immune defense.
For the experiments with the mice, the researchers used two groups of genetically modified mice. The first group carried the human ACE2 receptors in their brains while the second group had them only in their lungs. According to the results, the mice belonging to the first group lost weight quickly and died after six days. Those in the second group suffered no weight loss and survived.
According to the researchers, the results obtained with experiments with mice and organoids show that the coronavirus is very dangerous when it reaches the brain. The next steps are therefore to find out if it is the same for humans.
More studies on autopsied tissues of people with Covid-19 should therefore be undertaken to determine whether the preliminary results are still valid with a larger workforce.
Future research will also focus on whether the virus can affect the brainstem respiratory center, which could contribute to respiratory failure in people critically ill with Covid-19.
On the other hand, the fact that SARS-CoV-2 can disrupt the senses of taste and smell in patients indicates that it could also invade the brain via the nerves in the nose or through the bloodstream, especially by compromising the blood-brain barrier. The authors therefore stressed in their article that learning more about the route taken by the virus to reach the brain will be essential for the prevention and treatment of infection.
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