It has been shown for some time that the coronavirus can also damage the central nervous system. But so far it has been unclear how Sars-CoV-2 penetrates the brain. Now tissue analyzes of deceased Covid-19 patients provide an answer. Through them, researchers were able to detect the coronavirus in the olfactory sense cells and also in brain regions that are connected to the olfactory nerves. This confirms that the virus uses the nasal mucous membrane and the sensory cells and nerve processes it contains to spread to the brain via the olfactory nerve tracts. At the same time, this explains the frequent disorders of the sense of smell and taste with a corona infection.
The coronavirus not only attacks the airways and lungs, the cardiovascular system or the gastrointestinal tract, but can also affect the central nervous system: More than a third of Covid 19 patients report neurological symptoms such as odor and Loss of taste, headache, fatigue, dizziness and nausea. Occasionally, there are also other serious neurological diseases. Laboratory tests with brain organoids have also already confirmed that Sars-CoV-2 can directly attack and kill certain cell types in the human brain. So far, however, it was unclear how the virus got into the brain. Because normally our thinking organ is shielded from many pathogens by the blood-brain barrier.
Virus particles in the olfactory cells
That is why a research team led by Charité – Universitätsmedizin Berlin has now investigated whether the coronavirus may penetrate the central nervous system via the olfactory system. The nasal mucosa has long been considered one of the first points of attack of the virus in our body, because tests have shown the mucous membrane cells to be very susceptible to infection. First author Jenny Meinhardt from the Charité and her colleagues have now analyzed tissue samples from 33 people who had died as a result of a Covid-19 infection for traces of Sars-CoV-2. Using special histological detection methods, electron microscopy and PCR tests, they checked whether virus particles or parts of its genetic material were present in the mucous membranes of the eyes, mouth and nose, the olfactory bulb and olfactory tract and other parts of the brain.
The scientists actually found what they were looking for: They were able to detect the virus in various neuroanatomical structures that connect the eyes, mouth and nose to the brain stem. The highest viral load was found in the olfactory mucosa. There, the researchers made intact coronavirus particles visible for the first time, both inside nerve cells and on the extensions of the cover cells located there. “On the basis of this data, we assume that Sars-CoV-2 can use the olfactory mucous membrane as a gateway into the brain,” says senior author Frank Heppner from the Charité. “From the olfactory mucosa, the virus apparently uses neuroanatomical connections such as the olfactory nerve to reach the brain.”
Explanation also for the olfactory disorders
However, it has not yet been conclusively clarified how the virus moves from the nerve cells of the nasal mucosa. The researchers did not succeed in imaging viruses directly in the olfactory bulb or olfactory nerve. “However, our data suggest that the virus migrates from nerve cell to nerve cell in order to reach the brain,” explains Heppner’s colleague Helena Radbruch. In this way, several other viruses also attack the nervous system: “Other examples of this are herpes simplex viruses and the rabies virus, which causes rabies,” says the researcher. “The virus is presumably also transported via the blood vessel system at the same time, since the virus could also be detected in the vascular walls in the brain.”
According to the research team, their new findings also provide an explanation for why infection with Sars-CoV-2 so often causes olfactory disorders: “In our eyes, the SARS-CoV-2 infection of the nerve cells in the olfactory mucosa provides a good explanation for this typical neurological symptoms of Covid-19 sufferers, such as odor and taste disorders, “says Heppner. However, the detection of the coronavirus in some areas of the brain stem, including the respiratory center, is also interesting. “It cannot be ruled out that in the case of severe Covid-19 courses, the virus infection in these areas of the brain makes breathing more difficult – in addition to the impairment of respiratory function due to the virus infection of the lungs. The same can apply to the heart and circulation, ”says Heppner.
Source: Jenny Meinhardt (Charité – Universitätsmedizin Berlin) et al., Nature Neuroscience, doi: 10.1038 / s41593-020-00758-5