US and Belgian scientists explain why COVID-19 patients lose their sense of smell loss of smell in COVID-19. The researchers were unable to find evidence that the infection affects the sensory neurons of the olfactory epithelium or the nerve cells of the olfactory bulb. Thus, coronavirus is not a neurotropic virus, however SARS-CoV-2 infects a different type of cells in the olfactory organs. This is reported in an article published in the journal Cell.
When a cell is infected, SARS-CoV-2 binds to the ACE2 receptor on the cell membrane. This receptor is expressed on the surface of the sustentacular cells, but not on the olfactory sensory neurons, which transmit odor signals to the olfactory bulb. Sustentacular cells provide neurons with nutrients and maintain their structure. Both types of cells are continuously regenerated from stem cells of the olfactory epithelium throughout a person's life.
Researchers analyzed samples of the olfactory nasal mucosa from patients who died from COVID-19. As a control, tissue samples were taken from patients who died and for other reasons who were not infected with SARS-CoV-2. Mucosal tissues and samples of the olfactory bulb were obtained with endoscopes within 60-90 minutes after death.
The results show that SARS-CoV-2 infects the sustentacular cells in the olfactory epithelium of COVID-19 patients and actively replicates in these cells. However, viral RNA could not be detected in sensory neurons and neurons of the olfactory bulb. According to scientists, the cause of temporary olfactory dysfunction in COVID-19 is the infection of supporting cells, that is, the virus acts on neurons indirectly.