Loss of smell is one of the most common symptoms of SARS-CoV-2 infection. This type of symptom, although rarer in the case of respiratory viruses such as influenza, is nevertheless well known and is linked to the ability of these viruses to infect olfactory neurons. However, these neurons are exposed to the environment and connect directly to the central nervous system (CNS). A virus capable of infecting them will thus be able to pass in a privileged way to the CNS through “the olfactory rail”. A significant number of patients present with neurological manifestations, especially in the most severe cases of Covid-19, which suggests that SARS-CoV-2 can invade the CNS. In this context, it is important to understand the interactions between olfactory neurons and this virus.
SARS-CoV-2 enters cells through a specific receptor, called ACE2. The olfactory neurons present in the nose are surrounded by so-called sustentacular support cells which have this specific ACE2 receptor, while the neurons do not express it. Their work has shown that, in hamsters, SARS-CoV-2 massively infects these sustentacular cells but not the olfactory neurons. They found that in addition to the infection of the supporting cells, there was desquamation of the nasal mucosa, which could explain the loss of smell. Indeed, desquamation of the nasal mucosa leads to a loss of olfactory neurons responsible for detecting odors. If the same mechanism as in the infected hamster takes place in humans, it could be the cause of the anosmia observed and would prevent the virus from entering the CNS via the olfactory track as has been recently suggested (2) .
Fortunately, the nasal mucosa is able to regenerate throughout life thanks to pluripotent cells(3). In their experiments, the researchers thus observed a recovery of 50% of the initial structure of the nasal mucosa, and this 14 days after the start of the infection.
(1) Sia, S.F., Yan, L.M., Chin, A.W.H., Fung, K., Choy, K.T., Wong, A.Y.L., Kaewpreedee, P., Perera, R., Poon, L.L.M., Nicholls, J.M., Peiris, M., Yen, H.L., 2020. Pathogenesis and transmission of SARS-CoV-2 in golden hamsters. Nature.
(2) Romoli, M., Jelcic, I., Bernard-Valnet, R., Garcia Azorin, D., Mancinelli, L., Akhvlediani, T., Monaco, S., Taba, P., Sellner, J., 2020. A systematic review of neurological manifestations of SARS-CoV-2 infection: the devil is hidden in the details. European journal of neurology
(3) Cells capable of dividing throughout the life of an individual to renew the different types of cells, including the olfactory epithelium with its neurons and its sustentacular cells.
Reference :
Bertrand Bryche, Audrey St Albin, Severine Murri, Sandra Lacôte, Coralie Pulido, Meriadeg Ar Gouilh, Sandrine Lesellier, Alexandre Servat, Marine Wasniewski, Evelyne Picard-Meyer, Elodie Monchatre-Leroy, Romain Volmer, Olivier Rampin, Ronan Le Goffic, Philippe Marianneau, Nicolas Meunier, Massive transient damage of the olfactory epithelium associated with infection of sustentacular cells by SARS-CoV-2 in golden Syrian hamsters, Brain, Behavior, and Immunity,2020, doi.org/10.1016/j.bbi.2020.06.032.