Study shows how Covid-19 can cause loss of smell

Hyperaxion Jul 26, 2020

Researchers at Harvard Medical School have found that neurons related to smell are not vulnerable to Sars-CoV-2, but olfactory support cells are.

One of the main signs of Covid-19 is the temporary loss of smell: about 65% of patients with the disease have this symptom or loss of taste.

Study shows how Covid-19 can cause loss of smell
(Credit: Richárd Ecsedi / Unplash).

A new study by neuroscientists at the Harvard Medical School in the United States has managed to identify the olfactory cells most vulnerable to Sars-CoV-2 – and suggests that, in most cases, infection with the new coronavirus is unlikely to cause permanent loss of smell.

Published in the journal Science Advances, the research shows that olfactory sensory neurons do not express the gene that encodes the ACE2 receptor protein, which SARS-CoV-2 uses to enter human cells.

ACE2, instead, is expressed in cells that provide metabolic and structural support to olfactory sensory neurons and certain stem cells and blood vessel cells.

Because the findings indicate that the new coronavirus does not directly affect neurons, scientists believe that most people who lose their sense of smell when they have Covid-19 should regain that ability.

“I think it’s good news, because once the infection clears, olfactory neurons don’t appear to need to be replaced or rebuilt from scratch,” said Sandeep Robert Datta, co-author of the study and an associate professor of neurobiology at Harvard Medical School.

According to the researcher, however, more data needs to be collected and more research needs to be done to confirm this hypothesis.

(Credit: Battlecreek Coffee Roasters / Unsplash).

In the study, Datta and his team investigated the cell types most vulnerable to Sars-CoV-2 infection to try to understand how patients with Covid-19 can lose their sense of smell.

They focused on two genes: ACE2, found in the cells of the human respiratory tract and which encodes the protein to which Sars-CoV-2 binds to enter human cells, and TMPRSS2, which encodes an enzyme that is also considered important for the infection to take place.

The researchers eventually realized that both ACE2 and TMPRSS2 are expressed by cells in the olfactory epithelium, a type of tissue in the nasal cavity that is responsible for smell.

They also noted that none of these genes were expressed by olfactory sensory neurons. On the other hand, this type of neuron expressed genes associated with the ability of other types of coronaviruses to enter cells.

The scientists also analyzed the genes expressed in about 50,000 mouse olfactory bulb cells. They realized that this area of the animals’ brain, responsible for receiving signals from olfactory sensory neurons and doing initial odor processing, did not express ACE2.

This gene and the protein to which it is associated have only been found in blood vessel cells, especially pericytes – related to blood pressure regulation, the maintenance of the blood-brain barrier (a membrane that protects the central nervous system from potentially neurotoxic substances) and inflammatory responses. TMPRSS2 was not found in any type of olfactory bulb cell.

With the findings, Datta and his team believe that the loss of smell related to Covid-19 occurs due to a temporary loss of the function of support cells in the olfactory epithelium, which indirectly causes changes in olfactory sensory neurons. “We don’t fully understand what those changes are yet, however,” Datta said.

According to the scientist, although the loss of smell seems like a “curious phenomenon”, it can have devastating effects for some people. “It can have serious psychological consequences and could be a major public health problem if we have a growing population with permanent loss of smell,” he concluded.

Related topics:

Coronavirus Covid-19


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