A recent study has shed light on why the Covid virus impacts one’s sense of smell and taste. The research reveals that SARS-CoV-2, responsible for causing Covid-19, has the ability to infect sensory neurons, providing insight into the widespread reports of anosmia and ageusia among affected individuals.
Senses of smell and taste heavily rely on sensory neurons
As the Covid-19 pandemic emerged in 2020, it became evident that some of the most common symptoms were linked to the peripheral nervous system, the intricate network of nerves facilitating communication between the brain and the body. Importantly, the senses of smell and taste heavily rely on sensory neurons within this peripheral nervous system.
Initially, there were indications that the virus might not infect neurons or only do so infrequently. However, a team of researchers from the Whitehead Institute for Biomedical Research in the United States has now demonstrated that SARS-CoV-2 can indeed infect sensory neurons, causing alterations in the gene expression of these cells.
These findings offer a valuable explanation for how the virus triggers symptoms within the peripheral nervous system, providing a foundation for potential treatments. Rudolf Jaenisch, Founding Member of the Whitehead Institute and Professor of Biology at the Massachusetts Institute of Technology, commented on this discovery, stating, “There is clearly a clinical effect of SARS-CoV-2 infection on sensory neurons, such as on smell and taste, and we did not know what might be the cause. Knowing that the virus can infect and probably alter the function of the cells gives us a hint of what the cause might be.”
The study, published in the journal iScience, involved the differentiation of induced pluripotent stem cells into human sensory neurons within a laboratory setting. The researchers confirmed the identity of these cells by verifying their expression of key genes specific to sensory neurons. Subsequently, they examined whether the sensory neurons expressed ACE2, the gene responsible for encoding the protein used by SARS-CoV-2 to enter human cells. It was found that the sensory neurons indeed expressed ACE2, at levels comparable to other cell types known to be susceptible to the virus, suggesting that sensory neurons are likely susceptible to infection.
In further experiments, the researchers exposed these sensory neurons to three strains of SARS-CoV-2: the original WA1/2020 strain, as well as the Delta and Omicron variants responsible for surges in Covid infections. Sequencing RNA from the sensory neurons, the researchers discovered that all three strains of the virus infected a portion of these cells. Notably, Omicron exhibited the lowest infection rate within the same timeframe, potentially explaining why it results in lower rates of anosmia and ageusia compared to earlier strains. However, additional research is required to firmly establish this connection.