Three months after the first patients with a new type of coronavirus that defies our understanding, and which has spread rapidly across all continents with more than 100,000 infected worldwide, there are already more than 150 scientific articles dedicated to understanding SARS-CoV-2, its epidemiology, to find effective treatment and prevention routes.
One of them, published March 3 in the journal Intensive Care Med , offers interesting insights into how the virus attaches itself to human lung cells , as well as other organs that could also be a gateway. The key would be in some proteins, the so-called ACE2 receptors, that lung cells (and other body tissues) possess that would have a high affinity with the surface proteins of SARS-CoV-2.
Like a key in a lock
Using a computer model, the study authors discovered that the spikey proteins of SARS-CoV-2 and the coronavirus that causes severe acute respiratory syndrome (SARS) have nearly identical three-dimensional structures in the receptor-binding domain, or RBD, which it is what allows them to latch on and enter a cell. The pointed SARS proteins have a strong affinity for the ACE2 receptor on human lung cells; in turn, the also pointed proteins of SARS-CoV-2 share 76.5% of the amino acid sequences with those of SARS. As the study mentions: “The pointed proteins of SARS-CoV-2 and SARS are highly homologous.”
This means that SARS-CoV-2, like SARS, would ‘dock’ its proteins with the ACE2 receptor to facilitate entry of the virus into human lung cells, like a key in a lock. This metaphor can be used to understand other cellular processes, such as the entry of the sperm into the egg.
SARS-CoV-2 has an entry key ‘more efficient than that of SARS
But in addition, this discovery would explain why the virus has a high transmission rate. A subsequent analysis that we have described even suggested that SARS-CoV-2 recognizes human ACE2 more efficiently than SARS , increasing the ability of SARS-CoV-2 to transmit from person to person. R-0 is the term epidemiologists use to express the number of people who would be infected for each infected person. In the case of the new coronavirus or SARS-CoV-2, its R-0 ranges from 2 to 5 ; In other words, from each infected person, the virus would be transmitted to a minimum of 2 people and a maximum of 5.
Why is the lung the organ most affected by the infection?
Another key question about novel coronavirus infection is why the lung appears to be the most vulnerable target organ. One reason is that the vast surface area of the lung makes the lung highly susceptible to inhaled viruses; but this research there is also a biological factor.
Using normal lung tissue from eight adult donors, the researchers showed that 83% of the cells expressing the ACE2 protein were epithelial cells of the alveoli (bag-shaped structures that fill with air at the end of the bronchi) of the type II (AECII), suggesting that these cells may serve as a reservoir for viral invasion. Furthermore, the analysis showed that ACE2-expressing AECIIs have high levels of multiple genes related to viral processes , which would also facilitate the replication of the coronavirus in the lung.
The gut could also be a good entry point for SARS-CoV-2
Another of the most interesting conclusions that can be drawn from the study is that not only the respiratory tract would be the ideal entry point for this new coronavirus. ACE2 receptor expression is also found in many extrapulmonary tissues, including the heart, kidneys, endothelium, and intestine. Importantly, ACE2 is highly expressed on the surface of intestinal epithelial cells, functioning as a coreceptor for the absorption of nutrients, in particular for the reabsorption of amino acids from food.
This is how the scientists have expressed it in the publication: “We believe that the intestine could also be an important entry site for SARS-CoV-2 and that the infection could have been initiated by eating food from the Wuhan market, the supposed origin of the outbreak. ”.
If SARS-CoV-2 can actually infect human intestinal epithelium, this fact has important implications for fecal-oral transmission and containment of viral spread. However, according to the Centers for Disease Control and Prevention, it has not yet been confirmed whether a person can get COVID-19 by touching surfaces or objects that have viruses and then touching their mucous membranes, such as their nose, mouth, and eyes.
