Thank you very much for your keen interest in my article from last week in which I explained that SARS-CoV-2 binds to the ACE2 protein on our cell surfaces so that it can enter our cells together with ACE2 itself. Over the last few weeks you have probably read quite a few times that Covid-19 often isn't just cause an infection of the lungs, but probably affects many other tissues as well. Usually this is attributed to ACE2 being present in many other tissues. The idea behind it: all cells that carry ACE2 can be infected by SARS-CoV2. Well, today I would like to explain why it's not that simple. If we look at the viral entry process in more detail, we find that at first only a part (the S1 part) of the spike protein binds to the ACE2 receptor and then...

... nothing else happens at first! Only when a certain protease, i.e. a protein-cleaving enzyme, splits the spike protein at a very specific point and thus exposes its S2 part, does it continue from there. It is this exposed S2 part of the spike protein that allows the virus to fuse with the cell membrane of the host cell. This cleavage and the resulting activation of the spike protein is also called priming. So for a cell to get infected by SARS-CoV2, it requires to carry this special protease on its cell surface in addition to ACE2. Since this protease seems to cleave proteins only behind the amino acid serine, it was called Transmembrane Serine Protease 2, or TMPRSS2 for short.

Type II pneumocytes (unfortunately) have both ACE2 and TMPRSS2 on their surfaces 

In order to see which tissues can in principle be infected by SARS-CoV2, we have to look at which cell types have both ACE2 and TMPRSS2 on their surfaces. To which cell types in our body this applies, is currently being investigated intensively and it looks that it is probably quite a few. One of the cell types, carrying both ACE2 and TMPRSS2, are the type II pneumocytes in the alveoli of our lungs. There are two important types of cells in our alveoli: type I pneumocytes are directly involved in gas exchange, while the type II pneumocytes release a substance called surfactant. Surfactant is an artificial word that is made up from "Surface Active Agent". This mixture of fat and protein reduces the surface tension between the air we breathe and our wet lung lining. If the type II pneumocytes are damaged by the virus, surfactant is missing and breathing becomes difficult. Even worse: these infected cells release messenger substances that activate the immune system and cause the small blood vessels in our alveoli to expand and become more permeable. As a result, even more fluid flows into the alveoli, the surfactant is further diluted and the increasing pressure crushes the alveoli, leading to their collapse.

TMPRSS2 and male sex hormones

Like for almost all gene products in our body, the activity of the TMPRSS2 gene and therefore the amount of TMPRSS2 protein is controlled via signaling molecules. In the case of TMPRSS2, we know that it is upregulated by androgens. This could partially explain the observation that men, who have a much higher androgen levels than women, seem to be somewhat more affected by Covid-19. A study was published a few days ago that showed that men are more often affected by a severe course of the disease. There was one exception, however: men who received medication that lowers their androgen levels as part of their therapy against prostate cancer seemed to be significantly protected from severe Covid-19 infection. The connection between androgens, TMPRSS2 and Covid-19 is definitely exciting enough that it will be investigated in more detail over the months to come.

Direct inhibition of TMPRSS2?

Could we block TMPRSS2 directly to let the virus hang in front of our cells without it being able to enter? Yes, that might work, suggest researchers from the teams of Stefan Poehlmann (Goettingen) and perhaps the most famous virologist in Germany, Christian Drosten (Berlin). In an article that appeared a few weeks ago in the highly renowned magazine Cell, they were able to demonstrate thoroughly that ACE2 and TMPRSS2 are really needed for SARS-CoV2 to infect cells. That alone would have made the article quite relevant, but the authors describe another experiment that raises hope. When cultured human lung cells were treated with Camostat mesylate, a specific inhibitor against the activity of TMPRSS2, they could hardly be infected with SARS-CoV2 anymore.

Only after activation by TMPRSS2 can SARS-CoV2 enter our cells via ACE2. Image: Markus Hoffmann, German Primate Center. Original source: https://www.dpz.eu/de/infothek/wissen/coronaviren.html Used with kind permission! Thank you! 

Now you should justifiably react with restrained joy, because substances that are effective in cell cultures are by no means necessarily promising candidates for drug treatment. First of all, time-consuming studies need to assess how to administer the substance in such a way that it actually reaches the cells on which it is supposed to act. Furthermore, it must be ensured that it is not - or only to a limited extent - harmful to other tissues and organs. But in the case of Camostat mesylate, we are lucky because this substance has already passed many such tests and in Japan is approved as a drug, albeit for a completely different disease, namely for chronic inflammation of the pancreas. This would significantly shorten the route to approval of Camostat mesylate for the treatment of Covid-19. A 2015 study had already shown that Camostat mesylate can partially protect mice from untreated fatal infections with the first SARS-CoV. So there is a lot of hope for the ongoing clinical trials with Camostat mesylate. But like with so many things, there is still a small catch.

Are there any other proteases that can do what TMPRSS2 can do?

The spike proteins of both the first SARS-CoV and the second SARS-CoV2 can be activated not only by the serine protease TMPRSS2, but also by certain cysteine ​​proteases, called cathepsins. However, priming by cathepsins opens up a different path for the virus into the cell, which leads directly into a so-called endosome. An endosome is an inhospitable place in most cases. What is in an endosome is usually pretty effectively destroyed sooner or later. As a result, this endosomal path that the virus takes after cathepsin priming does not lead to successful virus replication. This has already been shown for SARS-CoV and is probably applicable to SARS-CoV2. The new SARS-CoV2 virus, however, could pose a new problem for us, because something has occured in its latest evolution that the first SARS-CoV did not have: binding sites for another class of proteases, furins. Such sites at which furin-like proteases can cleave the spike protein had already open up a new route for the MERS virus to get into the cells. From our perspective, perhaps the worst thing about this, is that some furins do not sit on the cell surfaces, but are in fact secreted from the cells. These soluble furins can then float around in their cellular neighborhoods and activate the spike proteins of SARS-CoV2, which are stuck to ACE2 proteins on cell surfaces and perhaps, in the absence of TMPRSS2, could not have penetrated these cells at first. However, the extent to which the furin-assisted entry path of SARS-CoV2 plays a role in Covid-19 definitely has to be examined in more detail first.