ACE2, the entry point of SARS-CoV2 – Part 1
Coronaviruses. For my part, I actually didn’t know anything about them, until the beginning of this very special year 2020. And then this year everything turned upside down and by now you have probably all heard enough of the latest coronavirus strain, SARS-CoV2. For those of you who spent the last months on Mars: SARS-CoV2 is the third strain of coronaviruses that recently expanded its range of host animals successfully to include humans and in this new host, us, can trigger serious respiratory diseases.
The first two coronaviruses to “achieve” this were SARS-CoV, which triggered the first pandemic of the 21st century in 2002/03, and MERS-CoV, which is around since 2012. Of these three viruses, MERS-CoV appears to be the deadliest. However, since it rarely transmits from person to person, it spreads very slowly (although there is a very real danger that this could change at some point). The first (2003) SARS-CoV in many aspects is very similar to the current SARS-CoV2. They are surrounded by a fatty shell, which is why they can be easily destroyed by soap or disinfectant. The so-called spike proteins (shown in the image in red) sit in this fatty sheath and can bind to certain proteins on cell surfaces in order to allow the virus to penetrate the host cell membranes. Both SARS coronaviruses bind with their spike protein to the same surface proteins on our cells: ACE2 and TMPRSS2, whereby – after a few rounds of mutations – the new SARS-CoV2 binds to ACE2 with 10-20-fold higher affinity than the original SARS-Virus did. This contributes significantly to the new SARS-CoV2 being so particularly infectious. Both ACE2 and TMPRSS2 thus are currently being researched intensively. This is done primarily with regard to possible therapies that could target these proteins. In this article I will introduce you to the ACE2 protein, next week I will present you TMPRSS2.
ACE2, the angiotensin-converting enzyme 2 and its physiological function
As the name suggests, ACE2 is an enzyme, which means it is a biologically active protein. What is special about the ACE2 enzyme is that it sits on the cell membrane, i.e. the envelope of the cell, with its biologically active center facing outwards. Cells that carry the ACE2 protein can thereby convert a substance that is in immediate vicinity but still outside the cell. ACE2 is an important component of the renin-angiotensin-aldosterone system, one of the key systems for regulating water balance and thus blood pressure. When our blood pressure drops, our kidney releases the enzyme renin. Renin binds and cleaves the liver-born angiotensinogen in the bloodstream to produce the biologically active angiotensin I, which is made up of only 10 amino acids.
Now the first angiotensin converting enzyme (ACE) comes into play. But be careful: this ACE protein usually does not have an additional number, because for a long time it has been the only such enzyme we knew. For clarity, I will call it ACE1 here. ACE1 now cleaves two amino acids from angiotensinogen, resulting in the eight amino acid long angiotensin II. Angiotensin II is a so-called vasoconstrictor, a messenger substance that leads to narrowing of blood vessels. The ACE2 protein is able to split off a single amino acid from angiotensin II, so that angiotensin1-7 is formed. With only one amino acid less, now seven amino acids long, angiotensin1-7 has the opposite effect than angiotensin II: it acts as a vasodilator, making the blood vessels wider.
The astonishingly short research history of the ACE2 gene
I find it particularly interesting that ACE2 has in fact not been known until quite recently. Even though the angiotensin system was described several decades ago, nothing was known about a second ACE gene for a long time. ACE2 was actually only discovered in 2000 when its sequence was published in parallel by two research groups (here and here). A functional characterization of ACE2 was not available until 2002, when the group of Josef Penninger published a Nature article. This characterization was largely based on a genetically modified mouse line in which the ACE2 gene was rendered non-functional. This led to heart defects and, notably, induced lung diseases in these mice to take a worse course. The normal ACE2 appears to protect the mice from acute lung failure. In terms of the impact of their research, Josef Penninger’s group was literally at exactly the right place at the right time, namely in Toronto, Canada. Because in spring 2003 Toronto became an epicenter of the first SARS coronavirus outbreak.
This first SARS-CoV triggered severe lung diseases and cost the lives of around 800 people worldwide. And after ACE2 was proposed as the entry point for the SARS-CoV in autumn 2003, Josef Penninger’s team sent the ACE2 knock-out mice they generated to China, where collaborative researchers infected them with SARS-CoV; or rather did not infect them, because these mice withstood the virus exposure – they were immune.
Soluble ACE2 as a therapy option for COVID-19?
Unfortunately, mice as well as humans with “healthy” ACE2 can very well get infected with the SARS corona viruses. After the virus has bound to ACE2, ACE2 is removed from the cell membrane and its functions are blocked. As a result, angiotensin II accumulates and the blood vessels contract more and more. This raises blood pressure, which can contribute to acute lung failure, which unfortunately is often observed in COVID-19 patients. In 2005, the team led by Josef Penninger was able to show that in mice artificially induced lung failure can be improved by administering recombinant human ACE2 (rhACE2). This soluble ACE2, which is not located on the cell surface, still appears to protect the mice from acute lung failure, regardless of whether the lung failure was caused by a virus or other factors. Penninger, who in the meantime became director of the Institute of Molecular Biotechnology (IMBA) in Vienna, founded the company Apeiron Biologics, which among other things aims to develop soluble rhACE2 as a drug (under the name APN01). This soluble ACE2 might thus be the stone that hits two birds, when used against the SARS coronaviruses: firstly, it protects against progressive lung damage and secondly, it intercepts the circulating viruses thereby preventing the virus from binding to the cell surface ACE2 and thus also from entering our cells and multiplying. Even though Josef Penninger has since returned to Canada to head the nation’s largest biomedical research facility, the UBC Life Science Institute in Vancouver, he is still involved in Apeiron Biologics – whose headquarters are in Vienna. After successfully completing the phase I clinical trial in 2009 already, Apeiron Biologics announced in early April 2020 that it has received approval for phase II clinical trials in Austria, Germany and Denmark to assess the efficacy of rhACE2 in the treatment of COVID-19 Patients.
ACE inhibitors as a risk for COVID-19 patients?
People who are prone to high blood pressure are often prescribed ACE inhibitors, to reduce vasoconstriction by lowering the levels of angiotensin II. It is important to note that ACE inhibitors only act on ACE1 and not on ACE2. Moreover, there have been cumulative evidence over the years that suggests ACE inhibitors actually increase the levels of ACE2. This triggered a vivid medical discussion in early March. The question was whether the intake of ACE inhibitors, through its role in increasing the amount of ACE2 proteins on the cells, could make patients more susceptible to the SARS-CoV2 infection, since the virus is offered more entry points. On the other hand, the studies by Josef Penninger’s team had shown that ACE2 basically protects mice against lung failure. So if ACE inhibitors upregulate ACE2, they could even have a positive impact on lung damage. Indeed, a 2012 meta-study showed that ACE inhibitors can have a positive effect when it comes to the severity of pneumonia. So ACE inhibitors could be a bit of a double-edged sword when it comes to SARS-CoV-induced lung diseases. Most scientists who are currently commenting on whether the negative or positive effects predominate, find that we cannot answer this question at the moment (see e.g. here and here and here) and therefore recommend to continue ACE inhibitor therapies, even if there is an impending or existing SARS-CoV infection.
