COVID-19 News: Canadian Researchers Discover That SARS-CoV-2 After Binding With ACE-2 Receptors Enter Host Cells Using Clathrin-Mediated Endocytosis
: Scientists from the Montreal Neurological Institute, McGill University have discovered that the SARS-CoV-2 may be using a unique method of clathrin-mediated endocytosis to gain entry into the host cells.
The research findings are published on a preprint server and are pending peer review.
The Canadian researchers say that the COVID-19 pandemic has affected almost all countries across the world and shows no sign of diminishing in its infectiousness. They call this virus, and this pandemic one of the “greatest challenges ever to the global scientific community and that it is vital to fully understand the biology of SARS-CoV-2”.
The spike glycoproteins on the surface of the novel coronavirus help the virus to interact with the surfaces of the host cells by way of the cell’s angiotensin-converting enzyme 2 (ACE2) receptors. Once this interaction takes place, the virus membrane and the cell membranes fuse, and this allows the virus to inject its RNA or genetic material into the host cell.
The ACE2 is a protein on the surface of many cell types. It is an enzyme that generates small proteins by cutting up the larger protein angiotensinogen that then go on to regulate functions in the cell.
Utilizing the spike-like protein on its surface, the SARS-CoV-2 virus binds to ACE2, like a key being inserted into a lock prior to entry and infection of cells. Hence, ACE2 acts as a cellular doorway ie a receptor for the virus that causes COVID-19 disease.
The study team added that this membrane system of the host cells or eukaryotic cells is an important cellular defense against invading viruses and microbes. However, the viruses tend to gain access into the cells by bypassing this complex mechanism. Antiviral drugs target this level of cellular entry of the virus, they wrote.
The researchers wrote that the numbers of confirmed CVOD-19 cases and the real number of people infected might be different. They said that antibody tests have revealed that 3 to 20 percent of certain communities have already been infected by the virus, indicating its high rates of infectiousness.
The researchers explained that the virus has a transmembrane spike (S) glycoprotein that is capable of forming “homotrimers.” The S protein has S1 and S2 subdomains. Here the S1 codes for the receptor-binding domain that allows the virus to bind to the host cells and the S2 subdomain have a transmembrane domain that allows the virus membrane to fuse with the host cell membrane. The receptor on which the S glycoprotein binds is the angiotensin-converting enzyme 2 (ACE2) that is present on the host cells. Once the S protein binds to the ACE2, it is broken into S1 and S2 by “type II transmembrane serine protease TMPRSS2”, a cleaving enzyme. Another enzyme furin activates the S2 subdomain that allows fusion of the viral and cellular membrane and entry of the viral RNA.
Interestingly, the coat proteins of the virus thus do not enter the host cell. What enters is viral RNA. Another theory is that the ACE2/SARS-CoV-2 bound complex on the cell membrane is engulfed as a who
le by the cell membrane by the process of endocytosis. This endocytosis includes capsid proteins.
Significantly, two methods of endocytosis of the whole virus/ACE2 receptor complex have been reported in the scientific literature. These are:
1.) Clathrin-mediated endocytosis (CME)
2.) Clathrin-independent process
In this research, the study team first took purified spike glycoprotein protein. They used lentivirus pseudotyped with spike glycoprotein. The whole simulation showed that the SARS-CoV-2 could undergo rapid endocytosis in the cell membrane after binding to the receptors.
The researchers then used specific chemical inhibitors to stop each of the steps. The results revealed that this entry of the virus and receptor complex is through clathrin-mediated endocytosis. The team wrote, “Thus, it appears that SARS-CoV-2 first engages the plasma membrane, then rapidly enters the lumen of the endosomal system, strongly suggesting that fusion of the viral membrane occurs with the lumenal membrane of endosomes.”
The study team said that Chlorpromazine, a drug used in psychiatric illness, is also known to disrupt this process of endocytosis. The researchers wrote that their finding is hugely significant because if chemical inhibitors to this clathrin mediated endocytosis could be developed, a possible drug to prevent the SARS-CoV-2 infection could be developed. They added that clathrin-mediated endocytosis “should be considered as a key cellular pathway in any future drug target screens” for COVID-19.
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