In this period all people are talking about Coronavirus, but it would be better to talk about Covid-19. This latter in fact belongs to the family of Coronaviruses, which includes also other viruses that account for up to 30% of upper respiratory tract infections in adults. Coronaviruses are classified together on the basis of the crown-like appearance of the envelope glycoproteins, and on characteristic features of chemistry and replication.
HOW DOES COVID-19 ENTER THE CELL?
The envelope glycoproteins are characterized by receptor binding domains (RBD) which are needed to bind host receptors and enter the cells to cause the infection. It is interesting to say that the RBD of the Covid-19 is similar to the one of SARS (severe acute respiratory syndrome), disease caused by another coronavirus whose envelope glycoproteins bind to the same functional host cell receptor as Covid-19 ones, angiotensin-converting enzyme 2 (ACE2).The differences between these two account for the higher binding affinity between Covid-19 RBDs and the ACE2 (10-20 fold higher than SARS’ one) and this could explain the ease of transmission of this form of virus from human to human through the droplets produced when an infected person sneezes or coughs. Other ways of transmission could be possible, since this receptor is not only expressed in lung cells (type 2 alveolar cells) but also in esophagus upper and stratified epithelial cells, enterocytes from ileum and colon, cholangiocytes, myocardial cells, kidney proximal tubule cells, and bladder urothelial cells.
HOW DOES IT REPLICATE?
When the virus attacks the cell, it releases its single RNA strand genome. This is then used to produce viral proteins through cellular machinery. The proteins produced include both non structural and structural proteins: among these last ones we can find the spyke glycoprotein, necessary for the virus entry in host cells. Once the virus is inside, millions of new copies of the virions are formed and released to infect other cells or end up in droplets that escape the lungs.
WHAT DOES IT CAUSE?
The infections by Covid-19 in most cases cause cold and fever: these symptoms are due to the reaction of immune system. Sometimes though, this reaction could be stronger and the immune system could start attacking lung cells, causing their death. The lungs then become obstructed with fluid and dying cells, making it difficult to breathe or in most severe cases it could lead to acute respiratory distress syndrome, and possibly death.