In this lecture we’re gonna discuss the replication cycle of coronavirus as well as
emerging targets for pharmacological antiviral interventions.
So without further ado let's get right into it.
The present outbreak of a coronavirus-associated acute respiratory disease is the third documented
spillover of an animal coronavirus to humans resulting in a major epidemic.
The Coronavirus Study Group of the International Committee on Taxonomy of Viruses formally
named the new coronavirus as “severe acute respiratory syndrome coronavirus 2,” or
SARS-CoV-2.
Human-to-human transmission of SARS-CoV-2 occurs primarily via respiratory droplets
from coughs and sneezes.
Like other coronaviruses, SARS-CoV-2 has four structural proteins, known as the "S" for spike,
"E" for envelope, "M" for membrane, and "N" for nucleocapsid, which encapsulate single-stranded viral RNA.
Now, spike is the main protein that interacts with the host cell receptors.
In the first step of receptor binding, spike protein is cleaved into S1 and S2 by the host
cell protease, one of which is transmembrane protease serine 2 abbreviated as TMPRSS2.
The main function of S1 subunit is to bind with the host cell surface receptors, while
the function of S2 subunit is to mediate membrane fusion.
So one of the potential therapeutic approaches is to either develop vaccine that contains
antigens derived from the spike protein, which can boost recognition of the virus by the
immune cells or to develop monoclonal antibodies that bind to the coronavirus spike protein
and block the interaction with human cells.
Another potential target is transmembrane protease serine 2, which appears to be essential
for entry and viral spread of the coronavirus.
One agent that is currently being tested, called Camostat Mesylate has shown to inhibit
TMPRSS2 activity and thus is being considered as a potential anticoronavirus candidate.
Now, recent protein modeling studies on the spike protein suggest that SARS-CoV-2 has
a strong binding affinity to human angiotensin converting enzyme 2 (ACE2) receptors and likely
uses them as a mechanism for cell entry.
One location where ACE2 receptors are highly expressed is type II alveolar cells, which
are found in the lungs.
However ACE2 receptors are also found in many extrapulmonary tissues including heart, kidney,
endothelium, and intestine.
So another potential drug targets could be the interaction sites between ACE2 and spike protein.
One compound that has already been found during recent computational study, which is predicted
to bind with the binding interface of the Spike–ACE2 complex, is a natural flavonoid
called Hesperidin.
Now studies in mice demonstrated that coronavirus binding of spike protein to ACE2, downregulates
the receptor, and thereby contributes to severe lung injury.
This suggests that delivery of excessive soluble form of ACE2 may competitively bind with SARS-CoV-2
and not only neutralize the virus but also rescue cellular ACE2 which regulates the renin-angiotensin
system to protect the lungs from injury.
One small study has already found recombinant human ACE2 to be safe, with no negative hemodynamic
effects in healthy subjects.
Now, moving on to the next replication step.
So after uncoating, first the genomic RNA of coronavirus acts as an mRNA for translation
of the replicase polyprotein 1a (pp1a) and 1ab (pp1ab).
Autoproteolytic cleavage of these polyproteins then produces number of non-structural proteins
including RNA-dependent RNA polymerase, helicase and nonstructural protein 3, 4, and 6.
These nonstructural proteins 3, 4, and 6 are thought to be responsible for anchoring the
coronavirus replication/transcription complex through recruitment of intracellular endoplasmic
reticulum membranes to form double membrane vesicles abbreviated as DMV.
RNA-dependent RNA polymerase (RdRp) and helicase localize to double membrane vesicles and drive
the production of subgenomic RNAs (sgRNAs) from which the structural and accessory proteins
are produced in the next phase of translation.
Now RNA-dependent RNA polymerase (RdRp) is a target of investigational drugs such as
Remdesivir and Favipiravir.
Preliminary research has shown that both of these agents inhibit RNA-dependent RNA polymerase
and thus might be useful in treatment of early or mid-stages of the coronavirus disease.
Allright, going back to the next replication step, once synthesized, transmembrane structural
proteins "S", "M", and "E" are inserted, and folded in the endoplasmic reticulum (ER) and then
transported to the Endoplasmic reticulum–Golgi intermediate compartment abbreviated as ERGIC.
The "N" proteins on the other hand bind the viral genomic RNA in cytoplasm to form nucleocapsid.
Once the final virion assembly occurs in the intermediate compartment, mature virions are
released via smooth-walled vesicles by exocytosis.
Now, in addition to the investigational agents that we discussed thus far, there are few
antiviral drugs that are already on the market that have been reported as potentially effective
against coronavirus.
One of them is antimalarial drug called Chloroquine, which is thought to exert its antiviral activity
in part by increasing the pH in host-cell lysosomes, which in turn inhibits hydrolytic
activity of protease enzymes that are required for processing of viral glycoproteins during infection.
Other drugs of interest are the combination of Lopinavir and Ritonavir, as well as Darunavir
and Cobicistat, which belong to class of drugs known as protease inhibitors.
These drugs were originally designed to block HIV viral replication.
However molecular docking studies have shown that they may also interact with some proteins
that are required for replication of coronavirus.
And with that I wanted to thank you for watching I hope you enjoyed this video and as always
stay tuned for more.