Figure 1.
Hypothesis of involvement of free S1 subunits of SARS-CoV-2 spike protein into COVID-19 induced
Binding to the receptor via their intact RBDs these molecules may induce ACE2 downregulation and the downstream deleterious effects as it was suggested by [9, 13].
If our hypothesis turns true, the release of free S1 particles from the infected cells and virions should reduce the virus infectivity towards the cells neighboring to the infected cells. Therefore, two stages of the SARS-CoV2 infection in the lungs can be predicted:
I. The virus infects certain loci where the production of free S1 protein causes the downregulation of ACE2 in the non-infected cells in the proximity of the viruse-infected cells. Simultaneously the expression of ACE2 on more distant cells, that did not yet encounter S1 particles, can be increases due to the effect of interferon produced in the infected cells that has been shown to stimulate ACE2 expression [14]. The local disbalance of the angiotensin II/ angiotensin (1-7) levels also may elicit compensatory ACE2 increase in the cells not yet affected by the virus (though such compensatory circuit is not yet described, it would be logical to suggest it may exist). Therefore, the virus spread for longer distances (e.g. to another alveolae) would be facilitated, however the virus induced tissue damage would be limited.
II. When larger amount of the tissue is infected, the greater quantities of free S1 is produced, and the RAS disbalance on the organ or organism level is induced. This causes the deleterious effects such as increased inflammation, thrombosis and pulmonary damage as it was suggested previously [9, 13]. Simultaneously the virus production should be decreased due to downregulation of ACE2 production in the infected areas of the lungs.
This model corresponds well to the known clinical traits of COVID-19 [15, 16]: involvement of large areas of the lungs with frequent bi-lateral pneumonia (stages I of our model), reported relatively good patients state despite significant part of the lungs involved according to radiological examination data that is followed by abrupt deterioration of the patients conditions in next hours or days (late stage I and stage II).
The proposed model suggests that the clinical interventions suggested to maintain RAS balance [17] should have synergistic effect with protease inhibitors, especially with furin inhibitors. Although the in vitro data suggest that lack of the furin pre-processing of SARS-CoV-2 S protein can be compensated by the post-attachment processing by other proteases [3], our model suggests that furin inhibition will not only decrease the virus infectivity but will also decrease the shed of the free S1 particles from both virions and infected cells and blunt the infection effect over RAS.
SARS-CoV-2 isolates encoding a D614G mutation in the viral spike (S) protein were found recently [18-22], and those mutant variants predominate over time in locales where it is found. It has been demonstrated that D614G mutation decreases the stability of the S protein trimer and also increases the transition of the RBDs into the open conformation [23]. This mutation increases overall viral infectivity and fitness [18-21, 24] offering higher affinity of furin binding to S protein [25] and viral replication enhancement [26]. However, there are no indications that D614G mutation is associated with more severe symptoms of the covid-19 disease. Surprisingly, despite the fact that this mutation destabilizes the trimer, it was shown that S1 shedding from the lentiviruses pseudotyped with D614G mutant SARS-CoV-2 spike protein [22] is reduced compared to the wild type D614 protein. Our hypothesis suggests that decreased S1 shedding may be one of the factors limiting the morbidity and mortality in D614G SARS-CoV-2 infections despite higher infectivity of the virus and higher viral loads [19] achieved with the mutant genotype.
The experiments that would allow to test the hypothesis of S1-dependent pathogenic effect are relatively simple and straightforward. It may be based on quantitative measurements of the S1 particles concentration in the SARS-CoV-2 infected cell cultures, production of S1 particles preparations by filtration of the culture supernatants through the membrane filters of 20 nm pore-size, experimental testing of the ability of such particles to bind to the ACE2 receptor and the effect of these particles over ACE2 expression and RAS parameters both in cell cultures and in animal models.
It also should be mentioned that free S1 molecules may represent a target for COVID-19 therapy or prevention. After the separation of S1 subunit from S2 stem the potential epitopes free of the glycans shielding the external surface of the complete S protein [27] become exposed. So, the immunization by the recombinant proteins that would elicit the antibodies response against these conserved and unprotected epitopes may lead to the sequestration of free S1 molecules to immune complexes and their elimination thus reducing the probability of severe COVID-19 pneumonia.
Acknowledgements
Authors are grateful to Ms. Ksenya Sayfulina from Moscow State University of Psychology and Education for the help with the figure creation.
Conflict of interest
Authors declare no conflict of interest.
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