Free SARS-CoV-2 Spike Protein S1 Particles May Play a Role in the Pathogenesis of COVID-19 Infection

The imbalance of the renin-angiotensin system is currently considered as a potentially important factor of the pathogenesis of COVID-19 disease. It has been shown previously in the murine model, that the expression of angiotensin-converting enzyme 2 (ACE2) on the cell surface is downregulated in response to the infection by SARS-CoV virus or recombinant spike protein (S protein) alone. In the case of natural infection, circulation of the S protein in a soluble form is unlikely. However, in SARS-CoV-2, a large fraction of S protein trimers is pre-processed during virion morphogenesis due to the presence of furin protease cleavage site between the S1 and S2 subunits. Therefore, S protein transition into the fusion conformation may be accompanied by the separation of the S1 subunits carrying the receptor-binding domains from the membrane-bound S2 subunits. The fate of the S1 particles shed due to the spontaneous "firing" of some S protein trimers exposed on the virions and on the surface of infected cells has been never investigated. We hypothesize that the soluble S1 subunits of the SARS-CoV-2 S protein shed from the infected cells and from the virions in vivo may bind to the ACE2 and downregulate cell surface expression of this protein. The decrease in the ACE2 activity on the background of constant or increased ACE activity in the lungs may lead to the prevalence of angiotensin II effects over those of angiotensin (1-7), thus promoting thrombosis, inflammation, and pulmonary damage. This hypothesis also suggests the association between less pronounced shedding of the S1 particles reported for the S protein carrying the D614G mutation (vs. the wild type D614 protein), and lack of increased severity of the COVID-19 infection caused by the mutant (D614G) SARS-CoV-2 strain, despite its higher infectivity and higher in vivo viral load.