The SARS-CoV-2 spike (S) protein is crucial for understanding the molecular mechanisms of SARS-CoV-2 infection. It consists of an S1 and an S2 subunits that are separated by host cell proteases. The receptor-binding domain (RBD) in the S1 subunit is responsible for attachment to the host cells. The S protein interacts not only with angiotensin-converting enzyme 2 (ACE2) receptors, but also with various other host receptors. The S protein can have serious effects not only on the pulmonary vasculature, but also on the vasculature of multiple organs, including the heart and the brain. The authors from the United States have shown in several publications that S1 subunit of the SARS-CoV-2 spike protein is able to induce cell signaling in cultured human vascular cells.
About the studies
The American scientists Suzuki Y and Gyschka SG in collaboration with other authors performed in vitro studies using various cultured blood vessels. A treatment of the smooth muscle cells and endothelial cells of the human pulmonary artery with full-length S1 subunit of the SARS-CoV-2 S protein for 0, 10, and 30 minutes demonstrated that S1 subunit, without the rest of the virus, at a concentration of 10 ng/ml (0.133 nM) strongly activated the MEK/ERK pathway in cultured human pulmonary artery cells. The MEK/ERK pathway is a well-known cellular growth mechanism and has been found to facilitate viral replication cycles. It is interesting to note that, in contrast to the full-length S1 subunit which strongly activated MEK, the recombinant protein that contains only the RBD did not activate MEK in cultured human pulmonary artery cells.
According to these results, the S1 subunit of the recombinant S protein (without the rest of the virus) was able to induce cell signaling in cultured human vascular cells. Furthermore, the results showed that regions other than the RBD region of the S1 subunit were required to trigger cell signaling for MEK phosphorylation. These results suggest that circulating S protein can induce various processes that promote pathological conditions.
The researchers then used the cultured smooth muscle cells of the rat pulmonary artery. The results showed species-specific differences in the function of the S1 subunit. Treatment of the rat pulmonary artery smooth muscle cells with the S1 subunit decreased MEK phosphorylation after only 10 minutes, and this dephosphorylation event was maintained for at least 60 minutes.
The researchers also proposed that the S protein-mediated cell signaling promotes the hyperplasia and/or hypertrophy of vascular smooth muscle and endothelial cells, and contributes to the complex cardiovascular outcomes. They performed a morphometric analysis of the pulmonary vessels of patients who died of SARS-CoV-2 or H1N1 influenza virus. The results revealed that the median wall thickness of the pulmonary arteries in patients with COVID-19 was 15.4 μm, while the median wall thickness of the pulmonary vessels in patients with H1N1 influenza was 6.7 μm. Pathohistological examination demonstrated a thickening of the vascular wall, mainly due to hypertrophy of the tunica media. Enlarged smooth muscle cells had become rounded, with swollen nuclei and cytoplasmic vacuoles.
A morphometric analysis of the placental arteries in women who developed COVID-19 during pregnancy, and gave birth to live full-term newborns showed a twofold increase in wall thickness and a fivefold reduction in the lumen area. Immunohistochemistry analysis demonstrated placental vascular remodeling associated with smooth muscle proliferation and fibrosis.
In particular, the authors pointed out a previous study by Chen et al, and very interesting results showing that ACE2 acts as a membrane receptor for cell signaling transduction in response to the S protein of SARS-CoV (now known as SARS-CoV-1) in the human lung alveolar epithelial cell line. Therefore, Gyschka, Suzuki et al considered the current view that the binding of the SARS-CoV-2 S protein to the host ACE2 receptor leads to viral entry into host cells, and that the cellular response is the result of viral infection, to be dogmatic.
Based on their experimental findings that showed that SARS-CoV-2 S1 subunit of the spike protein is able to induce cell signaling in cultured human vascular cells, Suzuki Y, et al presented an interesting theory of COVID-19 pathogenesis caused by viral protein fragment(s). According to this theory, the cell signaling mediated by the S protein is rather a growth factor/hormone-like specific cell signaling event that is well coordinated by the RBD, as well as other protein regions that facilitate cell signaling. The circulating fragment(s) of the S protein independently trigger(s) effects that ultimately lead to severe generalized pathological conditions. The signaling cascade triggered in the heart vasculature could cause coronary artery disease, and activation in the brain could lead to a stroke.
Also, Suzuki Y, et al suggested that a similar effect might occur in response to the mRNA vaccines, and warned of the possible long-term consequences for children and adults who have received COVID-19 vaccines based on the S protein.
Suzuki YJ et al. SARS-CoV-2 spike protein-mediated cell signaling in lung vascular cells. Vascular Pharmacology, 2021; Vol 137; 106823 https://www.sciencedirect.com/science/article/pii/S1537189120303281?via%3Dihub
Suzuki YJ. The viral protein fragment theory of COVID-19 pathogenesis. Medical Hypotheses, 2020, Vol 144; 110267. https://www.sciencedirect.com/science/article/pii/S0306987720326943?via%3Dihub
Suzuki, Y.J.; Gychka, S.G. SARS-CoV-2 Spike Protein Elicits Cell Signaling in Human Host Cells: Implications for Possible Consequences of COVID-19 Vaccines. Vaccines 2021, 9, 36. https://doi.org/10.3390/vaccines9010036
Gychka, S.G et al. Placental vascular remodeling in pregnant women with COVID-19. PLoS ONE 2022; 17(7): e0268591. https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0268591