The SARS-CoV-2 main protease doesn't induce cell death in human cells in vitro

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the cause of coronavirus disease 2019 (COVID-19) which has extremely rapidly spread worldwide. In order to develop the effective antiviral therapies, it is required to understand the molecular mechanisms of the SARS-CoV-2 pathogenesis. The main protease, or 3C-like protease (3CLpro), plays the essential role in the coronavirus replication that makes the enzyme a promising therapeutic target. Viral enzymes are known to be multifunctional. Particularly, 3CLpro of SARS-CoV was shown to induce apoptosis in addition to its main function. In the present study we analyzed the cytotoxicity of active SARS-CoV-2 3CLpro and its inactivated form upon their individual expression in four human cell lines. For this purpose, we constructed a protein biosensor which allows to detect the proteolytic activity of SARS-CoV-2 3CLpro and confirmed the expression of the active protease in all cell lines used. We studied viability and morphology of the cells and found that both active and inactivated enzyme variants induce no cell death in contrast to the homologous 3CL protease of SARS-CoV. These results indicate that SARS-CoV-2 3CLpro is unlikely contribute to the cytopathic effect observed during viral infection directly.

Cytotoxic effect of co-expression of human hepatitis A virus 3C protease and bifunctional suicide protein FCU1 genes in a bicistronic vector

Recent reports on various cancer models demonstrate a great potential of cytosine deaminase/5-fluorocytosine suicide system in cancer therapy. However, this approach has limited success and its application to patients has not reached the desirable clinical significance. Accordingly, the improvement of this suicide system is an actively developing trend in gene therapy. The purpose of this study was to explore the cytotoxic effect observed after co-expression of hepatitis A virus 3C protease (3C) and yeast cytosine deaminase/uracil phosphoribosyltransferase fusion protein (FCU1) in a bicistronic vector. A set of mono- and bicistronic plasmid constructs was generated to provide individual or combined expression of 3C and FCU1. The constructs were introduced into HEK293 and HeLa cells, and target protein synthesis as well as the effect of 5-fluorocytosine on cell death and the time course of the cytotoxic effect was studied. The obtained vectors provide for the synthesis of target proteins in human cells. The expression of the genes in a bicistronic construct provide for the cytotoxic effect comparable to that observed after the expression of genes in monocistronic constructs. At the same time, co-expression of FCU1 and 3C recapitulated their cytotoxic effects. The combined effect of the killer and suicide genes was studied for the first time on human cells in vitro. The integration of different gene therapy systems inducing cell death (FCU1 and 3C genes) in a bicistronic construct allowed us to demonstrate that it does not interfere with the cytotoxic effect of each of them. A combination of cytotoxic genes in multicistronic vectors can be used to develop pluripotent gene therapy agents.