Progress in Studies on Structural and Remedial Aspects of Newly Born Coronavirus, SARS-CoV-2

Engagement of the G3BP2-TRIM25 Interaction by Nucleocapsid Protein Suppresses the Type I Interferon Response in SARS-CoV-2-Infected Cells

The nucleocapsid (N) protein contributes to key steps of the SARS-CoV-2 life cycle, including packaging of the virus genome and modulating interactions with cytoplasmic components. Expanding knowledge of the N protein acting on cellular proteins and interfering with innate immunity is critical for studying the host antiviral strategy. In the study on SARS-CoV-2 infecting human bronchial epithelial cell line s1(16HBE), we identified that the N protein can promote the interaction between GTPase-activating protein SH3 domain-binding protein 2 (G3BP2) and tripartite motif containing 25 (TRIM25), which is involved in formation of the TRIM25-G3BP2-N protein interactome. Our findings suggest that the N protein is enrolled in the inhibition of type I interferon production in the process of infection. Meanwhile, upgraded binding of G3BP2 and TRIM25 interferes with the RIG-I-like receptor signaling pathway, which may contribute to SARS-CoV-2 escaping from cellular innate immune surveillance. The N protein plays a critical role in SARS-CoV-2 replication. Our study suggests that the N protein and its interacting cellular components has potential for use in antiviral therapy, and adding N protein into the vaccine as an antigen may be a good strategy to improve the effectiveness and safety of the vaccine. Its interference with innate immunity should be strongly considered as a target for SARS-CoV-2 infection control and vaccine design.

Mind the feline coronavirus: Comparison with SARS-CoV-2

Both feline coronavirus (FCoV) and SARS-CoV-2 are coronaviruses that infect cats and humans, respectively. However, cats have been shown to be susceptible to SARS-CoV-2, and FCoV also had been shown to infect human. To elucidate the relationship between FCoV and SARS-CoV-2, we highlight the main characteristics of the genome, the receptor usage, and the correlation of the receptor-binding domain (RBD) of spike proteins in FCoV and SARS-CoV-2. It is demonstrated that FCoV and SARS-CoV-2 are closely related to the main characteristics of the genome, receptor usage, and RBD of spike proteins with similar furin cleavage sites. In particular, the affinity of the conserved feline angiotensin-converting enzyme 2 (fACE2) receptor to the RBD of SARS-CoV-2 suggests that cats are susceptible to SARS-CoV-2. In addition, cross-species of coronaviruses between cats and humans or other domesticated animals are also discussed. This review sheds light on cats as potential intermediate hosts for SARS-CoV-2 transmission, and cross-species transmission or zoonotic infection of FCoV and SARS-CoV-2 between cats and humans was identified.

Both Baicalein and Gallocatechin Gallate Effectively Inhibit SARS-CoV-2 Replication by Targeting Mpro and Sepsis in Mice

The emergence of severe acute syndrome coronavirus 2 (SARS-CoV-2) in December 2019 has led to the global COVID-19 pandemic. Although the symptoms of most COVID-19 patients are mild or self-curable, most of severe patients have sepsis caused by cytokine storms, which greatly increases the case fatality rate. Moreover, there is no effective drug that can limit the novel coronavirus thus far, so it is more needed to develop antiviral drugs for the SARS-CoV-2. In our research, we employed the techniques of molecular docking to screen 35 flavonoid compounds among which 29 compounds have Z-scores lower than − 6. Then, ( −)-gallocatechin gallate, ( +)-gallocatechin and baicalein were identified to have potent inhibitory activity against SARS-CoV-2 M^pro with IC₅₀ values of 5.774 ± 0.805 μM, 13.14 ± 2.081 μM and 5.158 ± 0.928 μM respectively by FRET assay. Molecular docking results also showed that ( −)-gallocatechin gallate, ( +)-gallocatechin and baicalein can non-covalently bind to M^pro through π-π stacking and hydrogen bonds in the Cys145 catalytic site. We further evaluated the effect of ( −)-gallocatechin gallate and baicalein on cytokine storms using a mouse model of sepsis. ( −)-Gallocatechin gallate and baicalein significantly reduced sepsis of mouse models on weight, murine sepsis score, and survival rate and reduced the inflammatory factor levels, such as TNF-α, IL-1α, IL-4, and IL-10. Overall, ( −)-gallocatechin gallate and baicalein show certain potential of treatment against COVID-19.

Effect of dihydromyricetin on SARS-CoV-2 viral replication and pulmonary inflammation and fibrosis

BACKGROUND

COVID-19 (Coronavirus Disease-2019) has spread widely around the world and impacted human health for millions. The lack of effective targeted drugs and vaccines forces scientific world to search for new effective antiviral therapeutic drugs. It has reported that flavonoids have potential inhibitory activity on SARS-CoV-2 M^pro and anti-inflammatory properties. Dihydromyricetin, as a flavonol, also has antiviral and anti-inflammatory potential. However, the inhibition of dihydromyricetin on SARS-CoV-2 M^pro and the protective effect of dihydromyricetin on pulmonary inflammation and fibrosis have not been proved and explained.

PURPOSE

The coronavirus main protease (M^pro) is essential for SARS-CoV-2 replication and to be recognized as an attractive drug target, we expect to find the inhibitor of M^pro. Novel coronavirus infection can cause severe inflammation and even sequelae of pulmonary fibrosis in critically ill patients. We hope to find a drug that can not only inhibit virus replication but also alleviate inflammation and pulmonary fibrosis in patients.

METHODS

FRET-based enzymatic assay was used to evaluate the inhibit activity of dihydromyricetin on SARS-CoV-2 M^pro. Molecular docking was used to identify the binding pose of dihydromyricetin with SARS-CoV-2 M^pro. The protective effects of dihydromyricetin against BLM-induced pulmonary inflammation and fibrosis were investigated in C57BL6 mice. BALF and lung tissue were collected for inflammation cells count, ELISA, masson and HE staining, western blotting and immunohistochemistry to analyze the effects of dihydromyricetin on pulmonary inflammation and fibrosis. MTT, western blotting, reverse transcription-polymerase chain reaction (RT-PCR) and wound healing were used to analyze the effects of dihydromyricetin on lung fibrosis mechanisms in Mlg cells.

RESULTS

In this study, we found that dihydromyricetin is a potent inhibitor targeting the SARS-CoV-2 M^pro with a half-maximum inhibitory concentration (IC₅₀) of 1.716 ± 0.419 μM, using molecular docking and the FRET-based enzymatic assay. The binding pose of dihydromyricetin with SARS-CoV-2 M^pro was identified using molecular docking method. In the binding pocket of SARS-CoV-2 M^pro, the dihydrochromone ring of dihydromyricetin interact with the imidazole side chain of His163 through π-π stacking. The 1-oxygen of dihydromyricetin forms a hydrogen bond with the backbone nitrogen of Glu166. The 3-, 7-, 3'- and 4'-hydroxyl of dihydromyricetin interact with Gln189, Leu141, Arg188 and Thr190 through hydrogen bonds. Moreover, our results showed that dihydromyricetin can significantly alleviate BLM-induced pulmonary inflammation by inhibiting the infiltration of inflammation cells and the secretion of inflammation factors in the early process and also ameliorate pulmonary fibrosis by improving pulmonary function and down-regulate the expression of α-SMA and fibronectin in vivo. Our results also showed that dihydromyricetin inhibits the migration and activation of myofibroblasts and extracellular matrix production via transforming growth factor (TGF)-β1/Smad signaling pathways.

CONCLUSION

Dihydromyricetin is an effective inhibitor for SARS-CoV-2 M^pro and it prevents BLM-induced pulmonary inflammation and fibrosis in mice. Dihydromyricetin will be a potential medicine for the treatment of COVID-19 and its sequelae.

Purification and characterization of the receptor-binding domain of SARS-CoV-2 spike protein from Escherichia coli

SARS-CoV-2 is responsible for a disruptive worldwide viral pandemic, and renders a severe respiratory disease known as COVID-19. Spike protein of SARS-CoV-2 mediates viral entry into host cells by binding ACE2 through the receptor-binding domain (RBD). RBD is an important target for development of virus inhibitors, neutralizing antibodies, and vaccines. RBD expressed in mammalian cells suffers from low expression yield and high cost. E. coli is a popular host for protein expression, which has the advantage of easy scalability with low cost. However, RBD expressed by E. coli (RBD-1) lacks the glycosylation, and its antigenic epitopes may not be sufficiently exposed. In the present study, RBD-1 was expressed by E. coli and purified by a Ni Sepharose Fast Flow column. RBD-1 was structurally characterized and compared with RBD expressed by the HEK293 cells (RBD-2). The secondary structure and tertiary structure of RBD-1 were largely maintained without glycosylation. In particular, the major β-sheet content of RBD-1 was almost unaltered. RBD-1 could strongly bind ACE2 with a dissociation constant (KD) of 2.98 × 10-8 M. Thus, RBD-1 was expected to apply in the vaccine development, screening drugs and virus test kit.