A pan-coronavirus fusion inhibitor targeting the HR1 domain of human coronavirus spike

Xia, Shuai; Yan, Lei; Xu, Wei; Agrawal, Anurodh Shankar; Algaissi, Abdullah; Tseng, Chien-Te K.; Wang, Qian; Du, Lanying; Tan, Wenjie; Wilson, Ian A.; Jiang, Shibo; Yang, Bei; Lu, Lu

In its natural state, the S protein that is present on the CoV surface is inactive. After receptor binding target cell proteases activate the S protein by cleaving the exposed enzyme target sites, leaving the S2 subunit free to mediate viral fusion and entry. CoV can enter the target cell via two pathways: one is the endocytosis pathway and the other is direct fusion on the cellular surface. For example, when SARS-CoV enters the target cell through the endocytosis pathway, its S protein can be cleaved and activated by the pH-dependent cysteine protease cathepsin L in the endosome. On the other hand, recent studies have consistently reported that the SARS-CoV S protein can also be cleaved and activated by transmembrane protease serine 2 (TMPRSS2) and human airway trypsin-like protease, which are located on the cell surface, thus activating and allowing the S protein to mediate SARS-CoV infection at the plasma membrane (9) (10) (11) . In accord with such a finding, Matsuyama et al. (47) reported that the HR2 peptide efficiently inhibited SARS-CoV entry into cells, while lysosome-tropic reagents failed to inhibit at all. Similarly, TMPRSS2 also has the capacity to promote the entry of MERS-CoV through bypassing the endocytosis pathway and directly activating its S protein on the cellular surface (48) . Recently, several studies have reported that TMPRSS2 is highly expressed on human respiratory epithelial cells surface and was even associated with several CoV receptors, such as ACE2 and DPP4 (48) (49) (50) . Hence, the plasma membrane fusion pathway seems a preferred choice for HCoV infection in the human respiratory tract. Consistently, our previous study found that MERS-CoV-specific fusion inhibitor HR2PM2 effectively inhibited MERS-CoV infection in vivo by intranasal administration (33) . Other studies have also reported that the current clinical isolates of 229E and OC43 are very sensitive to cell surface TMRRSS2 but not to endosomal cathepsins (51, 52) . Overall, for current circulating HCoVs or emerging HCoVs, the cell surface fusion pathway in human respiratory tract would appear to be very important. Therefore, peptide fusion inhibitors and the strategy of intranasal administration are excellent choices for preventing HCoV infection via the airway, which is a key site for HCoV to rapidly establish infection and widely spread to other organs.