Evaluating MERS-CoV Entry Pathways

Abstract

Middle East respiratory syndrome coronavirus (MERS-CoV) is an emerging zoonotic pathogen with a broad host range. The extent of MERS-CoV in nature can be traced to its adaptable cell entry steps. The virus can bind host-cell carbohydrates as well as proteinaceous receptors. Following receptor interaction, the virus can utilize diverse host proteases for cleavage activation of virus-host cell membrane fusion and subsequent genome delivery. The fusion and genome delivery steps can be completed at variable times and places, either at or near cell surfaces or deep within endosomes. Investigators focusing on the CoVs have developed several methodologies that effectively distinguish these different cell entry pathways. Here we describe these methods, highlighting virus-cell entry factors, entry inhibitors, and viral determinants that specify the cell entry routes. While the specific methods described herein were utilized to reveal MERS-CoV entry pathways, they are equally suited for other CoVs, as well as other protease-dependent viral species.

Keywords: Coronavirus (CoV); Endosome; HR2 peptide; IFITM3; Middle East respiratory syndrome (MERS); Protease; Protease inhibitor; Pseudovirus; Spike (S); TMPRSS2; Transfection; Viral entry; Virus concentration; Virus purification.

Fig. 1

MERS-CoV enters host either at or near the plasma membrane or in the endosomes. The MERS-CoV spike (S) proteins (gray) engage human DiPeptidyl Peptidase 4 ( hDPP4, purple) via their receptor-binding domains (green). Receptor engagement exposes protease cleavage sites (blue stars) on S proteins. If cell surface proteases such as hTMPRSS2 (blue) are present, S proteins are cleaved and viral fusion occurs at or near the plasma membrane. If hTMPRSS2 or similar cell-surface proteases are not present, then MERS-CoV is endocytosed, and can be triggered by endosomal proteases such as cathepsin L (brown) to complete viral entry