In Vitro Models for Studying Entry, Tissue Tropism, and Therapeutic Approaches of Highly Pathogenic Coronaviruses

Abstract

Coronaviruses (CoVs) are enveloped nonsegmented positive-sense RNA viruses belonging to the family Coronaviridae that contain the largest genome among RNA viruses. Their genome encodes 4 major structural proteins, and among them, the Spike (S) protein plays a crucial role in determining the viral tropism. It mediates viral attachment to the host cell, fusion to the membranes, and cell entry using cellular proteases as activators. Several in vitro models have been developed to study the CoVs entry, pathogenesis, and possible therapeutic approaches. This article is aimed at summarizing the current knowledge about the use of relevant methodologies and cell lines permissive for CoV life cycle studies. The synthesis of this information can be useful for setting up specific experimental procedures. We also discuss different strategies for inhibiting the binding of the S protein to the cell receptors and the fusion process which may offer opportunities for therapeutic intervention.

Figure 1

Coronaviruses (CoVs) are enveloped nonsegmented positive-sense RNA viruses belonging to the family Coronaviridae. Four genera are known: alpha-, beta-, gamma-, and delta-CoVs. Beta-CoVs are accountable for zoonotic outbreaks as severe acute respiratory syndromes- (SARS-) CoV, Middle East respiratory syndrome virus- (MERS-) CoV, and now, the novel SARS-like coronavirus (SARS-CoV-2). CoVs encode several nonstructural and structural proteins as genome-associated nucleocapsid protein (N), envelope protein (E), and spike (S) glycoprotein. In particular, S protein is crucial for viral attachment, fusion, and entry into epithelial cells. SARS-CoV and SARS-CoV-2 bind to ACE2 receptor highly expressed on ciliated epithelial cells, whereas MERS-CoV enters nonciliated epithelial cells binding the CD26/DPP4 receptor. Several host cellular proteases are needed to activate the S protein (e.g., TMPRSS2, furin, and cathepsins) and viral entry into cells.