Coronavirus spike-receptor interactions

Abstract

Coronaviruses cause important diseases in humans and animals. Coronavirus infection starts with the virus binding with its spike proteins to molecules present on the surface of host cells that act as receptors. This spike-receptor interaction is highly specific and determines the virus’ cell, tissue and host tropism. In the study of doctoral candidate Huihui Mou, the receptors of two coronavirus species were investigated. Huihui Mou started her doctoral studies on the Middle East respiratory syndrome coronavirus (MERS-CoV). Reminiscent to the SARS-coronavirus, MERS-CoV causes a severe and often lethal respiratory infection in humans by transmission from an animal reservoir. In a collaborative effort with scientists of the Erasmus Medical Centre in Rotterdam, Huihui Mou identified the cell surface protein dipeptidyl peptidase 4 (DPP4) as the receptor for the MERS coronavirus. Based on the specific interaction between the spike protein and the receptor, she isolated DPP4 from the lysate of susceptible cells. The identification of a functional receptor for MERS-CoV turned out to be a key finding to understand how this coronavirus can cross species borders. In addition, the doctoral candidate mapped the receptor binding domain to a 231-amino acid fragment within the 1353-residues long spike protein and showed that this domain was efficient in eliciting neutralizing antibodies. Furthermore, a serological assay was developed using a recombinantly expressed and purified fragment of the spike protein of SARS-CoV and MERS-CoV that is highly immunogenic and displays high sequence diversity across different coronavirus species. This S1-based serological assay showed high specificity in the detection of antibodies against these human coronaviruses. Huihui Mou furthermore studied the receptor interaction of feline coronaviruses which are hypothesized to undergo a pathotype switch by mutation, thereby changing from an enteric virus (FECV) causing a mild or subclinical infection, to a systemically replicating and often fatal variant (FIPV). Recruitment of novel receptors might be one of the underlying mechanisms for this phenomenon. The DC-SIGN molecule has been demonstrated to function as a receptor for the macrophage-tropic FIPV strains. These cell surface molecules are present on immune cells including macrophages, but not on enterocytes, the primary target cell of FECV. Huihui Mou assessed whether FECV has the inherent ability to recruit DC-SIGN as a functional receptor or whether the virus requires adaptation towards this FIPV-receptor during the pathotype switch. Her results showed that FECV can - to some extent - recruit DC-SIGN for virus entry in a cell type dependent manner. Passaging of the virus in DC-SIGN expressing cells conferred increased infection capacity in DC-SIGN expressing cells, which correlated with a mutation in the receptor binding subunit of the virus’ spike protein. However, the ability of the cell-adapted virus to recruit DC-SIGN as a receptor appeared not to be sufficient for acquiring macrophage tropism, indicating that other factors may contribute to the cell tropism switch of feline coronaviruses. Mou’s research on coronavirus receptor interaction has significantly contributed to our understanding of how coronaviruses enter their host cell which is key to the development of prevention and therapeutic strategies against coronavirus infections.