Coronavirus receptor interactions and their role in cross-species tropism

This thesis focuses on the molecular determinants that govern cross-species transmission of the membrane enveloped coronaviruses (CoV), pathogens of both veterinary and medical importance. The coronavirus Spike protein mediates both receptor engagement and membrane fusion and is therefore considered the main determinant of CoV host range and transmission. In the experimental chapters of the thesis we studied the entry process of four distinct (groups of) coronaviruses, either known to have crossed the species border to the human population or displaying potential to do so. By studying their entry in molecular detail, we aimed to increase our understanding of the factors that drive emergence of coronaviruses in novel host species.
In the first chapter, we studied the zoonotic Middle East respiratory syndrome (MERS)-CoV, which is recurrently transmitted to humans from the dromedary camel reservoir since its identification in 2012. We discovered that MERS-CoV engages sialoglycans via its Spike protein, in addition to the earlier described DPP4 entry receptor. MERS-CoV S interaction with sialic acids was of low affinity but high specificity and aids during infection of highly glycosylated cells, such as those present in the respiratory tract of dromedary camels and humans. The reported resistance of horses to this pathogen (even though horse DPP4 is a functional entry receptor in vitro) corresponds to the inability of MERS-CoV to bind to a sialic acid subtype that is abundantly present in the respiratory tract of horses and indicates that the interaction with sialoglycans influences the host range and transmission of MERS-CoV.
In the second chapter we studied the receptor binding site on the Spike protein of a minor clade of coronaviruses, comprising two of the four coronaviruses that are endemic in humans (OC43 and HKU1). We identified the receptor binding site for OC43 and related animal viruses via comparative structural analysis and found that this binding site is conserved and functional also in HKU1. The binding affinity of HKU1 towards short sialoglycans is significantly lower than that of OC43, which we attribute to differences in local architecture and accessibility, and which may be indicative for differences between the two viruses in receptor fine-specificity.
In the third chapter we studied the porcine deltacoronavirus (PDCoV), a common enteropathogen of swine with global distribution that was identified in 2012. Because all currently known genetically related viruses have been found in birds, the mammalian PDCoV is anticipated to have originated from an avian reservoir. To gain insight into the requirements that enabled this remarkable cross-species transmission event, we tried to identify the PCoV receptor and found that it uses the APN host molecule -to which it binds via a cross-species conserved domain. We observed that PDCoV infects cells of different hosts (including those of humans and chicken) and is capable of using various receptor orthologous for entry, at least in vitro, and therefore has acquired the main feature for cross-species transmission.