On the entry and entrapment of picornaviruses

The family of Picornaviruses is one of the most diverse pathogenic families known to infect humans and animals. As described in chapter 1, these viruses have been studied for more than a century. Surprisingly, many aspects of the viral life cycle still remain unclear, and although for some members of this family vaccination campaigns are highly effective, medications to treat or prevent viral infection are not available. Chapter 2 describes how different genetic techniques that can be used to identify host and restriction factors. One of these approaches is mutagenesis-based genetics in haploid human cells.

The strength of this technique is illustrated in chapter 3, where this method is used to unravel the unknown biology of the heavily studied prototype picornavirus; poliovirus. This chapter describes a how multiple phenotypic genetic screens with poliovirus and other enteroviruses lead to the identification of the phospholipase PLA2G16 as a broadly required picornavirus host factor. Chapter 3 continues by applying a genetic suppressor screen in a PLA2G16-null background, which provides insight into why PLA2G16 is recruited to endosomal damage induced by entering picornaviruses. This suppressor screen highlights members of an anti-viral autophagic response pathway, which was previously described for bacterial infections, leading to the hypothesis that PLA2G16 is required for the virus to gain entry into the host before the virus gets sequestered by this previously unknown innate anti-viral immune response.

The potential of haploid genetics is further explored in chapter 4 and chapter 5. Chapter 4 presents a potential new receptor for enterovirus-D68 (EV-D68). In this part of the thesis, the identification of several host factors involved in sialic acid (Sia) biosynthesis leads to the identification of -2,6- and -2,3-linked Sia as receptors for EV-D68. Likewise, chapter 5 presents KREMEN1 as an entry receptor for multiple coxsackie A viruses (CV-A). KREMEN1 is known to be a transmembrane protein involved in the trafficking of Dickkopf through clathrin-mediated endocytosis. In this chapter, data is presented that demonstrates that KREMEN1 can directly interact with CV-A10 as a functional entry receptor. Furthermore, it is shown that KREMEN1 is required for virus infection both in vitro as well as in vivo.

The above-mentioned experimental chapters highlight the capability of genetic screens to elucidate the importance of host factor biology. Furthermore, these chapters have added to our understanding of the entry pathways used by these viruses and the restriction factors that try to counteract it. Chapter 6 explores the current knowledge of this host-pathogen interplay in a broader context, thereby providing an overview of strategies used by different virus species to escape from the endolysosomal compartment without getting detected. Finally, chapter 7 discusses future perspectives considering the work presented in this thesis.