Electrostatic interactions at the five-fold axis alter heparin-binding phenotype and drive EV-A71 virulence in mice

Abstract

Enterovirus A71 (EV-A71) causes hand, foot and mouth disease epidemics with neurological complications and fatalities. However, the neuropathogenesis of EV-A71 remains poorly understood. In mice, adaptation and virulence determinants have been mapped to mutations at VP1-145, VP1-244 and VP2-149. We hypothesized that heparin-binding phenotype shapes EV-A71 virulence in mice. We constructed six viruses with varying residues at VP1-98, VP1-145 (which are both heparin-binding determinants) and VP2-149 (based on the wild type 98E/145Q/149K, termed EQK) to generate KQK, KEK, EEK, EEI and EQI variants. We demonstrated that the weak heparin-binder EEI was highly lethal in mice. The initially strong heparin-binding EQI variant acquired an additional mutation VP1-K244E, which confers weak heparin-binding phenotype resulting in elevated viremia and increased brain inflammation and virus antigens in mice, with subsequent high virulence. EEI and EQI-K244E variants inoculated into mice disseminated efficiently and displayed high viremia. Increasing polymerase fidelity and impairing recombination of EQI attenuated virulence, suggesting the importance of population diversity in EV-A71 pathogenesis in vivo. Combining in silico docking and deep sequencing approaches, we inferred that virus population diversity is shaped by electrostatic interactions at the five-fold axis of the virus surface. Electrostatic surface charges facilitate virus adaptation by generating poor heparin-binding variants for better in vivo dissemination in mice, likely due to reduced adsorption to heparin-rich peripheral tissues, which ultimately results in increased neurovirulence. The dynamic switching from heparin-binding to weak heparin-binding phenotype in vivo explained the neurovirulence of EV-A71.\n\nAuthor summaryEnterovirus A71 (EV-A71) is the primary cause of hand, foot and mouth disease, and it can also infect the central nervous system and cause fatal outbreaks in young children. EV-A71 pathogenesis remains elusive. In this study, we demonstrated that EV-A71 variants with strong affinity to heparan sulfate (heparin) have a growth advantage in tissue culture, but are disadvantageous in vivo. When inoculated into mice, strong heparin-binding virus variants are more likely to be adsorbed to peripheral tissues, resulting in impaired ability to disseminate and being cleared from the bloodstream rapidly. The lower viremia level resulted in no neuroinvasion. In contrast, weak heparin-binding variants show greater levels of viremia, dissemination and subsequent neurovirulence in mice. We also provide evidence that the ability of EV-A71 to bind heparin is mediated by electrostatic surface charges due to amino acids on the virus capsid surface. In mice, EV-A71 undergoes adaptive mutation to acquire greater negative surface charges, thus generating new virulent variants with weak heparin-binding which allows greater viral spread. Our study underlines the importance of electrostatic surface charges in shaping EV-A71 virulence.