Membrane Hijacking: Biogenesis and Fate of Quasi-Enveloped Hepatovirus

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PROJECT ABSTRACT Hepatitis A virus (HAV), a member of the Picornaviridae (genus Hepatovirus), is an ancient human pathogen and a common cause of enterically-transmitted viral hepatitis globally. Long considered `nonenveloped', we discovered that HAV is released noncytolytically from infected cells both in vitro and in infected humans in vivo within extracellular vesicles that are similar in size and buoyant density to exosomes. The HAV capsid is completely cloaked by the membranes of these small vesicles, which nonetheless possess specific infectivity indistinguishable from naked HAV virions. The goal of this grant is to elucidate mechanisms underlying the biogenesis of these `quasi-enveloped' HAV (eHAV) virions and their role in the pathogenesis of hepatitis A. Extensive proteomics, virologic, and biophysical characterization of eHAV produced in cell culture demonstrate that viral capsids are selected for export in eHAV vesicles via a highly specific sorting process that is dependent on sequence in the pX domain of the VP1 capsid protein. We hypothesize (1) that the quasi- envelopment of eHAV results from budding of assembled VP1pX-containing capsids into endosomes (multivesicular bodies) in an ESCRT-dependent process mediated by specific interactions of capsid proteins with ESCRT complexes and facilitated by NEDD4-family E3 ubiquitin ligases; and (2) that cell entry by eHAV differs from naked virions and occurs within late endosome-lysosomes where membranes are hydrolyzed by lysosomal enzymes, providing capsid access to an unknown cellular receptor. In Specific Aim 1, we will map interactions of ESCRT components with VP1pX and ascertain the functional importance of ESCRT-0 and -I components in CRISPR/Cas9 knockout cells. Specific Aim 2 will determine whether catalytic activity of NEDD4-family ligases is required for eHAV biogenesis and contributes to ubiquitylation of pX or other capsid protein domains, and ascertain the extent to which core components of noncanonical `secretory autophagy' are involved in nonlytic eHAV egress. Specific Aim 3 will contrast mechanisms of eHAV vs. HAV entry, including endocytosis and endocytic transport, and define tissue tropisms of eHAV vs. HAV. Results from in vitro studies will be validated in vivo using a novel murine model of human hepatitis A. The discovery of eHAV has profoundly altered our understanding of the pathogenesis of this hepatotropic human virus and has substantial ramifications for the biology of other noncytopathic `nonenveloped' viruses, making this research of broad relevance to the field of virology.