Breaking the Species Barrier: Investigating the Host Proteins Involved in Human and Murine Hepatitis B Infection

Abstract

Hepatitis B virus (HBV) is a highly contagious pathogen that often leads to liver fibrosis, cirrhosis, and hepatocellular carcinoma (HCC), resulting in around 887,000 deaths each year. As more than 250 million people are chronically infected with HBV and there is no cure available, there is an urgent need to gain deeper insight into the molecular details of the viral life-cycle and to develop physiologically relevant platforms for studying hepatitis B in vivo. HBV exhibits a narrow species tropism, only infecting humans and great apes; thus, a mouse model with inbred of susceptibility to HBV infection would be a major asset for analyzing the complex interplay between this virus and the mammalian host. The barriers of interspecies transmission for HBV in mouse cells are incompletely understood. Prior work suggests that the viral life-cycle is blocked a step post entry but before the intranuclear replication stages. Thus, we hypothesized that nuclear import of the viral capsid and capsid disassembly are blocked in mouse cells, presumably due to the lack of human/primate specific host factor(s). To identify such factor(s), we conducted loss-of-function and gain-of-function experiments in human and murine hepatoma cells, respectively. We identified several candidates potentially implicated in HBV infection in human cell lines, including importin-α-1 (KPNA1) and 2 (KPNA2). Through confocal microscopy, we determined that these proteins are differentially localized in HBV-infected human and murine cells, with KPNA2 and importin-β (KPNB1) appearing to be restricted outside of the nucleus in murine cells upon HBV infection. We propose a mechanism for HBV nuclear entry block in murine cells due to the accumulation of karyopherins in the cytoplasm, and we identify several nucleoporins that could be implicated in the host restriction barrier that prevent mice from being infected. In an alternative, yet complementary approach, we are in the process of conducting an unbiased screen using a cDNA library of the human ORFeome, with preliminary results suggesting that expression of a small proportion of unidentified human factors increases support of HBV infection in murine cells. Further testing is required to confirm these results, but our findings contribute significantly to the search for the barriers in host tropism of this virus and the eventual development of an immunocompetent mouse model that is fully permissive to HBV infection and can facilitate testing of therapeutics for this deadly virus.