Fusion and cell entry by oncogenic sheep retroviruses

Jaagsiekte sheep retrovirus (JSRV) and enzootic nasal tumour virus (ENTV) are two highly related oncogenic retroviruses that induce a contagious cancer in sheep and goats respectively. Both JSRV and ENTV use hyaluronidase-2 (Hyal2) as a cell entry receptor, yet JSRV induces lung tumours and ENTV causes tumours in the nasal epithelium. Unlike most acutely transforming retroviruses, the genomes of JSRV and ENTV do not contain an oncogene derived from the host cell; instead, the viral envelope protein (Env) functions as an active oncogene in addition of mediating cell entry. JSRV and ENTV Env are first synthesized as precursors that are cleaved by a cellular protease into two functional subunits: the surface (SU) subunit that contains the receptor binding domain and the transmembrane (TM) subunit that mediates membrane fusion. While most of previous studies have focused on the oncogenic properties of these proteins, little was known about how they mediate membrane fusion and viral entry. The goal of my Ph.D study was to investigate the mechanisms of fusion and cell entry by JSRV and ENTV Env as well as their regulations. We found that, although most retroviruses are believed to use a pH-independent pathway for entry, JSRV requires an acidic pH for entry and fusion and that its fusogenicity was negatively regulated by its cytoplasmic tail. Unexpectedly, ENTV Env requires an unusually low pH (<4.5) for fusion activation. While an irreversible inhibitor, Bafilomycin A1, which prevents acidification in the endosomes and lysosomes inhibited entry of JSRV and ENTV, ENTV Env-mediated infection was considerably enhanced in the presence of lysosomotropic agents or leupeptin, suggesting that JSRV and ENTV likely fuse in distinct cellular compartments. Importantly, we found that SU also modulates the fusion activity of JSRV and ENTV Env, despite that TM dictates the differential pH requirements between JSRV and ENTV. The possible involvement of SU-receptor interaction in the fusion activation was further demonstrated by the fact that soluble receptor provided in trans can induce fusion of JSRV Env at low pH in normally fusion-inefficient cells, and that the soluble receptor induces SU shedding and modulates the TM conformational rearrangement induced by low pH. Taken together, our data indicate that JSRV/ENTV Env-mediated fusion is a controlled and multistep process, by which both receptor binding and low pH are required. The unique mechanism of JSRV/ENTV fusion activation and regulation might be important for their oncogenesis.