Roles of the Rabies Virus Phosphoprotein Isoforms in Pathogenesis

Abstract

Rabies virus (RABV) P gene mRNA encodes five in-frame start codons, resulting in expression of full-length P protein (P1) and N-terminally truncated P proteins (tPs), designated P2, P3, P4, and P5. Despite the fact that some tPs are known as interferon (IFN) antagonists, the importance of tPs in the pathogenesis of RABV is still unclear. In this study, to examine whether tPs contribute to pathogenesis, we exploited a reverse genetics approach to generate CE(NiP)⌬P2-5, a mutant of pathogenic CE(NiP) in which the P gene was mutated by replacing all of the start codons (AUG) for tPs with AUA. We confirmed that while CE(NiP) expresses detectable levels of P2 and P3, CE(NiP)⌬P2-5 has an impaired ability to express these tPs. After intramuscular inoculation, CE(NiP)⌬P2-5 caused significantly lower morbidity and mortality rates in mice than did CE ( Rabies virus (RABV), a member of the genus Lyssavirus of the family Rhabdoviridae, is a zoonotic agent that causes a lethal neurological disease in various mammal species, including humans. After transmission via a bite wound caused by an infected animal, RABV infects peripheral nerves and then spreads to and in the central nervous system (CNS), resulting in severe neurological symptoms with a high case fatality rate of almost 100% (reviewed in reference 1). Due to the absence of an effective cure and insufficient provision of postexposure prophylaxis, approximately 59,000 people die from rabies every year, mainly in developing countries (2). To establish an effective cure and also to develop a novel prophylaxis approach for rabies, it is necessary to understand the molecular mechanisms of the pathogenesis, including immune evasion, of RABV.The phosphoprotein (P protein) of RABV is a multifunctional protein that is indispensable not only for viral replication but also for evasion of host innate immunity. Specifically, this protein plays an essential role in viral RNA synthesis as a cofactor of viral RNA-dependent RNA polymerase (L protein) by bridging nucleoprotein (N protein), which directly binds to viral genomic RNA, and L protein in the ribonucleoprotein complex (reviewed in reference 3). In addition, P protein functions to antagonize the type I interferon (IFN)-mediated antiviral responses by inhibiting both signaling pathways for IFN induction and response (4-12). P protein suppresses activation of interferon regulatory factor 3 (IRF-3), which is an important transcription factor for IFN induction (5, 8). Also, P protein binds to the transcriptional factors signal transducers and activator of transcription 1 (STAT1) and STAT2, which play a key role in the IFN response by activating expression of IFN-stimulated genes (ISGs), and inhibits their nuclear translocation and DNA binding (6,10,11).In RABV-infected cells, mRNA of the P gene is translated from five in-frame start codons by a ribosomal leaky scanning mechanism, resulting in expression of full-length P protein (P1; 297 amino acids) and also less abundant expression of N-terminally truncated P proteins (t...

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