Picornaviruses cause several diseases, not only in humans but also in various animal hosts. For instance, human enteroviruses can cause hand-foot-and-mouth disease, herpangina, myocarditis, acute flaccid paralysis, acute hemorrhagic conjunctivitis, severe neurological complications, including brainstem encephalitis, meningitis and poliomyelitis, and even death. The interaction between the virus and the host is important for viral replication, virulence and pathogenicity. This article reviews studies of the functions of viral and host factors that are involved in the life cycle of picornavirus. The interactions of viral capsid proteins with host cell receptors is discussed first, and the mechanisms by which the viral and host cell factors are involved in viral replication, viral translation and the switch from translation to RNA replication are then addressed. Understanding how cellular proteins interact with viral RNA or viral proteins, as well as the roles of each in viral infection, will provide insights for the design of novel antiviral agents based on these interactions.

The roles of virus-derived small RNAs (vsRNAs) have been studied in plants and insects. However, the generation and function of small RNAs from cytoplasmic RNA viruses in mammalian cells remain unexplored. This study describes four vsRNAs that were detected in enterovirus 71-infected cells using next-generation sequencing and northern blots. Viral infection produced substantial levels (>105 copy numbers per cell) of vsRNA1, one of the four vsRNAs. We also demonstrated that Dicer is involved in vsRNA1 generation in infected cells. vsRNA1 overexpression inhibited viral translation and internal ribosomal entry site (IRES) activity in infected cells. Conversely, blocking vsRNA1 enhanced viral yield and viral protein synthesis. We also present evidence that vsRNA1 targets stem-loop II of the viral 5′ untranslated region and inhibits the activity of the IRES through this sequence-specific targeting. Our study demonstrates the ability of a cytoplasmic RNA virus to generate functional vsRNA in mammalian cells. In addition, we also demonstrate a potential novel mechanism for a positive-stranded RNA virus to regulate viral translation: generating a vsRNA that targets the IRES.

Far-upstream element-binding protein 2 (FBP2) is an internal ribosomal entry site (IRES) trans-acting factor (ITAF) that negatively regulates enterovirus 71 (EV71) translation. This study shows that EV71 infection cleaved FBP2. Live EV71 and the EV71 replicon (but not UV-inactivated virus particles) induced FBP2 cleavage, suggesting that viral replication results in FBP2 cleavage. The results also showed that virus-induced proteasome, autophagy, and caspase activity co-contribute to EV71-induced FBP2 cleavage. Using FLAG-fused FBP2, we mapped the potential cleavage fragments of FBP2 in infected cells. We also found that FBP2 altered its function when its carboxyl terminus was cleaved. This study presents a mechanism for virus-induced cellular events to cleave a negative regulator for viral IRES-driven translation. Enterovirus 71 (EV71), an RNA virus that belongs to the family Picornaviridae, has caused several outbreaks worldwide and often results in severe neurological complications and high mortality in patients (1-10). Picornavirus infection can affect host mechanisms, such as host cap-dependent translation (11, 12), transcription (13,14), and immune responses (15-17). FBP2, also called the KH-type splicing regulatory protein (KSRP), was first identified as a single-strand DNA-binding protein (37). FBP2 positively regulates c-myc transcription (37), enhances the splicing of the neuron-specific c-src N1 exon (38, 39), edits apolipoprotein B (apoB) mRNA (40), and is associated with mRNA decay (41,42). FBP2 is also a component of the Dicer and Drosha complexes and regulates let-7 microRNA (miRNA) biogenesis (43-45). A previous study has shown that FBP2 binds to the EV71 5= untranslated region (UTR), which contains an IRES, and downregulates IRES activity by competing with other, positive ITAFs, such as PTB (35).In this study, we show that FBP2, a negative ITAF of EV71, is cleaved in EV71-infected cells. We also demonstrate that EV71-induced caspase activity, autophagic activity, and proteasome activity are involved in virus-induced cleavage of FBP2. Moreover, we found a cleavage product, FBP2 , that loses its carboxyl terminus and positively regulates the IRES activity of EV71. MATERIALS AND METHODS Cell lines and virus infection.Human embryonal rhabdomyosarcoma (RD) and HeLa cells were maintained in Dulbecco's modified Eagle medium (DMEM) (GIBCO) containing 10% fetal bovine serum (FBS; GIBCO) at 37°C. RD cells were grown to 80% to 90% confluence and were infected with enterovirus 71 strain Tainan/4643/98 at a multiplicity of infection (MOI) of 40 PFU per cell in serum-free DMEM. Virus was adsorbed at 37°C for 1 h. After adsorption, the cells were washed with phosphate-buffered saline (PBS) and were incubated with a medium containing 2% FBS. In several experiments, RD cells were infected with EV71 for 1 h, washed with PBS, and incubated with a medium containing 2% FBS and various inhibitors. UV-inactivated EV71 was prepared by following a method described elsewhere (46). In other experiments, RD cells were tran...