A 3C(pro)-dependent bioluminescence imaging assay for in vivo evaluation of anti-enterovirus 71 agents

Cleavage of Stau2 by 3C protease promotes EV-A71 replication

BACKGROUND

Enterovirus A71 (EV-A71), as a neurotropic virus, mainly affects infants and young children under the age of 5. EV-A71 infection causes hand-foot-mouth disease and herpetic angina, and even life-threatening neurological complications. However, the molecular mechanism by which EV-A71 induces nervous system damage remains elusive. The viral protease 3C plays an important role during EV-A71 infection and is also a key intersection of virus-host interactions. Previously, we used yeast two-hybrid to screen out the host protein Double-stranded RNA-binding protein Staufen homolog 2 (Stau2), an important member involved in neuronal mRNA transport, potentially interacts with 3C.

METHODS

We used coimmunoprecipitation (Co-IP) and immunofluorescence assay (IFA) to confirm that EV-A71 3C interacts with Stau2. By constructing the mutant of Stau2, we found the specific site where the 3C protease cleaves Stau2. Detection of VP1 protein using Western blotting characterized EV-A71 viral replication, and overexpression or knockdown of Stau2 exhibited effects on EV-A71 replication. The effect of different cleavage products on EV-A71 replication was demonstrated by constructing Stau2 truncates.

RESULTS

In this study, we found that EV-A71 3C interacts with Stau2. Stau2 is cleaved by 3C at the Q507-G508 site. Overexpression of Stau2 promotes EV-A71 VP1 protein expression, whereas depletion of Stau2 by small interfering RNA inhibits EV-A71 replication. Stau2 is essential for EV-A71 replication, and the product of Stau2 cleavage by 3C, 508-570 aa, has activity that promotes EV-A71 replication. In addition, we found that mouse Stau2 is also cleaved by EV-A71 3C at the same site.

CONCLUSIONS

Our research provides an example for EV-A71-host interaction, enriching key targets of host factors that contribute to viral replication.

TRAF3IP3 Is Cleaved by EV71 3C Protease and Exhibits Antiviral Activity

Enterovirus 71 (EV71) is one of the major pathogens of hand, foot, and mouth disease, which poses a major risk to public health and infant safety. 3C protease (3C^pro), a non-structural protein of EV71, promotes viral protein maturation by cleaving polyprotein precursors and facilitates viral immune escape by cleaving host proteins. In this study, we screened for human proteins that could interact with EV71 3C^pro using a yeast two-hybrid assay. Immune-associated protein TRAF3 Interacting Protein 3 (TRAF3IP3) was selected for further study. The results of co-immunoprecipitation and immunofluorescence demonstrated the interaction between TRAF3IP3 and EV71 3C^pro. A cleavage band was detected, indicating that both transfected 3C^pro and EV71 infection could cleave TRAF3IP3. 87Q-88G was identified as the only 3C^pro cleavage site in TRAF3IP3. In Jurkat and rhabdomyosarcoma (RD) cells, TRAF3IP3 inhibited EV71 replication, and 3C^pro cleavage partially resisted TRAF3IP3-induced inhibition. Additionally, the nuclear localization signal (NLS) and nuclear export signal (NES) of TRAF3IP3 were identified. The NES contributed to TRAF3IP3 alteration of 3C^pro localization and inhibition of EV71 replication. Together, these results indicate that TRAF3IP3 inhibits EV71 replication and 3C^pro resists such inhibition via proteolytic cleavage, providing a new example of virus-host interaction.

TAR DNA-Binding Protein 43 is Cleaved by the Protease 3C of Enterovirus A71

Enterovirus A71 (EV-A71) is one of the etiological pathogens leading to hand, foot, and mouth disease (HFMD), which can cause severe neurological complications. The neuropathogenesis of EV-A71 infection is not well understood. The mislocalization and aggregation of TAR DNA-binding protein 43 (TDP-43) is the pathological hallmark of amyotrophic lateral sclerosis (ALS). However, whether TDP-43 was impacted by EV-A71 infection is unknown. This study demonstrated that TDP-43 was cleaved during EV-A71 infection. The cleavage of TDP-43 requires EV-A71 replication rather than the activated caspases due to viral infection. TDP-43 is cleaved by viral protease 3C between the residues 331Q and 332S, while mutated TDP-43 (Q331A) was not cleaved. In addition, mutated 3C which lacks the protease activity failed to induce TDP-43 cleavage. We also found that TDP-43 was translocated from the nucleus to the cytoplasm, and the mislocalization of TDP-43 was induced by viral protease 2A rather than 3C. Taken together, we demonstrated that TDP-43 was cleaved by viral protease and translocated to the cytoplasm during EV-A71 infection, implicating the possible involvement of TDP-43 in the pathogenesis of EV-A71infection.

Reevaluation of the efficacy of favipiravir against rabies virus using in vivo imaging analysis

Rabies virus (RABV) is a highly neurotropic virus and the causative agent of rabies, an encephalitis with an almost 100% case-fatality rate that remains incurable after the onset of symptoms. Favipiravir (T-705), a broad-spectrum antiviral drug against RNA viruses, has been shown to be effective against RABV in vitro but ineffective in vivo. We hypothesized that favipiravir is effective in infected mice when RABV replicates in the peripheral tissues/nerves but not after virus neuroinvasion. We attempted to clarify this point in this study using in vivo bioluminescence imaging. We generated a recombinant RABV from the field isolate 1088, which expressed red firefly luciferase (1088/RFLuc). This allowed semiquantitative detection and monitoring of primary replication at the inoculation site and viral spread in the central nervous system (CNS) in the same mice. Bioluminescence imaging revealed that favipiravir (300 mg/kg/day) treatment commencing 1 h after intramuscular inoculation of RABV efficiently suppressed viral replication at the inoculation site and the subsequent replication in the CNS. However, virus replication in the CNS was not inhibited when the treatment began 2 days after inoculation. We also found that higher doses (600 or 900 mg/kg/day) of favipiravir could suppress viral replication in the CNS even when administration started 2 days after inoculation. These results support our hypothesis and suggest that a highly effective drug-delivery system into the CNS and/or the enhancement of favipiravir conversion to its active form are required to improve favipiravir treatment of rabies. Furthermore, the bioluminescence imaging system established in this study will facilitate the development of treatment for symptomatic rabies.

Coxsackievirus B3 Responds to Polyamine Depletion via Enhancement of 2A and 3C Protease Activity

Polyamines are small positively-charged molecules abundant in eukaryotic cells that are crucial to RNA virus replication. In eukaryotic cells, polyamines facilitate processes such as transcription, translation, and DNA replication, and viruses similarly rely on polyamines to facilitate transcription and translation. Whether polyamines function at additional stages in viral replication remains poorly understood. Picornaviruses, including Coxsackievirus B3 (CVB3), are sensitive to polyamine depletion both in vitro and in vivo; however, precisely how polyamine function in picornavirus infection has not been described. Here, we describe CVB3 mutants that arise with passage in polyamine-depleted conditions. We observe mutations in the 2A and 3C proteases, and we find that these mutant proteases confer resistance to polyamine depletion. Using a split luciferase reporter system to measure protease activity, we determined that polyamines facilitate viral protease activity. We further observe that the 2A and 3C protease mutations enhance reporter protease activity in polyamine-depleted conditions. Finally, we find that these mutations promote cleavage of cellular eIF4G during infection of polyamine-depleted cells. In sum, our results suggest that polyamines are crucial to protease function during picornavirus infection. Further, these data highlight viral proteases as potential antiviral targets and highlight how CVB3 may overcome polyamine-depleting antiviral therapies.

Development of a luciferase-based biosensor to assess enterovirus 71 3C protease activity in living cells

Enterovirus 71 (EV71) is a major pathogen of hand, foot, and mouth disease (HFMD). To date, no antiviral drug has been approved to treat EV71 infection. Due to the essential role that EV71 3 C protease (3Cpro) plays in the viral life cycle, it is generally considered as a highly appealing target for antiviral drug development. In this study, we present a transgene-encoded biosensor that can accurately, sensitively and quantitatively report the proteolytic activity of EV71 3Cpro. This biosensor is based on the catalyzed activity of a pro-interleukin (IL)-1β-enterovirus 3Cpro cleavage site-Gaussia Luciferase (GLuc) fusion protein that we named i-3CS-GLuc. GLuc enzyme is inactive in the fusion protein because of aggregation caused by pro-IL-1β. However, the 3Cpro of EV71 and other enteroviruses, such as coxsackievirus A9 (CVA9), coxsackievirus B3 (CVB3), and poliovirus can recognize and process the canonical enterovirus 3Cpro cleavage site between pro-IL-1β and GLuc, thereby releasing and activating GLuc and resulting in increased luciferase activity. The high sensitivity, ease of use, and applicability as a transgene in cell-based assays of i-3CS-GLuc biosensor make it a powerful tool for studying viral protease proteolytic events in living cells and for achieving high-throughput screening of antiviral agents.

Protease 2A induces stress granule formation during coxsackievirus B3 and enterovirus 71 infections

Background

Stress granules (SGs) are granular aggregates in the cytoplasm that are formed under a variety of stress situations including viral infection. Previous studies indicate that poliovirus, a member of Picornaviridae, can induce SG formation. However, the exact mechanism by which the picornaviruses induce SG formation is unknown.

Method

The localization of SG markers in cells infected with coxsackievirus B3 (CVB3) or enterovirus 71 (EV71) and in cells expressing each viral protein was determined via immunofluorescence assays or plasmid transfection. Eight plasmids expressing mutants of the 2A protease (2A^pro) of CVB3 were generated using a site-directed mutagenesis strategy. The cleavage efficiencies of eIF4G by CVB3 2A^pro and its mutants were determined via western blotting assays.

Results

In this study, we found that CVB3 infection induced SG formation, as evidenced by the co-localization of some accepted SG markers in viral infection-induced granules. Furthermore, we identified that 2A^pro of CVB3 was the key viral component that triggered SG formation. A 2A^pro mutant with the G122E mutation, which exhibited very low cleavage efficiency toward eIF4G, significantly attenuated its capacity for SG induction, indicating that the protease activity was required for 2A^pro to initiate SG formation. Finally, we observed that SGs also formed in EV71-infected cells. Expression of EV71 2A^pro alone was also sufficient to cause SG formation.

Conclusion

Both CVB3 and EV71 infections can induce SG formation, and 2A^pro plays a crucial role in the induction of SG formation during these infections. This finding may help us to better understand how picornaviruses initiate the SG response.

Electronic supplementary material

The online version of this article (doi:10.1186/s12985-014-0192-1) contains supplementary material, which is available to authorized users.

AUF1 is recruited to the stress granules induced by coxsackievirus B3

Stress granules (SGs) are cytoplasmic granules that are formed in cells when stress occurs. In this study, we found that SGs formed in cells infected with coxsackievirus B3 (CVB3), evidenced with the co-localization of some accepted SG markers in the viral infection-induced granules. We further discovered that adenosine-uridine (AU)-rich element RNA binding factor 1 (AUF1), which can bind to mRNAs and regulate their translation, was recruited to the SGs in response to high dose of CVB3 by detecting the co-localization of AUF1 with SG markers. Similar results were also observed in the enterovirus 71 (EV71)-infected cells. Finally, we demonstrated that AUF1 was also recruited to arsenite-induced SGs, suggesting that the recruitment of AUF1 to SG is not a specific response to viral infection. In summary, our data indicate that both CVB3 and EV71 infections can induce SG formation, and AUF1 is a novel SG component upon the viral infections. Our findings may shed light on understanding the picornavirus-host interaction.