Foot-and-Mouth Disease Virus 3Cpro Cleaves BP180 to Induce Blister Formation

Foot-and-mouth disease (FMD) is mainly characterized by blister formation (vesicles) in animals infected with foot-and-mouth disease virus (FMDV). However, the molecular basis of the blister formation in FMD is still unknown. BP180 is one of the main anchoring proteins connecting the dermal and epidermal layers of the skin. Previous studies have shown that the cleavage of BP180 by proteases produced by the inflammatory cells and the resulting skin loosening are major causes of the blister formation in bullous pemphigoid (BP) disease. Similar to BP, here we have demonstrated that, among the FMDV-encoded proteases, only FMDV 3C^pro contributes to the cleavage of BP180 at multiple sites, consequently inducing the degradation of BP180, leading to skin loosening. Additionally, we confirmed that FMDV 3C^pro interacts directly with BP180 and the FMDV 3C^pro C142T mutant, known to have reduced protease activity, is less effective for BP180 degradation than wild-type FMDV 3C^pro. In conclusion, for the first time, our results demonstrate the function of FMDV 3C^pro on the connective-tissue protein BP180 associated with blister formation.

Foot-and-Mouth Disease Virus 3C Protease Antagonizes Interferon Signaling and C142T Substitution Attenuates the FMD Virus

3C protease (3Cpro), a chymotrypsin-like cysteine protease encoded by the foot-and-mouth disease virus (FMDV), plays an essential role in processing the FMDV P1 polyprotein into individual viral capsid proteins in FMDV replication. Previously, it has been shown that 3Cpro is involved in the blockage of the host type-I interferon (IFN) responses by FMDV. However, the underlying mechanisms are poorly understood. Here, we demonstrated that the protease activity of 3Cpro contributed to the degradation of RIG-I and MDA5, key cytosolic sensors of the type-I IFN signaling cascade in proteasome, lysosome and caspase-independent manner. And also, we examined the degradation ability on RIG-I and MDA5 of wild-type FMDV 3Cpro and FMDV 3Cpro C142T mutant which is known to significantly alter the enzymatic activity of 3Cpro. The results showed that the FMDV 3Cpro C142T mutant dramatically reduce the degradation of RIG-I and MDA5 due to weakened protease activity. Thus, the protease activity of FMDV 3Cpro governs its RIG-I and MDA5 degradation ability and subsequent negative regulation of the type-I IFN signaling. Importantly, FMD viruses harboring 3Cpro C142T mutant showed the moderate attenuation of FMDV in a pig model. In conclusion, our results indicate that a novel mechanism evolved by FMDV 3Cpro to counteract host type-I IFN responses and a rational approach to virus attenuation that could be utilized for future vaccine development.

Natural Phytochemicals, Luteolin and Isoginkgetin, Inhibit 3C Protease and Infection of FMDV, In Silico and In Vitro

Foot-and-mouth-disease virus (FMDV) is a picornavirus that causes a highly contagious disease of cloven-hoofed animals resulting in economic losses worldwide. The 3C protease (3Cpro) is the main protease essential in the picornavirus life cycle, which is an attractive antiviral target. Here, we used computer-aided virtual screening to filter potential anti-FMDV agents from the natural phytochemical compound libraries. The top 23 filtered compounds were examined for anti-FMDV activities by a cell-based assay, two of which possessed antiviral effects. In the viral and post-viral entry experiments, luteolin and isoginkgetin could significantly block FMDV growth with low 50% effective concentrations (EC50). Moreover, these flavonoids could reduce the viral load as determined by RT-qPCR. However, their prophylactic activities were less effective. Both the cell-based and the fluorescence resonance energy transfer (FRET)-based protease assays confirmed that isoginkgetin was a potent FMDV 3Cpro inhibitor with a 50% inhibition concentration (IC50) of 39.03 ± 0.05 and 65.3 ± 1.7 μM, respectively, whereas luteolin was less effective. Analyses of the protein-ligand interactions revealed that both compounds fit in the substrate-binding pocket and reacted to the key enzymatic residues of the 3Cpro. Our findings suggested that luteolin and isoginkgetin are promising antiviral agents for FMDV and other picornaviruses.