Replication and Inhibitors of Enteroviruses and Parechoviruses

Abstract

The Enterovirus (EV) and Parechovirus genera of the picornavirus family include many important human pathogens, including poliovirus, rhinovirus, EV-A71, EV-D68, and human parechoviruses (HPeV). They cause a wide variety of diseases, ranging from a simple common cold to life-threatening diseases such as encephalitis and myocarditis. At the moment, no antiviral therapy is available against these viruses and it is not feasible to develop vaccines against all EVs and HPeVs due to the great number of serotypes. Therefore, a lot of effort is being invested in the development of antiviral drugs. Both viral proteins and host proteins essential for virus replication can be used as targets for virus inhibitors. As such, a good understanding of the complex process of virus replication is pivotal in the design of antiviral strategies goes hand in hand with a good understanding of the complex process of virus replication. In this review, we will give an overview of the current state of knowledge of EV and HPeV replication and how this can be inhibited by small-molecule inhibitors.

Keywords: antiviral; enterovirus; human parechovirus; inhibitor; replication; small molecules.

Figure 1

Classification of the virus family Picornaviridae. The clinically most important genera are depicted. For a selection of these, the species and some examples of genotypes/serotypes are given.

Figure 2

Enterovirus genome. (A) Depicted is a schematic representation of the enterovirus genome on scale. The enterovirus genome encodes a single polyprotein divided into a P1, P2, and P3 area. At the 5′- and 3′-end the genome contains untranslated regions (UTR), which are highly structured. The 5′-UTR contains an internal ribosomal entry site for cap-independent translation. At the 5′-end, the RNA genome is covalently bound to the viral protein VPg which is used as a primer during RNA replication; (B) The polyprotein is processed into the viral proteins and some stable precursors by the viral proteases 2A^pro and 3C^pro (and its precursors).

Figure 3

Enterovirus replication cycle. The Enterovirus replication cycle is initiated by binding of the virus to the receptor and internalization into the cell. Subsequently, the viral RNA genome is released from the virion and translated into a single polyprotein which is then processed by the viral proteases to release the viral proteins. Next, the nonstructural proteins mediate the replication of the RNA genome via a negative-stranded intermediate. This takes place on replication organelles that are formed as a result of a rearrangement of cellular membranes. Newly synthesized positive-stranded RNA molecules can then either enter another round of translation and replication (not depicted) or they can be packaged into the viral capsid proteins to form new infectious virus particles which are released upon cell lysis and through several non-lytic mechanisms.

Figure 4

Extensive membrane rearrangements in Enterovirus-infected cells. An electron tomographic slice through a serial tomogram, bar = 500 nm (A); and top and side views of the surface-rendered model of the boxed area (B) show the presence of single-membrane tubules (green), open (orange) and closed (yellow) double-membrane vesicles in a cell infected with coxsackievirus B3 at 5 h post infection. The ER is depicted in blue. Reprinted from Limpens et al. [70], mBio 2011 with permission from the authors, © 2011 by the American Society for Microbiology.

Figure 5

The proposed role of Golgi-localized host factors in Enterovirus replication. Upon infection, the viral 3A protein recruits GBF1 and indirectly Arf1 to the replication organelles. As a result, COP-I is lost from the membranes. At the same time, PI4KIIIβ is recruited by 3A through a GBF1/Arf-independent mechanism, resulting in an increase in PI4P lipids. OSBP then binds to the PI4P lipids and mediates a PI4P/cholesterol counterflow between the membranes of the replication organelles and the ER.

Figure 6

Morphogenesis of enteroviruses and targets for assembly inhibitors. Hsp90 ensures the proper folding of the P1 precursor protein enabling the cleavage by 3CD^pro into capsid proteins VP0, VP3, and VP1 which then form protomers. For part of the EVs, glutathione (GSH) is required either for the transition of protomers into pentamers or for the stabilization of pentamers. Twelve pentamers plus the viral genome (in red) combine to form a provirion, followed by a maturation step in which the VP0 protein is cleaved into VP4 and VP2. Treatment with Hsp90 inhibitors or glutathione depletors results in impaired morphogenesis.