Enterovirus Replication Organelles and Inhibitors of Their Formation

Abstract

Enteroviral replication reorganizes the cellular membrane. Upon infection, viral proteins and hijacked host factors generate unique structures called replication organelles (ROs) to replicate their viral genomes. ROs promote efficient viral genome replication, coordinate the steps of the viral replication cycle, and protect viral RNA from host immune responses. More recent researches have focused on the ultrastructure structures, formation mechanism, and functions in the virus life cycle of ROs. Dynamic model of enterovirus ROs structure is proposed, and the secretory pathway, the autophagy pathway, and lipid metabolism are found to be associated in the formation of ROs. With deeper understanding of ROs, some compounds have been found to show inhibitory effects on viral replication by targeting key proteins in the process of ROs formation. Here, we review the recent findings concerning the role, morphology, biogenesis, formation mechanism, and inhibitors of enterovirus ROs.

Keywords: biogenesis; enteroviruses; inhibitors; lipid metabolism; replication organelles.

FIGURE 1

The morphology of enterovirus ROs. (A) Tomographic slice through the serial tomogram of a CVB3-infected cell at 5 h post-infection, with clusters of SMT and sparsely embedded DMVs. Scale bar is 500 nm. (B) Top and side views of a surface-rendered model of the region boxed in panel (A) showing SMT (green), open DMVs (orange), closed DMVs (yellow), and ER (blue). (C) Pattern about of RO morphology. Enterovirus ROs are generated from the ER and the Golgi. During the early stages of infection, enteroviruses produce ROs with single-membrane tubule (SMT) morphology. SMTs transform into double-membrane vesicles (DMVs) and multilamellar vesicles (MVs) with the progression of infection. (A,B) Adapted from Limpens et al. (2011). The original images have been published under the Creative Commons Attribution-Non-commercial-Share Alike 3.0 Unported license. We have obtained author’s permission.

FIGURE 2

PC recruitment on enterovirus ROs. FAs are precursors for PC synthesis. Enteroviruses increase the level of FAs via two ways. One is to recruit FANS to enterovirus RO to synthesize FAs and then FAs convert to PC on the membrane of ROs. The other is to uptake FAs from the extracellular medium which depends on ACSL3. The imported FAs are targeted to TG synthesis and storage in LDs. FAs are released from LDs by lipolysis. CCTα translocates from the nucleus to the membrane of ROs, where CCTα synthesizes PC using FAs released from LDs.

FIGURE 3

PI4P generation of enterovirus ROs. Enterovirus 3A recruits PI4KB to RO membrane through indirect interaction with PI4KB to phosphorylate PI to PI4P. Two intermediate factors have been found to be related to this process. One is GBF1. The viral 3A protein binds and regulates GBF1/Arf1, increasing the accumulation of PI4KB over COPI on RO. The other is ACBD3. ACBD3 not only is an intermediary regulator of 3A and PI4KB but also participates in the correct localization of viral 3A protein in the nucleus. PI4KB can recruit c10orf76 to the c10orf76-dependent virus RO membrane via the c10orf76-PI4KB interface. c10orf76 on the ROs may contribute to proper Arf1 activation and increases the PI4P level of ROs. PI4P is related to transporting cholesterol to ROs and recruiting 3D to generate vRNA on ROs (Sasvari and Nagy, 2010; McPhail et al., 2020).

FIGURE 4

Cholesterol homeostasis of enterovirus ROs. Different enteroviruses utilize multiple mechanisms to enrich cholesterol on ROs. HRV-A16 is dependent on HSL to hydrolyze cholesterol ester stored in LDs. PV and CVB3 activate CME to redistribute cholesterol from the plasma membrane and extracellular medium to ROs and harness Rab11 recycling endosomes to target cholesterol to ROs. Enteroviruses such as PV, CVB3, and HRV can also use OSBP and other host factors driving PI4P-cholesterol countercurrents to increase the level of cholesterol in ROs. OSBP mediates the transfer of PI4P from the ROs to the ER with cholesterol transfer from the ER to the ROs. PI4P is dephosphorylated to PI by Sac1 on the ER membrane. PI is transported to enterovirus ROs from the ER by PITP-b. On the membrane of enterovirus ROs, PI is phosphorylated to PI4P by PI4KB (Roulin et al., 2014; Nchoutmboube et al., 2015).