The Autophagic Machinery in Enterovirus Infection

Abstract

The Enterovirus genus of the Picornaviridae family comprises many important human pathogens, including polioviruses, rhinovirus, enterovirus A71, and enterovirus D68. They cause a wide variety of diseases, ranging from mild to severe life-threatening diseases. Currently, no effective vaccine is available against enteroviruses except for poliovirus. Enteroviruses subvert the autophagic machinery to benefit their assembly, maturation, and exit from host. Some enteroviruses spread between cells via a process described as autophagosome-mediated exit without lysis (AWOL). The early and late phases of autophagy are regulated through various lipids and their metabolizing enzymes. Some of these lipids and enzymes are specifically regulated by enteroviruses. In the present review, we summarize the current understanding of the regulation of autophagic machinery by enteroviruses, and provide updates on recent developments in this field.

Keywords: antiviral; autophagosome maturation; autophagy; enterovirus; lipids; picornavirus; replication.

Figure 1

The autophagic machinery and enteroviruses. Autophagy begins with the formation of phagophores that originate from either the endoplasmic reticulum (ER), Golgi, mitochondria, or plasma membrane, or are synthesized de novo, and undergo sealing to form autophagosomes. Autophagosomes then undergo a maturation process via fusion with endosomes to form acidic amphisomes. In EV-A71, HRV-A2, HRV-B14, and foot-and-mouth disease virus (FMDV) infections, upon internalization of the virus-receptor complex into endosomes, the acidified endosomes will then trigger viral uncoating [24,25,26,27]. In contrast, poliovirus, echovirus 1, and coxsackievirus A9 do not require endosomal acidification for viral entry [28,29]. During late autophagy, amphisomes fuse with lysosomes to form autolysosomes, where autophagic flux/degradation of sequestered organelles occurs. LC3-II and p62, both markers of autophagic membranes, are also degraded in autolysosomes by lysosomal proteases. Enteroviruses positively (grey arrows) and negatively (grey T arrow) regulate multiple steps of the autophagic machinery for their assembly, maturation, and exit from the host.

Figure 2

Subversion of autophagosome maturation by enteroviruses. As recently described, autophagosomes fuse with lysosomes via the SNARE complex consisting of STX17, SNAP29, and VAMP8. Alternatively, autophagosome-lysosome fusion can be achieved via LC3-II, PLEKHM1, and Rab7. The engulfed cytoplasmic organelles are then degraded along with the inner membrane of the autolysosome, in a process termed autophagic flux. Cholesterol is commonly found on the membranes of autolysosome. Fusion between autophagosomes and endosomes forms amphisomes bearing viral particles, which can fuse with the plasma membrane to secrete enteroviruses via a proposed process known as autophagosome-mediated exit without lysis (AWOL). Enteroviruses positively (grey arrows) and negatively (grey T arrow) regulate autophagosome maturation to facilitate their own assembly, maturation, and exit from the host.

Figure 3

Modes of action of lipids in enterovirus replication. (A) The processes of lipid signal transduction regulated by enteroviruses. Class I PI3Ks and their downstream product PI(3,4,5)P₃ regulate mTORC1 signals. The phosphorylation of PI(4,5)P₂ via PI3K activates the AKT signaling pathway. AKT blocks the inhibitory effect of TSC1-TSC2 complex on Rheb. Activated Rheb in turn promotes mTORC1 complex signaling. Rapamycin inhibits the mTORC1 complex (violet T arrow) and further enhances production of enteroviruses, which also have an inhibitory effect on mTORC1 (grey T arrow). 3-MA inhibits the class I PI3K and shuts off its downstream signaling (violet T arrow); (B) Picornaviruses recruit PI3P effectors (grey arrow) to the outer membranes of autophagosomes. PI3P lipids on autophagosome membranes directly interact with PI3P effectors to induce signals for membrane budding, membrane fusion, and vesicular transport of autophagosomes; 3-MA inhibits the recruitment of PI3P effectors to PI3P lipids (violet T arrow); (C) Enteroviruses recruit PI4KIIIβ (grey arrow) via Golgi-localized host factors, resulting in an increase of PI4P lipids. These lipids are commonly found to localize on autolysosome membranes and are converted to PI(4,5)P₂ by PIP5K for autophagic lysosome reformation. Enviroxime inhibits PI4KIIIβ and itraconazole inhibits cholesterol and PI4P transfers (violet T arrow).