Return of the Neurotropic Enteroviruses: Co-Opting Cellular Pathways for Infection

Abstract

Enteroviruses are among the most common human infectious agents. While infections are often mild, the severe neuropathogenesis associated with recent outbreaks of emerging non-polio enteroviruses, such as EV-A71 and EV-D68, highlights their continuing threat to public health. In recent years, our understanding of how non-polio enteroviruses co-opt cellular pathways has greatly increased, revealing intricate host-virus relationships. In this review, we focus on newly identified mechanisms by which enteroviruses hijack the cellular machinery to promote their replication and spread, and address their potential for the development of host-directed therapeutics. Specifically, we discuss newly identified cellular receptors and their contribution to neurotropism and spread, host factors required for viral entry and replication, and recent insights into lipid acquisition and replication organelle biogenesis. The comprehensive knowledge of common cellular pathways required by enteroviruses could expose vulnerabilities amenable for host-directed therapeutics against a broad spectrum of enteroviruses. Since this will likely include newly arising strains, it will better prepare us for future epidemics. Moreover, identifying host proteins specific to neurovirulent strains may allow us to better understand factors contributing to the neurotropism of these viruses.

Keywords: CRISPR and haploid screens; EV-A71; EV-D68; antivirals; enteroviruses; host-directed therapeutics; neuropathogenesis; picornaviruses; viral tissue tropism; virus–host interactions.

Figure 1

Main receptors for human enteroviruses. Enteroviruses A–D contain neurotropic viruses that are associated with encephalitis, meningitis, and acute flaccid paralysis. Rhinoviruses A–C contain respiratory viruses that are the causative agents of the common cold. Representative EV serotypes are named here.

Figure 2

Speculative model of EV-A71 spread from the primary site of replication in the intestine to the CNS. There are three proposed phases of EV-A71 systemic infection: (1) initial replication in the gut, (2) dissemination to the blood compartment or immune cells, and (3) subsequent invasion of the CNS by infecting extraneural tissues to access motor neurons or by directly crossing the BBB. EV-A71 utilizes many receptors to promote its infection of various tissues. SCARB2 and PSGL-1 are the primary receptors. SCARB2 is broadly required for dissemination to many tissues, while PSGL-1 is found primarily on leukocytes and is important for the viremia phase of infection. Additional receptors including Vimentin and Prohibitin have been implicated in EV-A71 infection of the CNS, while the microbiome plays a role in initial infection of the gut leading to widespread EV-A71 infection.

Figure 3

Cellular factors required for viral replication. (1) Genome replication takes place on virus induced membranous replication organelles (ROs). To establish the massive web of replication organelles during infection, EVs upregulate phospholipid biosynthesis. Upon infection, the import of free fatty acids (FFAs) and their storage in lipid droplets (LDs) is increased. FFAs serve as substrates for phospholipid biosynthesis. EVs scavenge FFAs from lipid droplets to sustain the production of new lipids that drive RO biogenesis. (2) EV viral proteins recruit host proteins to generate a favorable lipid environment on developing ROs for viral replication. Viral proteins recruit PI4KB to enrich replication organelle membranes with the phospholipid PI4P. OSBP is redirected to ROs to transport cholesterol from the ER by shuttling PI4P down its concentration gradient. At the ER membrane, Sac1 hydrolyzes PI4P to phosphatidylinositol (PI), which is transferred back to the ROs by PITPb. (3) The viral RNA polymerase 3D^pol replicates the viral genome in coordination with several host and viral proteins. The viral genome is bound by the viral protein VPg (3B), which is required as a primer for replication. Viral proteins 2C and 3CD aid in replication by serving as a helicase and circularizing the RNA. Genome replication by 3D^pol begins with the synthesis of a negative-strand (−) RNA that serves as a template for the synthesis of new positive-strand (+) RNA. The host factors SETD3 and UGGT1 interact with viral proteins to promote RNA replication; however, their exact mechanisms are unknown.