EV-A71 Mechanism of Entry: Receptors/Co-Receptors, Related Pathways and Inhibitors

Abstract

Enterovirus A71, a non-enveloped single-stranded (+) RNA virus, enters host cells through three stages: attachment, endocytosis and uncoating. In recent years, receptors/co-receptors anchored on the host cell membrane and involved in this process have been continuously identified. Among these, hSCARB-2 was the first receptor revealed to specifically bind to a definite site of the EV-A71 viral capsid and plays an indispensable role during viral entry. It actually acts as the main receptor due to its ability to recognize all EV-A71 strains. In addition, PSGL-1 is the second EV-A71 receptor discovered. Unlike hSCARB-2, PSGL-1 binding is strain-specific; only 20% of EV-A71 strains isolated to date are able to recognize and bind it. Some other receptors, such as sialylated glycan, Anx 2, HS, HSP90, vimentin, nucleolin and fibronectin, were discovered successively and considered as "co-receptors" because, without hSCARB-2 or PSGL-1, they are not able to mediate entry. For cypA, prohibitin and hWARS, whether they belong to the category of receptors or of co-receptors still needs further investigation. In fact, they have shown to exhibit an hSCARB-2-independent entry. All this information has gradually enriched our knowledge of EV-A71's early stages of infection. In addition to the availability of receptors/co-receptors for EV-A71 on host cells, the complex interaction between the virus and host proteins and various intracellular signaling pathways that are intricately connected to each other is critical for a successful EV-A71 invasion and for escaping the attack of the immune system. However, a lot remains unknown about the EV-A71 entry process. Nevertheless, researchers have been continuously interested in developing EV-A71 entry inhibitors, as this study area offers a large number of targets. To date, important progress has been made toward the development of several inhibitors targeting: receptors/co-receptors, including their soluble forms and chemically designed compounds; virus capsids, such as capsid inhibitors designed on the VP1 capsid; compounds potentially interfering with related signaling pathways, such as MAPK-, IFN- and ATR-inhibitors; and other strategies, such as siRNA and monoclonal antibodies targeting entry. The present review summarizes these latest studies, which are undoubtedly of great significance in developing a novel therapeutic approach against EV-A71.

Keywords: Enterovirus 71; entry; inhibitors; receptors/co-receptors; signaling pathways.

Figure 1

Enterovirus A71: mode of entry, virus–host interaction and viral life cycle. (A) Schematic illustration of the 3 different modes of entry identified to date for EV-A71 to invade cells. EV-A71 enters cells through receptor-mediated endocytosis, which includes clathrin-mediated (I), caveolae-mediated (II) and endophilin-A2-mediated (III). (I) In presence of the main receptor, hSCARB2, EV-A71 binds to it with or without the help of one or more co-receptors. The binding induces recruitment of adaptor proteins on the receptor cytoplasmic tail, which afterward bind to clathrin and form “a clathrin-coated pit” (CCP), leading to EV-A71 entry through clathrin-mediated endocytosis. hSCARB2 delivers β-GC from the ER to the lysosomes under physiological conditions. hSCARB2 is abundant in the lysosomal and endosomal compartments, and it also shuttles to the plasma membrane where it encounters EV-A71. After the binding of the virus on the cell surface, the virus–receptor complex is internalized. In the endosome or lysosome, where the pH is low, the virus initiates a conformational change that leads to uncoating. (II) In an alternative process, caveolins are the proteins binding to PSGL-1 cytoplasmic tail in the presence of actin cytoskeleton. Thus, this entrance way is called caveolae-mediated endocytosis. PSGL-1 can bind to EV-A71 and internalize via caveolin-mediated endocytosis, but PSGL-1 cannot initiate uncoating. (III) Recently, endophilin-A2-mediated endocytosis was identified. However, how receptor/co-receptor mediates this kind of entry is not yet elucidated. Once endocytosis initiates, the virus is internalized and delivered to early endosome for translocation, which is assured by the endosomal sorting complex required for transport to multivesicular bodies (ESCRT-MVBs). (B) EV-A71 promotes its production through interaction with intracellular signaling pathways. Once the virus is captured by main or co-receptors, several intracellular signaling pathways related to immune response are activated in order to eliminate the viral infection. However, EV-A71 has to escape immune response and overcome cellular apoptosis/autophagy for its survival by interfering with these pathways by interacting with host proteins. MAPK signaling cascade is activated after release of IL-2, IL-4, IL-10 and TNF-α. Subsequently, together with activated P13K/Akt pathway, MAPK downregulates GSK3, resulting in the delay of apoptosis. Multiple proteins involved in IFN-, apoptosis- and autophagy-related pathways are also regulated, such as JAK/STAT, TRAFs, p53, bax and mTOR. Consequently, many aspects of viral production are promoted, including viral polyprotein processing, RNA synthesis, assembly and maturation and release of new virions.