Making sense of flavivirus non-strctural protein 1 in innate immune evasion and inducing tissue-specific damage

Abstract

Flaviviruses include medically important mosquito-borne pathogens, such as Zika virus (ZIKV), Japanese encephalitis virus (JEV), dengue virus (DENV) and West Nile virus (WNV), that cause hundreds of millions of infections each year. Currently, there are no approved effect therapies against mosquito-borne flaviviruses. The flaviviruses encoded nonstructural protein 1 (NS1) is a secreted glycoprotein widely involved in viral replication, immune evasion, and directly causing tissue-specific damage during flaviviruses infection. Upon viral infection of host cell, NS1 can be found in multiple oligomeric forms and include a dimer on the cell surface, and a soluble secreted hexameric lipoparticle. In the recent decade, the detailed crystal structure of several flaviviruses NS1 have been determined and unraveled its broader and deeper functions. Consistent with the potential immune function revealed by its structure, NS1 is involved in the escaping of host signal immune pathway mediated by pattern recognition receptors (PRRs), including RIG-I-like receptors (RLR_S) and Toll-like receptors (TLRs). Moreover, the flavivirus NS1 is efficiently secreted by infected cells and circulates in the blood of the host to directly induce specific tissues damage. The NS1 of ZIKV, JEV and WNV changes the permeability of brain microvascular endothelial cell to cause endothelial cell dysfunction and promote virus pathogenesis. DENV NS1 can induce systemic tissues damage in humans through multiple strategies. Mutations of several key amino acids in NS1 can reduce the neurovirulence of the flavivirus. In this article, we provide an overview of the latest research on this fascinating protein in these disparate areas.

Keywords: Crystal structure; Flavivirus; Innate immune evasion; Nonstructural protein 1; Tissue-specific damage.

Fig. 1

Model of flavivirus genome and encoded NS1 three-dimensional structure. (A) The genome and encoded proteins of flaviviruses. (B) NS1 dimer: Represented by the 3D structure of the DENV NS1 dimer, one of the monomers is colored gray, and the other monomer is divided into three colors according to the domain: β-roll (red), wing (green), and β-ladder (blue). Those amino acid sites thought to be involved in innate immune evasion and induction of tissue-specific damage are annotated with labels representing the colors of the different virus strains. Structure factor files have been deposited in the RCSB Protein Data Bank (PDB) database under the accession codes 4O6B for DENV-2 NS1, 5GS6 for ZIKV NS1, and 4O6C for WNV NS1. Molecular graphics were performed using the PyMOL Molecular Graphics system.

Fig. 2

Schematic of evasion of the host immune response by flavivirus NS1. Schematic representation of the mechanisms utilized by flaviviruses NS1 to inhibit the immune response. Viral components of flaviviruses were sensed via RIG-I and TLR3/4/7/8. TLR3 and TLR4/7/8 recruit TRIF and MyD88 adaptor respectively, while RIG-I interacts with MAVS adaptor. These adaptors were then activate downstream TBK1 and IKKε kinases, which then leads to phosphorylation of IRF3. The phosphorylated IRF3 translocates to the nucleus, thus inducing the production of type I IFNs. NS1 of ZIKV inhibits IFN-β production by preventing TBK1 phosphorylation, and promotes the clevage of cGAS by inhibiting the proteasomal degradation of caspase-1. JEV NS1′ inhibits the production of IFN-β by reducing MAVS expression. NS1 of WNV interact with RIG-I and MDA5 receptors and degrade their expression to inhibit IFN-β production, and simultaneously to inhibit TLR3-mediated IFN-β expression. DENV NS2B directly target cGAS and causing its degradation.

Fig. 3

Model of flavivirus NS1 to induce tissue-specific damage and its contribution to virus pathogenesis. Flavivirus infections can induce specific tissues damage, including brain, lung, liver, vascular endothelium, and the placenta during pregnancy, to cause neurotropic-encephalitic or systemic diseases. ZIKV NS1 causes hyperpermeability of HBMEC and HUVEC. NS1 of JEV and WNV mainly induces the permeability change of HBMEC. YFV NS1 mainly causes the dysfuntion of HLSEC. NS1 of DENV can induce systemic tissues damage in humans. ZIKV NS1 phosphorylate β-catenin and VE-cadherin via increasing ROS expression, and simultaneously to inhibit VE-cadherin and claudin-5 expression to destroy the adhesion junctions of HBMEC. DENV NS1 recruits MMP-9 to interact with β-catenin and ZO-1 to disrupt cells tight junctions (TJs) and adhesion junctions (AJs). DENV NS1 also promotes MIF and induces cell autophagy, leading to intercellular connections disruption. NS1 of ZIKV, JEV, and DENV down-regulates the sialic acid on the surface of endothelial cells, and can up-regulate the expression of cathepsin L and heparanase to destroy EGL and lead to endothelial cell damage. DENV NS1 N207Q mutant abolish cell endocytosis and EGL disruption. The N-linked glycosylation sites mutation of WNV and YFV NS1 reduces the neurovirulence of the mutant virus. NS1-P250L and NS1-P101K mutation of WNV reduce the virus neuroinvasiveness.