Stress granules: potential therapeutic targets for infectious and inflammatory diseases

Abstract

Eukaryotic cells are stimulated by external pressure such as that derived from heat shock, oxidative stress, nutrient deficiencies, or infections, which induce the formation of stress granules (SGs) that facilitates cellular adaptation to environmental pressures. As aggregated products of the translation initiation complex in the cytoplasm, SGs play important roles in cell gene expression and homeostasis. Infection induces SGs formation. Specifically, a pathogen that invades a host cell leverages the host cell translation machinery to complete the pathogen life cycle. In response, the host cell suspends translation, which leads to SGs formation, to resist pathogen invasion. This article reviews the production and function of SGs, the interaction between SGs and pathogens, and the relationship between SGs and pathogen-induced innate immunity to provide directions for further research into anti-infection and anti-inflammatory disease strategies.

Keywords: infection; inflammation; innate immunity; stress granules; therapeutic targets.

Figure 1

The main components of SGs and the factors that induce SGs formation. SGs comprise the following 7 components: (1) mRNAs that are protected from degradation. (2) TIFs targeting mRNAs, such as eIF4E, elF3, PABPC1, p-elF2α and elF5a; (3) RNA-binding proteins (RBPs) that regulate translation and protect mRNA stability, such as TIA-1, TIAR, HuR/ELAVL1, FMRP and PUM1; (4) mRNA metabolism-related proteins, such as G3BP1, G3BP2, DDX6, PMR1, SMN, STAU1, DHX36, caprin-1, ZBP1, HDAC6 and ADAR1; (5) signaling proteins, such as mTOR, RACK1 and TRAF; (6) expression products of interferon-stimulated genes (ISGs), such as PKR, RIG-I, MDA5 and LGP2, RNase L and OAS; and (7) regulatory proteins in SGs formation, such as APOBEC3G, Ago2, BRF1, DDX3, FAST and TTP. Eight factors induce SGs formation: (1) endogenous stressors, such as low-level nutrients, hypoxia, and osmotic shock; (2) environmental stressors, such as heat shock, UV radiation, arsenic compounds, H₂O₂, menadione, diethyl maleate (DEM), paraquat, and brain ischemia; (3) overexpressed G3BP1, TIA-1, TZAR, TTP, FMRP, CPEB, SMN, DYRK3, tRNA fragments, and Aβ42; (4) translation modulators, such as puromycin, salubrinal, cycloheximide, emetine, and ISRIB; (5) proteasome inhibitors, such as MG 132 and lactacystin; (6) ER stressors, such as DTT and thapsigargin; (7) mitochondrial poisons, such as FCCP, clotrimazole, and sodium azide; and (8) other compounds, such as sorbitol, pateamine A, hippuristanol, silvestrol, 15d-prostaglandin J2 (15d-PGJ2); prostaglandin A1, selenite, salicylate, and A769662.

Figure 2

The formation of SGs is dependent or independent of eIF2α phosphorylation. (Left) Via different external stimuli, such as amino acids, ER stress (ERS), oxidative double-stranded RNA (dsRNA), and changes in heme heavy metal levels, four kinases, GCN2, PERK, PKR and HRI, can be activated. The α subunit of eIF2 is phosphorylated, preventing its separation from eIF2B. Subsequently, translation initiation is blocked. The mRNAs and certain proteins involved in the translation process aggregate to form SGs. Before recruiting the 60S ribosome to start translation, the eIF2-GTP-tRNA^Met trimer complex is recruited to the mRNA/eIF4F complex bound to the 40S ribosomal subunit, presenting messenger RNA linked to the translation initiation amino acid to the 40S ribosomal subunit. However, when eIF2 and eIF2B are separated, the ternary complex of eIF2-GTP-tRNA^Met is formed. The complex then binds to mRNA and 40S and 60S ribosomes and participates in translation initiation. Phosphorylated eIF2α interferes with the formation of the eIF2-GTP-tRNAMet complex, resulting in the retention of a large number of translation initiation complexes in the cytoplasm and ultimately induces SGs formation. (Left) The p-eIF2α-independent pathway formed by SGs requires disruption of the eIF4F complex to interfere with RNA helicase eIF4E activity and inhibit its interaction with eIF4G. Maintaining the structural and functional integrity of eIF4F is required for translation initiation and can induce SGs formation through the following three mechanisms: ① Inhibition of the interaction between eIF4E and eIF4G. The recruitment of eIF4F to mRNA may be regulated by mTOR. When cells are starved or damaged by drugs, mTOR is inactivated, resulting in the accumulation of unphosphorylated 4EBP in cells. Unphosphorylated 4EBP competes with eIF4F to bind eIF4E, preventing the formation of eIF4F. Therefore, inhibiting the initiation of translation results in a reduction in the number of polysomes, and translationally stopped preinitiation complexes (PICs) accumulate in the cell and are thus assembled into SGs; ② Inhibition of the interaction between eIF4A and eIF4G or inhibition of the RNA helicase activity of eIF4A. ③ Destruction of the eIF4G structure. For example, after CVB infection, the 2A protein cleaves eIF4G.

Figure 3

The relationship between SGs and antiviral innate immunity. After viral invasion into host cells, viral RNA (dsRNA or 5’pppRNA) activates the PKR kinase, resulting in eIF2α phosphorylation and SGs assembly. SGs contain the RIG-I-like receptors MDA5 and RIG-I, which can interact with MAVS on the mitochondrial membrane to activate the IFN signaling pathway and promote IFN production. After binding to IFN receptors on the cell membrane, interferon-stimulated genes (ISGs) induce the inhibition of viral RNA replication through the JAK/STAT pathway. After viral infection of cells, viral genomic RNA activates PKR, phosphorylates eIF2α, and inhibits the initiation of translation, thereby inhibiting viral protein synthesis and inducing SGs formation. SGs are composed of diverse components, including viral RNA, RNA-binding proteins (RBPs), and translation initiation factors, and especially, many innate immune pattern recognition molecules, such as MDA5, RIG-I, LDP2, and RNase L. These pattern recognition molecules bind viral RNA, which in turn transmits signals to MAVS on the outer mitochondrial membrane, activates TBK1/IKK phosphorylation, and allows transcription factors IRF3/7 and NF-κB to enter the nucleus, where they activate the expression and secretion of type I interferons and certain inflammatory factors. IFN activates intracellular STAT1/2 phosphorylation after autocrine or paracrine binding to cell membrane interferon receptors. Subsequently, p-STAT1/2 enters the nucleus together with IRF9, recognizes internal ribosome entry site (IRSE) regions, activates the expression of various ISGs, and produces antiviral effects.