Uncoupling Stress Granule Assembly and Translation Initiation Inhibition

Abstract

Cytoplasmic stress granules (SGs) are specialized regulatory sites of mRNA translation that form under different stress conditions known to inhibit translation initiation. The formation of SG occurs via two pathways; the eukaryotic initiation factor (eIF) 2alpha phosphorylation-dependent pathway mediated by stress and the eIF2alpha phosphorylation-independent pathway mediated by inactivation of the translation initiation factors eIF4A and eIF4G. In this study, we investigated the effects of targeting different translation initiation factors and steps in SG formation in HeLa cells. By depleting eIF2alpha, we demonstrate that reduced levels of the eIF2.GTP.Met-tRNAi(Met) ternary translation initiation complexes is sufficient to induce SGs. Likewise, reduced levels of eIF4B, eIF4H, or polyA-binding protein, also trigger SG formation. In contrast, depletion of the cap-binding protein eIF4E or preventing its assembly into eIF4F results in modest SG formation. Intriguingly, interfering with the last step of translation initiation by blocking the recruitment of 60S ribosome either with 2-(4-methyl-2,6-dinitroanilino)-N-methylpropionamideis or through depletion of the large ribosomal subunits protein L28 does not induce SG assembly. Our study identifies translation initiation steps and factors involved in SG formation as well as those that can be targeted without induction of SGs.

Nuclear accumulation of mRNAs underlies G4C2 repeat-induced translational repression in a cellular model of C9orf72 ALS

“…; n53. granules are formed when the availability of eIF2a, as well as other translational factors, is decreased by small interfering RNA (siRNA) (Mokas et al, 2009), a situation that might be mimicked by RNA repeat sequestration. As an alternative, decreased mRNA export from cell nuclei, as suggested by the clear accumulation of poly(A) RNA in the nuclei of cells expressing (G4C2) 31 repeats, might explain, at least in part, this phenomenon.…”

Role of oxygen consumption in hypoxia protection by translation factor depletion

“…Components of the translation machinery have been found to be aggregated after hypoxia and this aggregation may promote or reduce cell death depending on the experimental model (Jamison et al, 2008;Buchan and Parker, 2009). Further, depletion of specific translation factors has differential effects on translation machinery aggregation (Mokas et al, 2009). One reasonable hypothesis is that reduction of some translation factors such as RARS-1 strongly alters the level of translation machinery aggregation produced by hypoxia in a way that improves recovery after hypoxia and increases resistance to protein misfolding agents.…”

Arginylated Calreticulin at Plasma Membrane Increases Susceptibility of Cells to Apoptosis

“…Other examples include the histone deacetylase 6 activity in SG formation (47), the inhibition of the ubiquitin proteasome system in SG disassembly (51), and the demonstration that SGs contain methylarginine proteins (52). Interestingly the trigger for SGs formation under arsenite treatment involves the phosphorylation of eukaryotic initiation factor 2␣, a fundamental regulatory mechanism that controls global rates of protein synthesis (53,54), which is also critical for CRT exposure (55).…”