Stress Granule Induction after Brain Ischemia Is Independent of Eukaryotic Translation Initiation Factor (eIF) 2α Phosphorylation and Is Correlated with a Decrease in eIF4B and eIF4E Proteins

Abstract

Stress granules (SGs) are cytoplasmic ribonucleoprotein aggregates that are directly connected with the translation initiation arrest response to cellular stresses. Translation inhibition (TI) is observed in transient brain ischemia, a condition that induces persistent TI even after reperfusion, i.e. when blood flow is restored, and causes delayed neuronal death (DND) in selective vulnerable regions. We previously described a connection between TI and DND in the hippocampal cornu ammonis 1 (CA1) in an animal model of transient brain ischemia. To link the formation of SGs to TI and DND after brain ischemia, we investigated SG induction in brain regions with differential vulnerabilities to ischemia-reperfusion (IR) in this animal model. SG formation is triggered by both eukaryotic translation initiation factor (eIF) 2α phosphorylation and eIF4F complex dysfunction. We analyzed SGs by immunofluorescence colocalization of granule-associated protein T-cell internal antigen-1 with eIF3b, eIF4E, and ribosomal protein S6 and studied eIF2 and eIF4F complex. The results showed that IR stress induced SG formation in the CA1 region after 3-day reperfusion, consistent with TI and DND in CA1. SGs were formed independently of eIF2α phosphorylation, and their appearance was correlated with a decrease in the levels of eIF4F compounds, the cap-binding protein eIF4E, and eIF4B, suggesting that remodeling of the eIF4F complex was required for SG formation. Finally, pharmacological protection of CA1 ischemic neurons with cycloheximide decreased the formation of SGs and restored eIF4E and eIF4B levels in CA1. These findings link changes in eIF4B and eIF4E to SG induction in regions vulnerable to death after IR.