Molecular Pathways of Protein Synthesis Inhibition during Brain Reperfusion: Implications for Neuronal Survival or Death

Abstract

Protein synthesis inhibition occurs in neurons immediately on reperfusion after ischemia and involves at least alterations in eukaryotic initiation factors 2 (eIF2) and 4 (eIF4). Phosphorylation of the alpha subunit of eIF2 [eIF2(alphaP)] by the endoplasmic reticulum transmembrane eIF2alpha kinase PERK occurs immediately on reperfusion and inhibits translation initiation. PERK activation, along with depletion of endoplasmic reticulum Ca2+ and inhibition of the endoplasmic reticulum Ca2+ -ATPase, SERCA2b, indicate that an endoplasmic reticulum unfolded protein response occurs as a consequence of brain ischemia and reperfusion. In mammals, the upstream unfolded protein response components PERK, IRE1, and ATF6 activate prosurvivial mechanisms (e.g., transcription of GRP78, PDI, SERCA2b ) and proapoptotic mechanisms (i.e., activation of Jun N-terminal kinases, caspase-12, and CHOP transcription). Sustained eIF2(alphaP) is proapoptotic by inducing the synthesis of ATF4, the CHOP transcription factor, through "bypass scanning" of 5' upstream open-reading frames in ATF4 messenger RNA; these upstream open-reading frames normally inhibit access to the ATF4 coding sequence. Brain ischemia and reperfusion also induce mu-calpain-mediated or caspase-3-mediated proteolysis of eIF4G, which shifts message selection to m 7 G-cap-independent translation initiation of messenger RNAs containing internal ribosome entry sites. This internal ribosome entry site-mediated translation initiation (i.e., for apoptosis-activating factor-1 and death-associated protein-5) can also promote apoptosis. Thus, alterations in eIF2 and eIF4 have major implications for which messenger RNAs are translated by residual protein synthesis in neurons during brain reperfusion, in turn constraining protein expression of changes in gene transcription induced by ischemia and reperfusion. Therefore, our current understanding shifts the focus from protein synthesis inhibition to the molecular pathways that underlie this inhibition, and the role that these pathways play in prosurvival and proapoptotic processes that may be differentially expressed in vulnerable and resistant regions of the reperfused brain.

Role of the Endoplasmic Reticulum Unfolded Protein Response in Glomerular Epithelial Cell Injury

“…Phosphorylation of PERK and eIF2␣ and reduction in protein synthesis during ischemia-reperfusion in GECs are in keeping with the up-regulation of bip and grp94 expression demonstrated previously (10). Similar to complement, eIF2␣ phosphorylation occurred early, whereas bip and grp94 expression was evident at 24 h. Our results in GECs are in keeping with previous studies (41)(42)(43), which showed that PERK and eIF2␣ became phosphorylated after experimental brain ischemia. However, unlike GECs, another study reported that brain ischemia is associated with activation of cPLA 2 (44).…”

Double-stranded RNA-dependent Protein Kinase Phosphorylation of the α-Subunit of Eukaryotic Translation Initiation Factor 2 Mediates Apoptosis

“…Agents that inhibit eIF2␣ phosphorylation could promote survival under desired conditions, for example to inhibit macrophage apoptosis upon viral infection (9) or prevent ischemic cell injury (71). Alternatively, direct targeting of therapeutic agents to induce eIF2␣ phosphorylation may accentuate apoptosis of virus-infected cells (72).…”

Ischemia-induced neuronal cell death is mediated by the endoplasmic reticulum stress pathway involving CHOP

“…12 In brain ischemia, neuronal cells cannot receive sufficient glucose and oxygen required for modification of proteins in ER, and ER stress may occur. 23 The induction of CHOP mRNA by brain ischemia or hypoxia has been reported. [24][25][26][27] However, the exact role of CHOP in ER stress-mediated neuronal apoptosis in brain ischemia or hypoxia remains to be clarified.…”