Activation of GCN2 kinase by ribosome stalling links translation elongation with translation initiation

Ribosome stalling during translation has recently been shown to cause neurodegeneration, yet the signaling pathways triggered by stalled elongation complexes are unknown. To investigate these pathways we analyzed the brain of C57BL/6J-Gtpbp2(nmf205-/-) mice in which neuronal elongation complexes are stalled at AGA codons due to deficiencies in a tRNA(UCU)(Arg) tRNA and GTPBP2, a mammalian ribosome rescue factor. Increased levels of phosphorylation of eIF2 alpha (Ser51) were detected prior to neurodegeneration in these mice and transcriptome analysis demonstrated activation of ATF4, a key transcription factor in the integrated stress response (ISR) pathway. Genetic experiments showed that this pathway was activated by the eIF2 alpha kinase, GCN2, in an apparent deacylated tRNA-independent fashion. Further we found that the ISR attenuates neurodegeneration in C57BL/6J-Gtpbp2(nmf205-/-) mice, underscoring the importance of cellular and stress context on the outcome of activation of this pathway. These results demonstrate the critical interplay between translation elongation and initiation in regulating neuron survival during cellular stress.