Selective and competitive functions of the AAR and UPR pathways in stress-induced angiogenesis

The amino acid response (AAR) and unfolded protein response (UPR) pathways converge on eIF2 alpha phosphorylation, which is catalyzed by Gcn2 and Perk, respectively, under different stresses. This close interconnection makes it difficult to specify different functions of AAR and UPR. Here, we generated a zebrafish model in which loss of threonyl-tRNA synthetase (Tars) induces angiogenesis dependent on Tars aminoacylation activity. Comparative transcriptome analysis of the tars-mutant and wild-type embryos with/without Gcn2- or Perk-inhibition reveals that only Gcn2-mediated AAR is activated in the tars-mutants, whereas Perk functions predominantly in normal development. Mechanistic analysis shows that, while a considerable amount of eIF2 alpha is normally phosphorylated by Perk, the loss of Tars causes an accumulation of uncharged tRNA(Thr), which in turn activates Gcn2, leading to phosphorylation of an extra amount of eIF2 alpha. The partial switchover of kinases for eIF2 alpha largely overwhelms the functions of Perk in normal development. Interestingly, although inhibition of Gcn2 and Perk in this stress condition both can reduce the eIF2 alpha phosphorylation levels, their functional consequences in the regulation of target genes and in the rescue of the angiogenic phenotypes are dramatically different. Indeed, genetic and pharmacological manipulations of these pathways validate that the Gcn2-mediated AAR, but not the Perk-mediated UPR, is required for tars-deficiency induced angiogenesis. Thus, the interconnected AAR and UPR pathways differentially regulate angiogenesis through selective functions and mutual competitions, reflecting the specificity and efficiency of multiple stress response pathways that evolve integrally to enable an organism to sense/respond precisely to various types of stresses.

Automated summary

The study reveals that under a specific stress condition, Gcn2-mediated amino acid response (AAR) is crucial for angiogenesis induced by the loss of threonyl-tRNA synthetase (Tars), while Perk predominantly functions in normal development.