Upstream open reading frames differentially regulate genespecific translation in the integrated stress response

Abstract

Gene expression is a highly coordinated process that relies upon appropriate regulation of translation for protein homeostasis. Regulation of protein synthesis largely occurs at the initiation step in which the translational start site is selected by ribosomes and associated initiating factors. In addition to the coding sequences (CDS) for protein products, short upstream open reading frames (uORFs) located in the 5’-leader of mRNAs are selected for translation initiation. While uORFs are largely considered to be inhibitory to translation at the downstream CDS, uORFs can also promote initiation of CDS translation in response to environmental stresses. Multiple transcripts associated with stress adaptation are preferentially translated through uORF-mediated mechanisms during activation of the Integrated Stress Response (ISR). In the ISR, phosphorylation of α subunit of the translation initiation factor eIF2α (eIF2α~P) during environmental stresses results in a global reduction in protein synthesis that functions to conserve energy and nutrient resources and facilitate reprogramming of gene expression. Many key regulators of the ISR network are subject to preferential translation in the response to eIF2α-P. These preferentially translated genes include the pro-apoptotic transcriptional activator Chop that modifies gene expression programs, feedback regulator Gadd34 that targets the catalytic subunit of protein phosphatase 1 to dephosphorylate eIF2α~P, and glutamyl-prolyl tRNA synthetase Eprs that increases the charged tRNA pool and primes the cell for resumption of protein synthesis after stress remediation. Ribosome bypass of at least one inhibitory uORF is a common theme between Chop, Gadd34, and Eprs, which allows for their regulated expression in response to cellular stress. However, different features encoded within the uORFs of the Chop, Gadd34, and Eprs mRNAs provide for regulation of their inhibitory functions, illustrating the complexities of uORF-mediated regulation of gene-specific translation. Importantly, preferentially translated ISR targets can also be transcriptionally regulated in response to cellular stress and misregulation of transcriptional or translational expression of Gadd34 can elicit maladaptive cell responses that contribute to disease. These mechanisms of translation control are conserved throughout species, emphasizing the importance of translation control in appropriate gene expression and the maintenance of protein homeostasis and health in diverse cellular conditions.