Plasticity of the Mammalian Integrated Stress Response [eIF2B5_eIF4E]

Decreased nucleotide exchange activity of the eukaryotic translation initiation factor eIF2B coupled with increased phosphorylation of eIF2alpha (eIF2alpha-p) is a hallmark of the “canonical” integrated stress response (c-ISR). In mammals, however, it is unclear whether decreased eIF2B activity in absence of alterations in eIF2alpha-p which occurs in human disease including leukodystrophies, is synonymous to c-ISR. Herein, we describe a previously unknown mechanism of adaptation to decreased eIF2B activity, distinct from c-ISR, which we term “split” ISR (s-ISR). We demonstrate that s-ISR comprises translation reprogramming of only a subset of c-ISR mRNA targets which is accompanied by distinct transcriptomes. In contrast to c-ISR, s-ISR requires eIF4E-dependent translation of the upstream open reading frame 1 and subsequent stabilization of ATF4 mRNA. This is followed by altered expression of a subset of metabolic genes (e.g., PCK2), resulting in metabolic adaptations to maintain cellular bioenergetics under conditions of low eIF2B activity. Overall, these data demonstrate hitherto-unappreciated plasticity of the mammalian ISR, whereby the loss of eIF2B activity in the absence of increased eIF2-p, activates an eIF4E/ATF4/PCK2 axis to maintain energy homeostasis.