Small molecule ISRIB suppresses the integrated stress response within a defined window of activation
Significance The integrated stress response (ISR) protects cells from a variety of harmful stressors by temporarily halting protein synthesis. However, chronic ISR activation has pathological consequences and is linked to several neurological disorders. Pharmacological inhibition of chronic ISR activity emerges as a powerful strategy to treat ISR mediated neurodegeneration but is typically linked to adverse effects due to the ISR's importance for normal cellular function. Paradoxically, the small molecule ISR inhibitor ISRIB has promising therapeutic potential in vivo without overt side effects. We demonstrate here that ISRIB inhibits low level ISR activity, but does not affect strong ISR signaling. We thereby provide a plausible mechanism of how ISRIB counteracts toxic chronic ISR activity, without disturbing the cytoprotective effects of a strong acute ISR. Activation of the integrated stress response (ISR) by a variety of stresses triggers phosphorylation of the a subunit of translation initiation factor eIF2. P eIF2a inhibits eIF2B, the guanine nucleotide exchange factor that recycles inactive eIF2*GDP to active eIF2*GTP. eIF2 phosphorylation thereby represses translation. Persistent activation of the ISR has been linked to the development of several neurological disorders, and modulation of the ISR promises new therapeutic strategies. Recently, a small molecule ISR inhibitor (ISRIB) was identified that rescues translation in the presence of P eIF2a by facilitating the assembly of more active eIF2B. ISRIB enhances cognitive memory processes and has therapeutic effects in brain injured mice without displaying overt side effects. While using ISRIB to investigate the ISR in picornavirus infected cells, we observed that ISRIB rescued translation early in infection when P eIF2a levels were low, but not late in infection when P eIF2a levels were high. By treating cells with varying concentrations of poly(I:C) or arsenite to induce the ISR, we provide additional proof that ISRIB is unable to inhibit the ISR when intracellular P eIF2a concentrations exceed a critical threshold level. Together, our data demonstrate that the effects of pharmacological activation of eIF2B are tuned by P eIF2a concentration. Thus, ISRIB can mitigate undesirable outcomes of low level ISR activation that may manifest neurological disease but leaves the cytoprotective effects of acute ISR activation intact. The insensitivity of cells to ISRIB during acute ISR may explain why ISRIB does not cause overt toxic side effects in vivo.
Proceedings of the National Academy of Sciences