We not only found that the ISR
We not only found that the ISR leads to attenuation of conventional cap-dependent translation with no change in RAN translation of the G4C2 expanded repeat, but also that GA translation from the expanded repeat (but not from 5 G4C2 repeats, perhaps because the protein product does not aggregate) is associated with induction of the ISR. These observations suggest that the following events could occur to enhance pathology in C9ORF72-induced ALS and FTD. Oxidative stress as well as misfolded/aggregated DPRs and TDP43 in neural Carbenicillin receptor of ALS and FTD patients could trigger the ISR to phosphorylate eIF2α and favor the use of eIF2A over eIF2α in translation initiation, a step that is critical in translation regulation. This preferential use of eIF2A in translation initiation as a result of phosphorylation of eIF2α could lead to a relative increase in DPR synthesis compared to the synthesis of other proteins dependent on conventional translation from a capped mRNA. The decrease in conventional cap-dependent translation could lead to a decrease in proteins that normally limit DPR aggregate formation. Furthermore, DPR synthesis may increase because various stresses can lead to increases in eIF2A protein (Kwon et al., 2017; Starck et al., 2012, Starck et al., 2016), and because DPRs are reported to interfere with nucleocytoplasmic transport, thereby potentially bringing more TDP-43 into the cytoplasm over time as well as more eIF2A from its normal predominantly nuclear localization to the cytoplasm (Kim et al., 2011). The increase in DPR aggregation over time, especially in non-replicating motor neurons, could then lead to further ER stress, induction of the ISR, as well as further DPR interference with nucleocytoplasmic transport (resulting in more TDP43 aggregation and more eIF2A in the cytoplasm) in a continuing pathogenic cycle. Eventually, when the ISR is overwhelmed by oxidative stress and misfolded proteins, apoptosis could ensue from overexpression of C/EBP homologous protein or HIPK2 (Lee et al., 2016). In addition, UPR/ISR activation of NF-κB could occur, which has been reported to be pathogenic in ALS (Picher-Martel et al., 2015; Swarup et al., 2011). The recognition that eIF2A plays a potentially important role in DPR translation and pathogenicity in ALS and FTD suggests that this initiation factor and the unconventional translation of the G4C2 expanded repeat could be targets for therapy.
Subsequent to the submission of the present study, three other investigations were published describing DPR translation from the C9ORF72 expanded repeat (Cheng et al., 2018; Green et al., 2017; Tabet et al., 2018). There were a number of similarities in all of these publications, but also some differences. Two publications as well as the present study identified the same translation initiation codon for GA, a CUG upstream of the repeats (Green et al., 2017; Tabet et al., 2018). Tabet et al. found that translation of GP and GR was affected by this CUG, suggesting that there was frameshifting. Two publications (Cheng et al., 2018; Green et al., 2017) as well as the present study identified a relationship between DPR translation and the ISR and suggested that there was a self-sustaining feedforward loop. Two of the publications found that translation of the DPRs was cap-dependent (Green et al., 2017; Tabet et al., 2018). Although the third study (Cheng et al., 2018) found that DPR translation was cap-independent, the authors did not rule out the possibility that there may be additional cap-dependent DPR translation.
Conflict of interests
Acknowledgments RPR, YS, GG and KM are funded by a grant from the ALS Association and gifts from Steps 4 Doug, Families Unite Against ALS, John and Patricia McDonald, Gerry Kaufman, Arnold Sarnoff, Susan Warso, and Marc and Barbara Posner. This work was also partly supported by grants to EF from NYSTEM CO29559. AS is currently supported by the People Programme (Marie Curie Actions) of the European Union 7th Framework Programme FP7 under REA grant agreement no. 629861. EF is an Investigator of the Howard Hughes Medical Institute.