Regulation of UsnRNP trafficking by the Integrated Stress Response is compromised by mutant ALS proteins

Activation of the integrated stress response (ISR), alterations in nucleo-cytoplasmic transport and changes in alternative splicing regulation are all involved in the pathogenesis of Amyotrophic Lateral Sclerosis (ALS). However, whether these processes act independently from each other, or are part of a coordinated mechanism of gene expression regulation that is affected in pathogenic conditions, is still rather undefined. To answer these questions, in this work we set out to characterise the functional connections existing between ISR activation and nucleo-cytosol trafficking and nuclear localization of spliceosomal U-rich small nuclear ribonucleoproteins (UsnRNPs), the core constituents of the spliceosome. Activation of the ISR induces a profound reorganization of nuclear Gems and Cajal bodies, membrane-less particles where the SMN complex and assembled UsnRNPs are accumulated for further maturation or storage, and to changes in alternative splicing. Notably, these effects are reversed by both inhibiting the ISR or modulating UsnRNP import receptors, indicating that the regulation of nucleo-cytoplasmic trafficking of UsnRNPs might control alternative splicing in response to stress. Importantly, dismantling of nuclear Gems and Cajal bodies by ALS-linked mutant proteins is not halted by inhibition of the ISR. This indicates that changes in the nuclear localization of the UsnRNP complexes promoted by ALS proteins are uncoupled from ISR activation, and that defects in the nucleo-cytoplasmic trafficking of UsnRNPs might play a role in ALS pathogenesis