Disruption of nucleocytoplasmic transport in SOD1 models of ALS

Mutations in superoxide dismutase (SOD1) cause amyotrophic lateral sclerosis (ALS), a fatal neurodegenerative disease caused by the progressive loss of motor neurons in the brain and spinal cord. Nucleocytoplasmic transport (NCT) dysfunction has been implicated as a possible target for toxicity by several studies in C9orf72, TDP43 and FUS-related ALS as well as in other neurological diseases. However, whether misfolded SOD1, which accumulates in the motor neurons, affects this pathway, is still unknown. Our data using a shuttle-GFP reporter (S-GFP) show that misfolded/mutant SOD1 disrupts the nuclear pore normal function in regulating active import/export from the nucleus in a human neuronal cell line (SH5Y5). Following previous findings, misfolded SOD1 exposes an NES-like sequence, which results in its export from the nucleus via recognition by CRM1. We hypothesize that this association and export of misfolded SOD1 from the nucleus could trigger its contribution to the NCT disruption. Indeed, we showed that SH5Y5 cells transfected to express the double mutant SOD1^G93A/L38R, which mutates the NES-like sequence and subsequently disrupts the binding to CRM1, are protected from NCT disruption. Moreover, we revealed that CRM1 and Ran-Gap1, another key regulator of NCT, are sequestered and mislocalized in the spinal cord of symptomatic mutant SOD1 transgenic mice and mutant SOD1 overexpressing SH5Y5 cells, respectively. Finally, we show that macrophage migration inhibitory factor )MIF) chaperone function can rescue the NCT disruption caused by misfolded SOD1, and we suggest that it may serve as a potential therapeutic candidate targeting misfolded SOD1 toxicity in ALS.