Uncovering Molecular Mechanisms of C9orf72- and NEK1-related ALS Pathogenesis Using iPSC-Derived Motor Neurons

Daley, Elizabeth

doi:https://doi.org/10.21985/n2-m7t0-kf15

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Uncovering Molecular Mechanisms of C9orf72- and NEK1-related ALS Pathogenesis Using iPSC-Derived Motor Neurons

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Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disease characterized by motor neuron (MN) degeneration and resulting in progressive paralysis and death. ALS is genetically heterogeneous, disease pathophysiology is not completely understood, and there are no effective drug therapies. To develop broadly applicable therapeutics, we examine disease mechanisms in the context of specific causal genes to identify convergent pathways in MN dysfunction.The first part of this work interrogates the subcellular redistribution of proteins due to the presence of the hexanucleotide repeat expansion (HRE) in the first intron of the C9orf72 (C9) gene, which is the most common genetic cause of ALS. Among identified redistributed proteins was eRF1, which regulates translation termination and RNA degradation through nonsense-mediated decay (NMD). We found that eRF1 accumulates within elaborate nuclear envelope invaginations in patient induced pluripotent stem cell (iPSC) neurons and postmortem tissue and mediates a protective shift from protein translation to NMD-dependent mRNA degradation. These findings highlight eRF1 and NMD as therapeutic targets in C9-ALS. The second part of this work interrogates how mutations in the NIMA-related kinase 1 (NEK1) gene cause MN dysfunction. Here, we used proteomics to identify 15 unique NEK1 interactors, enriched for function in the cytoskeleton, nucleocytoplasmic transport, and proteostasis, three pathways highly associated with ALS pathogenesis. We identified defects in the microtubule (MT) network and the nucleocytoplasmic transport pathway using two iPSC-neuron models of NEK1 haploinsufficiency. Critically, we found that the addition of MT-stabilizing drug paclitaxel restored these defects in both models. Our findings highlight the relationship between two salient features of neurodegenerative disease, and we propose renewed efforts to target MT homeostasis as an early and primary event in ALS pathophysiology. Collectively, this work supports the hypothesis that there are converging mechanisms of MN pathophysiology, and future research should pursue therapeutic targets that are relevant across subtypes. This work also supports the view that iPSC-neuron platforms are an invaluable tool for deciphering convergent disease mechanisms, enabling studies such as this to be rapidly tested in different ALS subtypes and sporadic ALS cases.

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