Human Motor Neurons Elicit Pathological Hallmarks of ALS and Reveal Potential Biomarkers of the Disease in Response to Prolonged IFNγ Exposure

Amyotrophic lateral sclerosis (ALS) is a debilitating neurodegenerative disorder marked by progressive motor neuron degeneration and muscle denervation. A recent transcriptomic study integrating a wide range of human ALS samples revealed that upregulation of p53, a downstream target of inflammatory stress, is commonly detected in familial and sporadic ALS cases by a mechanism linked to TDP-43 dysfunction. In this study, we show that prolonged IFNγ treatment of human iPSC-derived spinal motor neurons results in severe cytoplasmic aggregation of TDP-43. Either TDP-43 dysfunction resulting from IFNγ exposure or from an ALS-associated TDP-43 mutation was associated with activation of the p53 pathway. This was accompanied by hyperactivation of neuronal firing, followed by the complete loss of their electrophysiological function. Through comparative single-cell transcriptome analysis, we have identified significant alterations in ALS-associated genes in motor neurons exposed to IFNγ, implicating their direct involvement in ALS pathology. Interestingly, IFNγ was found to induce significant levels of PD-L1 expression in motor neurons without affecting the levels of any other immune checkpoint proteins. This finding suggests a potential role for excessive PD-L1 expression in ALS development, given that PD-L1 was recently reported to impair neuronal firing ability in mice. Our findings suggest that exposing motor neurons to IFNγ could directly derive ALS pathogenesis, even without the presence of inherent genetic mutation or functional glia component. Furthermore, this study provides a comprehensive list of potential candidate genes for future immunotherapeutic targets with which to treat sporadic forms of ALS, which account for 90% of all reported cases. Significance statement ALS is currently an incurable neurodegenerative disease that primarily damages motor function. 90% of the reported cases have unknown reasons, but their progression is extremely fast once triggered. A pathologic hallmark of ALS is an aggregation of RNA/DNA binding protein, TDP-43, but its pathologic link to the disease is yet to be elucidated. In this study, we found the Interferon-gamma (IFNγ), an immune-derived cytokine activates p53 pathway which appears in common in post-mortem ALS tissue. Importantly, IFNγ triggered a significant cytoplasmic TDP-43 aggregation and impaired electrophysiological function of human motor neurons. Furthermore, we found that ALS risk genes related to the mitochondrial dysfunction are aberrantly expressed under the IFNγ exposure, which constitutes potential therapeutic targets of immune-dysregulated neurodegeneration in ALS.