Unfolded protein Response is a major hurdle in direct neuronal reprogramming of human astrocytes

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Mitochondria account for essential cellular pathways, from ATP production to nucleotide metabolism, and their deficits lead to neurological disorders and contribute to the onset of age-related diseases. Direct neuronal reprogramming aims at replacing neurons lost in such conditions, but almost nothing is known about the impact of mitochondrial dysfunction in human cell direct reprogramming. Here we explore the defects and how to improve the neuronal reprogramming of iPSC-derived astrocytes carrying mutations in NDUFS4 gene, important for Complex I and associated to Leigh Sydrome. This led to the identification of the unfolded protein response as a major hurdle in the direct neuronal conversion of not only patient, but also control astrocytes and fibroblasts. Our data highlight mitochondria-ER stress-mediated inhibition of translation as a key hurdle in direct neuronal reprogramming, emphasizing how disease modelling using patient cells can contribute to unravel novel pathways in human glia cell reprogramming.