Understanding the molecular mechanisms of TDP‐43 neurotoxicity in ALS and FTD to inspire targeted therapies

Abstract ALS and FTD are fatal and incurable neurodegenerative diseases, characterized by accumulation of pathologic forms of RNA‐binding proteins, predominantly TDP‐43. The functional consequences and potential neurotoxic effects of the highly heterogeneous TDP‐43 aggregates observed in postmortem brains are debated. Combining structural protein analysis with cellular systems we uncovered a novel and unexpected mechanism that counteracts pathologic aggregation of TDP‐43. More recently, we described two distinct mechanisms of aggregation resulting in either nuclear of cytoplasmic TDP‐43 aggregates, resembling the neuropathological heterogeneity described in FTD patients and potentially unravels the origins of heterogeneous pathological species occurring in TDP‐43 proteinopathies. We also showed that FTD heterogeneity is associated with alternate pathological TDP‐43 conformations, reminiscent of prion strains. We used advanced microscopy techniques to explore the differences in the physical and seeding properties between these pathological TDP‐43 aggregates. We found that the subcellular environment and organization of TDP‐43 aggregates differs in patient brains in a subtype‐specific manner. When isolated and introduced in cells, these distinct TDP‐43 assemblies triggered neoaggregate formation with seeding potencies and morphologies that mimicked human brain pathology. To study neurodegeneration phenotypes in human neurons, we developed a new methodology for generating human neuronal networks with remarkable maturity, longevity and reproducibility. Combining high density multielectron arrays and single cell RNA sequencing, we demonstrated the potential of these cultures for modeling neurodegeneration. In this new model, we identified novel RNA targets of TDP‐43, which we found abnormally accumulated in neurons with TDP‐43 pathology in human brains of ALS and FTD patients. The implications of these studies for understanding the mechanisms of neurodegeneration and inspiring therapeutic approaches will be discussed.