Concentration-dependent cytoplasmic phase separation of TDP-43 drives aggregation and proteinopathy.

TDP-43 mislocalization and aggregation are common features of several neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD). However, the mechanisms underlying the transition of nuclear TDP-43 to cytoplasmic aggregates, and their contribution to disease pathogenesis, remain poorly understood. To address this gap, we present a methodology to chemically control the assembly and disassembly of cytoplasmic TDP-43 condensates. By fusing TDP-43 to a phase separation-prone protein scaffold, we can induce the formation of cytoplasmic TDP-43 condensates or, conversely, promote nuclear localization upon addition of a disassembly molecule. TDP-43 accumulates into various assemblies, ranging from submicrometric puncta to larger aggregate-like structures that display hallmarks of proteinopathy in a concentration-dependent manner. Furthermore, oxidative stress drives the maturation of TDP-43 assemblies from puncta into aggregates through interactions with stress granule components. Finally, we show that cytoplasmic TDP-43 aggregates deplete nuclear endogenous TDP-43 and induce cytotoxicity. Collectively, these findings highlight the local cytoplasmic concentration of TDP-43 and stress exposure as key determinants in the onset of TDP-43 proteinopathy, providing a relevant model to study pathological TDP-43 aggregation.