TDP-43 is a nuclear protein that can bind RNA and have an important role in regulating various RNA-related biological processes. Under disease conditions, such as amyotrophic lateral sclerosis (ALS) and frontotemporal dementia, TDP-43 translocates from the nucleus to the cytoplasm and forms aggregates. However, the precise locations and mechanisms behind TDP-43 aggregation remain unclear.

Now, Yan et al. have found that TDP-43 aggregation begins with TDP-43 demixing in stress granules, followed by a liquid-to-solid transition into aggregates. Using a cytoplasmic expression system, the team revealed that TDP-43 has different modes of distribution in the cytoplasm, which vary depending on its concentration. At low concentrations, TDP-43 partitions into stress granules, and so is kept in a dispersed state. However, once a critical concentration threshold is exceeded, TDP-43 in stress granules undergoes intra-condensate demixing, leading to the formation of a TDP-43-rich phase, which ages over time and eventually forms aggregates. This demixing process is facilitated by two key factors: the enrichment provided by stress granules and oxidative stress. Yan et al. showed that TDP-43 failed to form aggregates in cells that cannot form stress granules. Furthermore, that oxidative stress induces TDP-43 intermolecular interactions, mainly mediated by the two cysteine residues in the RRM1 domain and the hydrophobic patch in the C-terminal domain. TDP-43 variants with mutations in these residue positions or lacking the hydrophobic patch region were also unable to form aggregates.