Heat shock chaperone HSPB1 regulates cytoplasmic TDP-43 phase separation and liquid-to-gel transition

While acetylated, RNA binding deficient TDP-43 reversibly phase separates within nuclei into complex droplets (anisosomes) comprised of TDP-43-containing liquid outer shells and liquid centers of HSP70 family chaperones, cytoplasmic aggregates of TDP-43 are hallmarks of multiple neurodegenerative diseases, including ALS. Here we show that transient oxidative stress, proteasome inhibition, or inhibition of HSP70’s ATP-dependent chaperone activity provokes reversible cytoplasmic TDP-43 de-mixing and transition from liquid to gel/solid, independent of RNA binding or stress granules. Proxmity labeling coupled with quantitative mass spectrometry is used to identify that phase separated cytoplasmic TDP-43 is bound by the small heat shock protein HSPB1.