NOMIS Awardee Don Cleveland and colleagues have identified how phase separation of the RNA-binding protein TDP-43 can be regulated through RNA binding, disease-causing mutation, posttranslational modification, or chaperone activity inside cells.
The RNA binding protein TDP-43 forms intranuclear or cytoplasmic aggregates in age-related neurodegenerative diseases. In this NOMIS-supported study, Cleveland and colleagues found that RNA binding-deficient TDP-43 (produced by neurodegeneration-causing mutations or posttranslational acetylation in its RNA recognition motifs) drove TDP-43 demixing into intranuclear liquid spherical shells with liquid cores. These droplets, which the authors named “anisosomes”, have shells that exhibit birefringence, thus indicating liquid crystal formation. Guided by mathematical modeling, the research team identified the primary components of the liquid core to be HSP70 family chaperones, whose adenosine triphosphate (ATP)-dependent activity maintained the liquidity of shells and cores. In vivo proteasome inhibition within neurons, to mimic aging-related reduction of proteasome activity, induced TDP-43-containing anisosomes. These structures converted to aggregates when ATP levels were reduced. Thus, acetylation, HSP70, and proteasome activities regulate TDP-43 phase separation and conversion into a gel or solid phase.
RNA binding protein TDP-43 forms aggregates in degenerating neurons, a pathological feature associated with aging, genetic, and/or environmental factors. Although naturally demixed, RNA binding–proficient TDP-43 is largely soluble in the nucleus, with a small proportion demixed (A and B). Its RNA affinity is eliminated by acetylation, which drives most TDP-43 into anisosomes—intranuclear membraneless compartments with symmetrically aligned shells and cores in which RNA-free TDP-43 is enriched in the shells. HSP70 stabilizes RNA-free TDP-43 and is enriched in the anisosomal core (C). When ATP-dependent chaperone activity of HSP70 is reduced by ATP depletion, TDP-43 anisosomes collapse into gels (D), which may be precursors of intranuclear and cytoplasmic aggregates observed in degenerating neurons (E).
The study, “HSP70 chaperones RNA-free TDP-43 into anisotropic intranuclear liquid spherical shells,” was published in Science on Feb. 5, 2021.
