NIH Research Festival
GRN gene mutations cause loss of expression of the intra-endolysosomal protein progranulin, lysosomal dysfunction, and frontotemporal dementia (FTD) with pathologic aggregates of the ribonuclear protein (RNP)-granule protein TDP-43. Using proximity-labeling proteomics in human iPSC-derived neurons, we found that GRN-KO neurons had diminished endolysosomal recruitment of ANXA11, a nucleocytosolic protein of unknown function recently implicated in familial ALS. FLIM-FRET and photoconversion microscopy studies showed that ANXA11 is dynamically recruited to endolysosomal membranes in a GRN-dependent manner. In addition to C-terminal annexin repeats, which interact with membrane lipids and calcium, ANXA11 has an intrinsically disordered prion-like domain in its N-terminus. Recombinant ANXA11 underwent liquid-liquid phase separation in vitro, consistent with self-organization into into liquid droplets. Live cell FLIP microscopy revealed a stable pool of ANXA11 that co-localized with stress granule markers, including TDP-43, as well as SYTO-tagged RNA. Light-activated phase-separation of ANXA11-cry2 fusion proteins caused bulk redistribution of ANXA11 to endolysosomes, including ANXA11 optodroplets that co-localized with stress-granule markers. Taken together, our findings suggest that the combination of membrane-binding and intrinsically disordered-domains of ANXA11 enable it to function as a molecular bridge between endolysosomal membranes and phase-separated ribonuclear protein complexes, and that loss of GRN-mediated ANXA11 activity may play an important role in FTD pathogenesis.
Scientific Focus Area: Neuroscience
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