Mislocalization of FTD3-associated mutant CHMP2B to the nucleus of human neurons due to loss of a nuclear export signal

Frontotemporal dementia linked to chromosome 3 (FTD3) is caused by a splice site point mutation in CHMP2B, resulting in the production of mutant proteins CHMP2B^In5 and CHMP2B^Δ10. Here, we found that wildtype CHMP2B (CHMP2B^WT) is mostly present in the cytoplasm, but CHMP2B^In5 is mislocalized to the nucleus of human induced pluripotent stem cell (iPSC)-derived cortical neurons. To understand the underlying mechanism, we identified a previously unreported nuclear export signal (NES) in the C-terminus of CHMP2B. Functional assays, including CRM1 inhibition and site-directed mutagenesis of key hydrophobic residues, demonstrated that this NES motif is both necessary and sufficient for nuclear export of CHMP2B^WT and ALS-associated CHMP2B^Q206H, and its loss in CHMP2B^In5 is responsible for the observed nuclear mislocalization. CHMP2B^Δ10 remains in the cytoplasm due to the presence of an artificial NES in the C-terminus. These results reveal the presence of an NES in CHMP2B and highlight the need to dissect the gain-of-toxic nuclear functions of CHMP2B^In5 in FTD3 pathogenesis.