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.