Phosphosite mapping on Vimentin under hypotonic stress

Giant Axonal Neuropathy (GAN) is a pediatric neurodegenerative disease caused by loss-of-function mutations in the E3 ubiquitin ligase adaptor gigaxonin, which is encoded by the GAN (KLHL16) gene. Gigaxonin regulates the degradation of multiple intermediate filament (IF) proteins, including neurofilaments, peripherin, GFAP, and vimentin, which aggregate in GAN patient cells. Understanding how IFs and their aggregates are processed under stress can reveal new GAN disease mechanisms and potential targets for therapy. Here we tested the hypothesis that hypotonic stress-induced vimentin proteolysis is impaired in GAN. In this mass spectrometry-based proteomics analysis, phosphorylation at Ser-412, located at the junction between the central “rod” domain and C-terminal “tail” domain on vimentin, was identified to be involved in this stress response. Over-expression studies using either phospho-deficient (S412A) or phospho-mimic (S412D) mutants revealed that Ser-412 is important for filament organization, solubility dynamics, and cleavage of vimentin upon hypotonic stress exposure. Collectively, our work reveals that osmotic stress induces calpain- and proteasome-mediated vimentin degradation and IF network breakdown.