[Heterogeneity in the regulation of cellular stress responses by FUS gene mutations associated with amyotrophic lateral sclerosis].

OBJECTIVES: Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease characterized by the selective death of motor neurons, exhibiting marked clinical heterogeneity and lacking effective treatment. The etiology and pathogenic mechanisms remain incompletely understood. The FUS (fused in sarcoma) gene is one of the key causative genes in ALS. Pathogenic mutations in the encoded protein are predominantly clustered in the C-terminal nuclear localization signal (NLS) region, and distinct NLS mutation sites show considerable differences in pathogenic potency, clinical phenotypes, and molecular mechanisms. This study focuses on 2 representative pathogenic NLS mutations of FUS (FUSR514S and FUSP525L) to investigate their differential regulation of cellular stress responses and explore the underlying mechanisms.

METHODS: Multiple sequence alignment of FUS protein homologs from 12 species was performed using an online tool from the National Center for Biotechnology Information (NCBI) to determine the evolutionary conservation of residues R514 and P525. The three-dimensional (3D) structure of the nuclear transport receptor-FUS complex [Protein Data Bank (PDB) ID: 5YVG] was analyzed and visualized using PyMOL. Structure of FUS mutants were generated using the mutation wizard tool in PyMOL by selecting the target conformational isomer and executing the mutation workflow. Tet-on inducible expression cell models for FUS wild-type (WT) and mutant FUS (FUSR514SS and FUSP525L) were established in human embryonic kidney 293T (HEK293T) cells. Protein expression levels and subcellular localization of FUS were assessed by Western blotting and immunofluorescence assay, respectively. FUS aggregation states were compared between WT and mutant FUS using a digitonin-based permeabilization and extraction assay, followed by sodium dodecylsulfate-polyacrylamide gel electrophoresis-polyacrylamide gel electrophoresis (SDS-PAGE) and Western blotting analysis. Blue native PAGE (BN-PAGE) was used to evaluate the stability of FUS-containing complexes. Mitochondrial membrane potential and reactive oxygen species (ROS) levels were measured by flow cytometry. Stress granule (SG) formation was induced using sodium arsenite, and the effects of WT and mutant FUS on SG dynamics were analyzed by immunofluorescence assay. Protein expression changes of mitochondrial function-related proteins [translocase of outer membrane 20 kD subunit (Tom20) and voltage-dependent anion channel 1 (VDAC1)] and key molecules of the integrated stress response (ISR) pathway [phosphorylated-eukaryotic initiation factor 2 alpha (p-eIF2α) and activating transcription factor 4 (ATF4)] were examined by Western blotting.

RESULTS: Sequence alignment revealed that R514 and P525 are highly conserved across FUS homologs from 12 species. Structural analysis indicated that the FUSR514S and FUSP525L mutations disrupt hydrogen bonding or hydrophobic interactions between FUS and importin-β2, weakening the stability of these interactions. Western blotting confirmed the successful establishment of inducible WT and mutant FUS expression cell models, and exogenous FUS expression slightly suppressed endogenous FUS protein levels. Immunofluorescence assay demonstrated that WT FUS is predominantly localized in the nucleus, whereas both FUSR514S and FUSP525L mutants mislocalize to the cytoplasm with a punctate, granular distribution. Compared with WT FUS, neither mutant significantly affected mitochondrial membrane potential, ROS levels, or the homeostasis of mitochondrial function-related proteins (all P>0.05). Upon sodium arsenite exposure, mutant FUS formed SGs more rapidly, generated SGs with larger diameters, and displayed distinct intracellular distribution and aggregation patterns relative to WT (P>0.05). After drug withdrawal, WT and mutant FUS showed no significant difference in their effects on SG disassembly (P<0.05). Under basal conditions, FUSR514S exhibited significantly higher eIF2α phosphorylation levels than WT, and ATF4 protein levels also showed an increasing trend (P<0.05). No statistically significant difference was observed between FUSP525L and WT FUS in these measures (P>0.05). Sodium arsenite treatment increased eIF2α phosphorylation across all groups, eliminating inter-mutant differences.

CONCLUSIONS: Distinct pathogenic NLS mutations of FUS differentially regulate cellular stress responses through different mechanisms, contributing to ALS initiation and progression. Among these, FUSP525L promotes the formation of larger stress granules, whereas FUSR514S more readily activates the cellular ISR.