LRSAM1, an E3 ubiquitin ligase, modulates TDP-43 proteostasis in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis is a fatal neurodegenerative disease characterized by the cytoplasmic mislocalization, aberrant phosphorylation and pathological aggregation of TDP-43, a nuclear RNA-binding protein essential for RNA metabolism. Despite its central involvement in ALS pathogenesis, the molecular mechanisms that govern TDP-43 proteostasis remain incompletely understood. Here, we investigated the role of LRSAM1, an E3 ubiquitin ligase, in regulating TDP-43 localization and degradation. Postmortem spinal cord tissues from five patients with sporadic amyotrophic lateral sclerosis and five healthy controls were examined. Immunohistochemical analyses revealed that LRSAM1 preferentially colocalized with diffusely mislocalized cytoplasmic TDP-43 but not with dense cytoplasmic aggregates, suggesting a potential role in early-stage pathological processing. To model these effects in vitro, induced pluripotent stem cell-derived motor neurons from three patients with amyotrophic lateral sclerosis and four healthy control participants were used. Knockdown of LRSAM1 in induced pluripotent stem cell-derived motor neurons from patients with amyotrophic lateral sclerosis significantly increased cytoplasmic TDP-43 accumulation and phosphorylation, as measured by immunofluorescence imaging. Moreover, STMN2 mRNA splicing, a well-established TDP-43 functional readout, was disrupted upon LRSAM1 depletion. Conversely, overexpression of LRSAM1 ameliorated stress-induced TDP-43 mislocalization and phosphorylation in induced pluripotent stem cell-derived motor neurons from patients with amyotrophic lateral sclerosis but had minimal impact in control cells, indicating disease-specific vulnerability. Mechanistically, LRSAM1 interacted directly with the aggregation-prone C-terminal fragments of TDP-43 via the RRM2 domain of TDP-43 and promoted their proteasomal degradation. This selective interaction reduced the accumulation of toxic TDP-43 species and supported cytoplasmic proteostasis, particularly under conditions of cellular stress. In summary, our study identifies LRSAM1 as a novel modulator of cytoplasmic TDP-43 dynamics in amyotrophic lateral sclerosis. By facilitating degradation of pathogenic TDP-43 fragments, LRSAM1 preserves neuronal RNA processing and mitigates molecular hallmarks of amyotrophic lateral sclerosis. These findings enhance our understanding of post-translational control of TDP-43 and highlight LRSAM1 as a promising therapeutic target for modifying disease progression in amyotrophic lateral sclerosis.