SASP-Secreted MMP-9 from Senescent Microglia Disrupts TDP-43 Nuclear Retention Leading to Cytoplasmic Mislocalization in ALS

The hypothesis proposes that MMP-9 (matrix metalloproteinase-9), secreted via the senescence-associated secretory phenotype (SASP) from senescent microglia, disrupts TDP-43 nuclear retention by degrading nuclear import machinery components and nuclear envelope proteins, leading to pathological cytoplasmic mislocalization of intact TDP-43 that drives ALS pathology. Rather than directly cleaving TDP-43, MMP-9 targets key nuclear transport proteins including importin-α and nucleoporin components, as well as lamin proteins that maintain nuclear envelope integrity. High-strength evidence from TDP-43 ALS mouse models demonstrates that reactive microglia expressing MMP-9 remodel perineuronal nets around motor neurons, and genetic reduction of MMP-9 protects motor neurons from TDP-43-triggered degeneration in the rNLS8 ALS model, consistent with MMP-9 disrupting nuclear-cytoplasmic compartmentalization. Human ALS tissue shows characteristic cytoplasmic TDP-43 mislocalization with nuclear clearance, and MMP-9 substrates include multiple nuclear envelope and transport proteins that could explain this redistribution pattern. Cell-model evidence confirms that cytoplasmic TDP-43 aggregates readily and injures neurons, supporting the pathogenic relevance of mislocalized TDP-43. However, mechanistic specificity remains uncertain: comprehensive biomarker studies show MMP-9 alterations in serum and CSF of ALS patients alongside other neuroinflammatory markers, but direct evidence of MMP-9-mediated nuclear transport disruption is lacking. Furthermore, MMP-9 and TIMP inhibitor elevations occur in multiple motor neuron diseases, suggesting non-ALS-specific effects. The evidence supports MMP-9 involvement in TDP-43 pathology through nuclear-cytoplasmic transport disruption rather than direct proteolysis, but definitive proof of this mechanism in human disease requires demonstration of MMP-9-mediated nuclear import machinery degradation.